Cheat Sheet

For some good general notes on designing spacecraft in general, read Rick Robinson's Rocketpunk Manifesto essay on Spaceship Design 101. Also worth reading are Rick's essays on constructing things in space and the price of a spaceship.

For some good general notes on making a fusion powered spacecraft, you might want to read Application of Recommended Design Practices for Conceptual Nuclear Fusion Space Propulsion Systems. There are also some nice examples on the Realistic Designs page.

For less scientifically accurate spacecraft design the Constant Variantions blog has a nice article on historical trends in science fiction spacecraft design.

Like any other living system, the internal operations of a spacecraft can be analyzed with Living Systems Theory, to discover sources of interesting plot complications.


The improvised space warcraft are the type that seems to hold the most story potential.  These would, as mentioned, likely be built by colonies that are in conflict.  As they do not have to operate in an atmosphere, and are built by relatively poor colonies, they are likely to be rather crude.  The basic components required are structure, propulsion, weapons, life support, power, sensors, control, and communications, and each will be briefly discussed in turn.

There are two methods of assembling an improvised warcraft, either adapting an existing vessel, or constructing a new one from parts.  The use of an existing vessel removes the need for some, though not all, of the various components.  The structure will obviously be preserved, and propulsion and life support are almost certain to remain unchanged as well.  Weapons would obviously be added, along with their associated control equipment.  Existing power supplies might or might not be sufficient, depending on the weapons fit chosen.  Sensors and communications are gray areas, depending on the tasks required of the vessel, and the existing fits in these areas.

Structure is one of the easiest components to create.  So long as the builder does not mind the craft being somewhat heavy, slapping a few beams together should be sufficient.  Existing structures, such as cargo containers, could easily be modified, or simple new ones fabricated.  In any case, this is not likely to be a driving factor in construction or conversion of vessels.  Any group incapable of creating basic structure is also almost certainly incapable of surviving if it were to win a rebellion.

Propulsion for improvised craft is likely to be chemfuel due to the fact that it is by far the simplest to implement, and has sufficient delta-V for any operations that do not involve transiting deep space.  It is entirely possible that a colony will have standard chemfuel engines used in various places, and one of them, with appropriate fuel tanks, would be fitted to the vessel.  Nuclear propulsion is much more expensive, and might well involve detailed engineering to avoid killing the crew.  The performance advantage over chemfuel for nuclear-thermal is probably not significant enough to justify the problems involved, and nuclear-electric is only practical for vessels intended for deep space use.  

Weapons are a tricky issue. These are likely to be improvised as well, and would fall into the same categories already discussed.  Improvised lasers are highly unlikely.  Industrial lasers lack the optics trains required for weapon use, while any optical trains available (probably from astronomical or other scientific sources) are unlikely to be able to handle the high powers output by the lasers.  With some time, an appropriate optical train could be designed and mated to an industrial laser, and it is even possible that colonies could design and test such things in case of war.  Kinetic projectors are in largely the same boat.  While small mass drivers could be adapted to such a task, it is difficult to see a role for such devices on a colony.  There is also the issue of targeting, which, while not insoluble, requires good pointing accuracy and possibly the creation of guided projectiles, which have even less peacetime use then the launchers themselves.  

This leaves three options, missiles, lancers, and unguided kinetics.  Unguided kinetics can be as simple as junk thrown out of the airlock, but they are of very limited effectiveness, as shown in Section 8.  Missiles and lancer projectiles face many of the same issues, and the only practical difference is the motor, which should be relatively easy to improvise.  A missile or lancer will require sensors, thrusters, and guidance logic of its own.  As this force is presumably facing another more or less improvised one, complicated guidance logic is probably unnecessary, and proportional navigation is quite easy to implement.  The sensor might well be adapted from another role, which means that the likely problem is in the thrusters.  These must be well-balanced and integrated with the guidance logic.  Depending upon the materials available, this could range from very easy (if there are large numbers of small, self-propelled objects that can be used as warheads lying around) to extremely difficult (if the entire system must be designed from scratch).  Small thrusters themselves are an unknown.  There are some systems that might use small thrusters, such as thruster packs for spacewalkers, in which case the actual integration is all that is required.  However, the number available might well be strictly limited, forcing the builder to start from scratch.  Note that this is not as easy as it seems.  While a primitive kinetic could probably be built with the simplest of systems, it would be inefficient, of dubious reliability, and probably quite large.  In the end, this is an area in which the specific situation plays a very large role, leaving us unable to anticipate exactly what might occur.  

Life support should be straightforward to build into a vessel.  Any space colony will undoubtedly have small, portable habs that can be used for surface expeditions or what have you.  Mounting one of those would be relatively simple, and the actual mechanisms for short-term life support are fairly rudimentary, easing implementation if for some reason a hab had to be constructed from scratch.

Power is a rather tricky proposition.  Unless a nuclear propulsion system is used, power is likely to be at a premium.  Most non-nuclear power studies assume that solar panels will be used, but these have significant drawbacks for space warfare.  The biggest problem is that solar panels are vulnerable to damage from opponent’s lasers or powder weapons, and cannot be angled for protection, unlike radiators.  Radiators, discussed in Section 7, are both somewhat less vulnerable to damage, and can be kept edge-on to the enemy.  A clever opponent could manage to create a dilemma between getting power and preserving the panels from damage.  Alternatives include fuel cells and batteries.  Fuel cells are the current solution for short-duration spacecraft, due to their possessing higher specific energy than batteries.  The problem is that fuel cells are somewhat involved to manufacture and are not likely to be common on space colonies, unless the colony is far enough from the Sun that solar panels are not effective.  Batteries are somewhat more likely to exist, but are heavy for their power output.  The only bright side is that a truly improvised spacecraft is unlikely to need much in the way of power, particularly when compared to a nuclear-electric laserstar.

Sensors are probably the biggest unknown.  A proper space warcraft needs some form of active sensor to localize the enemy, although it is possible that a simple passive sensor would be adequate for simple missiles.  The sensors might or might not be readily available.  Sensor suites for existing spacecraft are the most likely source, although cobbling sensor suites together from other uses might be possible.

Control is mostly a matter of systems integration.  Depending on the nature of the systems involved, control setup could range from simple running of cables and plugging together a few modules, to having to write all of the code to make everything talk, or simply doing without an integrated control system.  While it is certain that some systems will have to talk (sensors and weapons spring to mind) a large portion of the integration could be skipped, with a resulting loss of efficiency due to the crew having to move around.

Communications is fairly simple, as one thing people in space will have to do is talk.  This should ensure the availability of communications modules, which can be attached to the vessel.  The most likely cause of problems is lack of strong encryption and particularly electronic warfare capability in such modules.  Depending upon the capability of the opposition, this may or may not be a serious hindrance.  The encryption capabilities should be a reasonably simple fix, involving mostly software updates.  The EW work will be harder, as there are likely to be physical changes required to ensure freedom from interference by enemy radio transmissions.  This is not likely to be a problem if the communications module is radio-based.

by Byron Coffey (2016)


RocketCat sez

This is the living breathing core of all rocket design. Delta Vee equals Vee Ee times Natural Log of Arr. This is the secret that makes rocket design possible. Now it is time to see the practical application of the key to rocketry.

Everything about fundamental spacecraft design revolves around the Tsiolkovsky rocket equation.

Δv = Ve * ln[R]

The variables are the velocity change required by the mission (Δv or delta-V), the propulsion system's exhaust velocity (Ve), and the spacecraft's mass ratio (R). Remember the mass ratio is the spacecraft's wet mass (mass fully loaded with propellant) divided by the dry mass (mass with empty propellant tanks).

The point is you want as high a delta-V as you can possibly get. The higher the delta-V, the more types of missions the spacecraft will be able to perform. If the delta-V is too low the spacecraft will not be able to perform any useful missions at all.

Looking at the equation, the two obvious ways of increasing the delta-V is to increase the exhaust velocity or increase the mass ratio. Or both. Turns out there are two more sneaky ways of dealing with the problem which we will get to in a moment.

Historically, the first approach has been increasing the exhaust velocity by inventing more and more powerful rocket engines. Unfortunately for the anti-nuclear people, chemical propulsion exhaust velocity has pretty much hit the theoretical maximum. The only way to increase exhaust velocity is by using rockets powered by nuclear energy or by power sources even more frightful and ecologically unsound. And you ain't gonna be able to run a large thrust ion-drive with solar cells.

The second approach is increasing the mass ratio by reducing the spacecraft's dry mass. This is the source of the rule below Every Gram Counts. Remember that the dry mass includes a spacecraft's structure, propellant tankage, lifesystem, crewmembers, consumables (food, water, and air), hydroponics tanks, cargo, atomic missiles, toilet paper, clothing, space suits, dental floss, kitty litter for the ship's cat, the ship's cat itself, and other ship systems. Everything that is not propellant, in other words. All of it will have to be trimmed.

To reduce dry mass: use lightweight titanium instead of heavy steel, shave all structural members as thin as possible while also using lightening holes, make the propellant tanks little more than foil balloons, use inflatable structures, make the floors open mesh gratings instead of solid sheets, hire short and skinny astronauts, use life support systems that recycle, impose draconian limits on the mass each crewperson is allowed for personal items, and so on. Other tricks include using Beamed Power so that the spacecraft does not carry the mass of an on-board power plant, and avoiding the mass of a habitat module by hitching a ride on an Aldrin Cycler. Finally the effective mass ratio can be increased by multi-staging but that should be reserved for when you are really desperate.

The third approach is trying to reduce the delta-V required by the mission. Use Hohmann minimum energy orbits. If the destination planet has an atmosphere, use aerobraking instead of delta-V. Get more delta-V for free by exploiting the Oberth Effect, that is, do your burns while very close to a planet. Instead of paying delta-V for shifting the spacecraft's trajectory or velocity, use gravitational slingshots. NASA uses all of these techniques heavily.

If your technology is high enough, use space tethers, launch catapults, and MagBeams.

The fourth and most extreme approach is to cheat the equation itself, to make the entire equation not relevant to the spacecraft. The equation assumes that the spacecraft is carrying all the propellant needed for the mission, this can be bent several ways. Use Sail Propulsion which does not use propellant at all. Use propellant depots and in-situ resource utilization to refuel in mid-mission. The extreme case of ISRU is the Bussard Ramjet which scoops up propellant from the thin interstellar medium, but that only works past the speed of 1% lightspeed or so.

In our Polaris example, given the mass ratio of 3, we know that the Polaris is 66% propellant and 33% everything else. Give the total mass of 1188.9 tons means 792.6 tons of propellant and 396.3 tons of everything else. Since each GC engine is 30 tons, that means 150 tons of engine and 246.3 of everything else.

Every Gram Counts

RocketCat sez

Listen up, rocket designers. Write these words in letters of fire on your cerebellum. Every Gram Counts! Add an extra gram and you will pay for it with extra propellant as if the Mafia loan shark wants you to pay up with liquid hydrogen. With interest compounded hourly.

The only exception is if you are dealing with an Orion drive spaceship or other torchship.

The most fundamental constraint on designing a rocket-propelled vehicle is Every Gram Counts.

Why? Short answer: This is a consequence of the equation for delta-V.

Why? Slightly longer answer: As a general rule, a rocket with the highest delta-V capacity is going to need three kilograms of propellant for every kilogram of rocket+payload. The lower the total kilograms of rocket+payload, the lower the propellant mass required. This relates to the second strategy of rocket design mentioned above.

Why? Long Answer:

Say the mission needs 5 km/s of delta-V. Each kilogram of payload requires propellant to give it 5 km/s.

But that propellant has mass as well. The propellant needed for that original kilogram of payload will require a second slug of propellant so that it too can be delta-Ved to 5 km/s.

And the second slug of propellant has mass as well, so you'll need a third slug of propellant for the second slug of propellant — you see how it gets expensive fast. So you want to minimize the payload mass as much as possible or you will be paying through the nose with propellant.

This is called The Tyranny of the Rocket Equation.

Even worse, for a given propulsion system, the easiest way to increase the delta-V you can get out of that system is by increasing the mass ratio. It probably is not economical to push the mass ratio above 4.0, which translates into 3 kg of propellant for every 1 kg of rocket+payload. And it is nearly impossible to push the mass ratio above 20. Translation: spacecraft with a mass ratio of 20 or above are basically constructed out of gossamer and soap bubbles.

This is why rocket designers are always looking for ways to conserve mass.

Orion drive spacecraft and other torchships are not subject to this constraint, because they are unreasonably powerful.

It also does not apply to "stationary" items such as space stations and planetary bases, since they do not move under rocket propulsion. In fact, the added mass might be useful to stablize a space station's orbit, or as additional radiation shielding. Rocket vehicles might use aluminium, titanium, magnesium, or other lightweight metal as their structural material; but a space station would be better off using heavy iron or Invar.

The only consideration is if the station or base components have to be transported to the desired site by a rocket-propelled transport. Then it makes sense to make the components low mass, because then the station bits are payload. It makes even more sense to construct the space station or base on site using in-situ resources, so you don't have to eat the transport costs at all.

Everything Is Connected

Like aircraft and sea-going warship design, one soon discovers that everything is connected to everything else. When the designer changes one aspect of the design this causes a series of related changes to ripple through the rest of the design.

For instance, if the designer reduces the propellant tank capacity by 5% this has implications for the spacecraft's mass ratio. If it is important for the spacecraft's delta V to stay the same, the payload will have to be reduced by the same amount. This might cut into the amount of life support consumables carried, which will reduce the number of days a mission can last. If the same amount of scientific observations have to be done in the reduced time, another crew member might have to be added. This will decrease the mass available for consumables even more. And so on.

The technical term is "cascading changes." The only thing worse is cascading failures.

As an example, I have some notes of graphing various space warship optimizations with examples of how various warship classes map onto the graph.


(ed note: Captain Randall, his crew, and his submarine were inadvertently put into suspended animation in 1945. They have been awakened one thousand years later, and must go into battle with alien invaders. Captain Randall is talking over the refurbishment of his submarine with a robot named Austin. The discussion is about limitations in submarine design, but the "everything is connected to everything" principle still holds true.)

     "I'll take your word for it, Austin. At the moment there are more important things. What is the state of the ship?"
     "As on the day of trials, sir, without fault, and completely refitted. The experts have many ideas for improvements, sir, which will be submitted to you in due course."
     "And the munitions?"
     "None, sir. They had deteriorated beyond safe use. However, they can be replaced or, if you prefer it, something more violent substituted."
     "One thing at a time. At the moment, have them replaced. Does this age understand explosives?"
     "Chemical history does, sir. Your ammunition holds can be filled in approximately half an hour and that includes the torpedo tubes. Will you continue with ancient compressed air, underwater missiles, sir? We have some smaller and far more efficient."
     "Normal torpedoes with a normal warhead, please, Austin."
     "As you wish, sir."
     Randall smiled faintly. "You sound disappointed, Austin. So, since you have a logical mind, I will be logical. You probably have in mind more advanced and far more destructive missiles which came into being just before man decided on peace for all time. No doubt these missiles are faster, have a greater range, are self-seeking and are several times more destructive. How big are they, Austin?"
     "About the quarter of the size of yours sir. I estimate you can carry—"
     "They won't fit our torpedo tubes, will they? This will mean interior adaptation, rebuilding, all of which will have to be related to buoyancy, speed—above and below surface—diving and surfacing. Which method is going to be the quickest?"
     "I had overlooked that angle, sir." There was considerable respect in the robot's voice.
     "That is because you are not yet a sailor—and let me assure you. You cannot become a seaman from text books or even convenient memory banks: It's a question of experience. No doubt, Ordinary Seaman Austin, your mind is brimming with ideas for replacing the ship's motors with something modern, smaller, a hundred times more efficient and one tenth the weight. This would play hell with our diving, upset our displacement and require the alteration of our tank capacity to meet our increased buoyancy. Am I making myself plain?"
     "Too plain, sir. With respect, I am learning a great deal."
     "Good. Don't misunderstand me, Austin. I want your ideas and I shall ask for them, but only when you, yourself, can relate them to experience."

From THE TIME MERCENARIES by Philip E. High (1968)

Design Humor

Fundamental Design

As mentioned in Rick Robinson's Spaceship Design 101, all spacecraft are composed of two sections: the Propulsion Bus and the Payload Section.

The Propulsion Bus has the propulsion system, propellant tankage, fuel container (if any), power plant, power plant heat radiator (if any), anti-radiation shadow shield (if any), and a keel-structure to hold it all together. Sometimes the keel is reduced to just a thrust-frame on top of the engine, with the other components stacked on top.

The Payload Section is what the propulsion bus is pushing from planet to planet. It can include crew, flight control station, propulsion/power plant control station and maintenance center, astrogation station, detection and communication equipment, habitat module with life support equipment (including environmental heat radiators) and consumables (air, food, water), space taxis, space pods, and docking ports.

But most importantly, the payload section must contain the reason for the spacecraft's existence. This might be organized as a discrete mission module, or it might be several components mounted around the payload section.


I will argue that deep space craft have essentially two sections that can largely be treated separately from one another. One section is the propulsion bus — drive engine, reactor if any, solar wings or radiator fins, propellant tankage, and a keel structure to hold it all together. The other is the payload section that it pushes along from world to world.

There are both conceptual and economic reasons to treat them separately. Conceptually, because a propulsion bus might push many different payloads for different missions, such as light payloads on fast orbits versus heavy payloads on slow orbits. A little noticed but important feature of deep space craft is that you cannot overload them. They do not sink, or crash at the end of the runway, or even bottom out their suspension. They merely perform more sluggishly, with reduced acceleration and (for a given propellant supply) less delta v.

Conceptual logic is also economic logic. The outfits that build drive buses would like to sell them to lots of different customers for a broad range of assignments.

This is not necessarily an argument for true modular construction, with drive buses hitching up to payloads on an ad hoc basis like big-rig trucks and trailers. Building things to couple and uncouple adds complexity, mass, and cost — plug connectors, docking collars, and so forth. Moreover, drive buses intended for manned ships need to be human-rated, not just with higher safety factors but provision for supplying housekeeping power to the hab, etc. But these things, along with differing sizes or number of propellant tanks, and so forth, can all be minor variations in a drive bus design family.

The payload we are most interested in is, naturally, us. The main habitat section of a deep space ship closely resembles a space station. It is likely that habs intended for prolonged missions will be spun, for health, efficiency, and all round convenience. (Flush!) The design of a spin hab is dominated by the spin structure and — unless you spin the entire ship — the coupling between the spin and nonspin sections.

Because ships' spin habs have the features of stations they may be used as stations, and again we can imagine design families, with some variants intended for ships and others as orbital platforms having only stationkeeping propulsion. Habs are the one major part of a deep space ship that correspond fairly well to our concept of a hull. Spin habs are entirely different in shape, but the shape is constrained; once you build it you can't easily modify it, beyond adding another complete spin section.

For those with bank cards at the ready, buying a deep space ship might be not unlike buying a computer. If your mission needs are fairly standard, you check off options on a menu. Those with more specialized requirements can select major components — perhaps a drive bus from one manufacturer, a main crew hab from another, along with custom payload sections, service bays, and so forth, assembled to your specifications.

In fact, both technology and probable historical development suggest that fabrication and overall assembly will be two distinct phases, carried on in different places, quite unlike either shipyard or aircraft assembly practice. In the early days, large deep space craft will be built the way the ISS was, assembled on orbit out of modules built on Earth and launched as payloads. In time fabrication may move to the Moon, or wherever else, but final assembly (at least of larger craft) will continue to be done at orbital facilities. I call them cageworks, on the assumption that a cage or cradle structure provides handy anchoring points for equipment.

For game or sim purposes, my advice would be to treat drive buses and hab sections as the primary building blocks for ships, whether these components are permanently attached to each other or simply coupled together. Both approaches might be in use.

A couple of provisos. All of the above applies mainly to deep space craft, especially with high specific impulse drives. Ships for landing on airless planets have some similar features. Ships that use rapid aerobraking, however, are aerospace craft and broadly resemble airplanes, even if they never land or even go below orbital speed.

And I have said nothing of warcraft. Kinetics are essentially just another payload. Lasers, and other energy weapons such as coilguns, probably draw power from the drive reactor, calling for some modifications in the drive bus. These things don't much affect the overall configuration. Armor protection would, but discussions here have left me doubtful of its value against either lasers or kinetics. Laser stars and other major warcraft may not be dramatically different in appearance from civil craft of similar size.

Nyrath: I'm reminded of Sir Arthur C. Clarke's early space science books. He noted that a nuclear powered spacecraft would probably resemble a dumb bell, that is, two spheres connected by a stick. The hab module is the forward sphere, the nuclear drive is the rear sphere, the stick is long to provide some inverse square protection from radiation, and the propellant tanks would be on the stick, probably clustered near the nuclear drive.

One can also imagine modules designed by diverse corporations being incompatible with others on purpose. "Not invented here" syndrome.

One can also imagine a tramp freighter composed of incompatible modules, being held together with bailing wire and spit.

Qwert: The compatibility between modules will mostly depend on how the market develops. One extreme maybe Microsoft, a monopoly that basically sets the standard, as everything has to adhere to it. The other one extreme is the current hardware industry. Your memories and components have to fit on every motherboard if you want to sell them.

On the field of big aircraft manufacturing, standardisation dominates almost everything... excerpt the end product. If you manufacture engines, they have to fit on Airbus as well as on Boeing's aircraft. The components industry is dispersed and competition is intense.

On the other side, a pilot trained to flight with a Boeing can´t immediately switch to an Airbus without some training. Competition centers around two big players and nobody is interested in making life easier to the other.

In short: how spaceship components will be build, will mostly depend on how the industry evolves. A monopoly, strict government regulation, competition between many small producers or a highly dispersed specialised components industry may benefit a system of standards. On the other side competition between a small number of giants, may produce different incompatible systems.

Rick Robinson: True modularity is by no means a given. But some features of modularity, call it demi-modularity, are inherent to deep space technology.

You probably want to keep your propellant tanks separate from the corrosive, explosive stuff we breathe. Drive engines are essentially bolted onto the tail. Generally the major parts of a deep space ship don't have to fit together snugly. If you want to hang something out on a bracket you probably can.

Ferrell: I think that the engine package, mated to a suitable tank, mated to a hab module, mated to a mission module (a seperate entity from the hab module) would (as the last step in design) incorporate the heat management system suitable to the final design. Then construction/assymbly would occure.

If there aren't landers/shuttles at the final destination and you need them, then you can carry landers/shuttles in place of cargo. A mission module may be an extended docking module that a number of small modules 'plug' into, (or your transfer craft).

Manuvering thrusters should be at mutliple points/modules (distributed from the 'nose' to the 'tail').

Of course, you could stand it all on it's ear and have the mission module be on the inside of the ship, the hab ring be around the middle (with its radiators in arcs between its connecting pylons),with the engines, tanks, powerplants, radiators, and nav sensors clustered around both ends of the mission module; and any docking would be at the tips of the mission module, or on the inside walls of the mission module.

Nyrath: There might be a brisk trade in "interface modules", that would connect modules made by different manufacturers. I'm reminded of the Apollo ASTP Docking Module used in the Apollo-Soyuz mission. This was a tiny airlock module with a NASA style docking collar on one end, and a Soviet style docking collar on the other.

Not to mention the International Space Station Pressurized mating adapters.

Rick Robinson: Sabersonic — Yes, I'm gliding over a host of devils in the details. The payload section will surely have attitude thrusters, for example, and these must coordinate with attitude thrusters on the drive bus end.

Nyrath — 'Standards,' indeed! Again this is a promo for full service cageworks that will provide things like docking adapters.

Jean Remy: On standards:

Currently the main type of freight vessel is the container-carrier. They are favored because the containers can be loaded/unloaded easily, then simply popped on a freight train or a truck. I can see the same things for a cargo spaceship. There won't be a cargo "module" but rather anchoring hard points for standard containers. Those containers will be loaded on a booster to orbit, transferred onto a ship, then at the other end the container is loaded into a simple remote-controlled lifting body (for planets with an atmosphere, say, Mars) or just a simple frame with thrusters (for the Moon) where they will be loaded into maglev trains if needed, etc.

The command post/bridge:

I'd still put in in the non-rotational part of the ship, certainly on warships (you want it as deep inside the ship as you can for protection) Not only that, but I would stop rotation in combat: precession might alter your maneuverability, and damage might weaken the structure or cause a wobble which would rip your ship apart. I would also put it in the middle for a civilian vessel, as then the command module can double as storm cellar, or if there's a separate storm cellar, you'll still have access to the command post during a solar flare. It's also a good defense against micrometeorites. The command center is just too important to risk placing it on the outer rim of a ship, no matter how convenient or comfortable it would be to the crew.

Sabersonic: Even so, it would probably be prudent to not have all command, or at least navigation control, be monopolized by the CIC. At minimum and barring mass budgeting, there should be two for overall spacecraft control: One that is the CIC/Navigation primary with the second being the engine room for mostly emergency purposes. If only because for something as complex and (for the first few decades if not centuries of interplanetary travel, let alone interstellar) inevitably as fragile as a spacecraft, one should avoid "putting ones eggs into one basket" and to always "have a plan B". Space and interplanetary/interstellar travel is not a place for the ill-prudent, and that's just the natural dangers.

As for standards, well, before standards could ever become "standard" for lack of a better word, there lies the inevitable "Format War" in one form or another that would occur when one believes that their system is more efficient and reliable than the other or worse: a new industry/business standard that has the potential to supplant the jobs of numerous Dock workers or in this case "Cage Workers" that could drive hostility and perhaps a little bit of political pressure before the whole matter is settled in one way or another. It would not be a full parallel to the Teamsters Strike that is basically Teamsters vs Trucks, but it would not be a completely quiet deal when paychecks are involved.

Rick Robinson: My division of ships into drive buses and payload sections is more because of operational factors than manufacturing considerations.

For example, ships inherently have (at least) two big 'hull' structures, the crew hab and the main propellant tank. These are probably at very different temperatures, which right away is a big reason to keep them physically separate.

Using the propellant tankage as shielding is appealing, but even with a vacuum separation it means that your 290 K hab shell is dumping 350 W/m2 of waste heat right into your 20 K liquid hydrogen tanks.

Jean — The containership principle seems highly likely for most space cargo, with the standard pod being defined originally to fit Earth orbital shuttle bays.

In this case, your typical space freighter is a drive bus pushing a rack structure with clamps for pods. The rack might be configurable so that you can also carry 'oversize' loads.

Jean and Sabersonic — In a parallel discussion at SFConsim-l, the question was raised whether civil ships need a 'control room' at all, or whether people could just stand watch from their regular work stations.

I think that any spacecraft with a fair number of passengers or crew will have a watch at the main life support panel, because life support is always running, has constantly changing loads, and things can go very bad very quickly. The life support panel will almost certainly be in the spin section, because that is where the life support is.

You may as well put the engineering panel here as well. There's no reason for an 'engine room' in the maritime sense, since the drive is mounted externally, and if it is nuclear you don't want to go anywhere near it.

No doubt you could maneuver and navigate the ship from here as well. But en route there is very little of this to do.

The only time you really conn a (civil) spacecraft is during rendezvous and docking, or similar evolutions. At these times you surely de-spin, but you might want a separate control station next to the main airlock, with viewports for maximum situational awareness.

Because of its location, this station would also naturally serve as the ceremonial 'quarterdeck' where VIPs are greeted, and ordinary mortals report aboard.

Warcraft are a whole 'nother matter, with protective considerations arguing for a control room at the center of the hab section.

Jean Remy: Multiple Command posts: Yes. That's why I used the term command post rather than CIC, Bridge, DCC, Main Engineering. It was meant to be generic and refer to all these. There ought to be at least CIC and DCC, if not a redundant third post, all of them capable of doing every job, but each ideally suited to one. The ship is more efficient with all of them, but can still work with the loss of one or two.

Decentralized Command posts: Possibly, but I don't think it likely. First of all we could do that now, but we don't. Psychologically, I think crews want to have the "Captain on Bridge" so to speak. It is generally recognized that good officers are the ones who stick by their men when the going gets tough. Caesar rode into battle at Alesia, rallying his troops to victory when it looked uncertain. Washington rode with his men. Patton led from a tank. Now granted the captain of a ship can't go anywhere, but seeing him on the Bridge will be a boost to morale.

Rick Robinson: The question of having a control room at all was in the context of civil spacecraft. If they have an sudden emergency it is most likely to be a life support crisis such as fire, for which the classic 'bridge' functions are fairly irrelevant.

Military craft are a special case, and I'd certainly expect them to have a control center.

And maybe all ships, because to extend a point Jean makes, existing space programs are quasi-military in origin. The military outlook toward emergency response is coded into their DNA, so to speak.

All human carrying deep space ships will need a storm shelter in any case, and it would be fairly natural to configure this as an emergency control center.

Luke Campbell: Re: Shielding. It now seems likely that a plasma magnet generated by a low mass antenna could deflect any charged solar radiation, so the crew would be safe from flares and CMEs. It does not seem like a plasma magnet could stop galactic cosmic rays, GCRs are a steady source of background radiation, not the sort of thing that a "storm cellar" would help with. A plasma magnet also would not protect against neutral particle radiation, but the only neutral particle radiation likely to be a threat is man-made: neutron radiation from nukes and possibly high energy photons from x-ray or gamma ray lasers.

For those not in the know, a plasma magnet uses low frequency radio waves to produce a rotating field that induces a current in the surrounding solar wind plasma. This current forms a dipole magnetic field that deflects and reflects charged particles. The field is not strong enough to deflect solar wind protons, but it does deflect the electrons, leading to charge separation that pulls the protons back to the electron cloud before they reach the section being protected.

Yes I was trying to stop GCRs (but calculations showed it would need an unreasonably huge magnet). The plasma magnets wouldn't stop the solar wind protons either, when considered as individual particles — you need the plasma effects of the electrons to stop the solar wind. This lets you get by with a much smaller magnet.

However, there is one possible additional method of mitigating the GCR dose - medication. As we learn more about cellular repair and cell "suicide", new treatments may become possible for both chronic and acute radiation poisoning (and oddly, you are likely to want the opposite reaction in these two cases - for chronic exposure, you want the damaged cells to destroy themselves to prevent cancer; for acute exposure you want the damaged cells to repair themselves to prevent anemia, hemophilia, a compromised immune system, and digestive difficulties). Incidentally, it has been shown that vitamin D helps with chronic radiation exposure, although the mechanism is not clear.

Rick Robinson: To keep cryogenic propellant from boiling off on long missions you will need active refrigeration, pumping heat out of the tank. Otherwise heat buildup will be inexorable.

The ship will be designed to keep the propellant tanks away from the main radiator fins and such, and generally minimize heat absorption from the rest of the ship, so the heat you mainly have to deal with is from sunlight.

The tanks will be painted white or silvery to reflect away most sunlight. I assume that you can reflect about 90 percent. For a spherical tank at 1 AU, that means about 35 W/m2 of absorbed solar radiation that you'll have to pump out of the tank.

A 20 meter diameter tank holds about 250 tons of hydrogen, or 1500 tons of methane. Surface area is 1250 m2, so at 1 AU you'd need 44 kW of refrigerating capacity, i.e. heat extraction, to keep propellant cold.

At Jupiter distance, 5 AU, solar flux is reduced by 96 percent, and you only need 2 kW of refrigeration for hydrogen - none for methane, which will stay liquid or even tend to freeze.

Suppose you put a 10 meter diameter hab inside the tank {to give the hab some radiation protection}. (This only reduces tank capacity by 1/8.) But even with a vacuum layer you will have IR heating from the hab surface, at room temperature: 400 W/m2 * 314 m2 = ~125 kW.

So in this case putting the hab inside the fuel tank multiplies your propellant refrigeration bill by 4x. Which is a lot, but not horrible; the shielding might be worth it. But wrapping propellant around a spin hab is tougher.

Rick Robinson: Modular design is always a tradeoff. You get more operational flexibility, at cost of more complicated/heavier/weaker connections. Integral designs will be favored when the components will consistently be used together.

Much will depend on tech. Torch type drives and even 'conventional' nuke electric drives pretty much have to be mounted on a pylon, which sort of invites the option of unbolting it from the rest. OTOH, as you note, the drive section may well have its own control center. And since the rest of the ship sits on top of the pylon, it's a fine line between 'pylon' and 'chassis.'

On naming, I could also make a case that the crew hab compartment is the main component, and so would be named. Especially if it is a spin gravity structure. And 'spaceships' may end up having more than one name, just as a named train might included Pullman cars with names of their own.

And if ships are highly modular, some terms might be borrowed from railroading. For example, 'consist' as a noun (pronounced CON-sist) for the whole assemblage. Thus, 'The Ty Cobb departed Mars with a consist of [such and such modules].'

Amusing side note: modular spacecraft reverse the order of trains: the 'locomotive' or drive engine is at the back (more precisely the base), while the 'caboose' or control cabin might well be at the front/top.

From SPACESHIP DESIGN 101 by Rick Robinson (2009)

This section is intended to address some gaps in available information about spacecraft design in the Plausible Mid-Future (PMF), with an eye towards space warfare.  It is not a summary of such information, most of which can be found at Atomic Rockets.  The largest gap in current practice comes in the preliminary design phase.  A normal method used is to specify the fully-loaded mass of a vessel, and then work out the amounts required for remass, tanks, engine, and so on, and then figure out the payload (habitat, weapons, sensors, cargo, and so on) from there.  While there are times this is appropriate engineering practice (notably if you’re launching the spacecraft from Earth and have a fixed launch mass), in the majority of cases the payload mass should be the starting point.  The following equation can be used for such calculations:

Where P is the payload mass (any fixed masses, such as habitats, weapons, sensors, etc.), M is the loaded (wet) mass, R is the mass ratio of the rocket, T is the tank fraction (or any mass that scales with reaction mass) as a decimal ratio of such mass (e.g., 0.1 for 10% of remass), and E is any mass that scales with the overall mass of the ship, such as engines or structure, also as a decimal.  

This equation adequately describes a basic spacecraft with a single propulsion system.  It is possible to use the same equation to calculate the mass of a spacecraft with two separate propulsion systems.

The terms in this equation are identical to those in the equation above, with R1 and T1 representing the mass ratio and tank fraction for the (arbitrary) first engine, and R2 and T2 likewise for the second.  Calculate both mass ratios based on the fully-loaded spacecraft.  If both mass ratios approach 2, then the bottom of the equation will come out negative, and the spacecraft obviously cannot be built as specified.  Note that when doing delta-V calculations to get the mass ratio, each engine is assumed to expend all of its delta-V while the tanks for the other engine are still full.  In reality, the spacecraft will have more delta-V than those calculations would indicate, but solving properly for a more realistic and complicated mission profile requires numerical methods outside the scope of this paper.

One design problem that is commonly raised is the matter of artificial gravity.  In the setting under discussion, this can only be achieved by spin.  The details of this are available elsewhere, but these schemes essentially boil down to either spinning the entire spacecraft or just spinning the hab itself.  Both create significant design problems.  Spinning the spacecraft involves rating all systems for operations both in free fall and under spin, including tanks, thrusters, and plumbing.  The loads imposed by spin are likely to be significantly larger than any thrust loads, which drives up structural mass significantly.  This can be minimized by keeping things close to the spin axis, but that is likely to stretch the ship, which imposes its own structural penalties.  A spinning hab has to be connected to the rest of the spacecraft, which is not a trivial engineering problem.  The connection will have to be low-friction, transmit thrust loads, and pass power, fluids, and quite possibly people as well.  And it must work 24/5 for months.  All of this trouble with artificial gravity is required to avoid catastrophic health problems on arrival.  However, there is a potential alternative.  Medical science might someday be able to prevent the negative effects of Zero-G on the body, making the life of the spacecraft designer much easier.  

When this conclusion was put before Rob Herrick, an epidemiologist, he did not think it was feasible.

     “The problem is that they [the degenerative effects of zero-G] are the result of mechanical unloading and natural physiological processes. The muscles don't work as hard, and so they atrophy. The bones don't carry the same dynamic loads, so they demineralize. Both are the result of normal physiological processes whereby the body adapts to the environment, only expending what energy is necessary. The only way to treat that pharmacologically is to block those natural processes, and that opens up a really bad can of worms. All kinds of transporters would have to be knocked out, you'd have to monkey with the natural muscle processes, and God knows what else. Essentially, you're talking about chemically overriding lots of homeostasis mechanisms, and we have no idea if said overrides are reversible, or what the consequences of that would be in other tissues. My bet is bad to worse. As the whole field of endocrine disruptors is discovering, messing with natural hormonal processes is very very dangerous.

     Even if it worked with no off-target effects, you'd have major issues. Body development would be all kinds of screwed up, so it's not something you'd want to do for children or young adults. Since peak bone mass is not accrued until early twenties, a lot of your recruits would be in a window where they're supposed to still be growing, and you're chemically blocking that. Similarly, would you have issues with obesity? If your musculature is not functioning normally (to prevent atrophy), how will that effect the body's energy balance? What other bodily processes that are interconnected will be effected? Then you get into all the effects of going back into a gravity well. Would you come off the drugs (and thus require a washout period before you go downside, and a ramp-up period before you could go topside again)?

     Spin and gravity is an engineering headache, but a solvable one. Pharmacologically altering the body to prevent the loss of muscle and bone mass that the body seems surplus to requirements has all kinds of unknowns, off target-effects and unintended consequences. You're going to put people at severe risk for medical complications, some of which could be lifelong or even lethal.”

This is a compelling case that it is not possible to treat the effects of zero-G medically.  However, if for story reasons a workaround is needed, medical treatment is no less plausible than many devices used even in relatively hard Sci-Fi.

The task of designing spacecraft for a sci-fi setting is complicated by the need to find out all the things that need to be included, and get numbers for them.  The author has created a spreadsheet to automate this task, including an editable sheet of constants to allow the user to customize it to his needs.  The numbers there are the author’s best guess for Mid-PMF settings, but too complicated to duplicate here.

Rick Robinson’s general rule is that spacecraft will (in the sort of setting examined here) become broadly comparable to jetliners in cost, at about $1 million/ton in current dollars.  This is probably fairly accurate for civilian vessels, at least to a factor of 3 or so.  Warships are likely to be more expensive, as most of the components that separate warships from civilian ships are very expensive for their mass.  In aircraft terms, an F-16 is approximately $2 million/ton, as is the F-15, while the F/A-18E/F Super Hornet is closer to $4 million/ton.  This is certainly a better approximation than the difference between warships and cargo ships, as spacecraft and aircraft both have relatively expensive structures and engines, unlike naval vessels, where by far the most expensive component of a warship is its electronics.  For example, the ships of the Arleigh Burke-class of destroyers seem to be averaging between $150,000 and $250,000/ton, while various cargo ships seem to hover between $1000 and $5000/ton.

As mentioned in Section 5, some have suggested that the drive would be modular, with the front end of the ship (containing weapons, crew, cargo, and the like) built separately and attached for various missions.  This is somewhat plausible in a commercial context, but has serious problems in a military one.  However, the idea of buying a separate drive and payload and mating them together is quite likely, and could see military and civilian vessels sharing drive types.  (This is not as strange as present experience would lead us to believe.  It was only during WWII that military aircraft clearly separated from civilian ones in terms of performance and technology.)  This simplifies design of spacecraft significantly, as one can first design the engine, and then build payloads around it.

One common problem during the discussion of spacecraft design is the rating of the spacecraft.  With other vehicles, we have fairly simple specifications, such as maximum speed, range, and payload capacity.  However, none of these strictly applies in space, and the fact that spacecraft are not limited by gravity and movement through a fluid medium makes specifying the equivalents rather difficult.  Acceleration and delta-V obviously depend on the masses of the various components, which can be changed far more readily than on terrestrial vehicles, and cargo capacity is limited only by how long you’re willing to take to get where you’re going.  A replacement might be a series of standard trajectories, and the payload a craft can carry on them.  This works well if all of the spacecraft being rated are generally similar in terms of performance, and take similar trajectories in reality.  However, it does not work as well in a scenario where different types of ships take wildly different trajectories with different amounts of cargo.  In that scenario, ships might be rated by the minimum time for certain transfers (Earth-Mars at optimum, for instance) with a specified payload, either a fixed percentage of dry mass, or a series of specified masses for various sizes of ships.  This allows a comparison between ships of different classes, but within a class (liner, bulk cargo, etc.) the first method would probably be preferred.

A related problem is the selection of an appropriate delta-V during preliminary design.  In some cases, this is relatively easy, such as when a spacecraft is intended to use Hohmann or Hohmann-like trajectories, as numbers for such are easily available.  But such numbers are inadequate for a warship, or for any ship that operates in a much higher delta-V band, and unless the vessel has so much delta-V and such high acceleration that Brachistochrone approximations become accurate (and even then, if the vessel is not using a reactionless drive, the loss of remass can throw such numbers off significantly, unless much more complicated methods are used, numerical or otherwise).  The author has attempted to fill this gap by creating a series of tables of delta-Vs and transit times between various bodies, with the tables giving the percentage of the time that a vessel leaving one body can reach another within a specified amount of time with a given amount of delta-V.  The tables can be found at the end of this section.  Table 9 covers Earth-Mars transits, while Table 10 describes Earth-Jupiter transits.

The tables are generated in MATLAB by solving Lambert’s problem for a large number of departure days and transit times, and calculating the delta-V to go from stationary relative to the departure planet to stationary relative to the destination planet.  This involved assuming that there was a single instant delta-V burn at each end, which is a good approximation if the burn time is short compared to the transit time, as it would be for chemical or most fission-thermal rockets.  For systems which burn a significant amount of the time, this approximation is not as good, and the tables should only be used as a general guide to the required delta-V.  

Each table is the composite of 16 different tables generated with different starting geometry, and with each table containing data from at least one synodic period.  Note that this was all done in a sun-centric system, and that the delta-V necessary to deal with either planet’s gravity well was not included.  This will add some extra delta-V, the necessary amount shrinking in absolute terms as the overall delta-V is increased due to the Oberth effect.  The decision to not include escape and capture delta-V was made because to do otherwise would have involved specifying reference orbits to escape from and capture to, and would have added significant complexity to the program at a minimal gain in utility for most users.  

One thing that is apparent from these tables is the degree to which Jupiter missions are more hit-or-miss than Mars missions.  For Jupiter transfers, 84% of the options are either going to be viable all of the time, or not going to be viable at all.  For Mars, the equivalent value is only 56%.  Some of this is due to the much larger and more variable time increments used in the Jupiter calculations, but much of it is due to the fact that the geometry changes significantly less between Earth and Jupiter than it does between Earth and Mars.

It should also be noted that these tables are an attempt to find an average over all possible relative positions of the two bodies.  For the design of an actual spacecraft, analysis would instead start with modeling of geometries over the projected life of the spacecraft.  The approximations given here are reasonably close for theoretical use, but should not be used to plan actual space missions.  

Heat management is a vital part of the design and operation of a space vessel, particularly a warcraft.  Section 3 mentioned some of the issues with regards to stealth, but a more comprehensive analysis is necessary.  There are two options for dealing with waste heat in battle: radiators and heat sinks.  If the waste heat is not dealt with, it would rapidly fry the ship and crew.  

All space vessels will need radiators to disperse the heat they produce as part of normal operations.  If using an electric drive, power (and therefore waste heat) production will be no higher in battle then during cruise.  This would allow the standard radiators to be used indefinitely during battle without requiring additional cooling systems.  The problem with radiators is that they are relatively large and vulnerable to damage.  The best solution is to keep them edge-on to the enemy, and probably armor the front edge.  The problem with this solution is that the vessel is constrained in maneuver, and can only face one (or possibly two) enemy forces at once without exposing the radiator.  If the techlevel is high enough to make maneuver in combat a viable proposition, then radiators are of dubious utility in combat.  On the other hand, the traditional laserstar battle suits radiators quite well.  The faceplate and the forward edge of the radiators are always pointed at the enemy, and almost all maneuvers are made side-to-side to dodge kinetics.  The only problem is vulnerability to a direct kinetic hit.  If a projectile were to arrive precisely edge-on, it could tear the entire radiator in two.  Bending the radiator slightly would eliminate this vulnerability, but would also increase armor requirements.  However, even a bent radiator would still have issues with grazing impacts.  A projectile coming in very close to parallel with the radiator’s surface would tend to tear a long hole in it, as opposed to the small hole left by a projectile traveling perpendicular to the surface.  However, the low-incidence projectile would have to penetrate much more material, so kinetics designed for such attacks would naturally have more mass or fewer pieces of shrapnel than one designed for normal attacks.

Heat sinks avoid the vulnerability to damage of radiators, but have a drawback of their own.  By their very nature, they have a limited heat capacity, which places a limit on how much power a ship can produce during an engagement, and thus on the duration of an engagement.  If the heat sinks fill up, the ship would begin to fry unless the radiators were extended immediately.  In the game Attack Vector: Tactical, extending the radiators is used to signal surrender.  Obviously, the heat clock is a major disadvantage, but it is necessary when the vessel expects to expose several aspects to the enemy.  

One topic that briefly needs to be addressed is electric propulsion. In discussions, VASIMR-type engines are usually considered the baseline.  However, Dr. Joshua Rovey of Missouri S&T told the author that Hall Effect thrusters today are capable of the sort of performance that VASIMR is currently promising after development is finished.  VASIMR is apparently getting attention due to good marketing people.  

Another topic that deserves discussion is the effect of nuclear power on spacecraft design.  For large warcraft, nuclear power, both for propulsion and for electricity is a must-have.  Even if the design of solar panels advances to the point at which they become a viable alternative for providing electrical propulsion power in large civilian spacecraft, there are several major drawbacks for military service.  The largest is that solar panels only work when facing the sun, unlike radiators, which work best when not facing the sun.  The distinction between the two is important, as it is nearly always possible to find an orientation which keeps the radiator edge-on to the enemy and still operating efficiently, while a solar panel must be pointed in a single direction, potentially exposing it to hostile fire.  A solar panel is particularly vulnerable to laser fire, as it is by nature an optical device.  While hard numbers on this are surprisingly difficult to find, it appears that damage will probably occur to photovoltaics when exposed to intensities of around 300 KJ/m2, for short pulses (<10-4 seconds), with threshold requirements increasing from there as the pulse length increases.  For a CW laser (>1 second), the power flux required for damage is approximately 10 MW/m2.  Photovoltaics can also be attacked using small particles such as sand, as described in Section 7 for use against lasers.  While a full analysis of the potential damage is beyond the scope of this section, it appears that sand would be a reasonably effective means of attacking photovoltaics, particularly given the large area involved.  The size of a solar array also complicates maneuvering the panels edge-on to the incoming particles, and could potentially raise structural concerns.

Radiators, on the other hand, are more resistant to damage.  Firing lasers at them will only decrease the thermal efficiency of the reactor slightly, as the radiator is designed to disperse heat.  Particle clouds that are designed for surface effects would be ineffective against a properly-designed radiator, or at very best reduce the emissivity by a small amount.  Small pieces of shrapnel designed to pierce the radiator entirely would be the best means of attack (described above), as fully armoring a radiator is likely to be impossible because of the mass requirements.  

However, it could be argued that this ignores the vulnerability of the reactor itself to damage.  While in Hollywood, “They’ve hit the reactor!” is usually followed by a massive explosion, that is not the case in reality.  First, the reactor is a very small target, usually shielded by the bulk of the ship, so it’s unlikely to be hit in the first place.  Second, nuclear reactors simply do not turn into bombs under any circumstances, and particularly not random damage to the core.  The few cases in history in which a reactor has gone prompt critical (SL-1 and Chernobyl being the best-known) were caused by poor procedure, and are vanishingly unlikely to happen due to random damage.  

That said, it still seems a potentially bad idea to put all of one’s eggs in a single basket.  Solar panels are highly redundant, but the reactor could still be put out of action with a single hit.  The response to this is fairly simple.  First, there are reactor designs using heat pipes that have sufficient redundancy to continue operation even if the reactor core itself is hit.  The specific heat pipe will be put off line, but if the design has 150, that’s not a great worry.  Second, the reactor and associated gear (power converters and such) are buried deep in the ship, where they will be difficult to get at, and the power converters can be duplicated for redundancy.

One last concern is the ejection of reactor core material after a hit, and the potential for said material to irradiate the crew. This is also probably minor, as the crew is still being shot at, and the spacecraft will have some shielding against both background radiation and nuclear weapons. (Thanks to Dr. Jeffrey King of the Colorado School of Mines for providing much of the material on space nuclear power and propulsion.)

A couple of other issues with nuclear power are relevant and of interest.  The first is the choice of remass in nuclear-thermal rockets.  While hydrogen is obviously the best possible choice (the reasons for this are outside the scope of this paper, but the details are easy to find), it is also hard to find in many places.  With other forms of remass, the NTR does not compete terribly well with chemical rockets, but it can theoretically use any form of remass available.  The biggest problem with alternative remasses is material limits.  With most proposed materials, oxidizing remasses will rapidly erode and destroy the engine.  The alternatives to avoid this are rhenium and iridium, which are both very expensive, explaining why they are not in use today.  However, both elements are common in asteroids, making them viable choices in a setting with large-scale space industry.

As discussed in Section 7, vibration is a serious issue for laser-armed spacecraft.  Any rotating part will produce vibrations, and minimizing these vibrations is of interest to the designer.  While there is undoubted a significant amount that could be done to reduce the vibrations produced by conventional machinery (the exact techniques are probably classified, as their primary application is in submarine silencing), it seems simpler to use systems with no moving parts, which should theoretically minimize both vibration and maintenance.  Heat pipes, as mentioned above, are an entirely passive means of moving heat around, both from a reactor to an energy converter (which could mean a turbine, a thermocouple, or any of the other wonderful things engineers can think of) and from the energy converter to the radiator.  There are also electromagnetic pumps for liquid metal which while not entirely passive, but will cut down on the vibration load.

There are even proposed systems of energy conversion which are reasonably efficient and involve no moving parts.  The best-known of these proposed systems is probably the Alkali Metal Thermal-to-Electric Converter (AMTEC), which has been extensively studied.  However, a recent effort by NASA to bring the technology into deployment failed, giving the technology a bad name.  There are some, however, who believe it still holds promise.

If systems like AMTEC are not available, the spacecraft will have to use conventional hat engines.  These are likely to use one of the two standard thermodynamic cycles, the Brayton cycle (gas turbine), and the Rankine cycle (steam turbine).  The primary difference between the two is that in the Brayton cycle, the working fluid remains a gas throughout, while in the Rankine cycle, it moves from liquid to gas and back again.  In theoretical design, radiators are normally sized assuming constant temperature throughout, which is true for most Rankine cycle systems (as the radiators are where the fluid condenses at a constant temperature) and produces the well-known result that radiator area is minimized when the radiator temperature is 75% of the generation temperature.  However, this is not true for Brayton cycle radiators.  There is no convenient mechanism to release the necessary heat at a constant temperature, so the radiator performs differently as the gas cools.  There is not a simple formula here, but an iterative procedure can be used to minimize radiator area for an ideal system (which is close enough for our purposes).  Use of this method does require some knowledge of the basics of gas turbine propulsion, but it is not terribly esoteric. (Thanks to Dr. David Riggins of Missouri S&T for presenting this material in class. Those who have had more experience in propulsion and fluid dynamics might recognize some simplifications of the explanatory material, and some nomenclature changes. This was intended to hold down the length of this section and clarify it without sacrificing accuracy of the results.)

An ideal gas turbine can be thought of as being made of 4 separate stages.  First, isentropic compression, which means that there is no heat transfer and all energy put into the system by the compressor, is used to compress the gas instead of heating it.  Second, isobaric (constant-pressure) heat addition, which occurs in the reactor.  Third, isentropic expansion through a turbine, which outputs mechanical work (the goal of this whole process).  Lastly, isobaric heat rejection, through the radiator, which returns the working fluid to the condition it was at before entering the compressor.  The compressor and turbine are defined by their pressure ratios, written as πc and πt respectively.  The pressure ratio is the pressure of the fluid after the component divided by the pressure ahead of the component.

For this method, values for Cp, γ, ηc, ηt, and T3 must be selected.  Cp is the constant-pressure specific heat capacity of the fluid, while γ is the ratio of specific heats.  Definitions and values for various fluids can be found online.  ηc and ηt are the efficiencies for the compressor and turbine respectively.  Values between 0.8 and 0.9 are probably reasonable.  T3 is the temperature at the outlet of the heat addition stage, and is normally set by the design of the reactor itself.  Representative values might be 1600-1700 K for a conventional nuclear reactor, although higher values are possible.  (All temperature values throughout should be in Kelvin, not Celsius.)  

The first step is to select a value for πc, with anything from 2 to 10 being plausible.  Because it is a closed system, πt will be equal to 1/ πc.  Once this is known, it is possible to calculate T4 (temperature downstream of the turbine) using .

At this point, a value for T1 must also be selected.  This is the temperature at the entrance to the compressor.  Using this, the value for T2, the temperature at the compressor exit, can be calculated using .  This allows the overall efficiency of the power-generation system, &eta (work output/heat input), to be calculated using .  All of this information can then be used to find the radiator area per unit work output (A’, m2/W) with where σ is the Stefan-Boltzmann constant (5.670373×10-8) and ε is the emissivity of the radiator (0.9-1.0). Once you have this value, select a different value of T1, and repeat the rest of the paragraph.  When a minimum has been found, select a different value for πc and repeat the entire procedure until a global minimum has been found.  It would probably be a good idea to use a spreadsheet to automate this.

Table 9: Earth-Mars Transits
Maximum Transit Time (Must Arrive No Later Than, Days)
Table 10: Earth-Jupiter Transits
Maximum Transit Time (Must Arrive No Later Than, Days)
by Byron Coffey (2016)

Well, I was going to post the second part of The Shipping Trade today, except that writing it didn’t happen because of day job, and so forth. Then, I thought I might post a sketch of the ship involved, just to give y’all an idea of what you’ll be looking at, but then that would require me to go out and hire a scanner. That, and I made said sketch, and then looked at it, and then concluded that I couldn’t possibly inflict such a terrible picture on my readers…

So permit me, please, instead to sketch a verbal picture for you of the

CMS Greed and Mass-Energy

To start with, Greed and Mass-Energy is atypically large for a free trader; in those leagues, which principally deal in small, high-value-to-mass/volume cargoes, lugging around 40,000 tons displacement of cargo is huge. (It’s still not in the major freight line league, though; those guys can use freighters that are million ton-displacement behemoths.) Thus, the shipcorp that owns her (it’s essentially a syndicate of officers, crew, and former crew, with executive power vested in the captain-owner) is pretty prosperous to be able to cover her running costs. Dealing in brokered cargo actually isn’t her main business – she specializes in contracts like the RCS-assembly charter from Kerbol to Kythera she just left, but an empty hold is a hole that drinks money, so you take the cargo when you can get it.

Also, obviously, at a size like that, she’s not streamlined, or built to land planetside (gravity wells being acutely expensive); and is even rather more massy than anything that most stations like to have dock directly to them. Her cargo’s generally ferried to station, or upwell and downwell, by local lighters at each end of the trip. Rather, she’s built very much in the classic mode; a long, relatively thin, open-frame truss structure. Attached to that, going from fore to aft, we find these different sections of the ship:

Right at the bow, sitting on the end of the main truss, is the command capsule, an ellipsoid slightly stretched along the ship’s main axis, relatively tiny compared to the rest of the ship, and containing, for starters, the bridge and associated avionics systems. (The bridge is actually buried in the center of the capsule, for its protection; it’s displaced off to the front end of the ship, however, because the command capsule is also where the primary sensors are housed to keep them out of the way of cargo, fuel, and drive radiation, and this positioning cuts down on sensor lag. It’s still pretty safe; it’s not like anyone’s going to be shooting at them.) The first of the other two notable features it houses is docks and locks, right for’ard on the axis where it’s easiest to match thrust and spin, which usually houses a couple of cutters used for taking the crew ashore and for occasional maintenance, and a skimmer for in-field refueling. (The fuel itself doesn’t pass through here – the skimmer docks aft to offload what it scoops. No fuel for’ard of the support plate, that’s the general rule.) The second, aft by the truss, is the robot hotel for all the little space-rated utility spiders you may see now and them crawling about the structure doing maintenance, thus saving the engineering department any need to get suited up and go outside for routine work, although they still may need to do so from time to time.

Just aft of that, accommodations and secondary systems are housed in a toroidal gravity wheel. This is actually a very unusual design feature in an Imperial ship-class; just about everyone and especially the spacer-clades are genetically adapted to microgravity, and the spacer-clades prefer it, as a rule; but the Cheneos-class architects originally designed her class for near-frontier work, and included this for occasional passenger service. Greed and Mass-Energy only rarely carries passengers, so they keep it geared all the way down, producing only a tenth of a standard gravity, which doesn’t offend the spacer-clades all that much. There’s a second, smaller wheel rotating inside it to null out the gyroscopic effects; it’s used to house some other equipment that likes a little gravity, but for the most part, this one’s just a countermass.

(The wheel does, however, provide enough gravity to let the CELSS Manager run a pretty decent microbrewery in the spare volume, and perhaps more importantly, provides a place where you can drink it off-shift without suffering from a nasty case of the zero-g bloat. [Remember, folks, bubbles don’t rise in microgravity!] And apart from crew morale, having decent beer makes for good PR when traders meet.)

These areas, incidentally, are one of the few places on board where the really high-tech ontotechnological stuff makes an appearance, in the form of inertial damping. The people who built her liked microgravity, and weren’t all that keen on losing that while under thrust, especially since she was built to fly brachistochrones or near-brachistochrones (bulk tankers and ore freighters, etc., are usually built to fly economic minimum-delta/Hohmann transfers; no-one else wants to wait that long for their cargo) and so would be spending most of her time under thrust. The job of the inertial dampers is to apply the thrust of the drives evenly across the entire area’s structure and everything in it, thus ensuring that no-one actually feels any acceleration, and the lovely microgravity environment is preserved. (It also avoids having to come up with some wretchedly complicated gimbal arrangement for the already wretchedly complicated seals-and-bearings for the gravity wheel, no longer having to do which is something that made architects particularly grateful for this innovation.)

Behind this, the cargo. ‘Way back along the truss there is a very large, solid plate, the support plate. The cargo containers are simply stacked “atop” – by which we mean for’ard – of it, in six big blocks arranged around the axis with sixfold symmetry (this arrangement being a reasonable compromise between use-of-volume and convenient straight lines), and are designed to lock to the plate, the truss, and each other to form a solid interlocked structure. There’s no hold or other walls around the cargo; the containers are themselves spacetight when they need to be, and so lighters can just drop them into place and pick them up freely while in port.

The breakbulk cargo, on the other hand, is messy. It has to be podded up individually when not spacetight, and then individually lashed down and made secure atop the cargo container stacks. This annoys the cargomaster, which is why breakbulk is unpopular these days despite the fact that breakbulk shippers usually pay a premium in exchange for you having to do this (the “lash comp”). Actually, what really annoys the cargomaster is that she can punch a button and have the ship automatically query the v-tags on the container cargo for its mass stats, and so forth, whereas for breakbulk she’s got to recall her Academy training, dig out the spreadsheets, and work out the corrections to the center-of-mass-and-moment-of-inertia chart by hand. Well, still by computer, but you know what I mean.

Aft of the support plate, still in sixfold symmetry, you have the bunkerage – fuel tanks, stacked three deep in multiple rows, all filled with slush deuterium, running right to the stern, where they surround the cylindrical shroud of the mostly-unpressurized engineering hull (you can take a crawlway right back along the truss to the small, pressurized maneuvering room back this far, should you need to examine the drives close-up in flight, but the actual machinery space isn’t), which contains the interlinked systems of the main power reactors and the fusion torches themselves, strapped to the aftmost extent of the main truss.

And there are lots of fuel tanks. Even though said fusion torches are miracles of a mature nuclear technology, capable of achieving near-theoretical efficiencies and outputs and delta-v per unit fuel that routinely makes naval architects from less advanced civilizations throw down their slide rules in despair and weep into their terrible coffee-equivalents, the one unchangeable rule of space travel is that your mass ratio is always much, much less favorable than you might want it to be.

Good thing deuterium’s so cheap, isn’t it?

…and most prominently of all from a distance – dominating the entire view of the ship from a distance, by area as well as by temperature – sweeping out from among the fuel tanks (although comfortably retracted to sit alongside them, leaving approximately a sixth of their radiative area useful, while idling in dock – the vast panels and pipework of the heat radiators. Because the other one unchangeable rule of space travel is that you always have waste heat, too damn much waste heat, and you’ve got to get rid of it somehow. Especially once you fire up those fusion torches. (The radiators, however, unlike the rest of the ship, have only fourfold symmetry – so that they can be perpendicular to each other when unfolded, because there’s very little point in radiating heat right back at your own radiators.)

Rockets Are Not Hotels

RocketCat sez

See that space fantasy at the top? Yep, the good ol' Starship Enterprise. There are two glaring thing wrong with it, right off the bat.

First off, the direction of "down" is almost but not quite totally FUBAR. We'll get into that later.

But secondly, which of the other two ships look most like Enterprise? In terms of blue pressurized habitat module. Yep, the freaking Queen Mary, an ocean liner. Not the Lewis design, a nuclear thermal rocket spacecraft created for a Mars mission. I hope you see the problem.

If you look at most blueprints for the various iterations of the Starship Enterprise, you will notice that every single part of the spacecraft interior is pressurized, with doors, rooms, and toilets. The corridors are wide enough for five people to walk abreast on nice carpeted floors with indirect lighting.

This is ludicrously wrong. And it is not just Star Trek that does this, pretty much all of media science fiction has ships like this. TV Tropes calls this fallacy "Starship Luxurious".

This is an extension of the "Rockets are Boats" fallacy. Passenger aircraft and luxury liners have their entire interior pressurized because so is everything else at sea level on a planet with a breathable atmosphere. For free. So careless starship designers, without a thought, made the unconscious assumption that spacecraft would be totally pressurized as well.

Wrong. Tain't no air in space, and atmosphere is expensive when you have to cart it up out of Terra's gravity well. Not to mention the expensive pressurized hull that has to encase it.

And it is not just the cost of hauling it up the gravity well, the spacecraft's engine has to accelerate the mass of all that junk. Every Gram Counts, so every gram of carpeting, atmosphere, and pressure hull is one less gram of payload, i.e., the reason the spacecraft was created in the first place. Payload is what you are being paid to load, less payload means less pay. See The Tyranny of the Rocket Equation.

In the real world, spacecraft will be mostly tanks of propellant, propulsion system, payload bays, and a lacy lattice-work of support struts holding everything together. The part the people live in will be a tiny pressurized habitat module tucked away somewhere.

Ignorant starship designers have the unconscious assumption that the important part of a spacecraft is the crew, so they designed ships with their priorities reversed. Their ships were mostly gigantic habitat modules with a tiny engine stuck to the rear. Their ships are also ludicrously wrong. If the designers thought about it at all, they might grudgingly include a tiny fuel tank. Which is like the cherry on top of their big icecream sundae of Fail.

So quit drawing ship blueprints with every square inch pressurized and human-accessible. On a real spacecraft if the ship's engineer has to repair the propulsion system, heat radiators, power plant, propellant tanks, or anything like that, they will have to put on their space suit. They will not have the luxury enjoyed by Scotty the engineer, waltzing down a carpeted floor in a shirt-sleeve atmosphere.


The Enterprise's corridors seemed awfully roomy, they were about twice as wide as they should have been. In fact, the whole ship was too roomy. Space is at a premium in any kind of enclosed environment. Anyone who's ever been aboard a submarine—or even an aircraft carrier, for that matter—knows that they are designed for the maximum utilization of their volume. Efficiency is a necessity, and a spaceship is going to have to be designed the same way.

In fact, the requirements of a spaceship are much more stringent—for instance, the interior atmosphere must be maintained with the correct combination of gases, at the right temperature, pressure, humidity, and ionization, to maintain not just the lives, but the comfort as well, of the crew. The margins for deviation are narrow; therefore, every cubic inch of interior volume means airspace that must be maintained—and maintenance requires the expenditure of energy. When you have to conserve your ship's power, you don't waste airspace.

The reason for such broad corridors? They had to be wide enough for a camera dolly, cables and a film crew.

To attempt to show that the ship was cramped would have required the construction of cramped sets—which are harder to work with and would have meant much more in the way of production time.

(So, instead, we're told that the ship has power to waste—it's implied, not specifically said. But if that's so, then we should never see a story in which maintenance of life-support functions become a critical factor for building suspense. A ship can't be both wasteful and limited.)

Another example: the turbo-elevators. These were the machines that took the various crew members from one part of the ship to another. Cood idea; especially as we are told that the thickest part of the Enterprise‘s disc is twelve stories thick.

But—the elevators seemed to be the only way to get from one deck to another. If the ship's power supply were cut off, every deck would be separated from every other. Oh, well, not really—somebody could always crawl through the air vents, or through the Jefferies Tube, or down one of the ladders which we saw very infrequently. All except for the bridge. Cut off the turbo-elevators and you isolate the bridge. Tsk. That's bad designing. Illogical.

Another one: the Captain's cabin. Or anybody’s cabin for that matter. They were all redresses of the same set. If any of those cabins had a bathroom, it was never shown. There weren't even any doors to imply a bathroom. We were never shown the cleanup facilities on the whole ship—not even the sick bay. And the Enterprise was on a five-year mission—isn't that a long time to hold it? Isn't that carrying it a bit too far?

Also, about those cabins—all of the major officers aboard the ship had their own cabins, and roomy places they were too. No complaint here, but what about the crew's quarters? Those were never even shown or suggested. Did each member of the crew have a cabin too? That would have made the Enterprise more of a hotel than a starship. Or did they have bunkrooms?

If they did have bunkrooms, how come they were never mentioned or shown? How come we never got into the crew's lives?

Or was the crew just a collection of some 400-odd androids to walk up and down the halls—scenery behind the main characters, to be moved around as necessary, but not really important to the story except as another part of the background to support the overall illusion?

From THE WORLD OF STAR TREK by by David Gerrold (1973)

The ships above are tail-sitters, so properly avoid the "wrong way is down" problem. But the artist made the second problem much worse. Apparently they figured the entire interior of the spacecraft was for habitable volume. Notice what they got wrong? Well, where the heck is the space for the rocket engines? I lay the blame for this at the artist, I know from experience that writer Jack Williamson knew better.

Modular Construction

An attractive notion is the practice of constructing one's spacecraft out of mix-and-match replaceable components. So if your spacecraft needs to do a planetary landing you can swap the low thrust ion drive for a high thrust chemical rocket. In Charles Sheffield's The MacAndrews Chronicles, the protagonist just calls her ship "the assembly", customized out of whatever modules it needs for the current mission contract.

For detailed examples, see the Boeing Space Tug, NASA Space Tug, JPL Modular Hab System, Minimal Volume Spacecraft Cabin, and the Flyaway Engine.

This will also make ships basically immortal. It will also make it really easy for space pirates to fence their captured prize ships. All they have to do is get the prize ship to the spacecraft equivalent to an automobile chop-shop. There the ship vanishes as an entity, becoming an inventory of laundered easily sold anonymous ship modules with the serial numbers filed off.

One can also imagine junker spacecraft, lashed together out of salvaged and/or junk-heap spacecraft modules by stone-broke would-be ship captains down on their luck.

Or mechanically inclined teenagers who want a ship. This would be much like teens in the United States back in the 1960's used to assemble automobiles out of parts scavenged from the junkyard, since they could not afford to purchase a new or used car. Such teens would gain incredible practical skills as spacecraft mechanics. I wonder if this is how Kaylee from Firefly learned her trade.

Yet another scenario is Our Hero stranded in the interplanetary Sargasso Sea of lost spacecraft, trying to scavenge enough working modules from three broken spacecraft in order to make one working spacecraft.

Rick Robinson notes that attractive as the concept is, there are some practical drawbacks to extreme modularity:

Rick Robinson: This is not necessarily an argument for true modular construction, with drive buses hitching up to payloads on an ad hoc basis like big-rig trucks and trailers. Building things to couple and uncouple adds complexity, mass, and cost — plug connectors, docking collars, and so forth. Moreover, drive buses intended for manned ships need to be human-rated, not just with higher safety factors but provision for supplying housekeeping power to the hab, etc. But these things, along with differing sizes or number of propellant tanks, and so forth, can all be minor variations in a drive bus design family.

Rick Robinson: True modularity is by no means a given. But some features of modularity, call it demi-modularity, are inherent to deep space technology.

You probably want to keep your propellant tanks separate from the corrosive, explosive stuff we breathe. Drive engines are essentially bolted onto the tail. Generally the major parts of a deep space ship don't have to fit together snugly. If you want to hang something out on a bracket you probably can.

Nyrath (me): There might be a brisk trade in "interface modules", that would connect modules made by different manufacturers. I'm reminded of the Apollo ASTP Docking Module used in the Apollo-Soyuz mission. This was a tiny airlock module with a NASA style docking collar on one end, and a Soviet style docking collar on the other.

Not to mention the International Space Station Pressurized mating adapters.

Rick Robinson: Modular design is always a tradeoff. You get more operational flexibility, at cost of more complicated/heavier/weaker connections. Integral designs will be favored when the components will consistently be used together.

Much will depend on tech. Torch type drives and even 'conventional' nuke electric drives pretty much have to be mounted on a pylon, which sort of invites the option of unbolting it from the rest. OTOH, as you note, the drive section may well have its own control center. And since the rest of the ship sits on top of the pylon, it's a fine line between 'pylon' and 'chassis.'

On naming, I could also make a case that the crew hab compartment is the main component, and so would be named. Especially if it is a spin gravity structure. And 'spaceships' may end up having more than one name, just as a named train might included Pullman cars with names of their own.

And if ships are highly modular, some terms might be borrowed from railroading. For example, 'consist' as a noun (pronounced CON-sist) for the whole assemblage. Thus, 'The Ty Cobb departed Mars with a consist of [such and such modules].'

Amusing side note: modular spacecraft reverse the order of trains: the 'locomotive' or drive engine is at the back (more precisely the base), while the 'caboose' or control cabin might well be at the front/top.

From SPACESHIP DESIGN 101 by Rick Robinson (2009)

The ship had several “holds,” actually just enormous, detachable cylinders adapted to carry cargo or passengers. Some of these were sealed and Shaw was reluctant to reveal what was in them. For an unabashed smuggler, that suggested to Thor that some things were unacceptable, even in the freewheeling society of the space settlers. Drive, holds and control were all in separate modules, connected by struts and passage tunnels. It was a common system for ships never intended to make planetfall, allowing great flexibility of size and function. “Also,” Shaw told Thor with a sharklike grin, “it makes it very difficult to keep up with how many and what type of ships are out here. If the authorities were looking for Spartacus, I'd break her up and rearrange her modules with other ships. You can have as many ships as you have command and drive modules.”

It must be a nightmare for customs authorities,” Thor observed.

“We do our humble best. Hijacked ships are never found again because they’re broken up and utilized or sold off as modules. You’ll have to go to a ship sale some time. There’s no pirate hangout like in the holos. Word just gets passed that there’s going to be ship hardware for sale and everybody just sort of congregates at a certain set of coordinates that all the bartenders seem to know about. I've seen whole government military vessels broken up and sold, weaponry and all.”

“Military!” Thor said, aghast. “I thought that was supposed to be impossible. Are there hijackers powerful enough to attack a Space Service ship?”

“Who attacks?” Shaw said. “Usually, it’s just a matter of paying someone to look the other way. The degree of corruption in the higher echelons of the military is immense and has increased tremendously in the last fifteen years. It was historically inevitable. I’ll let you read my monograph on the subject. There are other ways that service vessels make it onto the black market. Sometimes, a whole crew will decide to take early retirement from the service and bring their ship along with them.”

“I think that society out there will be quite different from what I anticipated,” Thor mused.

“I can guarantee it,” Shaw said.

From THE ISLAND WORLDS by Erick Kotani and John Maddox Roberts (1987)

Here's a fat-ass merchant ship that prefers to hand off its cargo in orbit but of course needs to land now and then. It would be a standard hull in Cepheus Engine or MgT First Edition or a partially streamlined hull in CT. It has no business landing anywhere without a beacon, landing lights, and a level field. But of course sometimes it has to. To facilitate docking the original ship has a docking module stuck on the top.

Whether you have rockets, reactionless drives (boo!) or warp drive handwaving docking is a maneuver that gives pilots grey hairs early on. The docking module bears the brunt of it. It's easier and faster to replace a module than an airlock that requires welding a heat resistant hull.

Big companies usually stocked a few modules at starports so if their merchant ships called and were in a hurry or banged their module up they could just swap modules and be on their way to make that hot delivery by the contract deadline. Swap modules and jump the hell out.

As an aside the module also held 10 dTons of cargo for really fast transfer of priority deliveries. If you were lucky the shipment was throw pillows. If you were unlucky munitions but hey no pressure there.

Then some ship builder realized there were a bunch of these 'kegs' just dumped in starports awaiting repair or surplussed. He bought one, stuck a drive and power plant on it and voila! a 20 dTon launch!

The damned things proved popular. They could aid in docking maneuvers since their little bitty engines had more fractional control than the big ship engines. But wait there's more!

The 'caps' at both ends on the keg came off easily (for repairs ... not while docking, that would be a bad thing masquerading as a design feature).

People began modding the modules.

In the 'standard' configuration this keg has the top module fitted with proper landing legs. It docks "nose in" the the ship and of course she can't be used to dock with this feature. If you expect to land her on rough terrain, say in the case of a delivery to a mining outpost on a rocky moon, they provide more stability than those stubby docking clamps.

More modules will be forthcoming. Suggestions will be entertained. I'm pretty sure there's a market for a fuel scoop version with a streamlining module (I know it breaks the at-the-time-of-construction rule but I feel it's justified.) There will be other modules with waldos and power tools for salvage and mining.

(ed note: you might find some inspiration in the modification kits for the Boeing space tug and the NASA space tug)

From SORRY BUT ... by Rob Garitta (2016)

Ship of Theseus Paradox

I thought of a problem with modular designs, based on the ancient Ship of Theseus paradox.

This is my Grandfather's ax.
This is my Grandfather's ax.
My Father replaced the handle.
I replaced the ax-head.
This is my Grandfather's ax.

Is it really still Grandfather's ax or not?

Plutarch first wrote about the paradox in 75 CE. But it was that 17th-century smart-ass Thomas Hobbes who slipped the exploding cigar into the box. He asked the question: what if somebody saves the original discarded handle and ax-head, then assembled them into a second ax. Which one of the two axes is Grandfather's ax? Both, neither, the new one, the old one?

This sounds academic, until you apply it to modular spacecraft.

For purposes of insurance, liability, national registration, contract penalties, mortgages, and a host of other expensive issues; it is crucially important to know the identity of the spacecraft in question. Which ship exactly is being referred to in all those legal documents?

But what if the SS SkyTrash's modules are replaced and the old modules used to make a new ship? Legally which one is the SkyTrash? For that matter, intentionally making a stolen ship vanish by passing it through a spaceship chop-shop can make another set of legal headaches.

The problem of spacecraft identity has got to be legally nailed down.

Don't look to the Theseus Paradox for a solution. The problem was stated almost two thousands years ago and they are still arguing about it

Off-hand I'm not sure what a fool-proof solution would be. My first thought was to attach the identity of the spacecraft to some sine qua non "must-have" ship module. Unfortunately there does not seem to be any. Not all ships are manned, so the habitat module won't work. The only must-have module I see is the propulsion bus (otherwise you have a space station, not a spacecraft). However Captain Affenpinscher might find it strange that the identity of her ship has changed just because she swapped out the propulsion module.

I had a discussion on Google Plus with some of my brain-trust:

Winchell Chung (me): I was mulling over modular spacecraft design, when I suddenly realized I had a "Ship of Theseus" paradox on my hands. Does anybody have any bright ideas about where the legal identity of a spacecraft resides in?

Ray McVay: Keel? Drive core? The route, like railroad trains? In Black Desert, it would be with the AI (spacecraft's installed artificial intelligence)... Annabelle Li (a ship AI) has been two kinds of Heinlein and a CASSTOR, but kept her name. Possibly valid inspection certs for the given configuration is the legal identity of a ship... After all, no transit authority will let one boost in a franken-rocket without giving it a once-over...

John Reiher: I had a thought. There's one component that never changes on any ship. It can be updated, but it can't be replaced, otherwise the Insurance companies will void your policies.

What is it?

The Vehicle Identification Number (VIN) Box. It's a transponder with a unique ID and call sign. It can be customized at time of purchase, but it is the "ship". Anything attached to it becomes the ship.

No VIN Box, no ship.

Two or more VIN Boxes on a single ship, you're breaking the law and you have to inactivate all but one of them.

This is something the Banking and Insurance companies would come up with. They need something that can be unique to each ship, and nothing is more unique than a government issued, sealed, black box VIN Box.

William Black: Modular freighters in my future history setting the Command Module (CMOD) is what the freighter captain actually owns — in a single owner operator context, a transportation company being a single individual or corporate entity that owns many individual CMOD's. However I like +John Reiher's suggestion that the VIN box be the source of identity, and +Raymond McVay's suggestion that identity is locked to the ships AI has obvious merit especially in regards to spacecraft-as-characters. I may steal borrow either or both concepts with a note of attribution.

Raymond McVay +William Black, I proposed something similar in terms of Command Modules myself back in the day. The article actually illustrates +Winchell Chung 's point rather well.

John Reiher Nice thing about a VIN Box is that the issuing authority can put a time limit on how long it's good for before you have to go and get it renewed. Think of it as license plates for spaceships.

Winchell Chung Wow, lots of good ideas here.

I had dismissed the idea of using the habitat module for ship ID because a robot unmanned ship would not have one. But +William Black idea of command module has some appeal. If you narrow it to the module that has actuators and computers controlling the various other modules. So the command module is a box with cables or BlueTooth connections to control all the other ship modules. Plus an I/O port for the captain to issue commands to the command module, and so the CMOD can report status reading to the captain.

This would indeed make the CMOD the sine qua non of a spacecraft, worthy to be indelibly embossed with a serial number usable as the ship's identity.

Plug in a human usable control panel into the CMOD I/O for human manned ships.
Plug in an AI interface connecting the AI computer to the CMOD I/O for AI manned ships.
Plug in a sequencer interface connecting a moron computer to the CMOD I/O allowing the moron computer to execute a pre-programmed set of commands as if it were a space-going player piano.
Plug in a radio interface connecting a radio to the CMOD I/O for a remote-controlled drone ship.
Or any combination of the above

+Raymond McVay's idea of the ID of the ship tied to its AI has merit. The only thing is AIs are absurdly easy to clone (once you crack the DRM copy protection). It is hard to stamp a serial number on software running in a computer. Especially if the AI software is self-modifying, as human being are. How does one distinguish one AI from another?

+John Reiher idea for a VIN Box is probably my favorite. It is very much the sort of thing that banking and insurance companies would come up with. And the requirement for renewability makes perfect sense. However it must always contain a legal ID, for liability purposes. Much like automobiles. If somebody crashes their car into a building or something else expensive, then flees the scene on foot, the car's license plate may be expired but it still allows the police and building owners to discover who is liable for the damages.

This also vaguely reminds me about ship transponders in the Traveller role playing game. They constantly broadcasts the ship's unique ID and location. Civilian starships are legally required to have transponders always turned on, unless there are extenuating situations. Such as pirate corsair ships in the area, using the transponder to home in on their prey.

William Black I was thinking about that very same problem with AI's last night and this AM, as Winchell points out AIs are absurdly easy to clone. It is hard to stamp a serial number on software running in a computer. How does one distinguish one AI from another?

All I could come up with is this: A ship's AI is hardware configured so its inputs and outputs must plug in through an interface that is part of the VIN box, and these are highly tamper resistant. Both the AI and the VIN box are hardened in various ways, with both physical and with hardware/software safeguards against tampering.

In other words a ship's AI cannot function without a registered spacecraft with legal ownership.

Under the heading of future crime: Cracking the AI-VIN box security feature.

It's probably not impossible to do, but it might be very, very, difficult to do — a man with the right skill set could sell his labor at a high price on the black market. Skills would be comparable to a high-level safe-cracking expert combined with high level hardware/software expertise.

I was thinking of similar safeguards in relation to nuclear pulse systems in my setting as well.

John Reiher Actually, the current lines of research into AIs indicate that they will be very hard to "clone" as they will be as much hardware as human minds are. You can copy the data, but not the mind. (Unlike human brains, which have no I/O ports.)

But I do like +William Black's idea of tying the AI to the ship's VIN Box. That way the ship's AI is really part of the ship. But that lasts until AI's get equal rights. Then they will operate independently of a ship's VIN Box. Of course if they do get equal rights, there's nothing stopping one from buying their own VIN Box and leasing their "ship" to whoever can afford their terms.

Alistair “Cerebrate” Young I'm pretty sure in my 'verse the legal identity of a starship is vested in the leatherbound data rod/smart-paper folio in a safe in the captain's office (or welded in a suitable location on a drone ship), which is to say, the certificate of registry.

Components may come and components may go — and the Flight Administrator will faithfully update said folio's documentation of said components[1] each time or be hauled up in front of an Admiralty Court for seriously violating the Imperial Navigation Act — but CMS Gorram Freebies, Hull Number Eleven-Oh-Seven-Four-Two-Niner remains CMS Gorram Freebies, Hull Number Eleven-Oh-Seven-Four-Two-Niner whatever gets replaced up to and including said hull as long as it's operating under the same non-decommissioned certificate of registry.

(Of course, the bank that holds the note on your starship may have its own documented ideas on what exactly it holds the note on, and should the registry and the mortgage get out of sync on this point, your life may become... interesting.)

(ed note: a visit from the Repo Man)

((Equally of course, the spec plat in the engineering computers will also have its own documented ideas about what the ship's made of and can be expected to do, and when it gets out of sync, your life may also become interesting.

Also, depending on how out of sync it is with reality, potentially hot, noisy, and short.))

(ed note: spacecraft undergoes rapid explosive disassembly because the engineering computer's mathematical model of the spacecraft did not match reality)

[1] I like to imagine this as a nice hierarchically-organized document that begins with: Starship, free trader, Kalantha-class: 1
And ends with something like: Rivet, 4mm: 18,297

John Reiher Papers can be forged, data copied and manipulated. You need something that breaks if someone tries to open it. That's where the VIN Box comes in. Trying to crack one open is a operation in futility. The ID in the VIN Box is one half of a public encryption key that mates with a governmental key to validate your ship. If the two don't mate, you have a hacked box.

The Key that produces the public keys is private and in its own black box. The guys in the Ship Registry office just know that they have theses boxes that need ship's names, and that they already have IDs ready to go.

As an aside, you realize that we've been using modular space ships ever since we started building them. What do you think the Saturn V is? It's a one use modular rocket that you can put anything you want on top of the booster section and throw into orbit.

So that implies that a modular ship will have very good connections between the parts. On par with what a multi-stage rocket uses. Docking connections are just that, docking connections, and may or may not have the necessary structural strength for a space ship.

The type of connections that make up a modular ship most folks would call "permanent" but to folks in the business, they are temporary.

Alistair “Cerebrate” Young Side note on registries and transponders: of course, the Worlds being a not-exactly-unified group of polities, actual requirements on these points vary widely.

Even leaving aside the anarchic Rim Free Zone (the entirety of whose admiralty law could be summed up as "try not to hit anything that might complain"), the Accord on the Law of Free Space leaves it at the minimal "you should have a certificate of registry and a transponder that will squawk it out when queried". Local regulations, on the other hand...

(The Empire, for example, which holds that sovereignty begins with the individual, will happily accept self-signed registries and doesn't require much transponding, although lacking functionality in this area may leave you restricted to operating VFR and staying outside all regions of controlled space.

The other end of this particular bell curve is the Hope Hegemony, which wants transponders willing to disgorge pretty much any information you can think of on demand, including your code-signed visa from the Hegemony Bureau of Navigation and remote-slave ackles for your starship, and legally defines any vessel without such as "debris, subject to salvage and/or destruction at discretion".)

(ed note: "ackles" is Access-Control List. "Remote-slave ackles" means to grant the Hope Hegemony government the ability to seize command of your ship and fly it by remote control, at their whim)

John Reiher So +Alistair Young in this setting, if you want to be safe, you register your ship in the Hope Hegemony, which is the most restrictive polity and go elsewhere to do your business. (Or at least get a HH compatible VIN Box). 

Alistair “Cerebrate” Young Well, it depends on what you mean by "safe"...

Flying around most of the Worlds with an HH registry is kind of like sailing around Earth in a North Korean-flagged ship. It may make you welcome in Pyongyang, but the association with the local crazies may not do you any favors elsewhere...

(Common wisdom would probably suggest either a registry from one of the inoffensive, minor, single-system polities that hasn't really had the chance to offend anyone yet, or possibly an Imperial registry on the grounds that while they have offended lots of people, they're also notorious for sending gunboats after people who trouble them and theirs, so...

Neither of those is safe everywhere, though, which is why the Starfall Arc Free Merchant Confraternity suggests that the wise smuggler free trader goes to the trouble of discreetly procuring a number of suitable registries...)

+John Reiher I'll admit to being rather cynical where uncrackable devices are concerned, mostly because over my past IT career I've spent an awful lot of time watching notionally uncrackable software and hardware both be cracked, commonly within about a week of shipping...

So, the way I look at it is the standard software security way — never trust the client. If the papers and their cryptosignatures and the ship's "biometrics" and whatever other information you can gather match a recent update of the issuer's copy of the database, you can probably be sure that they are who they say they are, or at least who the issuer thinks they are. If you don't have that handy to verify against... well, it's time to take your best guess. 

Winchell Chung +John Reiher said: of course if they (AIs) do get equal rights, there's nothing stopping one from buying their own VIN box and leasing their "ship" to whoever can afford their terms.

Hmmmmm, interesting. I am reminded of the Spline aliens from Stephen Baxter's Xeelee novels. They are whale-like aliens who genetically engineered themselves to be exceptionally good spacecraft. They then rent themselves out to other races as spaceships for hire.

This also reminds me of the brain and brawn ships from Anne McCaffrey's The Ship Who Sang. The parents of severely deformed babies are given the option of having the baby engineered into becoming a shell person. They are encased in titanium shells and given a brain-computer link. Among other things they can be plugged into a starship, making a living ship. The process is expensive so the shell people come of age with heavy debts which they must work off in order to become free agents.

McCaffrey said: "I remember reading a story about a woman searching for her son's brain, it had been used for an autopilot on an ore ship and she wanted to find it and give it surcease. And I thought what if severely disabled people were given a chance to become starships? So that's how The Ship Who Sang was born."

John Reiher +Alistair Young it's a given that most hackers can crack the security of simple systems, but I'm talking about an ID that's at least 128 characters long, using the entirety of the UNICODE font character set, you have almost 39,000 glyphs. It will take the heat death of the universe to crack that code.

It would be easier to get a bootleg one that's already registered and in the system.

Also, I'm using a setting where while there are multiple governments, they do have treaties with each other and have agreed upon a common ship's registry system just to keep the confusion down and to prevent what you proposed: Smugglers with multiple transponders.

+Winchell Chung I'm also reminded of Eric the brain in the jar from Niven's Becalmed In Hell and The Coldest Place. He was the ship and could be put into any vessel as best as I can remember. 

Alistair “Cerebrate” Young +John Reiher Well, that depends on your methods. 128 UTF-16 characters — well, that's basically a 2048-bit key, and on average, sure, an RSS key of that length will take 6.4 quadrillion years to crack by brute force .

But we've cracked lots of them in reality by other methods: usually involving taking advantage of things like patterned data (even using only printing characters, rather than the entire code space, will halve the effective entropy of the key); flawed algorithm implementations; finding the government's or corporation's back-door they so often have left for themselves; or information leaks because there's always good old rubber-hose (torture) or brown-envelope (bribery) cryptanalysis.

(It also assumes no-one's invented quantum computing or found a constructive proof that P=NP.)

One advantage of checking against an external database is that, in theory, cracking the client won't do you any good because the database still won't match and cracking the db should be much harder without physical access, etc. It's just that that then begs the question of why it's worth bothering to secure the client in the first place.

...of course, verifying against an external db has its own issues of synchronization and light-lag, such as when Cap'n Harbatkin squawks an id that isn't in your local database and claims, on asking, that he updated his registry back on Flern and it's not his problem that you haven't got the updates yet. Is he a smuggler with a random number generator, or is he a legitimate trader whose lobby group will be screaming for your head on a spike if you hold him while you query Flern for verification and wait for an update to come back...?

John Reiher True, there is that, but still, you'd have to be a government or a corp to afford the computing power to crack one... OK, or have a botnet that doesn't go down because someone starts downloading pr0n.

But now we're falling into one-upmanship and that game never ends well. So I'll concede that there ain't no such thing as perfect security. Just good enough that only professionals and governments will bother to try to crack it.

Well, one solution is that your external db contains registry IDs that haven't been issued yet, but has a VIN Box waiting to be issued.

Of course that means if you can get one of those VIN Boxes illegally, then you got a valid ID. Or better yet, bribe the registrar of some backwater world to issue you three or four for use as you see fit.

From private conversation on Google Plus (2015)

If modular design is taken to its limit, "ships" will have no permanent existence. Instead they will be assembled out of modules and pods specifically for each run, much like a railroad train.

In that case, a ship's identity is attached to a service, not a physical structure. Example: the Santa Fe "Chief" was identified by a timetable and reputation, not a particular set of locomotive and cars.

From INTERSTELLAR TRADE: A PRIMER by Rick Robinson (2001)

(ed note: Veyndayk, Velmeran, Dveyella, and Keth are Starwolves. They are in the business of capturing warships of the stodgy bureaucratic interstellar Union and selling said warships back to the Union.)

     Soon they saw that it was Veyndayk, the cargo supervisor.
     "Business done," he said, stepping up to join Velmeran and Dveyella at the rail where they had been watching traffic pass on the level below.
     "Did you sell Keth back to the Sector Commander?" Velmeran asked.
     Veyndayk laughed. "No, although that might be a good use for old Starwolves. Farstell Freight and Trade bought back a shipment of clothing, conveniently packed in their own shipping containers. And (Union) fleet ordnance has just now payed us a finder's fee for an intact cutter."
     "A cutter?" Velmeran asked. Cutters were the smallest of the military ships, hardly bigger than a transport, and generally used only for police work.
     "My little joke," Veyndayk explained. "We took two intact cutters as riders on salvaged battleships, and one we have had sitting in a forward bay for the last year. We took them apart down to the smallest bolt and rebuilt the ships by taking parts at random. Now I am going to collect finder's fees on those ships in three different ports. That should give the boys in fleet ordnance fits, when they cross-check serial numbers of those parts."
     That appealed to Dveyella, who liked frustrating Union officials best of all. "You know, they will not be able to use those ships until they take them apart and rebuild them as they originally were."
     "You laugh, but that is probably the truth," the cargo officer said.

From THE STARWOLVES by Thorarinn Gunnarsson (1988)

Specialized Ship Types

This section has been moved here

Bare Bones Examples


      Now, to regress to the region of solar system transportation. I'm going to confine my discussion almost completely to gaseous fission rockets. The reason is not that I have decided that they are the things to be used rather than Orion or electrical rockets. The reason is simply that I have done more thinking about them. I think that I can see how to combine gaseous fission engines with advanced vehicles more efficiently than is possible with the other engine types. I believe that not enough thought has been given to the engine/vehicle interaction. There was a statement in the letter of invitation to this symposium to the effect that everybody knows what to do with 1800 seconds specific impulse. I happen to disagree. I don't think anybody knows what to do with 1800 seconds specific impulse, and I think we wouldn't know what to do with a good space engine if it walked up and bit us. I will try to prove those opinions as I go. I've used the "we" advisedly because I don't think I know either. Rather than getting into a big mish-mash by attempting to cover all various forms of propulsion, I will just stick with some mythical gaseous fission rockets. Presumably in the next 3 days we will discuss which engines you really should do this with, if you should do it at all.

     Figure 6 is a presentation of operating cost in dollars per pound of payload versus total velocity capability for single-stage vehicles with chemical, nuclear solid-core, and two different kinds of gaseous fission rocket engines. These curves were calculated four years ago when I was at Douglas for a paper by myself, Bill Mathiesen and Bob Trapp that was given at the I.A.F. Congress in Stockholm. We thought this was pretty interesting, but other than shocking an occasional person here and there, not very much has happened as a result. The thing that has been interesting to me is the fact that here was a clear indication that one could get out to extremely high velocities for a very low transportation cost. Velocities so high that you could open up the whole solar system for exploration with reasonable costs. Yet almost everyone believes it is extremely hard to do a little bit of space flight down in the low velocity region where we talk about just barely going to the moon.

     Figure 6 also is an interesting indication of the fact that you shouldn't drive a rocket faster than it wants to be driven. This is something that apparently a lot of people are forgetting. Theoretically, a rocket can go up to any velocity, not counting Einstein. But the way to make an inefficient rocket go to very high velocity is to stage it, and stage it, and stage it. One carries fuel to carry the fuel to carry the fuel that's going to be used later. Once the weight starts to pyramid, it's a logarithmic function and it just plain gets ridiculous in a hurry. So one has to be very careful about taking something like a solid-core nuclear rocket and deciding to perform missions at 200,000 fps. You can stack tip all that equipment if yqu want to do it, but it's a horrendous thing.

     A question that has bothered me quite a bit is why more attention was not paid to these curves. Consequently, I'm going to break down some of the assumptions used in these curves, then build new curves back up with this year's assumptions. Perhaps I can make the story more believable.

     Now, there are two big ringers in the curves of Figure 6. One is the obvious one. At that stage of the game, nobody had the foggiest idea of how to build a gaseous fission engine at all, let alone one with 5,000 to 20,000 seconds specific impulse. That, right off the bat, caused everybody to throw up their hands and forget it. The second ringer is that we used transport airplane operating cost assumptions. To put it mildly, we used recovery and reuse assumptions which were not the standard thing in rocket work. I'm going to examine both of these assumptions in today's light.

     Since so little was known of gaseous fission engines four years ago, the previous study assumed a thrust/weight ratio of 30 independent of specific impulse. This was recognized to be a very sporty assumption since, even if the containment problem coUld be solved, the achievement of specific impulses beyond about 3,000 seconds requires the use of a radiator to reject excess heat which cannot be handled by the thermal capacity of the propellant utilized (excess heat above what can be handled by open-cycle cooling). Although it was originally felt that such radiators would represent an intolerable decrease of thrust/weight ratio, it has since been pointed out that this is not true if high temperature radiators are used. Figure 7 shows a current estimate of the variation of the thrust/weight ratio with Isp achievable for a gaseous nuclear rocket system with radiator using as a basis an assumed thrust/weight ratio of 20, at an Isp of 2,500 seconds. The values fall off substantially at high specific impulses compared to the assumptions of 4 years ago, but are still greater than one to beyond 10,000 seconds specific impulse.

     It should be pointed out that the use of water, ammonia, methane, or other non-hydrogen working fluids should be seriously considered in gaseous fission engines from the start. Not only are better ship designs permissible due to small tankage sizes and ease of propellant storability, but the use of a higher density propellant might well ease the fuel containment problem if a vortex system is used. If so, it could result in smaller, lighter engines. It might also result in earlier development programs if the ability to prove adequate containment occurred at an earlier time.

(ed note: In thermal rockets, hydrogen produces the best specific impulse. But on the minus side: hydrogen tanks have to be huge because of low density, and it is a pain in the ass to store because is is cryogentic and needs lots of refrigeration)

     An interesting result in Figure 7 is that the value of thrust/weight ratio as the specific impulse approaches 10,000 seconds is independent of propellant used. This is because the reduced propellant flow at such a high specific impulse results in such small thermal capacity in the incoming fuel that the engine must be almost completely cooled by the radiator system. The propellant to fuel burned ratio required for a given specific impulse is independent of propellant used. Hence, the engine uses the same amount of energy to generate a given specific impulse, the same fraction of energy must be rejected by the radiator, and the radiator area is unaffected by the type of propellant used.

     It seems clear that an engine design cooled by radiator alone should be investigated. Such an engine might be easier to develop since a major interaction between propellant and cooling system would be severed. Furthermore, such an engine could more easily use a variety of propellants. This could be very helpful in early planetary exploring.

     A limitation on specific impulse of 10,000 seconds has been shown tentatively in Figure 7, assuming that the engine would be of the type which transfers heat from the fission plasma to the propellant by radiation. This is due to an unfortunate tendency of the propellants examined to date. Although adequately opaque to absorb the radiant energy at medium-high temperatures, they apparently become transparent at very high temperatures. At the moment seeding the flow, which is very effective at low temperatures, does not look promising at high temperatures.

(ed note: "seeding the flow" means putting tungsten dust in the propellant. Otherwise the heat radiated by the nuclear reaction fails to heat up the transparent propellant, and instead the rocket engine is destroyed)

     One other point of interest in connection with the thrust/weight ratios of gaseous fission engines is the power conversion weight thus achieved. Electrical propulsion enthusiasts feel extremely optimistic when power conversion weights of the order of 10 pounds per kilowatt are mentioned. A gaseous fission engine of 2,500 seconds Isp and T/W of 20 achieves about one-thousandth of a pound per kilowatt. In other words, gaseous fission engines are almost certain to be 10,000 times better than electrical rockets in power conversion weights. The fabulous effect of this number on spaceship design must be understood if anyone expects to make rational development decisions on future propulsion systems.

     I can defend the 1963 curves of Figure 7 today. Not very well, but at least I can begin to defend them. Four years ago, the 1960 assumption was nothing that could be defended at all. However, I have always liked the calculations we made then. It influenced me in feeling strongly that, at least theoretically, there was a great deal more which could be done with nuclear rockets than anyone realized, far more than just trivial improvements in our current systems.

     As part of the process of understanding spaceship operating costs, it is instructive to consider first only the fuel and propellant cost. This is true because this cost represents the minimum achievable. It is important to understand the mechanics of achieving a low fuel and propellant cost, particularly when truly reusable ships are used. In transport aircraft practice, the amount of reuse is so high that initial airframe costs are only a small fraction of the operating cost, and fuel costs represent about one-half the total. Thus, we shall examine fuel costs for their basic limitations on performance, and then see how closely these limits can be approached with reusable ships.

     Fuel and propellant costs as a function of total velocity increment for chemical, solid core nuclear, and gaseous nuclear rockets are shown in Figure 8. Compared to the assumptions of Figure 6, the specific impulse of the high energy chemical has been increased to represent a modern, high-pressure system: the solid core nuclear has been decreased in view of current development difficulties: and a number of different propellants and degrees of containment are shown for gaseous fission engines. All curves are for single-stage ships with structural assumptions more conservative than those of Figure 6 and each specifically sized for the velocity shown.

     It is evident that, on this basis alone, a gaseous fission engine without radiators and with separation ratio of 10-3 is not significantly better than a solid core engine. Gaseous engines with better containment would be much better. It is also evident that gaseous engines with space radiators, but with specific impulse limited to 10,000 seconds, can drive ships up to about one-half million feet per second and still maintain reasonable fuel cost. The attainment of a fuel separation ratio of 10-4 is almost as effective as perfect fuel containment.

     The optimum fuel cost curves for gaseous fission engines with radiators were obtained by determining the optimum specific impulse for each velocity and separation ratio. This is necessary since too low a specific impulse will result in excessive propellant cost while too high a specific impulse will result in excessive fuel cost. The optimum specific impulse is much higher than 10,000 seconds for all velocities beyond a few hundred thousand feet per second. Hence, these curves represent a future capability presently unattainable due to the propellant transparency problem at high temperatures previously mentioned. If it were not for this, gaseous fission ships could be driven to almost one million feet per second before fuel costs became a limitation.

     Under certain circumstances, a great deal can be learned about spaceship design without a detailed knowledge of the missions to be performed. In recent years, there has been a tendency to become so detail-mission-oriented that ship design is not even attempted until the exact mission is clearly understood. This may be a valid procedure when only one mission is in sight, although even then the inevitable lack of versatility usually leads to needless redesign much earlier than anticipated. In transport airplane design, the basic design procedure usually centers around the calculation of airplane operating characteristics as a function of range. The maximum range required comes from a knowledge of the total mission complex, but the airplane design is refined primarily by using general curves as a function of range, rather than by a detailed series of specific mission analyses.

     When considering total solar system transportation as we are, it is. clear that we face a variety of missions. It is also very unclear as to which of these will be paramount. One way of approaching the problem is to present the characteristics of the vehicle as a function of total velocity increment which the ship can achieve. This is exactly analogous to the use of range in aircraft design practice. In this way, an understanding of the ship's basic ability to deliver payload to a certain speed economically can be rather easily understood. The complex mission analyses, then, can be made to reflect the maximum design velocity increment required.

     This approach was actually used in the 1960 study, and Figure 6 represents one of the results. Figure 6, however, contains assumptions as to degree of reuse achieved by the vehicle which, although consistent with transport aircraft practice, may not apply to space transportation. At least, if they do, their application must be better documented.

     The 1960 study assumed a large number of reuses per vehicle, somewhat analogous to the number of times a transport airplane is reused. A transport aircraft is actually utilized about 50 percent of the time, and average flight durations are less than 4 hours. It is clear, therefore, that such vehicles are used over one thousand times per year. However, space travel durations are much longer, and it is obvious that the interaction between travel duration and number of reuses must be considered.

     For the lunar mission, it is clear that large numbers of reuses are feasible. Typically, 100 flights per year (50 each way) can be envisioned on the basis of 2-day travel times, one day turn around time at each terminal, with Sundays and 2 weeks off for vacation. Over a 10-year ship lifetime, 1,000 uses will be achieved.

     One can get a feeling for the number of interplanetary uses by assuming a certain ship total life. Typically, transport aircraft are designed for 40,000 hours (4.6 years) total life. On the basis of slightly less than 50 percent utilization, such a vehicle would last for 10 years. They always last much longer, but the amortization time of the airframe is usually about 40,000 hours, since new equipment always becomes available in even shorter time.

     Selecting a suitable lifetime for a spaceship presents a considerable technical dilemma. One viewpoint would simply take 10 years as above. An even shorter lifetime might technically be justified due to the severe aerodynamic environments associated with atmospheric entries, and the generally unknown operational environment of space. This type of assumption has become standard in this country recently. If you don't understand the problem, assume it's horrible.

     It may well be, however, that spaceships will last much longer than transport aircraft. The transport has its main propulsion system operating continually during flight, and is also continually facing the temperatures and gust loads within our atmosphere. The question is whether spaceship operating life should be determined by the total time of operation, or only by the times during which the main engines operate and/or it is within an atmosphere. In other words, is a spaceship coasting between planets actually operating in the aircraft transport sense, or is it merely parked in space, breathing quietly, waiting for its next mission.

     One can make an excellent case for the latter point of view in terms of the general environment that the ship faces, either from space or its own propulsion systems, while coasting. The ship would have to be on interplanetary runs for several centuries in order to build up 40,000 hours of engine and atmospheric operation. It is, however, bound to be replaced by better equipment within a few decades. As a base for calculations, this report assumes 25 years ship useful lifetime.

     The variation of various weights as a function of velocity is shown in Figure 9 for both specific impulse limited to 10,000 seconds and for the optimum specific impulse. These curves are for ships designed for 20 percent payload, then operated at lower velocities by off-loading propellant and at higher velocities by off-loading payload. Thus, these curves represent a penalty for using a single ship for multiple missions, just as in other forms of transportation.

     By using suitable planetary travel time data, which is not yet easy to come by, the weight data of Figure 9, the same fuel plus propellant cost assumptions as in Figure 8, and assuming a vehicle cost of $100 per pound, the curves of Figure 10 were obtained. One hundred dollars per pound is the currently estimated cost of a supersonic transport. These curves show fuel cost plus amortized airframe cost as a function of design velocity increment for the missions selected.

     The lowest curves on Figure 10 are fuel cost only. Comparing them with the other curves show that for operations as far out as the planet Saturn, the structural costs are comparable to fuel costs. Further improvements in convenience of operation can be achieved with engines not limited to 10,000 seconds specific impulse. In that case, velocity increments beyond a half-million feet per second are economically reasonable.

     The average travel time between planets corresponding to the velocities of Figure 10 are shown in Figure 11. These two Figures taken together give a better feel for solar system transportation than Figure 6 alone. With specific impulse limited to 10,000 seconds, the solar system as far as Jupiter is available with travel times not exceeding 4 months. Inner solar system travel times need not exceed 2 months. The advantage of optimum specific impulse becomes more evident at Saturn and beyond.

     The curves of Figures 10 and 11 apply for a given ship design velocity only if the ship can be refueled at each terminal. If it must carry its own fuel for the return journey, then it must operate at half the total velocity shown. Except for Pluto, refueling bases at the major planets are much more needed than at the minor ones, as can be seen by Figure 10. Refueling bases could be expected to be located on the surfaces of all the minor planets, although it may require some design effort in the case of Venus and Pluto.

     The major planets are a different situation. Their surfaces are extremely forbidding as far as we know, to the extent that we are not even sure they have solid surfaces. It makes sense in that case to establish bases on one of the satellites of each of the four major planets. The curves are drawn with that assumption. If we do decide to penetrate to the surface of these planets, then the velocity requirements for doing this when operating from one of the satellites is a reasonable number. Thus, bases on the larger planets' satellites not only greatly facilitate the convenience of transportation, but also present a reasonable base for surface exploration, if required.

     These particular curves are also calculated for the average flight times involved in year around operations between all planets. There are no launch window restrictions. If you're ever going to have a transportation system, you're going to have to be able to go when you want to. You cannot spend most of the time waiting. A rough averaging process between the best and worst times of the year was used in. an attempt to make this a realistic transportation assumption.

     A few words about perspective on these curves are in order. There is nothing magic here except a ridiculous willingness to plot curves wherever the data is leading, rather than stopping somewhere. Both better and worse situations may well occur. Even the case of specific impulse limited to 10,000 seconds requires gaseous fission engines with radiators, and most people today would rather agree to engines without radiators. In that case, the velocity increment achieved will be only about 25 percent of the curves shown. Furthermore, the economic penalty of, if necessary, ejecting a critical mass of fuel in the process of shutting down the engine has not been included. This will be on the order of $100,000 per shutdown ($778,160 in 2017 dollars).

     On the other hand, perfect containment might be achieved. We might design ships for each velocity increment, rather than use the single design assumed here. Furthermore, one can get a greater utilization of vehicles by the expedient of refueling the vehicles which go on deep space missions. This is preferable to multi-stage vehicles, since a fleet of ships used for refueling can also be used for other missions. No attempt will be made here to present detailed curves showing the effects of refueling. Cursory checks show that over 200,000 fps can be added for reasonable cost with only two refuelings.

     The greatest conservatism of all in Figures 10 and 11 is, of course, in the magnitude of the ordinate scale. Costs beyond $12 per pound have not been plotted so that the entire set of curves is about 100 to 1,000 times lower than virtually all space cost analyses to date. This must be clearly remembered as we discuss the performance of these ships.

     I can't resist making one more solar system point here. So far, only travel between Earth and the other planets has been discussed. There is also the question of travel between planets other than Earth. The use of bases in other parts of the solar system to aid in the exploration of the even more remote portions should be considered. In fact, such considerations might well dictate the strategic location of bases.

     At first thought, it would seem to be a good idea, for instance, to use a base on one of the farther planets, say Saturn, to permit further exploration of the more remote planets like Pluto. Although this is an intriguing thought, such deep bases will have only limited utility. The reason is the extremely long synodic periods which exist among the outer planets since they move so slowly around the Sun. In the worst case of all, the synodic period between Neptune and Pluto is slightly over 500 years. In addition to the long synodic period, the difference between travel at the optimum time of the year and the worst time of the year becomes more extreme the farther the planet is located from the Sun.

     One way of illustrating this is shown in Figure 12, where the effects of basing on selected planets is shown for a constant ship velocity. It is true that a deep space base will be closer than Earth to the other deep space objects when in favorable position, but equally true that it will be much farther away during the worst conditions. Surprisingly enough, the base wants to be reasonably close to the Sun, once again emphasizing that the Sun is the center of the solar system. Although Mercury might be the best planetary base of all, the Earth is still sufficiently close to the Sun that it represents a pretty good compromise. Thus, the major space logistics support operations could, from a celestial mechanics viewpoint, be located efficiently on the Earth or its Moon. This is very convenient since the known industrial and research bases of the solar system also happen to be located in that vicinity.

     The slow movement of the outer planets leads to some interesting paradoxes. One would naturally assume that a base on Triton would be an excellent place from which to explore Pluto, since Neptune is at 30.09 A.U. from the Sun, while Pluto is 39.5 A.U. However, it turns out that Neptune at the moment is already leading Pluto around the Sun, and pulling away. In fact, in approximately 9 years, Neptune will be farther away from Pluto than Earth ever is. Furthermore, due to the long synodic period of Neptune and Pluto, that statement will be true for somewhat over the next 300 years. It would be nice to be sure that every other statement in this discussion will be true for that duration.

     I threw that in as a bit of tidbit. I' m gradually working back down to our more normal systems, and there's a point that I want to harp on further — that is, this whole question of structural reuse. As I indicated before, Figures 10 and 11 show costs that are less than $10 per pound throughout the entire solar system. Yet large and elaborate studies are made these days proving that it's going to take many hundred dollars per pound to go to the moon, no matter what we do with recovery, reuse, or anything else. It1 s quite clear that either I'm insane, or a lot of other people are, or we have to have an explanation. It was easier to give an explanation than to prove everyone else insane.

     Figure 6 was drawn with what I like to refer to as "transportation" type assumptions for operating cost. Maintenance costs, for instance, were taken from normal air transport practice. The vehicles were assumed to be as reusable as transport aircraft. This is not the fashion when calculating rocket operating costs today. Almost all of our rocket builders, including myself, have been doing nothing but "ammunition" work for a large number of years. It is axiomatic that, if you want to talk about recovery and reuse, you should not talk to an ammunition builder.

     To illustrate what can happen with different classes of maintenance and reuse assumption, I will shift gears drasticaily and discuss merely placing objects on Earth orbit with chemical propulsion. In Figure 13, I assumed, arbitrarily, about $300.00 per pound of payload as typical of current day orbital transportation systems with no reuse at all. It so happens, however, that if you consider the actual price of high energy fuel needed for orbital velocities with advanced rockets, it is only on the order of $1.00 per pound of payload. Figure 13 is simply a plot of operating cost as a function of recovery reliability and refurbishment cost with these assumptions.

     It is quite fashionable, whenever over-all system analyses for recoverable space vehicles are performed, to assume that recovery reliabilities will be around 75 percent. After all, that is the recovery experience to date. Also, refurbishment costs around 25 percent are quite likely to be used. The shaded region brackets these assumptions, and is typical of a good, solid, rational ammunition type analysis. It is evident that after spending that much money on refurbishment between flights with that low a recovery reliability, an improvement of at most two in over-all cost performance is the best to be expected.

     Also shown on Figure 13 is what had already been achieved many decades ago in air transportation. This is what happens when you think like a transportation man. The recovery reliability is so close to 1.00 that you can't possibly see it on this scale. The same is true of the maintenance cost, which is on the order of 0.04 percent. If anyone here thinks that a DC-8 is less complicated than a Thor, just take a good look at the inside workings of a DC-8 some day. Wonder, then, at the fact that a few people turn it around, give it some fuel, pat it on the head, and it takes off again. This is what we should be trying for in future spaceships. There is an improvement of a factor of 100 over current operations to be made. I do not want you to get the impression that I am all for airplane designers. I think they, too, are irrational conservatives. But if useful design techniques have been developed, I think they should be used in space.

     As a matter of fact, you can get rougher with this. You can make a calculation on what would have happened in our air transport system last year if the philosophy of our ammunition people had been used in running it. If you do that, you'd find that we would have killed 4 million people last year. You will also find that the attrition of equipment and refurbishment costs are so high that it would cost you $10,000 for a ticket to anywhere. The only way out of the ammunition dilemma, surprisingly enough, is the kind of advance propulsion we're planning to talk about the next few days. You have to get enough margin into the propulsion so that you can have extra weight available both for use and reuse.

     An interesting interaction exists between the containment capabilities of gaseous fission systems and the cost of boost to orbit. Attempts are frequently made to show that high fuel consumption gaseous fission systems (either Orion or co-axial systems) would be acceptable after all, since the extra economic penalty which they incur compared to the cost of chemical boost to orbit is relatively small. This conclusion would obviously be strongly influenced by the wide spread of orbital costs mentioned.

     Figure 14 shows typical interactions between containment of fuel and economics of boost to orbit. The point is obvious. Chemical take-off is not too bad. If chemical boost to orbit is tolerated at all, however, it must be of the economical "transportation" variety, or it will completely cripple the ability of gaseous fission engines to explore economically the solar system. This to me is the real challenge of advanced propulsion. This is also why I think there's a tremendous interaction between the engine and vehicle. It is not just a matter of sitting down with a specific impulse, and making one simple performance calculation. The big gain is made by the interaction of the engine and the development of transportation techniques.

     Figure 15 is a sketch of one result of using a gaseous fission engine to power a reusable spaceship. We all, by now, expect manned rockets to be hundreds of feet long. If drawn to the scale of Figure 15, Saturn V would be two pages long. It would have a little bit of payload on the front. If, however, we were to combine the kind of nuclear rocket engine we would like to have (running on water or ammonia rather than hydrogen) with a reusable structure for the entire ship, a possible result would be the ship shown. This is a typical case of about a million pounds gross weight with cargo weight on the order of 200,000 pounds.

     It turns out, not surprisingly, that for reasonable economy, large payload fractions, perhaps even more than 20 percent, are required. Note that 20 percent cargo at a density of 10 pounds per cubic foot (standard transport airplane practice) when combined with the required propellant results in a rocket vehicle with 60 percent of its length devoted to cargo and crew. The engine and propellant take up only a small portion at the rear, just like "Buck Rogers" has said it should all along.

     An interesting example of the change in design philosophy with such ships is in the matter of shielding. Indications are that about 20,000 pounds of shielding weight would be required. This is a severe penalty for most rockets. Since cargo itself is effective shielding material, however, by the simple expedient of never flying this ship with less than 10 percent cargo aboard, properly packaged, the shielding penalty is reduced effectively to zero.

     If such a ship were used without radiators on the engine but with hydrogen propellant, it would be able to generate about 80,000 feet per second. This is more than needed for a lunar round trip. We can hence examine the effect of this ship on a lunar run, performing like a normal transport airplane. Logical assumptions, as previously discussed, would lead to 50 flights a year, and the ship can carry 100 tons per flight. By maneuvering a little bit with that number, I concluded that one ship like this is equivalent to 300 Saturn V launches per year.

     This is the kind of thing that we're driving at. Incidentally, in our scientific operations in the Antarctic, we deliver to the Antarctic about 50,000 tons a year. Ten such ships, shuttling back and forth to the moon, could mount the same magnitude of operation on the moon as we mount in Antarctica. This is, to say the least, an interesting capability.

  • Fission Products Always in Vapor Form
    • No Burn-Up Problem as in Rover or Power Supplies
  • No Fission Products aboard upon Return
    • Never Overfly with Fission Product Load
    • Servicing Problem Not Great as in ANP Application
  • Fission Product Load small in case of Accident
    • Megapound Thrust is Only Kilo (Not Mega) Ton Equivalent
    • Only Millirem Exposure if Accident above 5000 Ft.
  • High Velocity Exhaust Jet
    • Velocity of at Least 100,000 ft/sec exceeds Earth Escape Speed
      • Most Products actually Ejected from Solar System
  • Characteristics also affect Development Procedures
    • Time between Static Tests Reduced
      • Engine does not contain Products after Test
      • Fuel Element Fabrication Delays Avoided
    • May Not be a Logical Extension of Solid Cores
      • Progress is Rarely Logical
      • ICBM was not Extrapolation of Winged Aircraft

Figure 16

     Some points should be made about the safety of gaseous fission spaceships. Contrary to most opinion, a gaseous fission rocket is probably a lot safer to use than a solid core rocket. Several reasons for this are listed on Figure 16. The fission products are always in vapor form, so there is never a fuel element burnup problem if emergency atmospheric entry is necessary. On return trips, one need never overfly a city with a fission product load, since the products can be ejected into space and the landing made aerodynamically. After landing, of course, the ship is radioactive only to the extent that any material has been locally activated. This will be very small with proper material selection and is certainly far lower than when the fission product load is a permanent feature of the structure. Hence, the servicing problem would be nowhere near as great as with the case of aircraft nuclear propulsion.

     Furthermore, in case of an accident, the fission product load is always small. A million pounds of thrust is, after all, only 1/2 kiloton of thrust, and the actual fission products created are comparable to those from kiloton, not megaton, bombs. As a matter of fact, some interesting calculations indicate that an accident as low as 5,000 feet in the air yields almost no exposure on the ground due to the effectiveness of atmospheric dispersal of the small fission product load on board.

     In addition to the fact that the fission product load is small, one can do interesting things by recognizing the fact that the exhaust jet velocity is actually higher than solar system escape speed. If trajectories are properly programmed, once out of the Earth's atmosphere, most of the fission products ejected with the exhaust will be thrown completely out of the solar system. This is the one way of not contaminating space. Space, incidentally, is a doggone big place, and the contamination of small local radiation belts, atmospheres, or planets should not be confused with all of space. Even our Sun, which is continually making a real attempt at space contamination compared to any puny spaceship, has not succeeded to any great distance.

     Some of the characteristics which make a gaseous fission rocket different from solid core propulsion systems also result in development differences. There is a tendency for many people to believe that gaseous fission engines are a logical extension of Rover. At the risk of losing a number of friends, I would like to point out that they probably are not a logical extension of Rover. For instance, the time between reactor tests should be greatly reduced for gaseous engines. One need not fabricate fuel elements between tests, and does not have to live with fission products imbedded within the engine. The handling advantages in operation previously mentioned also extend to the engine development process and the development program should be a lot easier to run than that of a solid core engine. Gaseous fission engines may not be a logical extension of solid core engines at all.

     A good space engine can also have a profound effect on spaceship development cost. It is not just a question of a high degree of reuse, there is also the effect of making the ship abortable during any part of the flight.

     Transportation systems not only achieve very high reuse, they contain sufficient redundancy to permit flying with partial equipment failures, and also have the ability to abort successfully from any flight condition. It is this last capability which is very important to the development program of such ships.

     The savings in development cost of not losing ships continually is obvious, yet our ammunition thinkers are so used to the massive throwaway that they usually claim that recoverable equipment would be more expensive to develop since it is more complicated. This might be true of the recovery of marginal performing rockets, but would not be true of a properly designed reusable spaceship which would not be marginal with a gaseous fission engine.

     It simply is not possible to overemphasize this difference in development philosophy. Commercial transports are extensively tested, and much of the equipment refined by flight tests. They become reliable pieces of equipment for expenditures very small in space budget terms, because it is possible to test the equipment over and over for a reasonable expenditure. If a high performance propulsion system can permit us for the first time to pursue a space vehicle program with the very efficient development techniques of transport aircraft systems, we will be very remiss if too blind to even consider these techniques.

     The use of a reusable and abortable ship from the start of the development program can well have a profound interaction on engine development tests. This interaction will be enhanced greatly if the engine is a "tractable" engine. A "tractable" engine is one which has "benign failure modes." In other words, it does not explode catastrophically when it fails. There is an excellent chance that gaseous fission engines will tend to go out rather than explode when trouble occurs.

     If the engine is tractable, and the ship abortable, then flight failures consist mostly of unscheduled landings. The ship is then capable of testing the main engine without the extreme sensitivity to component malfunction which exists in ammunition development programs. This can be a very large leverage on total development costs. Clearly, there must be extensive ground testing of the engines. However, it may be much easier to arrange partial duration ground runs in enclosed areas than it is to arrange total duration ground runs. The total duration runs could then be performed in the ship.

     Many engine developments in the past have made extensive use of flying test beds when ground facilities were not adequate. The technique should not be ignored if the flight vehicles are able to reintroduce it. This is one of the examples of development techniques available with transportation devices which are not within the realm of experience of ammunition developers.

     The final point I wish to make is, I'm against logical progress.

     A common mistake in development thinking seems to be a tendency to relate the basic performance achieved by a device with its development difficulty. It seems so logical to assume orderly progress in development programs. Actually, many major programs are not a result of orderly progress. One of the most recent interesting examples is the development of the ICBM. These ballistic missiles penetrate to their targets at a Mach number of 25. Orderly progress would have dictated that we build first fleets of supersonic bombers, then fleets of hypersonic bombers. Only after that would we consider whether or not Mach number 25 penetrators were desirable.

     The fact is that Mach number 25 ballistic missiles are considerably easier to build than hypersonic bombers (and evidently Mach number 3 bombers). Their performance is attained in a different manner, with different engines (not breathing air) and in a different flight region (out of the atmosphere). They are not a Mach number 25 airplane.

     The gaseous fission spaceship has many analogous elements. It is easy to achieve 500,000 fps out in space, as long as the engine is capable of it. A ship which never carries fission products aboard need never fight the safety problems of solid core nuclear rockets, or even the analogous problems of nuclear airplanes. An abortable transport rocket is a different development job than the building of larger ammunition. We must look at the gaseous fission ship in terms of its difficulty of development, not in awe of its possible accomplishments.

Space Transportation by M. W. Hunter II (1965)

(ed note: This is an interesting design example from the always worth reading Bootstrapping Space blog by Chris Wolfe. It is mostly centered around estimating a mission delta V and sizing a propulsion system to fit, but his thought processes are interesting.)

All previous posts described all-chemical systems that could be built and operated profitably in the near term. This one focuses on electrical propulsion systems.
The defining features of most electric propulsion:
 - High efficiency (high Isp)
 - Low thrust
 - High power requirements
 - Long trip times
 - Long operating life

     I chose a specific paper (Frisbee, Mikellides) to examine since the authors thoughtfully included most of the interesting parameters for a reusable Nuclear Electric Propulsion (NEP) Mars cargo tug. I don't really dive into how to calculate this for yourself because the problem is quite difficult without modeling software.

     It all comes down to the details; the question of NEP vs. Solar Electric Propulsion (SEP) vs. Chemical depends on the specific mission goals and technologies used.

The summary:
23 tons dry mass for a nuclear-electric tug of ~6 MW thermal / 1.2 MW electric
64 tons cargo capacity from low Earth orbit to Phobos-Mars orbit
Just under 40 tons of water propellant for the outbound trip and another 7.2 tons acquired at Phobos for the return
2.2 years outbound, slightly less inbound
Two round trips between thruster refits, five round trips between reactor refits

(skip to the next section if you are already familiar with electric propulsion)

     The general idea is to use electrical power to dump energy into a propellant and then release it at very high speed.

     The simplest of these is Electrothermal.
     An electric current produces heat and the propellant is passed through it. First up is the resistojet, where a resistor somewhat like an incandescent lightbulb filament is heated and then the propellant is pumped past it. These are common devices in the RCS systems of satellites. Second is arcjet, which passes an electric arc directly through the propellant instead of through a resistor. These can reach higher Isp because they can heat the propellant beyond material limits for resistor elements.
     Efficiency is moderate (Isp of 500 to 1000, well above any realistic chemical system but easily an order of magnitude lower than the most efficient electrics). Preferred fuels are low atomic weight with no particulates (hydrogen, water, ammonia, hydrazine). Thermal power is most efficient when there are few degrees of freedom for the molecules so more of the energy can be applied to macroscopic motion, so hydrogen or hydrogen-rich propellants are ideal.

     Next up is Electrostatic.
     These are the 'traditional' ion thrusters, NSTAR, Hall effect, etc. The propellant is ionized by a strong electrostatic field (with some variations) and then the ions are accelerated by a negatively-charged grid or electrode. An electron gun is used to neutralize the electric charge of the ion beam and keep the spacecraft electrically neutral.
     Efficiency is high. with Isp ranging from 1000 to 5000 for most designs and a few reaching as high as 10,000. Preferred fuels are high atomic weight gases or elements with a very low first ionization energy; argon and xenon are frequently used. Sometimes iodine or metals like tin, magnesium or sodium are used, while caesium and rubidium can be used in a FEEP thruster.

     Lastly, Electromagnetic.
     These are the 'new generation' plasma thrusters like VASIMR, MPD, PIT, etc. Note the idea is not new, it's just that these technologies have been getting a lot of press lately. In fact, a pulsed plasma thruster (from this family) was the first electric thruster flown in space. In these devices the propellant is ionized by arc discharge, microwave heating or other means and accelerated by a magnetic field rather than an electrostatic grid.
 Efficiency is typically variable but can be very high, with an Isp of 1000 to 30,000 (most commonly about 1500 to 6000). Preferred fuels are the same as electrostatic thrusters for the most part, with lithium making an appearance in MPD thrusters.

     All three families have been in use for decades, while each family has relatively recent members pushing the limits. All share common fundamental physics with regard to their efficiencies. Sources of loss are in the power processing units, ionization energy of the propellant, dissociation energy of the propellant if it is a molecule rather than a pure element, ion impacts with the grid or body and in the charge density and geometry of the exhaust.


     Of key concern for a reusable vehicle is the propellant should be available in space. Xenon, argon and nitrogen are available in the atmosphere of Mars. Small amounts of nitrogen may be available in lunar cold traps or bound in soils of Ceres and many C-type asteroids. Water appears to be widely available. Alkali metals would be available on the Moon and in most asteroids.

     Another critical factor is that the thruster system should be reliable over the long term.  Electrostatic systems have already demonstrated very long operational lifespans in the range of 5 to 10 years of active thrust. Electromagnetic systems don't yet measure up in demonstrated lifespan, but that is mainly because most systems of this type are low Isp / high thrust RCS components designed for relatively short operating times rather than main drive units designed to last a decade. As a practical engineering concern, there are difficult challenges in improving lifespan of systems with exposed electrodes or grids. Designs without grids, whether electrostatic or electromagnetic, have the potential for decades-long operation.

     A practical system will be able to service a cargo route in a reasonable time. To illustrate this, let's dive into a paper on a pulsed inductive thruster proposal that includes payload transits to Mars, Saturn and solar escape. A reusable cargo tug with about 64 tons of payload and a 2.2-year Mars transit would require about 2 megawatts of electricity and about 275 tons fueled mass at departure. Propellant would be plain water, though ammonia works as well. If propellant supplies are available at Mars then we require only 1.2 megawatts and 165 tons fueled mass. Of that, the tug itself is just under 23 tons. Enough spare components would be included to make two round trips between thruster refits. A refit would mean swapping out the entire thruster pod with a fresh assembly, quick and easy.
     2.2 years is a long trip, but it is right at the synodic period for Earth and Mars. A cargo tug launched at one opportunity would arrive just in time to go / no go the crew launch at the next opportunity. Part of the problem is that a low-thrust vehicle has to produce about 16 km/s of dV for this trip, far more than a high-thrust chemical system requires thanks to the Oberth effect. Assuming an Isp of 6000 seconds the fuel mass ratio is 23.81% or 39.285 tons of water. An empty return trip would require 7.2 tons of propellant. Put another way, each ton of fuel delivers 1.6 tons of payload. Contrast that with my chemical tug's ratio of 0.8 tons of payload per ton of fuel and you can see the advantage; twice the mass delivered for the same quantity of fuel. Of course, the two approaches trade off costs between dry structure and fuel; electric propulsion is not automatically better but it certainly lets you do a lot more with the same starting mass.

     Power can come from two sources, solar or nuclear. The tug described above is nuclear; its reactor should be good for five or possibly six round-trips out of the box but could be designed for a much longer lifespan of 40 to 50 years with a bit more mass. This is partly because the same propulsion system is intended for exploration missions to the outer planets, where solar power is minimal.

     A Mars cargo tug could certainly use solar power instead, with a lifespan in the 20 to 30 year range and less complicated refitting / disposal. The ~19 tons of reactor, radiators and conversion hardware would be replaced by very large solar panel arrays of about 30 tons, 3.0 MW at Earth beginning-of-life yielding 1.2 MW at Mars end-of-life, with a whole-system specific power of 100 W/kg, 20-year useful life, 20% degradation and 50% of Earth-normal power available at Mars. If Photovoltaics (PV) refits are available every two or three trips then the allowance for degradation can be reduced to perhaps 10%, saving about 3.5 tons.

     Reactors also degrade over time as their fuel decays; some designs such as traveling wave, drum reflector and pebble bed can level the power output over time by only burning part of the nuclear fuel load at any one time. These designs can be life-extended by including more fuel during construction and / or by replacing fuel elements. Since the nuclear fuel is only a tiny fraction of a reactor's mass, this life extension adds very little mass to the overall system. This is the same reason why increasing a reactor's power takes less mass than increasing a solar panel array's power; for low power outputs the solar panels are nearly always lighter due to the reactor's heavy power conversion equipment and shielding, but as the power output grows the reactor eventually beats PV. As you can see from the above example, 1.2 megawatts at Mars after 20 years is firmly in nuclear territory given current state of the art solar performance. Even so, I would bet there is room in the design space for solar PV to be competitive at this power level, design life and solar distance.

Future work:

     I think the next step is to work up some EML2 to Phobos tether-capture cargo runs and see how they compare to the baseline LEO to LMO mission. Keeping used nuclear reactors out of LEO is a good idea, as is keeping large solar arrays out of the Van Allen belts. I'll try for an electric interplanetary tug with similar payload to my chemical tug. These would have the added bonus of providing abundant power while parked; there may be a case for a set of tugs such that one is always at Phobos providing megawatt-scale electrical power.


     I need to continue on the topic of electric propulsion. The previous post was a lot of words but not a lot of meat. I felt it was too weak to stand alone, particularly as a part of this series where I am trying to focus on a realistic near-term plan for cargo transport. If you are interested in more background information I'd start with the Wikipedia page on electric propulsion and follow up with a look at the Atomic Rockets engine page. Another good look in the context of interplanetary travel is this paper (Hellin), while a deep look at relevant equations can be had in this paper (Keaton).

     One interesting result is a general rule to find required thrust given average acceleration. Google failed me on finding an exact solution, but it looks like there is a simple approach that is within 1% of the target value.

     I eventually settled on a design massing 33.4 tons, 1.6 MW solar-electric, Isp 6,000 and 40 N thrust using PIT thrusters with water propellant.

     Let's look at an electric tug with payload comparable to my reference tug, both a solar PV and a nuclear version. The main routes for this vehicle will be between LEO, GEO, EML1/2 and Mars orbit. Unlike the chemical tug we can't get much out of the Oberth effect, so the delta-V requirements are higher. Just like the chemical tug, the LEO to EML1 leg has the highest dV requirements (about 7km/s), so if we design for that case then the other trips will be faster, carry more cargo or burn less propellant.

     A key design factor here is trip time. If we throw enough power at the problem we can get to EML1 in the same amount of time as a chemical rocket, but that is a poor use of the mass. We need to decide how long we are willing to wait for the cargo and design enough thrust into the ship to make the trip in that span. I'm going to suggest four weeks to EML1 as a reasonable compromise, so let's see the consequences of that choice.

Estimating thrust requirements for average acceleration

     To apply 7km/s of delta-V in 28 days we need to make an average acceleration of 2.89 mm/s. To allow some wiggle room let's assume we can only thrust 90% of the time, meaning now we need 3.22 mm/s. Since this is our average acceleration, we need to find either the initial or final acceleration to find the thrust of the propulsion system. To do that we will first need to know the vehicle's propellant mass fraction, so let's take a few test cases at Isp of 3000, 6000 and 10,000.

     Propellant mass fraction (Mf) is equal to 1 - e ^ (- dV / Ve), where dV in this case is 7000 m/s and Ve is Isp * g. See the rocket equation page for more details.
Isp 3000 -> Mf of 0.21175
Isp 6000 -> Mf of 0.11217
Isp 10000 -> Mf of 0.06890

     It would be nice if the average acceleration also matches up with the midpoint of fuel consumption, but somehow I doubt it. Let's find out.

     Given a dry mass of, say, 10 tons and an Isp of 3000, the fueled mass is ( 1 / ( 1 - Mf ) ) * dry mass, or 12.686 tons. When half of the fuel is burned the craft masses 11.343 tons. The target acceleration is 0.00322 m/s, so the required thrust is 36.52 newtons. Thrust is mass-flow (mdot) times exhaust velocity, so mdot is 1.2414 grams per second. That rate of propellant consumption would require 25 days to empty the tank, or 27.825 days after accounting for our 90% duty cycle.

     That surprises me. It's not exact but it is close enough for exploratory work. The case of 10,000 Isp works out to 27.947 days, so it looks like this general rule is valid across a fair range of Isp values. I also spot-checked some different mission dV values and found similar agreement, always within 1%. If anyone out there knows of an exact solution I would love to hear it.

     To calculate this yourself you need your mission dV, Isp, thrust duration and a test mass. If you set the test mass to 1kg (or 1t) then you can find a multiplier to use for different dry masses. The relationships are linear.

The required acceleration a will be dV in meters per second divided by thrust duration in seconds.
First, find fuel mass fraction, which is 1 - e ^ (- dV / Ve).
Convert to dry mass fraction Md, which is -Mf + 1
Convert to the 'gear ratio', which is simply 1 / Md
Multiply by dry mass M1 to get fueled mass M0 and note this value.
Find the fuel mass by taking M0 - M1 and note this value.
Find the 'halfway point', which is half the fuel mass plus M1; let's call this Mh.
Find the thrust F, which is Mh * a. This is the value you are looking for.
Find mdot, which is thrust divided by exhaust velocity, or F / (g * Isp ).
Find the real thrust duration, which is fuel mass divided by mdot. This should be within 1% of your stated thrust duration; if it is then the average acceleration value is accurate enough to use.

     If you have a known spacecraft (known dry mass and fuel mass, known thrust), you can use thrust divided by (dry mass plus half the fuel mass).

Electric tug design

     To align with the chemical tug, let's target a payload of 40 tons from LEO to EML1. Note that EML2 is a better target, but for purposes of comparison I'm using the LEO to EML1 trip as the most costly trip in the set. As mentioned above, we need to deliver in 28 days or provide an average acceleration of 3.22 mm/s. I don't have an exact solution, so I can't solve the problem in a single step. That's fine; spacecraft design is an iterative process.

     Let's assume an electric thruster at Isp = 6000 and mission dV of 7000 m/s. Also assume a one-way trip (meaning fuel is available at both endpoints). Power alpha is assumed to be 18 kg/kW, whether that be nuclear or long-life solar. Thrusters will be the NuPIT design shown in the last post, using the design values for the 5 N, 200 kW unit at 2.75 kg/kW (550 kg per thruster).
     As a first guess let's try eight thrusters, 1.6 MW. That's 33.2 tons, for a dry mass without tanks of 73.2 tons. We will need approximately 10 tons of liquid water propellant; using a tankage fraction of 2% would be reasonable in this case, so tack on 200kg for tanks for a total dry mass of 73.4 tons. Actual propellant load is 9,273 kg, so tankage is sufficient. The half-fueled mass is 78,037 kg and approximate average acceleration is 0.513 mm/s. We're not even close. Trip time would be 157.9 days, or 2.3 one-way trips per year.
     Maybe 20 thusters / 4 MW? Power alpha would improve to about 15, yielding 71 tons of power and propulsion. 40 tons of payload and perhaps 0.4 tons of tankage gives a dry mass of 111.4 tons, fuel mass of 14.1 t and average acceleration of 0.844 mm/s. This is clearly not going our way. Trip time would be 106.3 days, or 3.4 one-way trips per year.
     Let's aim much higher, 50 thrusters / 10 MW. Power alpha would continue to improve to about 14, yielding 167.5 t of power and propulsion. 40 tons of payload and 0.6 tons of tankage gives a dry mass of 208.1 tons, 26.29 tons of fuel and an average acceleration of 1.13 mm/s. Trip time would be 71.7 days or 5.1 one-way trips per year.

     Clearly, short trip times require increasingly absurd power levels. Matching the payload size of a chemical thruster with the 1.6 MW version means only making one round-trip per year. In fact, looking at that version of the ship, if we eliminate the payload entirely the highest acceleration the ship can make is 1.2 mm/s on its last gasp of propellant. Since fuel mass, dry mass, power and thrust are all linear relationships* that means no matter how we scale up the ship it can never get better than this. (The power system alpha does actually get better as we scale up, but moving a ship that masses several times your payload is inefficient and extremely expensive.)

     One thing we can do is increase the thrust of each propulsion unit, which usually means decreasing the Isp significantly. Let's look at a VASIMR thruster for comparison, since I have some data on performance at different Isp levels handy. A VASIMR thruster at 200 kW and 6000 Isp produces about 4.75 N of thrust, a fairly close match to the NuPIT. We need about six times that thrust (28.5 N), which occurs right at an Isp of 1000. That would bring the 8-thruster 1.6 MW vessel up to about 230 N of thrust. However, dropping the Isp so dramatically brings the fuel fraction just over 50%. That pushes our dry mass up to 75t, fuel mass to 78.1t and nets us only 2.0 mm/s average acceleration. It's a 40.5 day trip or 9 trips per year, but now we are burning more fuel than the chemical tug thanks to our drastically higher dry mass. Still no net benefit to be had.

Putting the tug to work

     Let's look at what an electric tug actually saves: propellant. In a fully functional ecosystem of cis-lunar services propellant is fairly plentiful. The speed, convenience and throughput of chemical vehicles far outweighs the efficiency of ion vehicles in this environment. Where an electric tug shines is in the buildup phase, where all of the propellant is coming from Earth. The tug would save money during a critical part of the project. What that means is we do not need to survive dozens of Van Allen belt transits over two decades, we just need to make a reasonable number of trips over two or three years. We also don't need to standardize on the same payload sizes as the chemical tug, nor do we need to make trips in 1 month. I would say that using the same power system alpha for the solar version as I do for the nuclear version is very pessimistic; these vessels would not need to function at Mars orbit, though they do need significantly thicker front-glass shielding on the panels than other craft.

     So, a lunar ISRU plan would still start with a single chemical tug / lander as described in part 1. Using performance for the detailed reference tug, a 15-ton package can be delivered from LEO direct to the lunar surface. This will be 12.4 tons of ISRU equipment and 2.6 tons of spares (2.1 year supply). Refilling tug 1 will take 6 months, after which it can deliver 33 tons to EML1.
     In the meantime, an electric tug (call it tug A) will deliver a 9-ton fuel depot (135 ton capacity) to EML1. Let's use our 40 N / 1.6 MW / 33.4t / 6000 Isp vehicle from above. It does the job in about 92 days, which means there is a window of three months after the launch of the first ISRU package to get the 33.4t tug, 5.36t propellant and 9t payload into LEO.
     At the first lunar launch, 33 tons are delivered to the EML1 depot. Tug A will collect this and head to LEO, taking 127 days and consuming 7.45t of water. During this trip a LEO depot is launched, identical to the one at EML1. The tug turns back around and heads for EML1, taking 4.22t of water for the return trip and leaving 21.33t of cargo in LEO. This could be a mix of surface samples and water as desired. Let's assume five tons are samples and the rest is fuel.
     The return trip takes 72 days, during which tug 1 will have delivered another 33 tons to the EML1 depot. Tug A repeats its performance, returning to LEO with a full load of 23.78t water and another 5t of samples. At this point we are at 598 days elapsed since start of ISRU operations, which should be enough time to settle on and construct additional hardware to expand the lunar surface capacity.
     This is significantly longer than the all-chemical scenario and has an IMLEO of 129.71 tons, within a few tons of all-chemical. Hardware costs are higher since more of the mass is spacecraft and much less of it is fuel. The main benefit is that schedule pressures are greatly reduced; final design, construction and testing of the second round of ISRU plant is allowed more than a year and a half of time rather than two months. More operational data is available and the tolerance for mistakes or inefficiencies is higher. Another benefit is that this profile includes depots in LEO and at EML1; even if things do not progress beyond the first ISRU package the infrastructure is still useful for this and future projects.
     This baseline hardware could continue to deliver 21 tons of cargo to LEO every ~200 days for about a decade, eventually reaching 426 tons over 20 trips at a cost of 141 tons of Earth mass or a leverage of about 3 to 1. Things improve if we continue to expand, since about 44% of that mass was fuel to get the first ISRU plant in position; additional ISRU hardware is delivered using lunar propellant.

     The next phase would be to send more ISRU hardware. Tug A can pick up 33 tons at EML1, deliver 19.18t of net payload to LEO over 127 days, pick up a 17-ton package and head to EML1 in 109 days. All of the required propellant is lunar and picked up at EML1. Round trip time is 236 days (a bit under 8 months). The harvesting process run by tug 1 has a shorter turnover time of 6 months, so on average an extra 19 tons is accumulated at the depot. That's not quite enough to provide for a cargo landing, so tug A may not always be bringing a full load of cargo to LEO (meaning shorter round trips in practice).
     An alternative might be to use 12-ton packages that will fit into a Falcon 9 for cheaper launch costs; the delivery time for that is 98 days. If less cargo is returned to LEO then that trip time can be shortened as well; for example, 6 tons of return cargo plus round-trip fuel would make each leg of the trip take 98 days, or 196 days round-trip. Each 6.5-month trip would deliver another 10 tons of ISRU with two years of spares. Two electric tugs could deliver 80 tons of ISRU capacity in 26 months, roughly a single Mars synodic period. That would place 95 tons of ISRU with expected output of 950 tons of propellant annually at a cost of 9.5 tons of spares. Net propellant delivered to EML1 would be 505 tons annually, or could be 168 tons to LEO annually with chemical tugs. The annual demand for spares (both ISRU and depots) can be met in a single hardware run with minimal fuel costs, leaving 3/4 of the electric tug schedule open for assignments like delivering new chemical tugs or GEO debris retrieval (a mission that avoids the majority of the radiation belts and prolongs the tug's useful life).
     The total phase 2 IMLEO would be 134.4 tons, all of it hardware. Lunar mass to LEO during this period would only be another 48 tons since capacity is focused on buildup. This phase would run for 26 months, or a total of 46 months since first launch.
     Ongoing maintenance would require approximately 12 tons per year. The initial depots would be insufficient, so we need another 27t of hardware for fuel storage. If we rate all flight and depot hardware with a 10-year lifespan and pro-rate the replacement mass then we need an additional 12.2 tons annually (24.2t total). Depending on how the output is allocated, this could be considered an ongoing leverage of 23.5 tons in EML1 per ton IMLEO or 7.8 tons in LEO per ton IMLEO. Another way to look at it would be as a fuel supply for three manned Mars missions covering four synodic periods (104 months), or a full ISRU program length of 150 months (12.5 years). Overall Earth mass to LEO is then 511.15t to harvest 5,501.7 tons of gross lunar propellant, yielding 2,924.6 tons net lunar propellant at EML1. That's a gear ratio of about 5.7 to 1. If you are only interested in delivering fuel to LEO then you can net 972.9 tons, still a favorable 1.9 to 1 mass ratio. 511 tons is a lot of mass to launch, but only three payloads require a heavy lift vehicle: the initial chemical tug stack (62t fuel and 22t hardware, split across two Falcon H) and the two electric tugs (33.4t each, also requiring a Falcon H unless they can be built in parts and flown on two Vulcan launches). The remaining 360 tons would be delivered by 30 Falcon 9 launches, or by some combination of any price-competitive launchers with at least 12 tons of payload.

     Launch costs would be roughly $2.1 billion. Hardware would run another $6.7 billion (at $15m per ton). Operations might cost $125-$250 million. Call it a total of $9 billion over about 15 years (12.5 years of operation plus 2.5 years of r&d, manufacturing and testing). Overall cost of fuel at EML1 would be $3,094.44 per kg, about $3.1 million per ton and expected to decline to $0.9 million per ton in the long run. Savings are about the same as the all-chemical approach, a bit over $4.5 billion vs. NASA baseline. Additional savings could be realized by using the chemical tugs as cargo haulers to and from Mars as described in part 2, resulting in excess capacity that could be sold or used for other purposes. One of those purposes might be ISS reboost and water supply for life support. Another might be developing a significant water supply on the Moon for growing food, in support of manned missions.

Ship mass and size

Full load mass and physical size depends upon assumptions about fuel mass ration, fuel bulk, etc.


Deadweight (inert mass)117%
Cargo (payload)233%
Fuel (propellant)350%

Note that total mass is three times the cargo capacity. As you can see, deadweight is the ship proper, structure, engines, anything that is not cargo or propellant.

With this assumption, the big freighters will have a fully loaded mass of 60,000 tons. The largest ships might be twice as big: 120,000 tons.

Our building cost is $500,000 per ton of cargo capacity, the mass assumption makes a building cost equal to $1 million per ton of deadweight. Annual service cost is $100,000 per ton of cargo capacity, the mass assumption makes the annual service cost equal to $200,000 per ton of deadweight. The starship hulls are not cheaper, but they can carry more cargo in proportion to their structural mass.

Type of shipCargo capacityPurchase price
Large20,000 tons$20 billion
Medium5000 tons$2.5 billion
Small1500 tons$750 million

At $500,000 per ton of cargo capacity, largest giant freighter cost $20 billion to build, but it it has a cargo capacity of 200 Boeing 747 jets, and accounts for over one percent of whole fleet's cargo capacity all by itself. Small freighter costs $750 million, and has seven time the capacity of 747.

With a 30 year service life, the combined shipbuilding yards of the 12 planet trade network will turn out about 25 ships per year.

Hulls will last longer than 30 years but the equipment wears out and has to be replaced. Ships go back to the yards for an overhaul every decade or so, but eventually the cost of stripping everything and replacing it will exceed the value of the ship. Depending upon overhaul costs the shipyards may make more money on rebuilding than on constructing brand new ships. Some ships will stay in service for many decades. Others will be retained as the futuristic equivalent of naval hulks or the old passenger equipment that railroads use as work trains. Every big commercial space station will have a bunch of these old ships in the outskirts.

If modular design is taken to its limit, "ships" will have no permanent existence. Instead they will be assembled out of modules and pods specifically for each run, much like a railroad train. In that case, a ship's identity is attached to a service, not a physical structure. Example: the Santa Fe "Chief" was identified by a timetable and reputation, not a particular set of locomotive and cars.

Starship Performance

The analysis up until now focused on money and economics. Businessmen only care about how long it takes to deliver the cargo and how much transport costs, they could care less about the scientific details of the ship engines. But authors care.

As with everything else, it all depends upon the assumptions. Your assumptions will be different, so feel free to fiddle with these and see what the results are.

Assumption: the time spent in FTL transit is zero (jump drive). For the FTL segment of the transit you can use whatever you want, as long as the details do not affect the analysis. The main thing is that the required time spent in FTL transit will add to the total trip time, and thus the number of cargoes a starship can transport per year.

Assumption: starships use reaction drives for normal space travel.

We know that the mass ratio is 2.0. So the Tsiolkovsky rocket equation tells us that the starship's total delta V will be the propulsion system's exhaust velocity times 0.69 (i.e., ln(2.0) ). Since starships accelerate to half their delta V, coast, then decelerate to a halt, their maximum speed is half their delta V, or exhaust velocity times 0.35 (i.e., ln(2.0) / 2). In practice you would accelerate up to a bit less than half their delta V in order to allow a fuel reserve in case of emergency.

It will be even less if the FTL drive happens to use the same type of fuel that the reaction drive does. Basically part of the fuel mass will have to be considered as cargo, not propellant, which will alter the ship's mass ratio.

Reaction driveExhaust velocity
general rule
Nuclear powered Ion~100 km/s
Fusiona few thousand km/s
Beam core matter-antimatterabout 100,000 km/s
( 1/3 c )

We have assumed that the ship spends 27 days in route (with an instantaneous FTL jump), so the outbound and inbound legs are 13.5 days each (1.17 million seconds).

Assumption: the acceleration on each leg is constant. In reality at the same thrust setting the acceleration will increase as the ship's mass goes down due to propellant being expended. The thrust will probably be constantly throttled to maintain a constant acceleration. Makes it easier on the crew and easier on our analysis. The implication is that obviously the average speed will be half the maximum speed (which is half the delta V)

Reaction driveExhaust velocity
general rule
leg distance
Advanced Ion
or Early Fusion
400 km/s130 km/s75 million km
(1/2 AU)
0.01 g
Advanced Fusion10,000 km/s5000 km/s20 AU
0.44 g
c0.3 c350 AU
(x5 Pluto's orbit)
8 g !!!

These figures will be lower if time is consumed in FTL flight, maybe be only Terra-Luna distance

Propulsion system's thrust power is thrust times exhaust velocity, then divide by 2. To get the thrust, we know that thrust is ship mass times acceleration. The ship mass goes down as fuel is burnt. As a general rule for ship mass, figure that it only has 2/3rds of a propellant load. That is, multiply the total ship mass by 0.83. So our 120,000 metric ton ship would have a general rule mass of 120,000 * 0.83 = 100,000 metric tons (100,000,000 kilograms).

Reaction driveExhaust velocity
general rule
ThrustThrust power
Advanced Ion
or Early Fusion
400,000 m/s
(400 km/s)
0.108 m/s
(0.011 g)
1.08×107 N2.16×1012 W
(2 terawatts)
Advanced Fusion10,000,000 m/s
(10,000 km/s)
4.3 m/s
(0.44 g)
4.3×108 N2.15×1015 W
(2,000 terawatts)
3.0×108 m/s
76.5 m/s
7.65×109 N1.15×1018 W
(1 million terawatts)

Where does fuel come from and who does it get into the ship's fuel tanks? Easiest if it is obtained locally at the destination's solar system. The economics of interplanetary transport is same as interstellar (since we did a lot of work making interstellar a cheap as interplanetary).

if fuel from a gas giant at a distance comparable to Terra-Jupiter and round trip is to only take weeks, interplanetary tankers will need speeds of around 1000 km/s. So tankers will be almost as expensive as starships. If tankers use low speed (to make them cheaper), the round trip balloons to a year or more. To service the starship fleet's thirst for fuel, tankers will need to be huge or there will have to be a lot of them. Either way, fuel shipped from gas giants ain't gonna be cheap.

If we forgo interplanetary tankers and instead have starships make extra leg to the local gas giant to refuel, it will cost you more than you will save.

The alternative is shipping fuel up from destination planet. Yes, we know about how surface to orbit is "halfway to anywhere" in terms of delta V cost. But in order to colonize space at all, surface-to-orbit shipping cost will have to be cheap anyway. The industrialization of space will start with using space based resources, but eventually surface-to-orbit will have to be cheap or there is no rocketpunk future. Laser launch, Lofstrom loop, space elevator, something like that.

Assumption: surface-to-orbit shuttle economics are equivalent to current day airliner economics. Round trip to LEO and back is about two hours (not counting loading/unloading). With loading/unloading and maintenance, figure 4 flights a day. Implication is that a round trip passenger ticket is $250 and round trip freight service is $1000/ton (which is +10% added to interstellar transport costs)

Fuel is not round trip, it only goes from surface to orbit, but shuttles have to go orbit to surface in order to get the next load. You will have to streamline the process. High capacity pumps to minimize load/unload times, crew-less shuttle. You might be able to squeeze fuel lift cost to $500/ton. So if starships carry 1.5 tons of fuel per ton of cargo, surface-to-orbit fuel lift costs adds $750/ton to interstellar shipping cost.

So total surface-to-orbit overhead is $1000/ton + $750/ton = $1750/ton or 17.5%. This is an ouch but not a show-stopper.

Back to starships. How big are they?

Present-day maritime tonnage rule: 1 registered ton = ~3 cubic meters.

Assumption: 1 ton = 3 m3 applies to fuel and hull (e.g., crew quarters, engineering spaces, etc) as well as cargo. Therefore, if the absolutely hugest cargo starship in service has a cargo capacity of 40,000 tons (twice that of a large cargo starship), then:

Wet Mass
Payload mass to total mass ratio is 3. So wet mass is 3 * 40,000 = 120,000 tons
Starship Volume
1 ton of total ship mass = 3 m3 of volume. 120,000 * 3 = 360,000 cubic meters.

Volume of a sphere is 4/3πr3, so the radius of a sphere is 3√(v/(4/3π)) or

radius = CubeRoot( v / 4.189)

diameter = (CubeRoot( v / 4.189)) * 2

Assumption: a "cigar-shape" for a spacecraft is a six times as long as it is wide, with the proportions indicated in the diagram above. The center body is a cylinder 1 unit in diameter (0.5 units radius) and two units high. The two end caps are cones of 0.5 units radius and 2 units high.

If the monstrous cargo starship is spherical, it would have a diameter of 88 meters. If it is cigar shaped then length = 300 meters and diameter of 50 meters.

A 1500 ton cargo capacity tramp freighter would have a wet mass of 4500 tons and a volume of 13,500 m3. Spherical shape would have a diameter of 30 meters, cigar shaped length = 100 meters long and diameter of 17 meters.

Modular ships dimension would be similar but a bit larger due to being assembled out of component parts.


When a spacecraft built for humans ventures into deep space, it requires an array of features to keep it and a crew inside safe. Both distance and duration demand that spacecraft must have systems that can reliably operate far from home, be capable of keeping astronauts alive in case of emergencies and still be light enough that a rocket can launch it.

Missions near the Moon will start when NASA’s Orion spacecraft leaves Earth atop the world’s most powerful rocket, NASA’s Space Launch System. After launch from the agency’s Kennedy Space Center in Florida, Orion will travel beyond the Moon to a distance more than 1,000 times farther than where the International Space Station flies in low-Earth orbit, and farther than any spacecraft built for humans has ever ventured. To accomplish this feat, Orion has built-in technologies that enable the crew and spacecraft to explore far into the solar system.

Systems to Live and Breathe

As humans travel farther from Earth for longer missions, the systems that keep them alive must be highly reliable while taking up minimal mass and volume. Orion will be equipped with advanced environmental control and life support systems designed for the demands of a deep space mission. A high-tech system already being tested aboard the space station will remove carbon dioxide (CO2) and humidity from inside Orion. Removal of CO2 and humidity is important to ensure air remains safe for the crew breathing. And water condensation on the vehicle hardware is controlled to prevent water intrusion into sensitive equipment or corrosion on the primary pressure structure.

The system also saves volume inside the spacecraft. Without such technology, Orion would have to carry many chemical canisters that would otherwise take up the space of 127 basketballs (or 32 cubic feet) inside the spacecraft—about 10 percent of crew livable area. Orion will also have a new compact toilet, smaller than the one on the space station. Long duration missions far from Earth drive engineers to design compact systems not only to maximize available space for crew comfort, but also to accommodate the volume needed to carry consumables like enough food and water for the entirety of a mission lasting days or weeks.

Highly reliable systems are critically important when distant crew will not have the benefit of frequent resupply shipments to bring spare parts from Earth, like those to the space station. Even small systems have to function reliably to support life in space, from a working toilet to an automated fire suppression system or exercise equipment that helps astronauts stay in shape to counteract the zero-gravity environment in space that can cause muscle and bone atrophy. Distance from home also demands that Orion have spacesuits capable of keeping astronaut alive for six days in the event of cabin depressurization to support a long trip home.

Proper Propulsion

The farther into space a vehicle ventures, the more capable its propulsion systems need to be to maintain its course on the journey with precision and ensure its crew can get home.

Orion has a highly capable service module that serves as the powerhouse for the spacecraft, providing propulsion capabilities that enable Orion to go around the Moon and back on its exploration missions. The service module has 33 engines of various sizes. The main engine will provide major in-space maneuvering capabilities throughout the mission, including inserting Orion into lunar orbit and also firing powerfully enough to get out of the Moon’s orbit to return to Earth. The other 32 engines are used to steer and control Orion on orbit.

In part due to its propulsion capabilities, including tanks that can hold nearly 2,000 gallons of propellant and a back up for the main engine in the event of a failure, Orion’s service module is equipped to handle the rigors of travel for missions that are both far and long, and has the ability to bring the crew home in a variety of emergency situations.

The Ability to Hold Off the Heat

Going to the Moon is no easy task, and it’s only half the journey. The farther a spacecraft travels in space, the more heat it will generate as it returns to Earth. Getting back safely requires technologies that can help a spacecraft endure speeds 30 times the speed of sound and heat twice as hot as molten lava or half as hot as the sun.

When Orion returns from the Moon, it will be traveling nearly 25,000 mph, a speed that could cover the distance from Los Angeles to New York City in six minutes. Its advanced heat shield, made with a material called AVCOAT, is designed to wear away as it heats up. Orion’s heat shield is the largest of its kind ever built and will help the spacecraft withstand temperatures around 5,000 degrees Fahrenheit during reentry though Earth’s atmosphere.

Before reentry, Orion also will endure a 700-degree temperature range from about minus 150 to 550 degrees Fahrenheit. Orion’s highly capable thermal protection system, paired with thermal controls, will protect Orion during periods of direct sunlight and pitch black darkness while its crews will comfortably enjoy a safe and stable interior temperature of about 77 degrees Fahrenheit.

Radiation Protection

As a spacecraft travels on missions beyond the protection of Earth’s magnetic field, it will be exposed to a harsher radiation environment than in low-Earth orbit with greater amounts of radiation from charged particles and solar storms that can cause disruptions to critical computers, avionics and other equipment. Humans exposed to large amounts of radiation can experience both acute and chronic health problems ranging from near-term radiation sickness to the potential of developing cancer in the long-term.

Orion was designed from the start with built in system-level features to ensure reliability of essential elements of the spacecraft during potential radiation events. For example, Orion is equipped with four identical computers that each are self-checking, plus an entirely different backup computer, to ensure Orion can still send commands in the event of a disruption. Engineers have tested parts and systems to a high standard to ensure that all critical systems remain operable even under extreme circumstances.

Orion also has a makeshift storm shelter below the main deck of the crew module. In the event of a solar radiation event, NASA has developed plans for crew on board to create a temporary shelter inside using materials on board. A variety of radiation sensors will also be on the spacecraft to help scientists better understand the radiation environment far away from Earth. One investigation called AstroRad, will fly on Exploration Mission-1 and test an experimental vest that has the potential to help shield vital organs and decrease exposure from solar particle events.

Constant Communication and Navigation

Spacecraft venturing far from home go beyond the Global Positioning System (GPS) in space and above communication satellites in Earth orbit. To talk with mission control in Houston, Orion’s Orion will use all three of NASA’s space communications networks. As it rises from the launch pad and into cislunar space, Orion will switch from the Near Earth Network to the Space Network, made possible by the Tracking and Data Relay Satellites, and finally to the Deep Space Network that provides communications for some of NASA’s most distant spacecraft.

Orion is also equipped with backup communication and navigation systems to help the spacecraft stay in contact with the ground and orient itself if it’s primary systems fail. The backup navigation system, a relatively new technology called optical navigation, uses a camera to take pictures of the Earth, Moon and stars and autonomously triangulate Orion’s position from the photos. Its backup emergency communications system doesn’t use the primary system or antennae for high-rate data transfer.

Spacecraft Parameters

For an given type of automobile, there are parameters that tell you what kind of performance you can expect. Things like miles per gallon, acceleration, weight, and so on.

Spacecraft have parameters too, it is just that they are odd measures that you have not encountered before. I am going to list the more important ones here, but they will be fully explained on other pages. Refer back to this list if you run across an unfamiliar term.

Habitat Module
The pressurized part of the spaceraft where people live. Included in Payload Section. Remember that Rockets Are Not Hotels. Unlike the Starship Enterprise a real spacecraft is a huge expanse of airless machinery with a tiny pressurized habitat module tucked away in a corner where people can walk around without spacesuits.
The part of the spacecraft that is its reason for existance. For a satellite booster, the payload is the satellite it is lifting into orbit. For a transport ship: habitat module, passengers, ship controls. For a warship: habitat module, crew, weapons, defenses, ship controls. For a robot freighter: robot controls and cargo. Some payload like cargo and crew are removable from the spacecraft. Some payload like weapons and habitat modules are fixed parts of the spacecraft. Included in Payload Section.
Engine or Thruster
The rocket engine that moves the spacecraft, and the empty propellant tanks. Included in Propulsion Bus.
Power Plant
Part that generates electricity. Included in Propulsion Bus.
Struture is the skeleton and skin of the spacecraft. Included in both Propulsion Bus and Payload Section.
Propellant and Fuel
Propellant or Reaction mass (remass) is what the thruster fires out the exhaust nozzle to create thrust. Fuel is the source of energy used to propel the propellant. Remember that Fuel Is Not Propellant. In chemical rockets, the chemicals are both propellant and fuel. In nuclear rockets the liquid hydrogen is the propellant and the uranium is the fuel. Included in Propulsion Bus.

Payload Mass (Mpl)
Mass of all the payload. For NASA vessels this is typically 26.7% of Dry Mass. Note that for many spacecraft there is no specific set maximum payload mass. You can strap as much payload to the ship's nose as you want. Understanding that the price will be reducing the spacecraft's delta-V due to the increase in payload degrading the spacecraft's mass ratio.
Payload Fraction (λ)
Payload mass as percentage of wet mass. Mpl / M
Structural Mass (Mst)
Mass of all the struture. For NASA vessels this is typically 21.7% of Dry Mass.
Propellant Mass (Mpt)
The mass of all the propellant in the spacecraft's propellant tanks. Does not include fuel that is retained after it is burnt, e.g., uranium fissioned inside a solid core reactor. For some calculations, you will use instead the mass of propellant that will be expended in a given maneuver.
Power Plant Mass (Mpp)
The mass of the electrical generation system. Includes any heat radiators. For NASA vessels this is typically 28% + 3.4% of Dry Mass
Thruster System Mass (Mts)
The mass of the rocket engines, including the empty propellant/fuel tanks. For NASA vessels this is typically 3.7% of Dry Mass
Propulsion System Mass (Mps)
Thruster System Mass + Power Plant Mass.
Inert Mass (Mi)
Mass of spacecraft with no propellant and no payload. Propulsion System Mass + Structural Mass.
Inert Mass Fraction (δ)
Inert mass as percentage of wet mass. Mi / M
Dry (Empty, Burnout) Mass (Me)
Mass of spacecraft with no propellant but with payload. Propulsion System Mass + Structural Mass + Payload Mass.
Wet (Total, Ignition) Mass (M)
Total mass of spacecraft. Propellant Mass + Propulsion System Mass + Structural Mass + Payload Mass.
Mass Ratio (R)
Ratio of wet mass to dry mass. Wet Mass / Dry Mass.
Parametric Mass Ratio (r)
λ + δ.
Propellant Fraction (Pf, PMF, or ζ)
Percentage of wet mass that is propellant. 1 - ( 1 / MassRatio )

Propellant Mass Flow (mDot or )

How quickly does the Thruster System drain the propellant tanks? Rated in kilograms per second.

mDot constrains the amount of thrust the propulsion system can produce. Changing the propellant mass flow is a way to make a spacecraft engine shift gears.

Exhaust-Velocity (Ve)

How fast does the propellant shoot out the exhaust nozzle of the Thruster System? Rated in meters per second. Exhaust velocity (and delta V) is of primary importance for space travel. For liftoff, landing, and dodging hostile weapons fire, thrust is more important.

Broadly exhaust velocity is a measure of the spacecraft's "fuel" efficiency (actually propellant efficiency). The higher the Ve, the better the "fuel economy".

Generally if a propulsion system has a high Ve it has a low thrust and vice versa. The only systems where both are high are torch drives. Some spacecraft engines can shift gears by trading exhaust velocity for thrust.

For a more in-depth look at exhaust velocity look here

Specific Impulse (Isp)
Another way of stating exhaust velocity. Exhaust Velocity / 9.81 where 9.81 = acceleration due to gravity on Terra in meters per second. Specific Impulse is rated in seconds. It is also a broad measure of the spacecraft's "fuel" efficiency.
Delta V or Δv

Spacecraft's total change in velocity capability. This determines which missions the spacecraft can perform. Arguably this is the most important of all the spacecraft parameters. Rated in meters per second.

This can be thought of as how much "fuel" is in the tanks of the spacecraft (though it is actually a bit more complicated than that).

Velocity Ratio
Δv / Ve
Thrust (F)

Thrust produced by Thruster System. Rated in Newtons. Thrust is constrained by Propellant Mass Flow. Thrust (and acceleration) is of primary importance in liftoff, landing, and dodging hostile weapons fire. For space travel exhaust velocity (and delta V) is more important.

Generally if a propulsion system has a high Ve it has a low thrust and vice versa. The only systems where both are high are torch drives. Some spacecraft engines can shift gears by trading exhaust velocity for thrust.

Acceleration (A)

Spacecraft's current acceleration. Current total mass / Thrust. Rated in meters per second per second. Divide by 9.81 to get g's of acceleration.

In space, a spacecraft with higher acceleration will generally not travel to a destination any faster than a low acceleration ship. But a high acceleration ship will have wider launch windows for a given trajectory.

Note that as propellant is expended, current total mass goes down and acceleration goes up. If you want a constant level of acceleration you have to constantly throttle back the thrust.

5 milligee (0.05 m/s2) : General rule practical minimum for ion drive, laser sail or other low thrust / long duration drive. Otherwise the poor spacecraft will take years to change orbits. Unfortunately pure solar sails are lucky to do 3 milligees.

0.6 gee (5.88 m/s2) : General rule average for high thrust / short duration drive. Useful for Hohmann transfer orbits, or crossing the Van Allen radiation belts before they fry the astronauts.

3.0 gee (29.43 m/s2) : General rule minimum to lift off from Terra's surface into LEO.

For a more in-depth look at minimum accelerations look here.

Thrust Power (Fp)
Power produced by Thruster System. ( Thrust × Exhaust Velocity ) / 2. Rated in watts.
Specific Power (Fsp)
Power density of spacecraft. Thrust Power / Dry Mass. Rated in watts per kilograms.
Specific Mass
Alpha of Thruster System. Thruster System Mass / Thrust Power. Rated in kilograms per watt.


Typically the percentage of spacecraft dry mass that is structure is 21.7% for NASA vessels.

What is the structure of the ship going to be composed of? The strongest yet least massive of elements. This means Titanium, Magnesium, Aluminum, and those fancy composite materials. And all the interior girders are going to have a series of circular holes in them to reduce mass (the technical term is "lightening holes").

Spacecraft Spine

Many (but not all) spacecraft designs have the propulsion system at the "bottom", exerting thrust into a strong structural member called the ship's spine. The other components of the spacecraft are attached to the spine. The spine is also called a keel or a thrust frame. In all spacecraft the thrust frame is the network of girders on top of the engines that the thrust is applied to. But only in some spacecraft is the thrust frame elongated into a spine, in others the ship components are attached to a shell, generally cylindrical.

If you leave out the spine or thrust frame, engine ignition will send the propulsion system careening through the core of the ship, gutting it. Spacecraft engineers treat tiny cracks in the thrust frame with deep concern.


However, I'm debating if the structures you cite as "keels" make sense when cross-referenced with "thrust frame".

For instance, ISS' truss isn't really a thrust frame—the station is very rarely under thrust, and when it is, it's usually from spacecraft or its own thrusters on the end of the Russian segment, which would actually make the whole main line of modules (Zarya, Zvezda, Pressurized mating adapter-1, Unity, Destiny, Harmony) the main "keel". The job of the truss in such a case is just to stop itself from flexing and hold the solar "wings" in place.

Similarly, there's other space vehicles which lack such a "keel" entirely, such as the DTAL concept or the Altair ascent stage design. In both cases, an engine is basically mounted to a pressure vessel (a prop tank for DTAL and a crew cabin for Altair) and then the rest of the structure "hangs" off of that pressure hull.

I might also note that these kinds of {keel-less spacecraft} will, in a rocketpunk setting, likely be confined to special-purpose craft—landers or scooters, spaceplanes, dedicated fuel tankers, and such. Most "typical" ships will probably have a bit more of spiney spaceframe.

The distinction I might make is "primary structure" and "thrust structure". The thrust structure is just the structural system to distribute the force of the engine, such as the F9 Octaweb. On the other hand, the primary structure is anything that serves a major structural role in the ship, analyzed as a system. In DTAL, it'd be the engine, the thrust structure that mounts that engine to the tanks, the tanks, and then the landing gear and such. For ISS, it's the outer hulls of the core "line" of modules, plus the truss. This primary structure might also be called an airframe or spaceframe.

Engineer Rob Davidoff (2014)

OK, forget what I just said. On top of the engine will be the thrust frame or thrust structure. On top will be the primary structure or spaceframe. The thrust frame transmits the thrust into the spaceframe, and prevents the propulsion system careening through the core of the ship.

The spaceframe can be:

  • A long spine/keel with the propellant tanks and payload section bits attached in various places.
  • A large pressurized vessel, either propellant tank or habitat module. Other propellant tanks and payload section bits are attached to main tank or perched on top.
  • Something else.

The engineers are using a pressurized tank in lieu of a spine in a desperate attempt to reduce the spacecraft's mass. But this can be risky if you use the propellant tank. The original 1957 Convair Atlas rocket used "balloon tanks" for the propellant instead of conventional isogrid tanks. This means that the structural rigidity comes from the pressurization of the propellant. This also means if the pressure is lost in the tank the entire rocket collapses under its own weight. Blasted thing needed 35 kPa of nitrogen even when the rocket was not fueled.

As Rob Davidoff points out, keel-less ship designs using a pressurized tank for a spine is more for marginal ships that cannot afford any excess mass whatsoever. Such as ships that have to lift off and land in delta-V gobbling planetary gravity wells while using one-lung propulsion systems (*cough* chemical rockets *cough*).

This classification means that parts of the propulsion bus and payload section are intertwined with each other, but nobody said rocket science was going to be easy.

In von Braun Round the Moon Ship the thrust frame (dark blue) is right on top of the rocket motors. The spaceframe (light blue) is a cage attached to the thrust frame. The rocket motors push upwards on the thrust frame, which pushes upwards on the spaceframe. The personnel sphere, hydrazine tank, and nitric acid tank are all basically inflated balloons hung on the spaceframe.

Getting back to the spine. Remember that every gram counts. Spacecraft designers want a spine that is the strongest yet lowest mass structural member possible. The genius R. Buckminster Fuller and his science of "Synergetics" had the answer in his "octet truss" (which he called an "isotrophic vector matrix", and which had been independently discovered about 50 years earlier by Alexander Graham Bell). You remember Fuller, right? The fellow who invented the geodesic dome?

Each of the struts composing the octet truss are the same length. Geometrically it is an array of tetrahedrons and octahedrons (in terms of Dungeons and Dragons polyhedral dice it uses d4's and d8's).

Sometimes instead of an octet truss designers will opt for a weaker but easier to construct space frame. The truss of the International Space Station apparently falls into this category.


The artifact was the shell of a solid fuel rocket motor. Part of the Mariner XX, from the lettering.

The Mariner XX, the ancient Pluto fly-by. Ages ago the ancient empty shell must have drifted back toward the distant sun, drifted into the thin Trojan-point dust and coasted to a stop. The hull was pitted with dust holes and was still rotating with the stabilizing impulse imparted three generations back.

As a collector's item the thing was nearly beyond price. Brennan took phototapes of it in situ before he moved in to attach himself to the flat nose and used his jet backpac to stop the rotation. He strapped it to the fusion tube of his ship, below the lifesystem cabin. The gyros could compensate for the imbalance.

In another sense the bulk presented a problem.

He stood next to it on the slender metal shell of the fusion tube. The antique motor was half as big as his mining singleship, but very light, little more than a metal skin for its original shaped-core charge. If Brennan had found pitchblende the singleship would have been hung with cargo nets under the fuel ring, carrying its own weight in radioactive ore. He would have returned to the Belt at half a gee. But with the Mariner relic as his cargo he could accelerate at the one gee which was standard for empty singleships.

There are few big cargo ships in the Belt. Most miners prefer to haul their own ore. The ships that haul large cargoes from asteroid to asteroid are not large; rather, they are furnished with a great many attachments. The crew string their payload out on spars and rigging, in nets or on lightweight grids. They spray foam plastic to protect fragile items. spread reflective foil underneath to ward off hot backlighting from the drive flame, and take off on low power.

The Blue Ox was a special case. She carried fluids and fine dusts; refined quicksilver and mined water, grain, seeds, impure tin scooped molten from lakes on dayside Mercury, mixed and dangerous chemicals from Jupiter's atmosphere. Such loads were not always available for hauling. So the Ox was a huge tank with a small threeman lifesystem and a fusion tube running through her long axis; but, since her tank must sometimes become a cargo hold for bulky objects, it had been designed with mooring gear and a big lid.

Nilsson's own small, ancient mining ship had become the Ox's lifeboat. The slender length of its fusion tube, flared at the end, stretched almost the length of the hold. There was an Adzhubei 4-4 computer, almost new; there were machines intended to serve as the computer's senses and speakers, radar and radio and sonics and monochromatic lights and hi-fi equipment. Each item was tethered separately, half a dozen ways, to hooks on the inner wall.

Nilsson nodded, satisfied, his graying blond Belter crest brushing the crown of his helmet. "Go ahead, Nate."

Nathan La Pan began spraying fluid into the tank. In thirty seconds the tank was filled with foam which was already hardening.

"Close 'er up."

Perhaps the foam crunched as the great lid swung down. The sound did not carry. Patroclus Port was in vacuum, open beneath the black sky.

The captive ship was small. Phssthpok found little more than a cramped life support system, a long drive tube, a ring-shaped liquid hydrogen tank with a cooling motor. The toroidal fuel tank was detachable, with room for several more along the slender length of the drive tube. Around the rim of the cylindrical life support system were attachments for cargo, booms and folded fine-mesh nets and retractable hooks.

He did find inspection panels in the drive tube. Within an hour he could have built his own crystal-zinc fusion tube, had he the materials. He was impressed. The natives might be more intelligent than he had guessed, or luckier. He moved up to the lifesystem and through the oval door.

The cabin included an acceleration couch, banks of controls surrounding it in a horseshoe, a space behind the couch big enough to move around in, an automatic kitchen that was part of the horseshoe, and attachments to mechanical senses of types frequently used in Pak warfare. But this was no warship. The natives' senses must be less acute than Pak senses. Behind the cabin were machinery and tanks of fluid, which Phssthpok examined with great interest.

One thing he understood immediately.

He was being very careful with the instrument panel. He didn't want to wreck anything before he found out how to pull astronomical data from the ship's computer. When he opened the solar storm warning to ascertain its purpose, he found it surprisingly small. Curious, he investigated further. The thing was made with magnetic monopoles.

From PROTECTOR by Larry Niven (1973)

A bit more simplistic is a simple stack of octahedrons (Dungeons and Dragons d8 polyhedral dice). This was used for the spine of the Valley Forge from the movie Silent Running (1972), later reused as the agro ship from original Battlestar Galactica.

Saddle Truss

Spacecraft spines are generally down the center of the spacecraft following the ship's thrust axis (the line the engine's thrust is applied along, usually from the center of the engine's exhaust through the ship's center of gravity).

This can be a pain to spacecraft designers if they have anything that needs to be jettisoned. Such items will have to be in pairs on opposite sides of the spine, and jettisoned in pairs as well. Otherwise the spacecraft's center of gravity will shift off the thrust axis, and the next time the engines are fired up it's pinwheel time.

In a NASA study TM-1998-208834-REV1 they invent a clever way to avoid this: the Saddle Truss.

The truss is a hollow framework cylinder with a big enough diameter to accommodate standard propellant tanks, consumables storage pods, and auxiliary spacecraft. One side of the cylinder frame is missing. The thrust axis is cocked a fraction of a degree off-center to allow for the uneven mass distribution of the framework.

The point is that tanks and other jettison-able items no longer have to be in pairs if you use a saddle truss. When it is empty you just kick it out through the missing side of the saddle truss. No muss, no fuss, and no having to have double the amount of propellant plumbing and related items.

Examples of the saddle truss can be found in the Bimodal NTR and the SNRE Spacecraft.

Waterskiing Spacecraft

This is a quite radical method to drastically reduce the structural mass of a spacecraft, allowing a handsome increase in valuable payload mass. It also dramatically increase the separation between a dangerously radioactive propulsion system and the crew, allowing a drastic decrease in the radiation shadow shield mass. This allows yet more handsome increases in valuable payload mass. As the cherry on top of the cake, it allows using the tumbling pigeon method of spin gravity without the direction of gravity inverting.

Please note this has never actually been used in a serious nuclear spacecraft design due to its unorthodox nature.

And warships with such a design would have their manoeuvring critically handicapped (or it's "crack-the-whip" time and the cable breaks).

The concept comes from the observation that for a given amount of structural strength, a compression member (such as a girder) generally has a higher mass that a corresponding tension member (such as a cable). And we know that every gram counts.

Charles Pellegrino and Dr. Jim Powell put it this way: current spacecraft designs using compression members are guilty of "putting the cart before the horse". At the bottom is the engines, on top of that is the thrust frame, and on top of that is rest of the spacecraft held together with girders (compression members) like a skyscraper. But what if you put the engine at the top and have it drag the rest of the spacecraft on a long cable (tension member). You'll instantly cut the structural mass by an order of magnitude or more!

And if the engines are radioactive, remember that crew radiation exposure can be cut by time, shielding, or distance. The advantage of distance is it takes far less mass than a shield composed of lead or something else massive. The break-even point is where the mass of the boom or cable is equal to the mass of the shadow shield. But the mass of a shadow shield is equal to the mass of a incredibly long cable. The HELIOS cable was about 300 to 1000 meters, the Valkyrie was ten kilometers.

But keep in mind that this design has no maneuverability at all. Agile it ain't. If you turn the ship too fast it will try to "crack the whip" and probably snap the cable. This probably makes the design unsuitable for warships, who have to jink a lot or be hit by enemy weapons fire.

Examples include HELIOS, the Valkyrie Antimatter Starship, the ramrobots from Larry Niven's A Gift From Earth and the ISV Venture Star from the movie Avatar.

Certain propulsion systems incorporate the waterskiing concept in spacecraft that use the propulsion. The main one is the Medusa, which sets off nuclear explosions inside a huge parachute-shaped sail. The sail accelerates, and drags along the payload on a long cable. Long because the payload does not want to be any closer to a series of nuclear explosions than it has to be.

The various types of sail propulsion drag the payload with a long cable as well. But for them, the long cable is not because the sail is radioactive, just that it is typically several kilometers in radius.

Cosine Thrust Loss

If the exhaust is radioactive or otherwise dangerous to hose the rest of the spacecraft with, you can have two or more engines angled so the plumes miss the ship.

Angled engines do reduce the effective thrust by an amount proportional to the cosine of the angle but for small angles it is acceptable. The delta V of the spacecraft is also reduced by the same proportion.

Note in the HELIOS design Krafft Ehricke figured that the 300 meter separation was enough to render the exhaust harmless so it does not angle the engine at all. Krafft has a single engine blasting straight at the habitat module. The only concession to the exhaust is mounting the cables on outriggers, so the cables do not pass through the zillion degree nuclear fireball exhaust plume. It would be most embarassing if the cables melted.


The HELIOS has a thrust of 981,000 newtons. Say that Dr. Ehricke figured the exhaust would be dangerous to the habitat module, so the single engine would have to be replaced by two engines with 490,500 newtons each angled off-center by 5°. What would that do to the thrust?

CosineFactor = cos(OffAngle)
CosineFactor = cos(5°)
CosineFactor = 0.9962…

So both the thrust and delta V would be at 99.62%

Each engine has a thrust of 490,500 newtons, this would be reduced by the cosine thrust loss to:

EffectiveThrust = ActualThrust * CosineFactor
EffectiveThrust = 490,500 * 0.9962
EffectiveThrust = 488,636 newtons

488,636 newtons * 2 engines = 977,272 newtons. This means that angling the engines lowers the total thrust by 3,728 newtons, which is an ouch but not a show stopper. If the thrust absolutely has to be 981,000 newtons total, each engine would have to have its thrust increased from 490,500 to 492,371 newtons in order to compensate for the cosine loss.

If the HELIOS had a delta-V of 21,000 m/s, the cosine loss would reduce it to 21,000 * 0.9962 = 20,920 m/s. This is a loss of 80 m/s which is not negligible but not a show-stopper either.

Here's how we can shave off many tons of shielding.

Put the engine up front and carry the crew compartment ten kilometers behind the engine, on the end of a tether. Let the engine pull the ship along, much like a motorboat pulling a water skier, and let the distance between the gamma ray source and the crew compartment, as the rays stream out in every direction, provide part of the gamma ray protection - with almost no weight penalty at all. (ed. note: this should remind you of "Helios") We can easily direct the pion/muon thrust around the tether and its supporting structures, and we can strap a tiny block of (let's say) tungsten to the tether, about one hundred meters behind the engine. Gamma rays are attenuated by a factor of ten for every two centimeters of tungsten they pass through. Therefore, a block of tungsten twenty centimeters deep will reduce the gamma dose to anything behind it by a factor of ten to the tenth power (1010). An important shielding advantage provided by a ten-kilometer-long tether is that, by locating the tungsten shield one hundred times closer to the engine than the crew, the diameter of the shield need be only one-hundredth the diameter of the gamma ray shadow you want to cast over and around the crew compartment. The weight of the shielding system then becomes trivial.

The tether system requires that the elements of the ship must be designed to climb "up" and "down" the lines, somewhat like elevators on tracks.

We can even locate the hydrogen between the tungsten shadow shield and the antihydrogen, to provide even more shielding for both the crew and the antihydrogen.

There is an irony involved in this configuration. Our "inside-out" rocket, the most highly evolved rocket yet conceived, is nothing new. We have simply come full circle and rediscovered Robert Goddard's original rocket configuration: with engines ahead of the fuel tanks and the fuel tanks ahead of the payload.

From FLYING TO VALHALLA by Charles Pellegrino (1993)

Interstellar Ramscoop Robot #143 left Juno at the end of a linear accelerator. Coasting toward interstellar space, she looked like a huge metal insect, makeshift and hastily built. Yet, except for the contents of her cargo pod, she was identical to the last forty of her predecessors. Her nose was the ramscoop generator, a massive, heavily armored cylinder with a large orifice in the center. Along the sides were two big fusion motors, aimed ten degrees outward, mounted on oddly jointed metal structures like the folded legs of a praying mantis. The hull was small, containing only a computer and an insystem fuel tank.

(ed note: cosine of 10° is about 0.9848, so thrust and delta V would be reduced to 98.5%.)

Juno was invisible behind her when the fusion motors fired. Immediately the cable at her tail began to unroll. The cable was thirty miles long and was made of braided Sinclair molecule chain. Trailing at the end was a lead capsule as heavy as the ramrobot itself.

(ed note: "Sinclair molecule chain" is an unobtainium wire that is only one molecule thick and absurdly strong. The theoretical ultimate of low mass cable.)

On twin spears of actinic light the ramrobot approached Pluto's orbit. Pluto and Neptune were both on the far side of the sun, and there were no ships nearby to be harmed by magnetic effects.

The ramscoop generator came on.

The conical field formed rather slowly, but when it had stopped oscillating, it was two hundred miles across. The ship began to drag a little, a very little, as the cone scooped up interstellar dust and hydrogen. She was still accelerating. Her insystem tank was idle now, and would be for the next twelve years. Her food would be the thin stuff she scooped out between the stars.

From A GIFT FROM EARTH by Larry Niven (1968)
Pendulum Fallacy

Now those waterskiiing spacecraft designs look much like Robert Goddard's first liquid-fueled rocket "Nell " (i.e, engine on top). Don't be fooled, it is just a coincidence.

The waterskiing spacecraft designs are trying to subsitute almost mass-less engine standoff distance for ultra-penalty-mass radiation shield. Because on the one hand: Every Gram Counts, but on the other: nuclear radiation kills crewmembers dead.

Goddard's rocket was designed for totally different reasons (which you should have been tipped off by the fact it had no nuclear engine).

Goddard reasoned that if you put the rocket engine at the bottom and build the rest of the rocket on top of that, it would be as unstable as a waiter carrying a tall tippy bottle of wine on a tray held overhead by one hand. One minor shake of the hand and the wine goes crashing to the ground.

But if you put the rocket engine at the top and had it dragging the rest of the rocket, it would be as inherently stable as holding a pendulum by its string. If the engine tips over from its upward flight, the weight of the rest of the rocket will un-tip the engine. Right?

Nowadays rocket designers call this the Pendulum Rocket Fallacy. Meaning it looks good on paper, but it just doesn't work. Having the engines at the top is no more stable than at the bottom. A top-engine design superficially resembles a pendulum, but the system of forces acting on it are totally different.

Goddard discovered this the hard way with Nell's test flight. It rose barely 41 feet, tipped over, flew 184 feet horizontally, then augered into a cabbage field. All of his subsequent designs had the now-standard engine on the bottom arrangement.

Real spacecraft achieve stability by controlling some type of attitude actuator via some type of inertial measurement unit.

Space Trains and Truckers

This section has been moved here

Thermal Protection


Multi-layer insulation, or MLI, is thermal insulation composed of multiple layers of thin sheets and is often used on spacecraft. It is one of the main items of the spacecraft thermal design, primarily intended to reduce heat loss by thermal radiation. In its basic form, it does not appreciably insulate against other thermal losses such as heat conduction or convection. It is therefore commonly used on satellites and other applications in vacuum where conduction and convection are much less significant and radiation dominates. MLI gives many satellites and other space probes the appearance of being covered with gold foil.

Function and design

The principle behind MLI is radiation balance. To see why it works, start with a concrete example - imagine a square meter of a surface in outer space, held at a fixed temperature of 300 K, with an emissivity of 1, facing away from the sun or other heat sources. From the Stefan–Boltzmann law, this surface will radiate 460 W. Now imagine placing a thin (but opaque) layer 1 cm away from the plate, thermally insulated from it, and also with an emissivity of 1. This new layer will cool until it is radiating 230 W from each side, at which point everything is in balance. The new layer receives 460 W from the original plate. 230 W is radiated back to the original plate, and 230 W to space. The original surface still radiates 460 W, but gets 230 W back from the new layers, for a net loss of 230 W. So overall, the radiation losses from the surface have been reduced by half by adding the additional layer.

More layers can be added to reduce the loss further. The blanket can be further improved by making the outside surfaces highly reflective to thermal radiation, which reduces both absorption and emission.

The layers of MLI can be arbitrarily close to each other, as long as they are not in thermal contact. The separation space only needs to be minute, which is the function of the extremely thin scrim or polyester 'bridal veil' as shown in the photo. To reduce weight and blanket thickness, the internal layers are made very thin, but they must be opaque to thermal radiation. Since they don't need much structural strength, these internal layers are usually made of very thin plastic, about 6 μm (1/4 mil) thick, such as Mylar or Kapton, coated on one side with a thin layer of metal on both sides, typically silver or aluminium. For compactness, the layers are spaced as close to each other as possible, though without touching, since there should be little or no thermal conduction between the layers. A typical insulation blanket has 40 or more layers. The layers may be embossed or crinkled, so they only touch at a few points, or held apart by a thin cloth mesh, or scrim, which can be seen in the picture above. The outer layers must be stronger, and are often thicker and stronger plastic, reinforced with a stronger scrim material such as fiberglass.

In satellite applications, the MLI will be full of air at launch time. As the rocket ascends, this air must be able to escape without damaging the blanket. This may require holes or perforations in the layers, even though this reduces their effectiveness.

MLI blankets are constructed with sewing technology. The layers are cut, stacked on top of each other, and sewn together at the edges. Seams and gaps in the insulation are responsible for most of the heat leakage through MLI blankets. A new method is being developed to use polyetheretherketone (PEEK) tag pins (similar to plastic hooks used to attach price tags to garments) to fix the film layers in place instead of sewing to improve the thermal performance.

Additional properties

Spacecraft also may use MLI as a first line of defense against dust impacts. This normally means spacing it a cm or so away from the surface it is insulating. Also, one or more of the layers may be replaced by a mechanically strong material, such as beta cloth.

In some applications the insulating layers must be grounded, so they cannot build up a charge and arc, causing radio interference. Since the normal construction results in electrical as well as thermal insulation, these applications may include aluminum spacers as opposed to cloth scrim at the points where the blankets are sewn together.

From the Wikipedia entry for MULTI-LAYER INSULATION


There are some hazards to worry about with these space-age materials. Titanium and magnesium are extremely flammable (in an atmosphere containing oxygen). And when I say "extremely" I am not kidding.

Do not try to put out a magnesium fire by throwing water on it. Blasted burning magnesium will suck the oxygen atoms right out of the water molecules, leaving hydrogen gas (aka what the Hindenburg was full of). A carbon-dioxide fire extinguisher won't work either, same result as water except you get a cloud of carbon instead of hydrogen. Instead use a Class D dry chemical fire extinguisher or a lot of sand to cut off the oxygen supply. Oh, did I mention that burning magnesium emits enough ultraviolet light to permanently damage the retinas of the eyes?

The same goes for burning titanium. Except there is no ultraviolet light, but there is a chance of ignition if titanium is in contact with liquid oxygen and the titanium is struck by a hard object. It seems that the strike might create a fresh non-oxidized stretch of titanium surface, which ignites the fire even though the liquid oxygen is at something like minus 200° centigrade. This may mean that using titanium tanks for your rocket's liquid oxygen storage is a very bad idea.

An emergency crew at a spaceport, who has to deal with a crashed rocket, will need the equipment to deal with this.

And if the titanium, magnesium, or aluminum becomes powdered, you have to stop talking in terms of "fire" and start talking in terms of "explosion."


As an interesting side note, rockets constructed of aluminum are extremely vulnerable to splashes of metallic mercury or dustings of mercury salts. On aluminum, mercury is an "oxidizing catalyst", which means the blasted stuff can corrode through an aluminum beam in a matter of hours (in an atmosphere containing oxygen, of course). This is why mercury thermometers are forbidden on commercial aircraft.

Why? Ordinarily aluminum would corrode much faster than iron. However, iron oxide, i.e., "rust", flakes off, exposing more iron to be attacked. But aluminum oxide, i.e., "sapphire", sticks tight, protecting the remaining aluminum with a gem-hard barrier. Except mercury washes the protective layer away, allowing the aluminum to be consumed by galloping rust.

Alkalis will have a similar effect on aluminum, and acids have a similar effect on magnesium (you can dissolve magnesium with vinegar). As far as I know nothing really touches titanium, its corrosion-resistance is second only to platinum.


Corrosion in space is the corrosion of materials occurring in outer space. Instead of moisture and oxygen acting as the primary corrosion causes, the materials exposed to outer space are subjected to vacuum, bombardment by ultraviolet and X-rays, and high-energy charged particles (mostly electrons and protons from solar wind). In the upper layers of the atmosphere (between 90–800 km), the atmospheric atoms, ions, and free radicals, most notably atomic oxygen, play a major role. The concentration of atomic oxygen depends on altitude and solar activity, as the bursts of ultraviolet radiation cause photodissociation of molecular oxygen. Between 160 and 560 km, the atmosphere consists of about 90% atomic oxygen.


Corrosion in space has the highest impact on spacecraft with moving parts. Early satellites tended to develop problems with seizing bearings. Now the bearings are coated with a thin layer of gold.

Different materials resist corrosion in space differently. For example, aluminium is slowly eroded by atomic oxygen, while gold and platinum are highly corrosion-resistant. Gold-coated foils and thin layers of gold on exposed surfaces are therefore used to protect the spacecraft from the harsh environment. Thin layers of silicon dioxide deposited on the surfaces can also protect metals from the effects of atomic oxygen; e.g., the Starshine 3 satellite aluminium front mirrors were protected that way. However, the protective layers are subject to erosion by micrometeorites.

Silver builds up a layer of silver oxide, which tends to flake off and has no protective function; such gradual erosion of silver interconnects of solar cells was found to be the cause of some observed in-orbit failures.

Many plastics are considerably sensitive to atomic oxygen and ionizing radiation. Coatings resistant to atomic oxygen are a common protection method, especially for plastics. Silicone-based paints and coatings are frequently employed, due to their excellent resistance to radiation and atomic oxygen. However, the silicone durability is somewhat limited, as the surface exposed to atomic oxygen is converted to silica which is brittle and tends to crack.

Solving corrosion

The process of space corrosion is being actively investigated. One of the efforts aims to design a sensor based on zinc oxide, able to measure the amount of atomic oxygen in the vicinity of the spacecraft; the sensor relies on drop of electrical conductivity of zinc oxide as it absorbs further oxygen.

Other problems

The outgassing of volatile silicones on low Earth orbit devices leads to presence of a cloud of contaminants around the spacecraft. Together with atomic oxygen bombardment, this may lead to gradual deposition of thin layers of carbon-containing silicon dioxide. Their poor transparency is a concern in case of optical systems and solar panels. Deposits of up to several micrometers were observed after 10 years of service on the solar panels of the Mir space station.

Other sources of problems for structures subjected to outer space are erosion and redeposition of the materials by sputtering caused by fast atoms and micrometeoroids. Another major concern, though of non-corrosive kind, is material fatigue caused by cyclical heating and cooling and associated thermal expansion mechanical stresses.

From the Wikipedia entry for CORROSION IN SPACE

Protective Paint

If you want a World War II flavor for your rocket, any interior spaces that are exposed to rain and other corrosive planetary weather should be painted with a zinc chromate primer. Depending on what is mixed into the paint, this will give a paint color ranging from yellowish-green to greenish-yellow. In WWII aircraft it is found in wheel-wells and the interior of bomb bays. In your rocket it might be found on landing jacks and inside airlock doors.

Naturally this does not apply to strict orbit-to-orbit rockets, or rockets that only land on airless moons and planets. Well, now that I think about it, some of the lunar dust is like clouds of microscopic razor blades so they are dangerously abrasive.


Zinc chromate, ZnCrO4, is a chemical compound containing the chromate anion, appearing as odorless yellow powder or yellow-green crystals, but, when used for coatings, pigments are often added. It is used industrially in chromate conversion coatings, having been developed by the Ford Motor Company in the 1920s.


Zinc chromate’s main use is in industrial painting as a coating over iron or aluminum materials. It was used extensively on aircraft by the U.S. military, especially during the 1930s and 1940s, but is also used in a variety of paint coatings for the aerospace and automotive industries. Its use as a corrosion-resistant agent was applied to aluminium alloy parts first in commercial aircraft, and then in military ones. During the 1940 and 1950s it was typically found as the "paint" in the wheel wells of retractable landing gear on U.S. military aircraft to protect the aluminium from corrosion. This compound was a useful coating because it is an anti-corrosive and anti-rust primer. Since it is highly toxic it also destroys any organic growth on the surface. Zinc chromate is also used in spray paints, artists’ paints, pigments in varnishes, and in making linoleum.


Recent studies have shown that not only is zinc chromate highly toxic, it is also a carcinogen. Exposure to zinc chromate can cause tissue ulceration and cancer. A study published in the British Journal of Industrial Medicine showed a significant correlation between the use of zinc chromate and lead chromate in factories and the number of cases in lung cancer experienced by the workers. Because of its toxicity the use of zinc chromate has greatly diminished in recent years.

From the Wikipedia entry for ZINC CHROMATE

Rocket Tumble

The basic idea is that the Axis of Thrust from the engines had better pass through the the spacecraft's center of gravity (CG) or everybody is going to die. In addition, if the spacecraft is currently passing through a planet's atmosphere the axis of thrust had better be parallel to the aerodynamic axis or the same thing will happen.

Specifically, "everybody is going to die" means the spacecraft is going to loop-the-loop or tumble like a cheap Fourth-of-July skyrocket (Heinlein calls this a rocket "falling off its tail"). If this happens during lift-off the ship will auger into the ground like a nuclear-powered Dinosaur-Killer asteroid and make a titanic crater. If it happens in deep space, the rocket will spin like a pinwheel firework spraying atomic flame everywhere. This will waste precious propellant, give the spacecraft a random vector, and severely injure the crew with unexpected spin gravity. If they are lucky the crew's broken bones will heal about the same time that they run out of oxygen.

The axis of thrust is a line starting at the center of the exhaust nozzle's throat, and traveling in the exact opposite direction of the hot propellant. It is the direction that the thrust is pushing the rocket. As long as the axis of thrust passes through the CG, the spacecraft will be accelerated in that direction. If the axis of thrust is not passing through the CG, the spacecraft will start to spin around the CG. When done on purpose this is called a yaw or pitch maneuver. When this is done by accident, it is called OMG WE'RE ALL GOING TO DIE!

Some engines can be gimbaled, rotating their axis of thrust off-center by a few degrees. This is intended for yaw and pitch, but it can be used in emergencies to cope with accidental changes in the center of gravity (e.g., the cargo shifts).

When laying out the floor plan, you want the spacecraft to balance. This boils down to ensuring that the ship's center of gravity is on the central axis, which generally is the same as the axis of thrust. There are exceptions. The Grumman Space Tug has its center of gravity shift wildly when it jettisons a drop tank. To compensate, the engine can gimbal by a whopping ±20°.

Balancing also means that each deck should be "radially symmetric". That's a fancy way of saying that if you have something massive in the north-west corner of "D" deck, you'd better have something equally massive in the south-east corner. Otherwise the center of gravity won't be centered.

For a cargo ship, the Loadmaster has to ensure that the cargo is stored in a radially symmetric balance.

This is another reason to strap down the crew during a burn. Walking around could upset the ship's balance, resulting in the dreaded rocket tumble. This will be more of a problem with tiny ships than with huge cruisers, of course. The same goes for the cargo. The load-master better be blasted sure all the tons of cargo are nailed down so they don't shift. And be sure the cargo is evenly balanced around the ship's axis to keep the center of gravity in the center.

Small ships might have "trim tanks", small tanks into which water can be pumped in order to adjust the balance. The ship will also have heavy gyroscopes that will help prevent the ship from falling off its tail, but there is a limit to how much imbalance that they can compensate for.

Propellent Tankage

A cursory look at the rocket's mass ratio will reveal that most of the rocket's mass is going to be propellant tanks.

For anything but a torchship, the spacecraft's mass ratio is going to be greater than 2 (i.e., 50% or more of the total mass is going to be propellant). Presumably the propellant is inside a propellant tank (unless you are pulling a Martian Way gag and freezing the fuel into a solid block). Remember, RockCat said all rockets are giant propellant tanks with an engine on the bottom and the pilot's chair at the top.

If you have huge structure budget, you have a classic looking rocket-style rocket with propellant tanks inside. If you have a medium structure budget, you have a spine with propellant tanks attached. If you have a small structure budget, you'll have an isogrid propellant tank for a spine, with the rest of the rocket parts attached.

And if you are stuck with a microscopic structure budget, you'll have a foil-thin propellant tank stiffened by the pressure of the propellant, with the rest of the rocket parts attached. But the latter tends to collapse when the propellant is expended and the pressure is gone. This was used in the old 1957 Convair Atlas rocket, but not so much nowadays. You cannot really reuse them.


It had all seemed perfectly logical back on Mars, but that was Mars. He had worked it out carefully in his mind in perfectly reasonable steps. He could still remember exactly how it went. It didn't take a ton of water to move a ton of ship. It was not mass equals mass, but mass times velocity equals mass times velocity. It didn't matter, in other words, whether you shot out a ton of water at a mile a second or a hundred pounds of water at twenty miles a second. You got the same velocity out of the ship.

That meant the jet nozzles had to be made narrower and the steam hotter. But then drawbacks appeared. The narrower the nozzle, the more energy was lost in friction and turbulence. The hotter the steam, the more refractory the nozzle had to be and the shorter its life. The limit In that direction was quickly reached.

Then, since a given weight of water could move considerably more than its own weight under the narrow-nozzle conditions, it paid to be big. The bigger the water-storage space, the larger the size of the actual travel-head, even in proportion. So they started to make liners heavier and bigger. But then the larger the shell, the heavier the bracings, the more difficult the weldings, the more exacting the engineering requirements. At the moment, the limit in that direction had been reached also.

And then he had put his finger on what had seemed to him to be the basic flaw—the original unswervable conception that the fuel had to be placed inside the ship; the metal had to be built to encircle a million tons of water.

Why? Water did not have to be water. It could be ice, and ice could be shaped. Holes could be melted into it. Travel-heads and jets could be fitted into it. Cables could hold travel-heads and jets stiffly together under the influence of magnetic field-force grips.

From THE MARTIAN WAY by Isaac Asimov (1952)

Our running example Polaris spacecraft has a gas core nuclear thermal rocket engine.

The fuel is uranium 235. It will probably be less than 1% of the total propellant load so we will focus on just the propellant for now.

Nuclear thermal rockets generally use hydrogen since you want propellant with the lowest molecular mass. Liquid hydrogen has a density of 0.07 grams per cubic centimeter.

The Polaris has 792.6 metric tons of hydrogen propellant. 792.6 tons of propellant = 792,600,000 grams / 0.07 = 11,323,000,000 cubic centimeters = 11,323 cubic meters . The volume of a sphere is 4/3πr3 so you can fit 11,323 cubic meters in a sphere about 14 meters in radius . Almost 92 feet in diameter, egad! It is a pity hydrogen isn't a bit denser.

If this offends your aesthetic sense, you'll have to go back and change a few parameters. Maybe a 2nd generation GC rocket, and a mission from Terra to Mars but not back. Maybe use methane instead of hydrogen. It only has an exhaust velocity of 6318 m/s instead of hydrogen's superior 8800 m/s, but it has a density of 0.42 g/cm3, which would only require a 1.7 meter radius tank. (Methane has a higher exhaust velocity than one would expect from its molecular weight, due to the fact that the GC engine is hot enough to turn methane into carbon and hydrogen. Note that in a NERVA style engine the reactor might become clogged with carbon deposits.)

Propellant Tank Mass

Robert Zubrin says that as a general rule, the mass of a fuel tank loaded with liquid hydrogen will be about 87% hydrogen and 13% tank. In other words, multiply the mass of the liquid hydrogen by 0.15 to get the mass of the empty tank (0.13 / 0.87 = 0.15).

So the Polaris' 792.6 tons of hydrogen will need a tank that masses 792.6 * 0.15 = 119 tons.

87% propellant and 13% tank is for a rocket designed to land on a planet or that is capable of high acceleration. An orbit-to-orbit rocket could get by with more hydrogen and less tank. This is because the tanks can be more flimsy since they will not have to endure the stress of landing (A landing-capable rocket that uses a propellant denser than hydrogen can also get away with a smaller tank percentage). Zubrin gives the following ballpark estimates of the tank percentage:

PropellentEngineTank %
ArgonIon rocket4
WaterNuclear salt water rocket4
HydrogenNTR / GCR10

But if you want to do this the hard way, you'd better warm up your slide rule.

The total tank volume (Vtot) of a tank is the sum of four components:

  1. Usable Propellant Volume (Vpu): the volume holding the propellant that can actually be used.
  2. Ullage Volume (Vull): the volume left unfilled to accomodate expansion of the propellant or contraction of the tank structure. Typically 1% to 3% of total tank volume.
  3. Boil-off Volume (Vbo): For cryogenic propellants only. The volume left unfilled to allow for the propellant that boils from liquid to gas due to external heat.
  4. Trapped Volume (Vtrap): the volume of unusable propellant left in all the feed lines, valves, and other components after the tank is drained. Typically the volume of the feed system.

Vtot = Vpu + Vull + Vbo + Vtrap

No, I do not know how to estimate the Boil-off Volume. A recent study estimated that in space cryogenic tanks suffered an absolutely unacceptable 0.1% boiloff/day, and suggested this had to be reduced by an order of magnitude or more. When the boil-off volume is full, a pressure relief valve lets the gaseous propellant vent into space, instead of exploding the tank.

Tanks come in two shapes: spherical and cylindrical. Spherical are better, they have the most volume for the least surface area, so are the lightest. But many spacecraft have a limit to their maximum diameter, especially launch vehicles. In this case cylindrical has a lower mass than a series of spherical tanks.

The internal pressure of the propellant has the greatest effect on the tank's structural requirements. Not as important but still significant are acceleration, vibration, and handling loads. Unfortunately I can only find equations for the effects of internal pressure. Acceleration means that tanks which are in high-acceleration spacecraft or in spacecraft that take-off and land from planets will have a higher mass than tanks for low-acceleration orbit-to-orbit ships. My source did say that figuring in acceleration, vibration, and handling would make the tank mass 2.0 to 2.5 times as large as what is calculated with the simplified equations below.

In the Space Shuttle external tank, the LOX tank was pressurized to 150,000 Pa and the LH2 tank was pressurized to 230,000 Pa.

The design burst pressure of a tank is:

Pb = fs * MEOP


Pb = design burst pressure (Pa)
fs = safety factor (typically 2.0)
MEOP = Maximum Expected Operating Pressure of the tank (Pa)

Tank Materials

Allowable Strength
Mass Factor
2219 - Aluminum2,8000.413
0.214 welded
4130 - Steel7,8300.86211.232,500
Graphite Fiber

Spherical Tanks

You have to make Vs so it is equal to Vtot, or at least equal to Vtot - Vtrap.

Vs = 4/3 * π * rs3

As = 4 * π * rs2

ts = (Pb * rs) / (2 * Ftu)

Ms = As * ts * ρ


rs = radius of sphere (m)
As = surface area of sphere (m2)
Vs = volume of sphere (m3)
ts = wall thickness of sphere (m)
Pb = design burst pressure (Pa)
Ftu = allowable material strength (Pa) from tank materials table
Ms = mass of spherical tank (kg)
ρ = density of tank structure material (kg/m3 from tank materials table

Cylindrical Tanks

Cylindrical tanks are cylinders where each end is capped with either hemispheres (where radius and height are equal) or hemiellipses (where radius and height are not equal). As it turns out cylindrical tanks with hemiellipses on the ends are always more massive than hemispherical cylindrical tanks. So we won't bother with the equations for hemielliptical tanks. In the real world rocket designers sometimes use hemielliptical tanks in order to reduce tank length.

What you do is calculate the mass of the cylindrical section of the tank Mc using the equations below. Then you calculate the mass of the two hemispherical endcaps (that is, the mass of a single sphere) Ms using the value of the cylindrical section's radius for the radius of the sphere in the spherical tank equations above. The mass of the cylindrical tank is Mc + Ms.

Vc = π * rc2 * lc

Ac = 2 * π * rc2 * lc

tc = (Pb * rc) / Ftu

Mc = Ac * tc * ρ


rc = radius of cylindrical section (m)
lc = length of cylindrical section (m)
Ac = surface area of cylindrical section (m2)
Vc = volume of cylindrical section (m3)
Pb = design burst pressure (Pa)
Ftu = allowable material strength (Pa) from tank materials table
ρ = density of tank structure material (kg/m3 from tank materials table
tc = wall thickness of cylindrical section (m)
Mc = mass of cylindrical tank section (kg)


When the rocket is sitting on the launch pad, the planet's gravity pulls the propellant down so that the pumps at the aft end of the tank can move it to the engine. When the rocket is under acceleration, the thrust pulls the propellant down to the pumps. Once the engines cut off and the rocket is in free fall, well, the remaining pooled at the bottom turns into zillions of blobs and starts floating everywhere. See the video:

This isn't a problem, up until the point where you want to start the engine up again. Trouble is, the propellant isn't at the aft pump, it is flying all over the place. What's worse, some of the liquid propellant might have turned into bubbles of gas, which could wreck the engine if they are sucked into the pump. Vapor lock in a rocket engine is an ugly thing.

In 1960 Soviet engineers invented the solution: Ullage Motors. These are tiny rocket engines that only have to accelerate the rocket by about 0.001g (0.01 m/s). That's enough to pull the propellant down to the pump, and to form a boundary between the liquid and gas portions. In some cases, the spacecraft's reaction control system (attitude jets) can operate as ullage motors.

In the Apollo service module, they use a "retention reservoir" instead of an ullage burn (but they have to burn anyway if the amount of fuel and oxidizer drops below 56.4%).

Liquid oxygen in the oxidizer storage tank flows into the oxidizer sump tank. During an engine burn, oxygen flows to the bottom of the sump tank, through an umbrella shaped screen, into the retention reservoir, then into a pipe at the bottom leading to the engine. The same system is used in the fuel tanks.

When the burn is terminated and the oxygen breaks up into a zillion blobs and starts floating everywhere, the oxygen under the screen umbrella cannot escape. Surface tension prevents it from escaping through the screen holes. The oxygen is trapped under the umbrella, inside the retention reservoir.

When the engines are restarted there is oxygen right at the pipe to feed into the engine, instead of a void with random floating blobs. The engine thrust then settles the oxygen in the sump tank for normal operation.

As near as I can figure, the 56.4% ullage limit happens when the storage tank is empty, so the sump tank is only partially full. But I'm not sure.

Heat Shield

Aerobraking is used to get rid of a portion of a spacecraft's velocity without using a rocket engine and reaction mass. Or as NASA thinks of it: "For Free!" This can be used for landing, for planetary capture, for circulating spacecraft's orbit, or other purposes.

Robert Zubrin says mass of the heat shield and thermal structure will be about 15% of the total mass being braked.

The general rule is that aerobraking can kill a velocity approximately equal to the escape velocity of the planet where the aerobraking is performed (10 km/s for Venus, 11 km/s for Terra, 5 km/s for Mars, 60 km/s for Jupiter).

This will mostly be used for our purposes designing a emergency re-entry life pod, not a Solar Guard patrol ship. With a sufficiently advanced engine it is more effective just to carry more fuel, so our atomic cruiser will not need to waste mass on such a primitive device.

NASA on the other hand uses aerobraking every chance it gets, since they do not have the luxury of using atomic engines. Many of the Mars probes use aerobraking for Mars capture and to circularize their orbit. Some use their solar panels as aerobraking drage chutes in order to make a given piece of payload mass do double duty. Some of the Space Tug designs listed in the Realistic Design section economize on reaction mass by using a ballute when returning to Terra orbit.

In the movie 2010, the good ship Leonov had a one-lung propulsion system, so they needed an aerobraking ballute to slow them into Jovian orbit. If you are thinking about aerobraking, keep in mind that many worlds in the Solar System do not have atmospheres.

You can find a more in-depth look at heat shields here.

Power Generation

If you cannot tap your propulsion system for electrical power, you will need a separate power plant (or it's going to be real dark inside your spacecraft).

Typically the percentage of spacecraft dry mass that is power systems is 28% for NASA vessels.

Spacecraft power systems have three subsystems:

  • Power Generation/ Conversion: generating power
  • Energy Storage: storing power for future use
  • Power Management and Distribution (PMAD): routing the power to equipment that needs it

There are a couple of parameters used to rate power plant performance:

  • Alpha : (kg/kW) power plant mass in kilograms divided by kilowatts of power. So if a solar power array had an alpha of 90, and you needed 150 kilowatts of output, the array would mass 90 * 150 = 13,500 kg or 13.5 metric tons
  • Specific Power : (W/kg) watts of power divided by power plant mass in kilograms (i.e., (1 / alpha) * 1000)
  • Specific Energy : (Wh/kg) watt-hours of energy divided by power plant mass in kilograms
  • Energy Density : (Wh/m3) watt-hours of energy divided by power plant volume in cubic meters

NASA has a rather comprehensive report on various spacecraft power systems here . The executive summary states that currently available spacecraft power systems are "heavy, bulky, not efficient enough, and cannot function properly in some extreme environments."

Scroll to see rest of infographic

Energy Harvesting

Energy Harvesting or energy scavenging is a pathetic "waste-not-want-not" strategy when you are desperate to squeeze every milliwatt of power out of your system. This includes waste engine heat (gradients), warm liquids, kinetic motion, vibration, and ambient radiation. This is generally used for such things as enabling power for remote sensors in places where no electricity is readily available.

Fuel Cells

The general term is "chemical power generation", which means power generated by chemical reactions. This is most commonly seen in the form of fuel cells, though occasionally there are applications like the hydrazine-fired gas turbines that the Space Shuttle uses to hydraulically actuate thrust vector vanes.

Fuel cells basically consume hydrogen and oxygen to produce low voltage electricity and water. They are quite popular in NASA manned spacecraft designs. Each PC17C fuel-cell stack in the Shuttle Orbiter has an alpha of about 10 kg/kW, specific power 98 W/kg, have a total mass of 122 kg, have an output of 12 kW, and produces about 2.7 kilowatt-hours per kilogram of hydrogen+oxygen consumed (about 70% efficient). They also have a service life of under 5000 hours. The water output can be used in the life support system.

Different applications will require fuel cells with different optimizations. Some will need high specific power (200 to 400 W/kg), some will need long service life (greater than 10,000 hours), and others will require high efficiency (greater than 80% efficient).

Solar Thermal Power

Back in the 1950's, on artist conceptions of space stations and space craft, one would sometimes see what looked like mirrored troughs. These were "mercury boilers", a crude method of harnessing solar energy in the days before photovoltaics. The troughs had a parabolic cross section and focused the sunlight on tubes that heated streams of mercury. The hot mercury was then used in turbines to generate electricity.

These gradually vanished from artist conceptions and were replaced by nuclear reactors. Generally in the form of a long framework boom sticking out of the hub, with a radiation shadow shield big enough to shadown the wheel.

The technical name is "solar dynamic power", where mirrors concentrate sunlight on a boiler. "Solar static power" is Photovoltaic solar cells.

Such systems are generally useful for power needs between 20 kW and 100 kW. Below 20 kW a solar cell panel is better. Above 100 kW a nuclear fission reactor is better.

They typically have an alpha of 250 to 170, a collector size of 130 to 150 watts per square meter at Terra orbit (i.e., about 11% efficient), and a radiator size of 140 to 200 watts per square meter.

He wasn't surprised when he was assigned to the job of helping paint the solar mirror. This was a big trough that was to run all around the top of the station, set to face the Sun. It was curved to focus the rays of the Sun on a blackened pipe that ran down its center. In the pipe, mercury would be heated into a gas, at a temperature of thirteen hundred degrees Fahrenheit. This would drive a highly efficient "steam" turbine, which would drive a generator for the needed power. When all its energy was used, the mercury would be returned to the outside, to cool in the shadow of the mirror, condensing back to a liquid before re-use.

It was valuable work, and the station badly needed a good supply of power. But painting the mirror was done with liquid sodium. It was a silvery metal that melted easily at a low temperature. On Earth, it was so violently corrosive that it could snatch oxygen out of water. But in a vacuum, it made an excellent reflective paint. The only trouble was that it had to be handled with extreme caution.

It was nasty work. A drop on the plasticized fabric of the space suits would burn a hole through them almost at once. Or a few drops left carelessly on the special gloves they wore for the job could explode violently if carried into the hut, to spread damage and dangerous wounds everywhere nearby.

Jim worked on cautiously, blending his speed with safety in a hard-earned lesson. But the first hour after the new man came out was enough to drive his nerves to the ragged edge. At first, the man began by painting the blackened pipe inside the trough.

Jim explained patiently that the pipe was blackened to absorb heat, and that the silver coating ruined it. He had to go back and construct a seat over the trough on which he could sit without touching the sodium, and then had to remove the metal chemically.

Finally, he gave up. The man was one of those whose intelligence was fine, but who never used it except for purely theoretical problems. He was either so bemused by space or so wrapped up in some inner excitement over being there that he didn't think—he followed orders blindly.

"All right," he said finally. "Go back to Dan and tell him Terrence and I can do it alone. Put your paint in the shop, and mark it dangerous. I'll clean up when I come in."

He watched the man leave, and turned to the boy who had been working with him.

Then suddenly Terrence dropped his brush into the sodium and pointed, his mouth open and working silently.

Jim swung about to see what was causing it, and his own mouth jerked open soundlessly.

The roof of the hut ahead of them was glowing hotly, and as they watched, it suddenly began crumbling away, while a great gout of flame rushed out as the air escaped. Oxygen and heat were fatal to the magnesium alloy out of which the plates were made.

The fire had been coming from the second air lock, installed when the hut was extended. The old one still worked, and men were inside the hut, laboring in space suits. An automatic door had snapped shut between the two sections at the first break in the airtight outer sheathing. But there were still men inside where the flames were, and they were being dragged out of a small emergency lock between the two sections.

One of them yanked off his helmet to cough harshly. His face was burned, but he seemed unaware of it. "Kid came through the lock with a can of something. He tripped, spilled it all over—and then it exploded. We tried to stop it, but it got away. The kid—"

He shuddered, and Jim found that his own body was suddenly weak and shaky. The third man must have done it. He'd taken the orders too literally—he'd gone to report to Dan first, before putting away the sodium. A solid hour's lecture on the dangers of the stuff had meant nothing to him.

From Step to the Stars by Lester Del Rey (1954)

Solar Photovoltaic Power

At Terra's distance to the sun, solar energy is about 1366 watts per square meter. This energy can be converted into electricity by photovoltaics. Of course the power density goes down the farther from the Sun the power array is located.

The technical name is "solar static power", where photovoltaic solar cells convert sunlight into electricity. "Solar dynamic power" is where mirrors concentrate sunlight on a boiler.

Solar power arrays have an alpha ranging from 100 to 1.4 kg/kW. Body-mounted rigid panels an alpha of 16 kg/kW while flexible deployable arrays have an alpha of 10 kg/kW. Most NASA ships use multi-junction solar cells which have an efficiency of 29%, but a few used silicon cells with an efficiency of 15%. Most NASA arrays output from 0.5 to 30 kW.

Some researchers (Dhere, Ghongadi, Pandit, Jahagirdar, Scheiman) have claimed to have achieved 1.4 kg/kW in the lab by using Culn1-×Ga×S2 thin films on titanium foil. Rob Davidoff is of the opinion that a practical design with rigging and everything will be closer to 4 kg/kW, but that is still almost three times better than conventional solar arrays.

In 2015 researchers at Georgia Institute of Technology demonstrated a photovoltaic cell using an optical rectenna. They estimate that such rectennas could have a power conversion efficiency of up to 40% and a lower cost than silicon cells. No word on the alpha, though.

The International Space Station uses 14.5% efficient large-area silicon cells. Each of the Solar Array Wings are 34 m (112 ft) long by 12 m (39 ft) wide, and are capable of generating nearly 32.8 kW of DC power. 19% efficiency is available with gallium arsenide (GaAs) cells, and efficiencies as high as 30% have been demonstrated in the laboratory.

To power a ion drive or other electric propulsion system with solar cells is going to require an array capable of high voltage (300 to 1000 volts), high power (greater than 100 kW), and a low alpha (2 to 1 kg/kW).

Obviously the array works best when oriented face-on to the sun, and unshadowed. As the angle increases the available power decreases in proportion to the cosine of the angle (e.g., if the array was 75° away from face-on, its power output would be Cos(75°) = 0.2588 or 26% of maximum). Solar cells also gradually degrade due to radiation exposure (say, from 8% to 17% power loss over a five year period if the panel is inhabiting the deadly Van Allen radiation belt, much less if it is in free space).

Typically solar power arrays are used to charge batteries (so you have power when in the shadow of a planet). You should have an array output of 20% higher voltage than the battery voltage or the batteries will not reliably charge up. Sometimes the array is used instead to run a regenerative fuel cell.

Solar Power
PlanetSol Dist
☿ Mercury0.3876.6779,121
⊕ Terra1.0001.0001,366
⚶ Vesta2.3620.179245
⚵ Juno2.6700.140192
⚳ Ceres2.7680.131178
⚴ Pallas2.7720.130178
Start LILT3.0000.111152
♃ Jupiter5.2000.03751
♄ Saturn9.5800.01115
♅ Uranus19.2000.0034
♆ Neptune30.0500.0012


Like all non-coherent light, solar energy is subject to the inverse square law. If you double the distance to the light source, the intensity drops by 1/4.

Translation: if you travel farther from the sun than Terra orbit, the solar array will produce less electricity. Contrawise if you travel closer to the sun the array will produce more electricity. This is why some science fiction novels have huge solar energy farms on Mercury; to produce commercial quantities of antimatter, beamed power propulsion networks, and other power-hungry operations.

As a general rule:

Es = 1366 * (1 / Ds2)


  • Es = available solar energy (watts per square meter)
  • Ds = distance from the Sun (astronomical units)
  • 1366 = Solar Constant (watts per square meter)

Remember that you divide distance in meters by 1.496e11 in order to obtain astronomical units. Divide distance in kilometers by 1.496e8 to obtain astronomical units


What is the available solar energy at the orbit of Mars?

Mars orbits the sun at a distance of 2.28e11 meters. That is 2.28e11 / 1.49e11 = 1.52 astronomical units. So the available solar energy is:

  • Es = 1366 * (1 / Ds2)
  • Es = 1366 * (1 / 1.522)
  • Es = 1366 * (1 / 2.31)
  • Es = 1366 * 0.423
  • Es = 591 watts per square meter

This means that the available solar energy around Saturn is a pitiful 15 W/m2. That's available energy, if you tried harvesting it with the 29% efficient ISS solar cell arrays you will be lucky to get 4.4 W/m2. Which is why the Cassini probe used RTGs.

Special high efficiency cells are needed in order to harvest worthwhile amounts of solar energy in low intensity/low temperature conditions (LILT). Which is defined as the solar array located at 3 AU from Sol or farther (i.e., about 150 watts per square meter or less, one-ninth the energy available at Terra's orbit).

Equation Derivation

If you are curious where the "1,366 W/m2" Solar Constant value in the equation came from (for instance, if you want to calculated it for another star), read on. Otherwise skip this section.

It starts with the Stefan–Boltzmann law:

j = σ * T4


j total energy radiated from black body (W/m2)
σ Stefan–Boltzmann constant (5.670367×10−8 W·m−2·K−4)
T thermodynamic temperature (K)

The Sun's thermodynamic temperature is 5,778 K (effective temperature in the photosphere). Doing the math reveals that j = 63,200,617 W/m2.

To calcuate what this is at Terra's orbit (1 Astronomical Unit) we use the Inverse-Square Law. For this purpose the equation is:

P1au = (Dss2 / Dau2) * j


P1au solar power at 1 AU or solar constant (W/m2)
Dss distance from center of sun to sun's surface, the sun's radius (AU)
Dau solar constant distance (AU) = 1 AU for all stars, by definition
j total energy irradiated, from first equation (W/m2)
x2 square of x, that is x * x

The Sun's radius is 696,342 km. Dividing by 1.496e8 tells us the Sun's radius is 0.00465 AU (because the equation wants both distances in AU). Plugging it all into the equation:

P1au = (Dss2 / Dau2) * j
P1au = (0.004652 / 12) * 63,200,617
P1au = (0.0000216225 / 1) * 63,200,617
P1au = 0.0000216225 * 63,200,617
P1au = 1,369 W/m2

which is close enough for government work to 1,366 W/m2.

To calculate this for other stars you will need that star's thermodynamic temperature and radius. If you do not want to do the math, I made a quick table for you.

  1. Refer to the Star Table
  2. Look up the star's Spectral Class (the Sun is a G2 star)
  3. For thermodynamic temperature T, use value for Teeff (G2 is 5770)
  4. For Dss take the value for R (G2 is 1.0) and multiply it by 0.00465 to get star's radius in AU
  5. Calculate P1au using the two equations above

The solar array drop off equation for that star will then be:

Es = P1au * (1 / Ds2)

For example, the star Sirus A is spectral class A0. From the table Teeff is 10,000 K, use that for T. From the table R is 2.7, times 0.00465 means Dss is 0.01257.

Doing the math, J = 567,036,700 and P1au = 89,561. So for Sirus the solar array drop off equation is

Es = 89,561 * (1 / Ds2)

This means that a spacecraft with a solar array orbiting 5 astronomical units from Sirius (orbital radius of Jupiter) could harvest 3,582 watts per square meter, or about 2.6 times as much as it could get in the solar system at Terra orbit.

A more exotic variant on solar cells is the beamed power concept. This is where the spacecraft has a solar cell array, but back at home in orbit around Terra (or Mercury) is a a huge power plant and a huge laser. The laser is fired at the solar cell array, thus energizing it. It is essentially an astronomically long electrical extension cord constructed of laser light. It shares the low mass advantage of a solar powered array. It has an advantage over solar power that the energy per square meter of array can be much larger.

It has the disadvantage that the spacecraft is utterly at the mercy of whoever is currently running the laser battery. It has the further disadvantage of being frowned upon by the military, since they take a dim view of weapons-grade lasers in civilian hands. Unless the military owned the power lasers in the first place.

Radioisotope Thermoelectric Generators

Radioisotope thermoelectric generators (RTG) are slugs of radioisotopes (usually plutonium-238 in the form of plutonium oxide) that heat up due to nuclear decay, and surrounded by thermocouples to turn the heat gradient into electricity (it does NOT turn the heat into electricity, that's why the RTG has heat radiator fins on it.).

There are engineering reasons that currently make it impractical to design an individual RTG that produces more than one kilowatt. However nothing is stopping you from using several RTGs in your power room. Engineers are trying to figure out how to construct a ten kilowatt RTG.

Current NASA RTGs have a useful lifespan of over 30 years.

Currently RTGs have an alpha of about 200 kg/kW (though there is a design on the drawing board that should get about 100 kg/kW). Efficiency is about 6%. The near term goal is to develop an RTG with an alpha of 100 to 60 kg/kW and an efficiency of 15 to 20%.

An RTG based on a Stirling cycle instead of thermionics might be able to reach an efficiency of 35%. Since they would need less Pu-238 for the same electrical output, a Sterling RTG would have only 0.66 the mass of an equivalent thermocouples RTG. However NASA is skittish about Sterling RTGs since unlike conventional ones, Sterlings have moving parts. Which are yet another possible point of failure on prolonged space missions.

Nuclear weapons-grade plutonium-239 cannot be used in RTGs. Non-fissionable plutonium-238 has a half life of 85 years, i.e., the power output will drop to one half after 85 years. To calculate power decay:

P1 = P0 * 0.9919^Y


  • P1 = current power output (watts)
  • P0 = power output when RTG was constructed (watts)
  • Y = years since RTG was constructed.

If a new RTG outputs 470 watts, in 23 years it will output 470 x 0.9919^23 = 470 x 0.83 = 390 watts

Wolfgang Weisselberg points out that this equation just measures the drop in the power output of the slug of plutonium. In the real world, the thermocouples will deteriorate under the constant radioactive bombardment, which will reduce the actual electrical power output even further. Looking at the RTGs on NASA's Voyager space probe, it appears that the thermocouples deteriorate at roughly the same rate as the plutonium.

Plutonium-238 has a specific power of 0.56 watts/gm or 560 watts per kilogram, so in theory all you would need is 470 / 560 = 0.84 kilograms. Alas, the thermoelectric generator which converts the thermal energy to electric energy has an efficiency of only 6%. If the thermoelectric efficiency is 6%, the plutonium RTG has an effective specific power of 560 x 0.06 = 30 watts per kilogram 238Pu (0.033 kilogram 238Pu per watt or 33 kgP/kW). This means you will need an entire 15.5 kilos of plutonium to produce 470 watts.

This is why a Sterling-based RTG with an efficience of 35% is so attractive.

Many RTG fuels would require less than 25 mm of lead shielding to control unwanted radiation. Americium-241 would need about 18 mm worth of lead shielding. And Plutonium-238 needs less than 2.5 mm, and in many cases no shielding is needed as the casing itself is adequate. Plutonium is the radioisotope of choice but it is hard to come by (due to nuclear proliferation fears). Americium is more readily available but lower performance.

At the time of this writing (2014) NASA has a severe Pu-238 problem. NASA only has about 16 kilograms left, you need about 4 kg per RTG, and nobody is making any more. They were purchasing it from the Russian Mayak nuclear industrial complex for $45,000 per ounce, but in 2009 the Russians refused to sell any more.

NASA is "rattled" because they need the Pu-238 for many upcoming missions, they do not have enough on had, and Congressional funding for creating Pu-238 manufacturing have been predictably sporadic and unreliable.

The European Space Agency (ESA) has no access to Pu-238 or RTGs at all. This is why their Philae space probe failed when it could not get solar power. The ESA is accepting the lesser of two evils and is investing in the design and construction of Americium-241 RTGs. Am-241 is expensive, but at least it is available.

Nuclear Fission Reactors

Los Alamos reactor
Fuel region157 kg
Reflector154 kg
Heat pipes117 kg
Reactor control33 kg
Other support32 kg
Total Reactor mass493 kg

For a great in-depth analysis of nuclear power for space applications, I refer you to Andrew Presby's engineer degree thesis: Thermophotovoltaic Energy Conversion in Space Nuclear Reactor Power Systems . There is a much older document with some interesting designs here .

As far as the nuclear fuel required, the amount is incredibly tiny. Which in this case means burning a microscopic 0.01 grams of nuclear fuel per second to produce a whopping 1000 megawatts! That's the theoretical maximum of course, you can find more details here.

Nuclear fission reactors are about an alpha of 18 kg/kW. However, Los Alamos labs had an amazing one megawatt Heat Pipe reactor that was only 493 kg (alpha of 0.493 kg/kW):

Fission reactors are attractive since they have an incredibly high fuel density, they don't care how far you are from the Sun nor if it is obscured, and they have power output that makes an RTG look like a stale flashlight battery. They are not commonly used by NASA due to the hysterical reaction of US citizens when they hear the "N" word. Off the top of my head the only nuclear powered NASA probe currently in operation is the Curiosity Mars Rover; and that is an RTG, not an actual nuclear reactor.

For a space probe a reactor in the 0.5 to 5 kW power range would be a useful size, 10 to 100 kW is good for surface and robotic missions, and megawatt size is needed for nuclear electric propulsion.

Here is a commentary on figuring the mass of the reactor of a nuclear thermal rocket by somebody who goes by the handle Tremolo:

Now, onto a more practical means for generation 1 MW of power using a Plutonium fission reaction.

To calculate the mass required to obtain a certain power level, we have to know the neutron flux and the fission cross-section. Let's assume the flux is 1E14 neutron/cm2/sec, the cross section for fast fission of Pu-239 is about 2 barns (2E-24 cm2), the energy release per fission is 204 MeV, and the Pu-239 number density is 4.939E22 atoms/cm3. Then the power is

P = flux * number density * cross section * Mev per fission * 1.602E-13 Watt/MeV

P = 1E14 * 4.939E22 * 2E-24 * 204 * 1.602E-13 = 323 W/cm3

So, for 1 MW, we need 1E6/323 = 3100 cm3. Given a density of 19.6 gm/cm3, this is 19.6*3100 = 60,760 gm or 60.76 kg.

The next question to ask is: how long do you want to sustain this reaction? In other words, what is the total energy output?

For example, a Watt is one Joule per second. So, to sustain a 1 MW reaction for 1 year, the total energy is 1E6 J/s * 3.15E7 s/year = 3.15E13 J.

For Pu-239, we have 204 Mev per fission and we have 6.023E23./239 = 2.52E21 atoms/gm. So, the energy release per gram is 2.52E21 * 204 Mev/fission * 1.602E-13 J/Mev = 8.24E10 J/gm.

Therefore, to sustain 1 MW for 1 year, we will use 3.15E13 J / 8.24E10 J/gm = 382 gm of Pu-239 or 0.382 kg. This is only a small fraction of the total 60.76 kg needed for the fission reaction.

Finally, this is thermal energy. Our current light water reactors have about a 35% efficiency for conversion to electric power. So, you can take these numbers and essentially multiply by 3 to get a rough answer for the total Pu-239 needed: 3 x 60.76 = 182 kg. Rounding up, you would need roughly 200 kg for a long term sustained 1 MW fission reaction with a 35% conversion efficieny.

These calculations assume quite a bit and I wouldn't use these numbers to design a real reactor, but they should give you a ballpark idea of the masses involved.


New reactors that have never been activated are not particularly radioactive. Of course, once they are turned on, they are intensely radioactive while generating electricity. And after they are turned off, there is some residual radiation due to neutron activation of the reactor structure.

How much deadly radiation does an operating reactor spew out? That is complicated, but Anthony Jackson has a quick-and-dirty first order approximation:

r = (0.5*kW) / (d2)


  • r = radiation dose (Sieverts per second)
  • kW = power production of the reactor core, which will be greater than the power output of the reactor due to reactor inefficiency (kilowatts)
  • d = distance from the reactor (meters)

This equation assumes that a 1 kW reactor puts out an additional 1.26 kW in penetrating radiation (mostly neutrons) with an average penetration (1/e) of 20 g/cm2.

As a side note, in 1950's era SF novels, nuclear fission reactors are commonly referred to as "atomic piles." This is because the very first reactor ever made was basically a precision assembled brick-by-brick pile of graphite blocks, uranium fuel elements, and cadmium control rods.


Space Nuclear Power Program

This program aims to develop high-power, safe, reliable nuclear energy sources for manned and deep-space missions. Nuclear electric propulsion will open a new frontier of exploration in the outer solar system and allow manned missions to Mars and other places with difficult solar power problems.

A starting point might be the direct gas reactor studied for the Prometheus project, a 1MWt/200kWe reactor at 40-50kg/kW (7.5-11t). Another might be the heatpipe reactor SAFE-400, a 400kWt/100kWe reactor at unknown specific mass. Sodium-cooled designs in the 70kg/kW range are also possible. (Masses include radiators, conversion and power conditioning.)

The SAFE-30 project demonstrated simple, affordable ground testing of non-nuclear components. The Prometheus project demonstrated productive cooperation with Naval Reactors and related organizations to tap their nuclear technology expertise. Joining these approaches will allow the project to proceed immediately into materials testing and design optimization. The most urgently needed component is an experimental fast reactor for materials testing. Also critical will be a design process that focuses on modular power units so the same basic design can be used for a wide range of missions, presumably in the 50-100kWe range.

Costs are not straightforward to estimate. One baseline figure is the $4.2 billion estimated to develop the Prometheus reactor system. Let’s assume a 50% increase on that figure and use $6.3 billion for the development program; further assume hardware costs of $5000 per watt. Three demonstration units will be built: one for flight test (possibly on a later carrier flight), one for a NEP asteroid capture and one as a base power supply for a manned mission. The goal is a 50kWe power unit massing 2,000kg or better (40kg/kW) with at least 20-year useful life. Individual units could power NEP asteroid retrieval tugs or small ISRU operations; sets of four could power manned bases or deep-space probes. A second phase using knowledge gained from the first generation reactor program would aim to build power units of 1MWe and 10t mass range (~10kg/kW) for use on deep-space and interstellar probes, permanent bases and orbital manufacturing facilities. All future NEP missions would be able to use a proven, existing design and avoid developmental uncertainties.

Estimated costs:

$6,300m development program
$750m flight hardware
$2,115m margin
$9,165m total cost ($611m per year)

Alternate scenario: A fast-spectrum reactor is made available by another country or organization for materials testing. Majority of the design, testing and construction is outsourced to Naval Reactors and experienced contractors. Additional funding is provided by ESA and allied space agencies in return for access to flight hardware. Development program costs cut in half and a fourth power unit is built for ESA use. New costs:

$3,150m development program
$750m flight hardware
$1,170m margin
$5,070m total cost ($338m per year)

 This is a subject that's been stewing for a while now. I often see debates in comment sections over whether or not nuclear electric power is feasible in space. Only rarely do those arguing hold the same assumptions about what nuclear power actually means. As a result, these debates rarely convince anyone of anything beyond the stubborn natures of their opponents.

 The goal of this post is to briefly cover the range of commercial, military and scientific nuclear power systems ranging from a few kilowatts to over a gigawatt. I will follow up the (hopefully) useful background information in a later post with some fanciful projections and my usual call for unlikely investments in space.

Very briefly:
Nuclear energy is produced by the fission (splitting) of certain heavy atoms. This fission produces radiation which becomes heat which is then turned into electricity. The leftover heat and spent nuclear fuel must be dealt with. Shielding must be provided.


 I won't get too deep into this subject, but there are several types of radiation. All of these types create challenges for material designs, since most materials become brittle with exposure to radiation. (Would you like to know more?)

 - Neutrons are nuclear particles emitted during fission; a certain amount of neutron radiation is needed to start up most nuclear reactors. Neutrons can be either fast (high energy) or thermal (fast neutrons that have been slowed by smashing into a moderator). Neutrons are a form of penetrating radiation; they are a neutral particle so electrical interactions have no effect, which means they can penetrate deep into many materials. Neutrons can also 'activate' other materials; once a neutron has been slowed down by many collisions with atoms, it eventually gets slow enough to be captured. This neutron capture process can form radioactive isotopes of common materials like iron or nitrogen. The best shielding for neutrons is either a lot of hydrogen (usually as water or polyethylene) or layers of neutron reflectors (lead, bismuth, beryllium; see below). It's important to note that the neutron environment inside a reactor must be carefully controlled for efficient operation, and there is definitely a lower limit as well as an upper limit for workable designs.
 - Gamma rays are very high energy photons (electromagnetic energy) produced either directly during fission, indirectly after a positron (anti-electron) is released and then annihilated with an electron, or indirectly by a beta particle colliding and emitting bremsstrahlung. Gamma is undesirable in a reactor because it is penetrating, very harmful and can be activating. Gamma rays can trigger the fission of deuterium, for example, causing the release of a moderate-energy neutron. The best shielding for gamma is a heavy metal like tungsten, but often a conductive liner (steel) and a bulk absorber (very thick concrete) are used.
 - Other particles (protons, alpha particles and heavier fission fragments) have different typical energy levels but are largely the same as far as a reactor is concerned. They are typically charged, can be slowed or stopped efficiently with metals and eventually become troublesome atoms trapped inside the fuel or coolant. Higher-speed fragments will also emit bremsstrahlung as they slow down, so essentially all nuclear reactors produce some level of gamma radiation.


 The simplest fission fuel is an unstable isotope that spontaneously decays. Plutonium-238 is probably the most common example; this is used in RTG (radioisotope thermoelectric generator) units and radioactive heater units on deep space probes. Strontium-90 is another example, widely used in the Soviet Union in space and on Earth as a reliable power source for remote outposts like lighthouses. Some additional possibilities are Polonium-210 (powerful, dangerous, short life) and Americium-241 (long life, relatively high penetrating radiation output). These decay fuels are usually used as a simple source of heat, either maintaining operating temperature for some other device or powering a thermoelectric generator. The ideal unstable fuel would be something that decays only into alpha particles and stable products, producing no penetrating or activating radiation while having a decay rate high enough to be reasonably energy-dense yet low enough to operate for a few decades. No such material is known.

 Next is fissile material. A fissile isotope is one that can capture a low-energy neutron and then split. The four main examples are uranium-235 (naturally occurring), uranium-233 (bred from thorium-232), plutonium-239 (bred from uranium-238) and plutonium-241 (bred from plutonium-239 by way of Pu-240). Fissile material is useful for making nuclear weapons, so the production and use of these isotopes is very tightly controlled. Inefficient early reactors couldn't use natural uranium because the fissile content was too low; the U-235 had to be separated (enriched) to produce a fuel that would work properly. The same technology is used to make highly-enriched material for weapons, so again enrichment technology is tightly controlled. More modern reactor designs are more neutron-efficient, so they can use fuel that is less enriched or not enriched at all. Note that highly-enriched fissile material is very dangerous to handle or transport; too much of it in one place or accidentally exposed to neutron flux could lead to a chain reaction, a sudden spike in radioactivity and heat.

 Last is fertile material. A fertile isotope is one that can capture a neutron and convert into a fissile isotope, which can then be split with another neutron. Examples are uranium-234 (natural, makes U-235), uranium-238 (natural, makes U-239), thorium-232 (natural, makes U-233), plutonium-238 (artificial, makes Pu-239) and plutonium-240 (artificial, makes Pu-241). Fertile materials are relatively stable; they are not particularly radioactive nor will they do anything dangerous if you put a lot of it in one place. Most of them are flammable metals, but that is a chemical hazard rather than a nuclear hazard; burning U-238 is no more dangerous than burning magnesium (though the results are a bit more toxic). Fertile materials (including natural uranium) are far easier to transport safely than fissile or unstable materials.

Fuel Cycles

 The fuel by itself is only part of the story. The full fuel cycle is important to consider. Earth-based commercial power reactors can rely on an extensive infrastructure of mining, refining, enrichment, fabrication, reprocessing and disposal. Space-based reactors will have none of those advantages.

 Most commercial reactors and some military reactors are thermal, meaning their fast neutrons are moderated down to an energy level that allows for efficient capture in fissile fuel. Most such reactors require enriched fuel, which means fuel elements would be shipped from Earth until nuclear materials processing infrastructure is established in space. This is politically, economically and environmentally difficult, so Earth-style thermal reactors are not likely to be used in space for a long time if ever. One possible exception is CANDU, a heavy water moderated thermal reactor that can burn natural uranium (and a lot of other radioactives) as fuel. Interestingly, ice on Mars is significantly richer in heavy water than on Earth thanks to atmospheric losses over the eons; this might be a reasonable medium-term approach, particularly since the design does not require massive pressure vessels.

 Many research and medical reactors and some military reactors are fast, meaning their neutrons are used as they are produced. Fast reactors are often called breeder reactors, because they turn fertile material into fissile material which is then split for energy. An initial 'spark plug' of fissile material is used to generate enough neutrons to get the reactor going, then the majority of the fuel is natural uranium, natural thorium or some other fertile material. The earliest breeder reactors were used to generate fissile plutonium for the production of nuclear weapons, but current designs using thorium are specifically intended to prevent any application to weapons (proliferation-safe). Small-scale research and medical reactors are used to irradiate materials to make useful isotopes for medical imaging, cancer radiation therapy and RTG power cores. Thorium-based reactors are particularly interesting for space colonization since they could be fueled using rudimentary refining techniques and produce little waste.

Moderators, Coolants, Poisons and Reflectors

 The neutron environment inside a reactor is critically important to safe and efficient operation. Four types of materials are present in most reactors and all of them affect how neutrons behave. Many materials have more than one property from this group.

 A moderator is some material that can absorb energy from neutrons without stopping them entirely. A coolant is something that can carry heat efficiently and hopefully is not too corrosive or degraded by radiation. By far the most common material in both cases is plain water thanks to its high hydrogen content, excellent heat capacity and reasonable thermal conductivity. Commercial power reactors are almost exclusively thermal, either pressurized water or boiling water types, which use purified light water to moderate neutrons and to carry heat out of the core. Care must be taken that the design is passively safe; that is, if the coolant were to boil suddenly then the reactor should naturally reduce its power output without intervention. For an example of passive safety, check out TRIGA (training, research, isotopes, General Atomics) reactors; operating safely since 1958 these are the only reactors licensed for unattended operation.

 The two other moderators in common use are heavy water (water made of oxygen and deuterium) and graphite (pure carbon). A third used in a handful of experimental and military reactors is lithium-7 (with or without beryllium), typically as part of a molten salt.
 The main heavy water reactor design is CANDU, which uses it as both moderator and coolant. Derivative designs use separate light and heavy water systems, with the heavy water providing mostly moderation and the light water providing mostly cooling. Heavy water is used because the hydrogen already has an extra neutron and is much less likely to capture another one. It does happen, so heavy water reactors produce small amounts of tritium.
 Graphite always uses a separate coolant since it is a solid. Graphite was used in the first reactor (the Chicago pile) and in many others since then due to its stability, mechanical strength, incredible temperature tolerance and ready availability. As a solid, graphite is susceptible to lattice defects called Wigner energy; this led to the Windscale fire before it was understood, though most modern reactors operate above the annealing temperature of carbon so this is not a concern.
 Beryllium is a suitable moderator if you only look at physics. Unfortunately it's expensive and extremely toxic, so it is not normally used on its own. In a mix with lithium-7 and fluorine it forms the coolant/moderator FLiBe used in molten salt reactors.

 Fast reactors need to have as little moderation as possible (or at least a predictable and controllable amount) inside the core. That means they need to use coolants that are poor moderators or are neutron-transparent. Common materials are sodium and lead (yes, lead; it's great at absorbing gamma radiation but it tends to reflect neutrons). Some molten salt reactors are also fast reactors and may use zirconium and sodium fluorides instead of beryllium and lithium fluorides in the salt mix. It's worth noting that some graphite-moderated reactors are cooled with molten lead or sodium, since using a coolant that is a poor moderator means the reactor's behavior is more predictable during transient problems with coolant flow.
 Carbon dioxide has been used as a coolant (with moderating properties) in the past, and may be used again as a supercritical fluid. This requires fairly high pressures, but learning how to handle supercritical CO2 would have useful applications for cooling or refrigeration elsewhere in space.
 Helium has also been used as a coolant and is proposed to be used in some very high temperature reactors as both the coolant and the working fluid for the turbine. Because it resists activation, if a reactor core uses fuel elements that trap their own fission products then the helium can pass directly through the core and into the generator turbine with no intermediate heat exchangers; this requires very high temperature turbine materials but leads to superior efficiency and compact, simple design.
 Zirconium is nearly transparent to neutrons. Many fuel assemblies use Zircalloy, an alloy that is at least 95% zirconium, to allow fast neutrons to escape the fuel pins and to allow moderated neutrons back into the fuel to trigger more fission. A common fuel is uranium zirconium hydride, with zirconium alloyed for structural strength and hydrogen adsorbed for inherent moderation.

 A poison is some material that absorbs neutrons very efficiently. Examples include lithium-6, boron, hafnium, xenon-135 and gadolinium. These are used in control rods and safety systems or are produced naturally by nuclear reactions within the core. Over time, neutron poisons build up in the fuel; the dynamics of this are complex but neutron poisons are the main reason why uranium fuels only burn about 2% of their potential in one pass through a reactor. The poison byproducts have to be removed for the fuel to become usable again. Xenon is the most important of these over short timescales.
 Hafnium, boron and gadolinium are common materials for control rods. These devices allow operators to precisely control how many neutrons are flying around at a given time inside the core and can also be used as an emergency shutdown device. Control rods may be suspended above the core by electromagnets; during a loss of electrical power the rods will naturally fall into the core and stop primary activity. Soluble boron salts are used as an emergency shutdown tool in water-moderated reactors; the salt is injected into the moderator or coolant loop, causing an immediate and dramatic reduction in neutron flux and stopping the reactor's primary activity. Radioactive byproducts will still produce significant heat and radiation for hours to days, so additional safety features like auxiliary cooling are required.

 A reflector is a material that reflects (elastically scatters) neutrons. Primary examples are beryllium, graphite, steel, lead and bismuth. This is another reason why graphite was used in early reactors: a layer of solid graphite blocks around the outside of the pile reflected neutrons back into the core, reducing the required size of the core and reducing the required neutron shielding.
 Many reactor designs intended for use in space rely on controllable reflectors rather than controllable poisons; the reactor core would be safe (subcritical) by design, only able to operate when neutron reflectors were properly placed. That allows a reactor to be launched before activation, meaning the potential radioactive release during a launch accident would be minimized.
Some other designs use reflectors to boost reactivity near the end of life for a given batch of fuel, or otherwise as an alternative to poisons for control. An example is the SSTAR design, which would use a movable reflector to move the active region of the reactor through a fuel load over the course of 30 years rather than refueling every ~18 months. If the reflector were to fail then the reactor's output would taper off to nearly nothing over a few days. By relying on reflectors rather than poisons, the reactor requires a lower level of neutron flux to operate and can use less efficient (less or not enriched) fuels.

Turning heat into electricity

 Once you have a steady supply of heat, you have to put it to use somehow. The laws of physics are singularly unforgiving about energy conversion. For every useful unit of electricity produced you will have to deal with two to five units of waste heat in any practical design. Less efficient options are always available.
 In space we don't have access to free-flowing rivers or oceans of water to use as coolant; without conduction or convection we can rely only on radiation. Thermal radiators are significantly more efficient at high temperature, so the higher our core reactor temperature the better for a free-flying spacecraft. (Radiative output scales as the fourth power of temperature, so a small increase in temperature causes a very large increase in radiator output.) The temperature limit for a reactor is usually based on either the primary coolant or the fuel material, around 900-1000 °C for zircalloy cladding and possibly higher for ceramic or carbide fuel elements. Molten salt or gas-cooled reactors could go higher, while water-cooled reactors are a fair bit lower. (Water-cooled reactors use water at high pressures, so the boiling point of the coolant is typically several hundred °C.) I won't get into the physics and mechanics of radiators here other than to say they are similar to solar panels in terms of areal density, pointing and deployment. The size of a radiator system depends very strongly on the temperature of the coolant and whether there is a large hot object (like Earth) nearby.
 For a surface base with access to a large thermal mass (dirt, ice, etc.) there may be the option of process heat. Some of the waste heat from the reactor can be used to do useful work like melting ice, heating greenhouses or powering thermochemical reactions like the sulfur-iodine process for producing hydrogen. From the perspective of the electrical generation system this is still waste energy, but these uses increase the overall efficiency of the system. This kind of cogeneration greatly increases the required radiator area in free space, so although it seems counterintuitive it may not be mass-efficient to use waste heat for chemical processes on an orbital station. Rather, it may actually take less mass to produce electricity (at 20-30% efficiency, but with high-temp radiators) and use it directly in electrochemical processes vs. thermochemical processes. Each individual mission / craft / architecture is unique and may come down on either side of the line.

 So, with a source of heat (reactor coolant loop) and a sink of heat (radiator coolant loop) we can put a heat engine between the two and extract useful energy. The most basic approach is to use the thermoelectric effect (like a Peltier cooler), directly converting heat into an electric current. These devices typically have no moving parts and are highly reliable, but are poorly scalable and only modestly efficient. RTGs use these, as have some flown reactors on Soviet satellites.
 By far the most common method on Earth is to use a steam turbine in the Rankine cycle. Heat from the reactor loop boils water into steam in a steam generator, which is passed through a turbine to rotate a shaft. The depleted steam is recondensed into water, passing low temperature waste heat into the cooling loop. This would be extremely inefficient in space as the low waste temperature would require enormous radiators.
 A promising technique is to use the Brayton cycle in a reactor with a gas coolant. The most likely of these is helium, since it is very stable and nearly impervious to neutrons. A space-optimized Brayton cycle reactor (see for example project Promethius) would circulate helium through the core and pass it directly through the turbine, with no intermediate loops or heat exchangers. This is possible only because helium does not become radioactive inside the core, but it also requires that the fuel elements contain all fission products; any fuel leak would contaminate the turbine. A cycle using steam without a condenser and boiler is also possible.
 Surface bases with abundant heatsink potential could use a Combined cycle. This is a high-temperature Brayton cycle turbine whose waste heat is still high enough to run a Rankine cycle turbine of one or two stages. The Rankine cycle exhaust heat is quite low temperature and would have to be rejected into a body of water (or some other liquid) or pumped into the ground like a reverse geothermal system. The best case would be a mixed-use system that provides electricity, industrial process heat for thermochemistry and ice melting, and life support heat for maintaining livable habitat conditions. Using an array of greenhouses as your low-temperature radiator system would be ideal. The drawbacks of a system like this are complexity, need for available heat sinks and the fact that each part of the process relies on all other parts maintaining a certain pace. If you want to have electricity while your industrial processes are not running then you need an alternate heat sink to replace those processes.

Dealing with waste

 Nuclear reactions produce radiation. Some of that radiation ends up activating parts of the reactor, which means those parts become radioactive themselves. Pumps, valves, pipes, pressure vessel walls, all of the structure in the core of a reactor will become radioactive over time. This material generally can't be reprocessed into a nonradioactive form. (It's possible but would be extremely expensive.) This is usually low to medium grade nuclear waste and the usual solution is to slag it, encase it in concrete and bury it. That probably works for surface bases on bodies with no 'weather' cycle, but it would be a no-go for active worlds like Titan / Io or icy worlds like Europa. Even then, there has to be some standardized way to indicate to future generations that there is something dangerous buried there. For craft and colonies that can't bury their waste, they would have to find some place to send it safely. This remains an unsolved problem on Earth; perhaps a waste repository and reprocessing center on the moon might some day be viable, provided shipments of waste are ever allowed to be launched.
 The fuel itself produces radioactive byproducts as a result of fission. These are mostly actinides, but there are some radioactive gases like iodine as well. On Earth we generally store fuel elements indefinitely in cooling ponds or eventually in dry casks. Fuel elements can be reprocessed, meaning the component materials are separated, byproducts are filtered out and the repurified fuel is recast into new fuel elements. The actinide wastes can be burned in certain types of reactor (usually the same sort that can burn thorium, but some fast spectrum reactors are designed for waste destruction). The old liners or shells and any equipment used in fuel processing will generally be considered high-grade nuclear waste; this is treated much like other types of waste but will be radioactive for a much longer time due to contamination with radioactive isotopes. Fuel reprocessing facilities are a proliferation concern because they allow for the extraction of weapons-grade plutonium from spent uranium fuels. Thorium cycle reactors would be politically easier because it is far more difficult to get anything of military interest out of the fuel.


 Radiation from an operational reactor is damaging to people, electronics and structures. Shielding must be provided to mitigate this damage. Earth reactors solve this problem using cheap, bulky, heavy material in abundance. Usually the reactor core is placed inside a containment building; the building is a thick stainless steel liner and several meters of concrete all around. Openings usually take sharp turns so there is no line of sight from the core to the outside world; radiation doesn't turn corners. (It does scatter, so it's still not simple.)
 Free-flying reactor designs don't have to worry about contaminating a planet full of voters during a system failure. These usually have the reactor at one end of the ship on a long truss, with a small shield plug (a shadow shield) that protects the rest of the spacecraft. Ships like these are easy to see coming if you have gamma detectors. They are great for deep space exploration, but they make bad neighbors and are difficult to handle for docking maneuvers since a small misalignment could kill everyone on the other ship.

Possible scenarios - surface base

 Let's look at the simplest case first. This is a manned surface colony with basic industry already online. Base metals (iron, nickel, aluminum) and bulk material (dirt) are available. First the coolant system is built (or installed) and tested. Next a containment pit is dug, then lined in concrete/sintered or pressed regolith/etc. Nickel-iron (simply iron from here on) blocks are piled up like bricks and welded together. A self-contained core unit is assembled on Earth and shipped in one piece, placed into the pit and connected to the radiator system. The pit is covered with iron sheets or beams with a layer of concrete/sinter/etc. then buried. The core unit is not activated until it is installed, so it is not radioactive and has no unusual handling restrictions. It would be designed to run for 20-30 years unattended, with no maintenance access possible; it would probably be limited to a few tons mass at most (~6-8t; 300-500kg fuel mass) and up to a few hundred kilowatts of electricity. New core assemblies would be shipped about every decade to maintain redundancy, more often if the colony's energy needs are growing. Cores would be in the few hundred million dollar range (plus shipping); comparable cores on Earth can be built for tens of millions but they don't need to survive a reentry accident and can be repaired on-site. Lifetime power generation (20 years, 95% availability) would be about 33 GWhr of electricity.
 The whole assembly would be several meters underground, safe to stand above while operating. A coolant failure would leave the reactor hot but safe, which means the coolant system could be rebuilt or replaced without needing to do anything to the reactor core. In the event of a serious problem like a core meltdown, any released radioactive gases would escape into space or be diffused through the (already unbreathable) atmosphere. Particles could be a bigger problem; on Mars they would be swept away in the next dust storm but on the Moon they would likely stick around for a while unless they were small enough for electrostatic scattering. Still, no crops would be contaminated.

 The next step would be an accessible reactor core that can be refueled. Fuel elements could be shipped from Earth or manufactured locally. The containment structure would not be much different, but the core could be bulkier; this would allow for things to be shipped in pieces and assembled on-site. Telerobotics would be ideal for this work, but the initial construction could be safely done in person. If the local industry is capable of building small superalloy pressure vessels then something like the CANDU approach can be used, where small tubes with fuel run through a large 'tub' of moderator+coolant at manageable pressure. Regardless, a gigawatt-sized pressure vessel is a tall order for local industry (many nations on Earth couldn't build a reactor pressure vessel today) and for in-space shipping; one way or another the approach will have to be modular and scalable. Perhaps an array of many reactor cores will feed a small number of high-power turbines. Core units will likely be in the range of a few hundred kW to about one MW each (5-25t including core coolant but not turbines).
 This modular approach would allow the colony to transition into locally-manufactured fuel elements and other parts. These might initially be reprocessed fuel from earlier cores or they could start right away with locally mined material.

 Beyond that, once the colony has the capacity to make high-performance turbines, pumps, pressure vessels, fuel assemblies, etc. then they will essentially be self-reliant.

Bimodal NTR

Nuclear Thermal Rockets are basically nuclear reactors with a thrust nozzle on the bottom. A concept called Bimodal NTR allows one to tap the reactor for power. This has other advantages. Since the reactor is running warm at a low level all the time (instead of just while thrusting) it doesn't have to be pre-heated if you have a burn coming up. This reduces thermal stress, and reduces the number of thermal cyclings the reactor will have to endure over the mission. It also allows for a quick engine start in case of emergency.

In the real world, during times of disaster, US Navy submarines have plugged their nuclear reactors into the local utility grid. This supplies emergency electricity when the municipal power plant is out. In the science fiction world, a grounded spacecraft with a bimodal NTR could provide the same service.

Dusty Plasma Fission Reactors

This is from A Half-Gigawatt Space Power System using Dusty Plasma Fission Fragment Reactor (2016)

Rodney Clarke and Robert Sheldon were working on a fission-fragment rocket engine when they noticed a useful side-benefit.

There is a remarkably efficient (84%) electrical power plant called a Magnetohydrodynamic Generator (MHD generator). They also have the virtue of being able to operate at high temperatures, and have no moving parts (which reduces the maintenance required and raises reliability). A conventional electrical power generator spins a conducting copper wire coil inside a magnetic field to create electricity. An MHD generator replaces the solid copper coil with a fast moving jet of conducting plasma.

Because many designs for fusion rocket engines and fusion power plants produce fast moving jets of plasma, MHD generators were the perfect match. Ground based power plants just sprayed the jet of fusion plasma into the MHD.

Fusion spacecraft could be bimodal. An MHD generator could be installed in the exhaust nozzle to constantly bleed off some of the thrust power in order to make electricity, this was popular with inertial confinement fusion which need to recharge huge capacitors before each fusion pulse. Alternatively the MHD generator could be installed at the opposite end of the fusion reaction chamber. The fusion plasma goes down out the exhaust nozzle for thrust, but it can be diverted upwards into an MHD generator for electrical power.

Finally getting to the point, Clarke and Sheldon realized that a fission-fragment rocket engine also produces a jet of plasma. Therefore, it too can be bimodal with the addition of an MHD generator.

Cutting to the chase, they would have a jaw-dropping specific power of 11 kWe/kg! The rough design they made had a power output of 448 megawatts and a total mass of 38,430 kg (38 metric tons).

Dusty Plasma Power Reactor
Power Output448 MW
Specific Power11 kWe/kg
Mass Schedule
U235 Fuel4.27 kg
Am242m Fuel1.25 kg
Moderator9,424 kg
Moderator Heat Radiator28,000 kg
Generator Heat Radiator1,000 kg
TOTAL38,430 kg

Nuclear MHD

This design combines open-cycle gas-core nuclear thermal rockets with the sophistication of a Magnetohydrodynamic (MHD) generator. OCGC NTRs can put out much more thermal energy than a solid core reactor, since the latter has to worry about melting. And MDH generator not only have great efficiencies and no moving parts, their core element is a stream of hot gas. The hotter the better.


      I hope this discussion of MHD energy conversion will not seem out of place in a meeting on advanced reactor concepts. The· fact is that to be useful in space, the MHD generator needs high temperatures of the sort that can only be produced by advanced types of reactors. I hope that today I can make the case, or. at least establish a reasonable possibility, that to be useful in space the advanced reactor concepts in turn need the MHD generator.

     Although I think many of -you know how an MHD generator works, let me review the basic principles briefly. Figure 1 illustrates the principle and compares it to that of a turbine generator. The basic principles are the same in the two cases. These are that expansion of a gas produces motion of a conductor and the motion of a conductor through a magnetic field generates an electromotive force. In the case of the MHD generator, the gas is itself an electrical conductor and is moved through the magnetic field. Observe that the MHD generator performs the function of both a turbine and a conventional generator. The function that it performs best is that of the turbine. In fact, it is really more useful to think of it as a high temperature turbine rather than as an electrical generator. In practice, an MHD generator would resemble a rocket nozzle with a field coil wrapped around it. It would have no hot, highly stressed, moving parts; no close tolerances; and the only solid parts, namely the walls, are readily accessible for external cooling, as are the walls of a conventional rocket. As a result, it can handle temperatures and pressures like a rocket nozzle and can stand erosive and corrosive atmospheres which would completely destroy any other type of energy conversion device in a very short time. Also as we will see later, it can produce very large amounts of power per unit volume and per unit weight.

     The primary limitation on how and where one uses an MHD generator is a low temperature limit. This is because at the present time the only way that we are sure is practical for rendering a gas conducting is introducing into it a few tenths of a percent of an alkali metal seed material and then heating it. Results obtained in combustion products, but typical for all gases, are shown on Figure 2. The points are experimental; the solid line is theoretical. Observe that the conductivity is a very steep function of temperature. In practice we find that below about 20000°K, the exact value depending upon just what gas is used, the conductivity becomes too low to be useful. Observe that a few hundred degrees change in temperature can bring about an order of magnitude change in conductivity. This in turn can bring about order of magnitude improvement in the performance of an MHD converter.

     This illustrates why you should not be misled by statements in the literature (or photographs of our devices) into assuming that MHD generators are intrinsically very large and heavy. In our struggle to fit MHD generators into existing technology, we do indeed make them as large and as heavy as the traffic will bear. But the technology which you people are discussing here can move us a very long way up this exponential curve. For example at just about 40000°K in hydrogen an MHD generator containing one cubic meter of volume could generate as much electric power as the sum total of all of the power plants in this country, i.e., about 200,000 megawatts.

     Now given a turbine which has no temperature or power limit and can handle any atmosphere, the next question is, how exactly can it be usefully employed in space? The answer (as is usual for questions of this nature) is that there are a virtual infinity of possibilities. The real problem of course is trying to decide which, if any, of these possibilities arc really worth pursuing. I obviously should not take the time here to discuss them all. So I will discuss rather briefly a few schemes which, I hope, illustrate the range of possibilities.

     In devising these propulsion schemes, one of the ground rules has been that it should not be necessary to retain fuel within the reactor. Desirable perhaps, but NOT necessary. Figure 3 illustrates a system in which it is obviously not necessary. What we have here is essentially a conventional closed thermodynamic power cycle which is using the propellant as its heat sink. In effect what happens is that heat is transferred between the reactor and the propellant by means of solid surfaces up to the maximum temperature that solids can be used, and above that it is transferred by means of the MHD generator and the accelerator. That is, energy is transferred by convection and conduction up to perhaps 2000°K; and above that, energy and also momentum is transferred electrically. Compare this with concepts such as the glow plug and the coaxial jet in which radiation is used to transfer energy at temperatures above the solids limit. In this respect, I think the MHD scheme has two things to its advantage. First of all, without trying very hard one can make the energy delivered at the electrode wall of an MHO generator at a given temperature be orders of magnitude greater than the energy per unit area delivered by even blackbody radiation. The second advantage of the MHD scheme is that wall materials and structures which are transparent to DC electric power arc a great deal easier to find than materials which are transparent to optical electromagnetic frequencies.

     Figure 4 is a simplification of the scheme shown in Figure 3. Here the same gas is used as working fluid in the reactor and MHD generator and as the propellant. As a result, the compressor and the heat exchanger are eliminated. Here we depend upon the fact that at a generator exit temperature of 2000 to 2500°K practically all of the fuel will be condensed and can be recovered by a gas-liquid separation technique without cooling the gas any further.

     Figure 5 illustrates the point that use of an MHO converter can do more than simply provide a way around the fuel containment problem. Shown here is the open cycle propulsion scheme illustrated in Figure 4, except that the power output of the generator is not put back into the propellant but rather used in an external air accelerating device. Obviously this is not a propulsion system for deep space. What we have here is the nuclear MHO analog of a turbo-rocket. The propulsive efficiency of such an arrangement is much higher than that of a rocket alone, assuming you are in the appropriate range of flight velocity through the atmosphere. In the case of a nuclear MHD turbo-rocket this appropriate velocity range could be from zero right up to the satellite velocity. Moreover, an electric ram jet might turn out to be a much easier device to get good performance out of than a comparable chemically fueled ram jet when operating in the hypersonic velocity range. There are a number of reasons for this, but what they all boil down to is just that electricity is a more highly organized or available form of energy than is chemical energy.

     Figure 6 shows the kind of specific impulse one might expect to get from the type of propulsion systems shown in Figures 4 and 5. This is shown as a function of the pressure ratio across the generator and the top temperature produced by the reactor. "Unaugmented rocket" corresponds to the system shown on Figure 4. The "augmented rocket" corresponds to the system shown on Figure 5. For the latter, the specific impulse shown is a weighted average over the flight velocity from zero up to the satellite velocity, and a range of values is shown corresponding to a range of assumptions about the efficiency of the electric air accelerator, or electric ram jet. ηa / ηt = 1 corresponds to an accelerator efficiency equal to the thermal efficiency of a rocket nozzle, that is about 70%; then ηa / ηt = 0.5 corresponds to an efficiency of about 35%.

     The closed cycle shown in Figure 3 may also be operated either as a pure rocket or as an air augmented rocket, and the performance that would result is shown in Figure 7. The closed cycle would be a good deal heavier than the open one. However, I believe that in sizes corresponding to a thrust level of 100 tons and up, both systems could be made to have a thrust to weight ratio substantially in excess of one.

     In order to get a specific impulse greater than 2000 to 3000 seconds in space it is necessary to consider a system which uses a radiator. This is true whether one is considering a nuclear-MHD scheme or a nuclear reactor working alone. As is well known, a key, if not the key, to making a system of this type with a reasonable thrust to weight ratio is attaining a high heat rejection temperature. In addition, gains of up to a factor of five can be made simply by making the cycle more efficient. Presently conceived space electric power supplies have a temperature limit set by their reactor and conversion device. By using a gas core reactor and an MHD generator there would no longer be a limit on top cycle temperature. Then the compressor and the radiator temperature could rise accordingly to what is now the top cycle temperature. Eventually it should be possible to also make an MHD compressor, and then only the radiator would be a solids limited device.

     However, even with "solids limited compressors" we can do orders of magnitudes better than presently conceived electric power systems as is shown on Figure 8. Here power per unit radiator area is displayed vs top cycle temperature for a variety of radiator and generator temperatures. It shows that we ought to be able to do at least a hundred times better than SNAP 8 in terms of power per unit radiator area. Assuming that the weight of all cycle components scales by the same factor, and there is reasonable grounds for supposing that it might, the result would be a propulsion system for· interplanetary flight whlch would be very hard to heat.


     Now that Tom Brogan has shown you something of our present. work, I would like to make one or two further comments arid then summarize.

     First of all I imagine that in the figures that were. shown you observed the very massive field coils in our present devices. I would like to assure this audience again that this is not an inevitable feature. First, the devices you have seen were designed for combustion product gases which produce a rather well defined temperature and hence conductivity. Now as we saw earlier, conductivity is an exponential function of temperature, and the size of the generator is pretty much proportional to the gas conductivity. Secondly, very large reductions in the size and weight of the magnet can be made by cryogenic cooling, and most of these propulsion systems would have an abundance of hydrogen available for this purpose.

     Figure 17 illustrates these points. On it coil mass is plotted as a function of the size of the generator in terms of gross megawatts of output. The top curve is for a combustion fired generator in which the coil dissipates 30% of the gross power or 6% if the coil is liquid oxygen cooled. The Mark V generator which Tom Brogan discussed falls slightly below this curve because its dissipation is closer to being 50%. You observe that it is a break-even generator. If it had been made much smaller, all of the copper in the world would not have made it self-excite. The lower curve shows what would happen if the gas conductivity and velocity is increased as it would be in a nuclear system at 2500°C using hydrogen as the working gas. Here the dissipation is 10% at room temperature, or 1% if the coil is cooled enough to produce a factor of 10 increase in conductivity. This could easily be accomplished using liquid hydrogen. In fact, much greater gains should be possible. Observe here that as long as the power level is greater than 10 megawatts the coil will weigh on the order of 1 ton. At power levels on the order of 1000 megawatts and up, this corresponds to an extremely small weight per kilowatt of energy handled.

     In summary then, there is no reason why an MHD generator cannot be made light enough. for the kind of high thrust propulsion systems which we have been discussing here.

     Figure 18 attempts to summarize the kind of systems that we think we can build using an MHD generator and advanced reactors on a map of specific impulse vs engine thrust to weight. The curve labeled "gas core propellant cooled" corresponds to systems as illustrated in Figures 3 and 4. The curve labeled "air breathers" corresponds to a system such as that shown on Figure 5, but includes also schemes using a closed as well as an open cycle. The vertical lines labeled "radiators" correspond to systems such as were discussed in connection with Figure 8. This figure gives the impression that for boosting off the surface of the earth, or any other body, air (or "atmosphere") breathers are hard to beat, and that for interplanetary flight into space, radiating systems are hard to beat if you can get up to power to weight ratios equal to, or exceeding 1 kilowatt per kilogram. However, the main point that I want to make with this curve is just, that by combining an MHD generator with advanced high temperature reactors, we can make propulsion systems whose performance is comparable to what you can hope to get in any other way. In particular they are comparable to, or perhaps better than, what you could hope to get with a gas core reactor alone…and you do not have to solve the fuel containment problem in order to get it!


Fusion Reactors

A fusion reactor would produce energy from thermonuclear fusion instead of nuclear fission. Unfortunately scientist have yet to create a fusion reactor that can reach the "break-even" point (where is actually produces more energy than it consumes), so it is anybody's guess what the value for alpha will be.

The two main approaches are magnetic confinement and inertial confinement. The third method, gravitational confinement, is only found in the cores of stars and among civilizations that have mastered gravidic technology. The current wild card is the Polywell device which is a type of inertial electrostatic confinement fusion generator.

Fusion is even more efficient than fission. You need to burn 0.01 grams of fission fuel per second to generate 1000 megawatts. But among the most promising fusion fuels, they start at 0.01 grams per second, and can get as low as 0.001 grams per second. You can find more details here.


There are five general methods for confining plasmas long enough and hot enough for achieving a positive Q (more energy out of a reaction than you need to ignite it, "break even"):

From HIGH FRONTIER by Philip Eklund

Lattice Confinement Fusion

Lattice Confinement Fusion is a theoretical way of creating fusion inside a metal alloy doped with deuterium. No, it ain't cold fusion, not even close. And not just because the majority of scientist find the evidence for cold fusion to be about as convincing as data from the Flat Earth Society. Cold fusion features two electrodes in some heavy water, all quiet like. Lattice confinement fusion has an erbium-titanium alloy savagely bombarded with x-rays from an electron particle accelerator.

As a power source, it is probably more like a strong RTG than anything else.


NASA researchers demonstrate the ability to fuse atoms inside room-temperature metals

      Nuclear fusion is hard to do. It requires extremely high densities and pressures to force the nuclei of elements like hydrogen and helium to overcome their natural inclination to repel each other. On Earth, fusion experiments typically require large, expensive equipment to pull off.
     But researchers at NASA’s Glenn Research Center have now demonstrated a method of inducing nuclear fusion without building a massive stellarator or tokamak. In fact, all they needed was a bit of metal, some hydrogen, and an electron accelerator.
     The team believes that their method, called lattice confinement fusion, could be a potential new power source for deep space missions. They have published their results in two papers in Physical Review C.
     “Lattice confinement” refers to the lattice structure formed by the atoms making up a piece of solid metal. The NASA group used samples of erbium and titanium for their experiments. Under high pressure, a sample was “loaded” with deuterium gas (D + D fusion fuel), an isotope of hydrogen with one proton and one neutron. The metal confines the deuterium nuclei, called deuterons, until it’s time for fusion.
     “During the loading process, the metal lattice starts breaking apart in order to hold the deuterium gas,” says Theresa Benyo, an analytical physicist and nuclear diagnostics lead on the project. “The result is more like a powder.” At that point, the metal is ready for the next step: overcoming the mutual electrostatic repulsion between the positively-charged deuteron nuclei, the so-called Coulomb barrier.

     To overcome that barrier requires a sequence of particle collisions. First, an electron accelerator speeds up and slams electrons into a nearby target made of tungsten. The collision between beam and target creates high-energy photons, just like in a conventional X-ray machine. The photons are focused and directed into the deuteron-loaded erbium or titanium sample. When a photon hits a deuteron within the metal, it splits it apart into an energetic proton and neutron. Then the neutron collides with another deuteron, accelerating it.
     At the end of this process of collisions and interactions, you’re left with a deuteron that’s moving with enough energy to overcome the Coulomb barrier and fuse with another deuteron in the lattice.
     Key to this process is an effect called electron screening, or the shielding effect. Even with very energetic deuterons hurtling around, the Coulomb barrier can still be enough to prevent fusion. But the lattice helps again. “The electrons in the metal lattice form a screen around the stationary deuteron,” says Benyo. The electrons’ negative charge shields the energetic deuteron from the repulsive effects of the target deuteron’s positive charge until the nuclei are very close, maximizing the amount of energy that can be used to fuse.
     Aside from deuteron-deuteron fusion, the NASA group found evidence of what are known as Oppenheimer-Phillips stripping reactions. Sometimes, rather than fusing with another deuteron, the energetic deuteron would collide with one of lattice’s metal atoms, either creating an isotope or converting the atom to a new element. The team found that both fusion and stripping reactions produced useable energy.

     “What we did was not cold fusion,” says Lawrence Forsley, a senior lead experimental physicist for the project. Cold fusion, the idea that fusion can occur at relatively low energies in room-temperature materials, is viewed with skepticism by the vast majority of physicists. Forsley stresses this is hot fusion, but “We’ve come up with a new way of driving it.”
     “Lattice confinement fusion initially has lower temperatures and pressures” than something like a tokamak, says Benyo. But “where the actual deuteron-deuteron fusion takes place is in these very hot, energetic locations.” Benyo says that when she would handle samples after an experiment, they were very warm. That warmth is partially from the fusion, but the energetic photons initiating the process also contribute heat.
     There’s still plenty of research to be done by the NASA team. Now they’ve demonstrated nuclear fusion, the next step is to create reactions that are more efficient and more numerous. When two deuterons fuse, they create either a proton and tritium (a hydrogen atom with two neutrons), or helium-3 and a neutron. In the latter case, that extra neutron can start the process over again, allowing two more deuterons to fuse. The team plans to experiment with ways to coax more consistent and sustained reactions in the metal.
     Benyo says that the ultimate goal is still to be able to power a deep-space mission with lattice confinement fusion. Power, space, and weight are all at a premium on a spacecraft, and this method of fusion offers a potentially reliable source for craft operating in places where solar panels may not be useable, for example. And of course, what works in space could be used on Earth.


NASA Detects Lattice Confinement Fusion

A team of NASA researchers seeking a new energy source for deep-space exploration missions, recently revealed a method for triggering nuclear fusion in the space between the atoms of a metal solid.

Their research was published in two peer-reviewed papers in the top journal in the field, Physical Review C, Volume 101 (April, 2020): Nuclear fusion reactions in deuterated metals” and “Novel nuclear reactions observed in bremsstrahlung-irradiated deuterated metals.”

Nuclear fusion is a process that produces energy when two nuclei join to form a heavier nucleus. “Scientists are interested in fusion, because it could generate enormous amounts of energy without creating long-lasting radioactive byproducts,” said Theresa Benyo, Ph.D., of NASA’s Glenn Research Center. “However, conventional fusion reactions are difficult to achieve and sustain because they rely on temperatures so extreme to overcome the strong electrostatic repulsion between positively charged nuclei that the process has been impractical.”

Called Lattice Confinement Fusion, the method NASA revealed accomplishes fusion reactions with the fuel (deuterium, a widely available non-radioactive hydrogen isotope composed of a proton, neutron, and electron, and denoted “D”) confined in the space between the atoms of a metal solid. In previous fusion research such as inertial confinement fusion, fuel (such as deuterium/tritium) is compressed to extremely high levels but for only a short, nano-second period of time, when fusion can occur. In magnetic confinement fusion, the fuel is heated in a plasma to temperatures much higher than those at the center of the Sun. In the new method, conditions sufficient for fusion are created in the confines of the metal lattice that is held at ambient temperature. While the metal lattice, loaded with deuterium fuel, may initially appear to be at room temperature, the new method creates an energetic environment inside the lattice where individual atoms achieve equivalent fusion-level kinetic energies.

A metal such as erbium is “deuterated” or loaded with deuterium atoms, “deuterons,” packing the fuel a billion times denser than in magnetic confinement (tokamak) fusion reactors. In the new method, a neutron source “heats” or accelerates deuterons sufficiently such that when colliding with a neighboring deuteron it causes D-D fusion reactions. In the current experiments, the neutrons were created through photodissociation of deuterons via exposure to 2.9+MeV gamma (energetic X-ray) beam. Upon irradiation, some of the fuel deuterons dissociate resulting in both the needed energetic neutrons and protons. In addition to measuring fusion reaction neutrons, the Glenn Team also observed the production of even more energetic neutrons which is evidence of boosted fusion reactions or screened Oppenheimer-Phillips (O-P) nuclear stripping reactions with the metal lattice atoms. Either reaction opens a path to process scaling.

Illustration of the main elements of the lattice confinement fusion process observed

     In Part (A), a lattice of erbium is loaded with deuterium atoms (i.e., erbium deuteride), which exist here as deuterons. Upon irradiation with a photon beam, a deuteron dissociates, and the neutron and proton are ejected. The ejected neutron collides with another deuteron, accelerating it as an energetic “d*” as seen in (B) and (D). The “d*” induces either screened fusion (C) or screened Oppenheimer-Phillips (O-P) stripping reactions (E).

     In (C), the energetic “d*” collides with a static deuteron “d” in the lattice, and they fuse together. This fusion reaction releases either a neutron and helium-3 (shown) or a proton and tritium. These fusion products may also react in subsequent nuclear reactions, releasing more energy.

     In (E), a proton is stripped from an energetic “d*” and is captured by an erbium (Er) atom, which is then converted to a different element, thulium (Tm). If the neutron instead is captured by Er, a new isotope of Er is formed (not shown).

     More details are in this paper

A novel feature of the new process is the critical role played by metal lattice electrons whose negative charges help “screen” the positively charged deuterons. Such screening allows adjacent fuel nuclei to approach one another more closely, reducing the chance they simply scatter off one another, and increasing the likelihood that they tunnel through the electrostatic barrier promoting fusion. This is according to the theory developed by the project’s theoretical physicist, Vladimir Pines, Ph.D, of PineSci.

“The current findings open a new path for initiating fusion reactions for further study within the scientific community. However, the reaction rates need to be increased substantially to achieve appreciable power levels, which may be possible utilizing various reaction multiplication methods under consideration,” said Glenn’s Bruce Steinetz, Ph.D., the NASA project principal investigator.

“The key to this discovery has been the talented, multi-disciplinary team that NASA Glenn assembled to investigate temperature anomalies and material transmutations that had been observed with highly deuterated metals,” said Leonard Dudzinski, Chief Technologist for Planetary Science, who supported the research. “We will need that approach to solve significant engineering challenges before a practical application can be designed.”

With more study and development, future applications could include power systems for long-duration space exploration missions or in-space propulsion. It also could be used on Earth for electrical power or creating medical isotopes for nuclear medicine.


      But as my old mucker Craig Buckley (28 years experience in the Hydrogen Storage research field) pointed out to me all those years ago at Salford, even at high pressure there simply aren't that many deuterons per metal atom.

     So the deuterons, which are very small, are nowhere near one another and will reside in the interatomic voids. If you want them to fuse you have to do something to smash them together. They are not stuffed together in the alloy so can't fuse because there are 10-10 m apart

tweet by Paul M. Cray (2020)

      Any reaction that needs solid state materials is at best a fancy RTG. It’s not a Propulsion system power source for anything other than slow electric drives.

tweet by Adam Crowl (2020)

Exotic power sources

There are all sorts of exotic power sources. Some are reasonably theoretically possible, others are more fringe science. None of them currently exist, and some never will.

Beamed Power

This is where the spacecraft receives its power not from an on-board generator but instead from a laser or maser beam sent from a remote space station. This is a popular option for spacecraft using propulsion systems that require lots of electricity but have low thrusts.

For instance, an ion drive has great specific impulse and exhaust velocity, but very low thrust. If the spacecraft has to power the ion drive using a heavy nuclear reactor with lead radiation shielding, the mass of the spacecraft will increase to the point where its acceleration could be beaten by a drugged snail. But with beamed power the power generator adds zero mass to the spacecraft, since the heavy generator is on the remote station instead of onboard and laser photons weigh nothing.

The drawback includes the distance decrease in power due to diffraction, and the fact that the spacecraft is at the mercy of whoever is running the remote power station. Also maneuvers must be carefully coordinated with the remote station, or they will have difficulty keeping the beam aimed at the ship.

The other drawback is the laser beam is also a strategic weapons-grade laser. The astromilitary (if any) take a very dim view of weapons-grade laser cannon in the hands of civilians. The beamed power equipment may be under the close (armed) supervision of the Laser Guard.

Antimatter Power

Any Star Trek fan knows that the Starship Enterprise runs on antimatter. The old term is "contra-terrene", "C-T", or "Seetee". At 100% of the matter-antimatter mass converted into energy, it would seem to be the ultimate power source. The operative word in this case is "seem".

What is not as well known is that unless the situation is non-standard, antimatter is not a fuel. It is an energy transport mechanism. Unless there exist "antimatter mines", antimatter is an energy transport mechanism, not a fuel. In Star Trek, I believe they found drifts of antimatter in deep space. An antimatter source was also featured in the Sten series. In real life, astronomers haven't seen many matter-antimatter explosions. Well, they've seen a few 511 keV gamma rays (the signature of electron-positron antimatter annihilation), but they've all been from thousands of light years away and most seem to be associated with large black holes. If they are antimatter mines, they are most inconveniently located. In Jack Williamson's novels Seetee Ship and Seetee Shock there exist commercially useful chunks of antimatter in the asteroid belt. However, if this was actually true, I think astronomers would have noticed all the antimatter explosions detonating in the belt by now.

And antimatter is a very inefficient energy transport mechanism. Current particle accelerators have an abysmal 0.000002% efficiency in converting electricity into antimatter (I don't care what you saw in the movie Angels and Demons). The late Dr. Robert Forward says this is because nuclear physicist are not engineers, an engineer might manage to increase the efficiency to something approaching 0.01% (one one-hundredth of one percent). Which is still pretty lousy, it means for every megawatt of electricity you pump in to the antimatter-maker you would only obtain enough antimatter to create a mere 100 pathetic watts. The theoretical maximum is 50% due to the pesky Law of Baryon Number Conservation (which demands that when turning energy into matter, equal amounts of matter and antimatter must be created).

In Charles Pellegrino and George Zebrowski novel The Killing Star they deal with this by having the Earth government plate the entire equatorial surface of the planet Mercury with solar power arrays, generating enough energy to produce a few kilograms of antimatter a year. They do this with von Neumann machines, of course.

Of course the other major draw-back is the difficulty of carrying the blasted stuff. If it comes into contact with the matter walls of the fuel tank the resulting explosion will make a nuclear detonation seem like a wet fire-cracker. Researchers are still working on a practical method of containment. In Michael McCollum's novel Thunder Strike! antimatter is transported in torus-shaped magnetic traps, it is used to alter the orbits of asteroids ("torus" is a fancy word for "donut").

Converting the energy from antimatter annihilation into electricity is also not very easy.

The electrons and positrons mutually annihilate into gamma rays. However, since an electron has 1/1836 the mass of a proton, and since matter usually contains about 2.5 protons or other nucleons for each electron, the energy contribution from electron-positron annihilation is negligible.

For every five proton-antiproton annihilations, two neutral pions are produced and three charged pions are produced (that is, 40% neutral pions and 60% charged pions). The neutral pions almost immediately decay into gamma rays. The charged pions (with about 94% the speed of light) will travel 21 meters before decaying into muons. The muons will then travel an additional two kilometers before decaying into electrons and positrons.

This means your power converter needs a component that will transform gamma rays into electricity, and a second component that has to attempt to extract the kinetic energy out of the charged pions and convert that into electricity. The bottom line is that there is no way you are going to get 100% of the annihilation energy converted into electricity. Exactly what percentage is likely achievable is a question above my pay grade.

The main virtue of antimatter power is that it is incredibly concentrated, which drastically reduces the mass of antimatter fuel required for a given application. And mass is always a problem in spacecraft design, so any way of reducing it is welcome.

The man known as magic9mushroom drew my attention to the fact that Dr. James Bickford has identified a sort of antimatter mine where antimatter can be collected by magnetic scoops (be sure to read the comment section), but the amounts are exceedingly small. He foresees using tiny amounts of antimatter for applications such as catalyzing sub-critical nuclear reactions, instead of just using raw antimatter for fuel. His report is here.

Dr. Bickford noted that high-energy galactic cosmic rays (GCR) create antimatter via "pair production" when they impact the upper atmospheres of planets or the interstellar medium. Planets with strong magnetic fields enhance antimatter production. One would think that Jupiter would be the best at producing antimatter, but alas its field is so strong that it prevents GCR from impacting the Jovian atmosphere at all. As it turns out, the planet with the most intense antimatter belt is Earth, while the planet with the most total antimatter in their belt is Saturn (mostly due to the rings). Saturn receives almost 250 micrograms of antimatter a year from the ring system. Please note that this is a renewable resource.

Dr. Bickford calculates that the plasma magnet scoop can collect antimatter about five orders of magnitude more cost effective than generating the stuff with particle accelerators.

Keep in mind that the quantities are very small. Around Earth the described system will collect about 25 nanograms per day, and can store up to 110 nanograms. That has about the same energy content as half a fluid ounce of gasoline, which ain't much. However, such tiny amounts of antimatter can catalyze tremendous amounts of energy from sub-critical fissionable fuel, which would give you the power of nuclear fission without requiring an entire wastefully massive nuclear reactor. Alternatively, one can harness the power of nuclear fusion with Antimatter-Catalyzed Micro-Fission/Fusion or Antimatter-Initiated Microfusion. Dr. Bickford describes a mission where an unmanned probe orbits Earth long enough to gather enough antimatter to travel to Saturn. There it can gather a larger amount of antimatter, and embark on a probe mission to the outer planets.

Vacuum energy

Vacuum energy or zero-point energy is one of those pie-in-the-sky concepts that sounds too good to be true, and is based on the weirdness of quantum mechanics. The zero-point energy is the lowest energy state of any quantum mechanical system, but because quantum systems are fond of being deliberately annoying their actual energy level fluctuates above the zero-point. Vacuum energy is the zero-point energy of all the fields of space.

Naturally quite a few people wondered if there was a way to harvest all this free energy.

Currently the only suggested method was proposed by the late Dr. Robert Forward, the science fiction writer's friend (hard-SF writers would do well to pick up a copy of Forward's Indistinguishable From Magic). His paper is Extracting Electrical Energy From the Vacuum by Cohesion of Charged Foliated Conductors, and can be read here.

How much energy are we talking about? Nobody knows. Estimates based on the upper limit of the cosmological constant put it at a pathetic 10-9 joules per cubic meter (about 1/10th the energy of a single cosmic-ray photon). On the other tentacle estimates based on Lorentz covariance and with the magnitude of the Planck constant put it at a jaw-dropping 10113 joules per cubic meter (about 3 quintillion-septillion times more energy than the Big Bang). A range between 10-9 and 10113 is another way of saying "nobody knows, especially if they tell you they know".

Vacuum energy was used in All the Colors of the Vacuum by Charles Sheffield, Encounter with Tiber by Buzz Aldrin John Barnes, and The Songs of Distant Earth by Sir Arthur C. Clarke.

Arguably the Grand Unified Theory (GUT) drives and GUTships in Stephen Baxter's Xeelee novels are also a species of vacuum energy power sources.


Casimir batteries and engines

A common assumption is that the Casimir force is of little practical use; the argument is made that the only way to actually gain energy from the two plates is to allow them to come together (getting them apart again would then require more energy), and therefore it is a one-use-only tiny force in nature. In 1984 Robert Forward published work showing how a "vacuum-fluctuation battery" could be constructed. The battery can be recharged by making the electrical forces slightly stronger than the Casimir force to reexpand the plates. (so it is more of an advanced capacitor or rechargable battery than it is a power source)

In 1995 and 1998 Maclay et al. published the first models of a microelectromechanical system (MEMS) with Casimir forces. While not exploiting the Casimir force for useful work, the papers drew attention from the MEMS community due to the revelation that Casimir effect needs to be considered as a vital factor in the future design of MEMS. In particular, Casimir effect might be the critical factor in the stiction failure of MEMS.

In 1999 Pinto, a former scientist at NASA's Jet Propulsion Laboratory at Caltech in Pasadena, published in Physical Review his Gedankenexperiment for a "Casimir engine". The paper showed that continuous positive net exchange of energy from the Casimir effect was possible, even stating in the abstract "In the event of no other alternative explanations, one should conclude that major technological advances in the area of endless, by-product free-energy production could be achieved." In 2001 Capasso et al. showed how the force can be used to control the mechanical motion of a MEMS device, The researchers suspended a polysilicon plate from a torsional rod – a twisting horizontal bar just a few microns in diameter. When they brought a metallized sphere close up to the plate, the attractive Casimir force between the two objects made the plate rotate. They also studied the dynamical behaviour of the MEMS device by making the plate oscillate. The Casimir force reduced the rate of oscillation and led to nonlinear phenomena, such as hysteresis and bistability in the frequency response of the oscillator. According to the team, the system’s behaviour agreed well with theoretical calculations.

Despite this and several similar peer reviewed papers, there is not a consensus as to whether such devices can produce a continuous output of work. Garret Moddel at University of Colorado has highlighted that he believes such devices hinge on the assumption that the Casimir force is a nonconservative force, he argues that there is sufficient evidence (e.g. analysis by Scandurra (2001)) to say that the Casimir effect is a conservative force and therefore even though such an engine can exploit the Casimir force for useful work it cannot produce more output energy then has been input into the system.

In 2008 DARPA solicited research proposals in the area of Casimir Effect Enhancement (CEE). The goal of the program is to develop new methods to control and manipulate attractive and repulsive forces at surfaces based on engineering of the Casimir Force.

A 2008 patent by Haisch and Moddel details a device that is able to extract power from zero-point fluctuations using a gas that circulates through a Casimir cavity. As gas atoms circulate around the system they enter the cavity. Upon entering the electrons spin down to release energy via electromagnetic radiation. This radiation is then extracted by an absorber. On exiting the cavity the ambient vacuum fluctuations (i.e. the zero-point field) impart energy on the electrons to return the orbitals to previous energy levels, as predicted by Senitzky (1960). The gas then goes through a pump and flows through the system again. A published test of this concept by Moddel was performed in 2012 and seemed to give excess energy that could not be attributed to another source. However it has not been conclusively shown to be from zero-point energy and the theory requires further investigation.

(ed note: see original article for links to references)

From the Wikipedia entry for ZERO-POINT ENERGY

8.19 The vacuum energy drive

     The most powerful theories in physics today are quantum theory and the theories of special and general relativity. Unfortunately, those theories are not totally consistent with each other. If we calculate the energy associated with an absence of matter—the "vacuum state"—we do not, as common sense would suggest, get zero. Instead, quantum theory assigns a specific energy value to a vacuum.

     In classical thinking, one could argue that the zero point of energy is arbitrary, so we could simply start measuring energies from the vacuum energy value. However, if we accept general relativity that option is denied to us. Energy, of any form, produces spacetime curvature, and we are therefore not allowed to redefine the origin of the energy scale. Once this is accepted, the energy of the vacuum cannot be talked out of existence. It is real, and when we calculate it we get a large positive value per unit volume.

     How large?

     Richard Feynman addressed the question of the vacuum energy value and computed an estimate for the equivalent mass per unit volume. The estimate came out as two billion tons per cubic centimeter. The energy in two billion tons of matter is more than enough to boil all Earth's oceans.

     Is there any possibility that the vacuum energy could be tapped for useful purposes? Robert Forward has proposed a mechanism, based upon a real physical phenomenon known as the Casimir Effect. I think it would work, but the energy produced is small. The well-publicized mechanisms of others, such as Harold Puthoff, for extracting vacuum energy leave me totally unpersuaded.

     Science fiction that admits it is science fiction is another matter. According to Arthur Clarke, I was the first person to employ the idea of the vacuum energy drive in fictional form, in the story "All the Colors of the Vacuum" (Sheffield, 1981). Clarke employed one in The Songs of Distant Earth (Clarke, 1986). Not surprisingly, there was a certain amount of hand-waving on both Clarke's part and mine as to how the vacuum energy drive was implemented. If the ship can obtain energy from the vacuum, and mass and energy are equivalent, why can't the ship get the reaction mass, too? How does the ship avoid being slowed when it takes on energy, which has an equivalent mass that is presumably at rest? If the vacuum energy is the energy of the ground state, to what new state does the vacuum go, after energy is extracted?

     Good questions. Look on them as an opportunity. There must be good science-fictional answers to go with them.

From BORDERLANDS OF SCIENCE by Charles Sheffield (1999)

      McAndrew laughed, a humorless bark. "I'll tell you why, Jeanie. You flew the Merganser. Tell me how the drive worked."

     "Well, the mass plate at the front balanced the acceleration, so we didn't get any sensation of fifty gee." I shrugged. "I didn't work out the math for myself, but I'm sure I could have if I felt like it."

     I could have, too. I was a bit rusty, but you never lose the basics once you have them planted deep enough in your head.

     "I don't mean the balancing mechanism, that was just common sense." He shook his head. "I mean the drive. Didn't it occur to you that we were accelerating a mass of trillions of tons at fifty gee? If you work out the mass conversion rate you will need, you find that even with an ideal photon drive you'll consume the whole mass in a few days. The Merganser got its drive by accelerating charged particles up to within millimeters a second of light speed. That was the reaction mass. But how did it get the energy to do it?"

     I felt like telling him that when I had been on Merganser there had been other details—such as survival—on my mind. I thought for a few moments, then shook my head.

     "You can't get more energy out of matter than the rest mass energy, I know that. But you're telling me that the drives on Merganser and Hoatzin do it. That Einstein was wrong."

     "No!" McAndrew looked horrified at the thought that he might have been criticizing one of his senior idols. "All I've done is build on what Einstein did. Look, you've done a fair amount of quantum mechanics. You know that when you calculate the energy for the vacuum state of a system you don't get zero. You get a positive value."

     I had a hazy recollection of a formula swimming back across the years. What was it? h/4πw, said a distant voice.

     "But you can set that to zero!" I was proud at remembering so much. "The zero point of energy is arbitrary."

     "In quantum theory it is. But not in general relativity." McAndrew was beating back my mental defenses. As usual when I spoke with him on theoretical subjects, I began to feel I would know less at the end of the conversation than I did at the beginning.

     "In general relativity," he went on, "energy implies space-time curvature. If the zero-point energy is not zero, the vacuum self-energy is real. It can be tapped, if you know what you are doing. That's where Hoatzin draws its energy. The reaction mass it needs is very small. You can get that by scooping up matter as you go along, or if you prefer it you can use a fraction—a very small fraction—of the mass plate."

From ALL THE COLORS OF THE VACUUM by Charles Sheffield (1981)

The first suggestion that vacuum energies might be used for propulsion appears to have been made by Shinichi Seike in 1969. (‘Quantum electric space vehicle’; 8th Symposium on Space Technology and Science, Tokyo.)

Ten years later, H. D. Froning of McDonnell Douglas Astronautics introduced the idea at the British Interplanetary Society’s Interstellar Studies Conference, London (September 1979) and followed it up with two papers: ‘Propulsion Requirements for a Quantum Interstellar Ramjet’ (JBIS, Vol. 33,1980) and ‘Investigation of a Quantum Ramjet for Interstellar Flight’ (AIAA Preprint 81-1534, 1981).

Ignoring the countless inventors of unspecified ‘space drives,’ the first person to use the idea in fiction appears to have been Dr Charles Sheffield, Chief Scientist of Earth Satellite Corporation; he discusses the theoretical basis of the ‘quantum drive’ (or, as he has named it, ‘vacuum energy drive’) in his novel The McAndrew Chronicles (Analog magazine 1981; Tor, 1983).

An admittedly naive calculation by Richard Feynman suggests that every cubic centimetre of vacuum contains enough energy to boil all the oceans of Earth. Another estimate by John Wheeler gives a value a mere seventy-nine orders of magnitude larger. When two of the world’s greatest physicists disagree by a little matter of seventy-nine zeros, the rest of us may be excused a certain scepticism; but it’s at least an interesting thought that the vacuum inside an ordinary light bulb contains enough energy to destroy the galaxy … and perhaps, with a little extra effort, the cosmos.

In what may hopefully be an historic paper (‘Extracting electrical energy from the vacuum by cohesion of charged foliated conductors,’ Physical Review, Vol. 30B, pp. 1700-1702, 15 August 1984) Dr Robert L. Forward of the Hughes Research Labs has shown that at least a minute fraction of this energy can be tapped. If it can be harnessed for propulsion by anyone besides science-fiction writers, the purely engineering problems of interstellar — or even intergalactic — flight would be solved.

From THE SONGS OF DISTANT EARTH by Sir Arthur C. Clarke (1985)

Ladderdown Reactors

Ladderdown transmutation reactors are fringe science invented by Wil McCarthy for his science fiction novel Bloom. It is certainly nothing we will be capable of making anytime soon, but it will take somebody more knowledgeable than me to prove it impossible. Offhand I do not see anything that straight out violates the laws of physics. Ladderdown is unobtainium, not handwavium

Basically ladderdown reactors obtain their energy the same way nuclear fission does: by splitting atomic nuclei and releasing the binding energy. It is just that the ladderdown reactor can work with any element heavier than Iron-56, and the splitting does not release any neutrons or gamma radiation. Nuclear fission only works with fission fuel, and any anti-nuclear activist can tell you horror stories about the dire radiation produced.

Apparently ladderdown reactors remove protons and neutrons from the fuel material one at a time, by quantum tunneling, quietly. Unlike fission, which shoots neutrons like bullets at nuclei, shattering the nucleus into sprays of radiation and exploding fission products.

As with fission the laddered-down nuclei releases the difference in binding energy and moves down the periodic table. The process comes to a screeching halt when the fuel transmutes into Iron-56, since it is at the basin of the binding energy curve (i.e., Iron-56 has the highest binding energy per nucleon). In the novel iron is the most worthless element for this reason, and so is used for cheap building material.

Ladderdown reactors can also take fuel elements that are lighter than Iron-56, and add protons and neutrons one at a time, to make heavier elements (called "ladderup"). This is the ladderdown version of fusion, except it will work with any element lighter than Iron-56 and there is no nasty radiation produced. This is handy because laddering down heavy elements produces lots of protons as a by product, which can be laddered up into Iron-56.

Late breaking news: as it turns out, Nickel-62 has microscopically more binding energy per nucleon than Iron-56. Actually not so much "late-breaking" as "totally ignored". This has been known since the 1960s.


"Now that we are dependent on heavy metals rather than fossil organics and sunlight, economics have simply gone away. You want a lesson in economics from a biophysicist's point of view? It works like ecology—it breeds and selects. Not that we actually carry them in our pockets, but the gram of uranium has become our most basic unit of currency. Thanks to chronic short-staffing, we consider it equivalent to half an hour of human labor, though its energy potential is some twenty-six million times greater. Aside from ourselves, it is the first driver of our economy, the reasons for which are not at all arbitrary."

"For energy reasons," I said.

He winced slightly, shifted position in his chair. "Energy? Well, yes and no. Energy is less important than transmutation potential. In rough terms, a fusion reactor cascading a gram of deuterium/tritium up into a gram of iron—the basin of the binding energy curve—will liberate enough energy to boil about twenty thousand tons of water. A gram of uranium in a ladderdown reactor produces approximately the same. And yet, the uranium is worth ten thousand times more, because in laddering it down, we don't have to sink all the way to iron. We can stop anywhere along the way, and our waste products are isotopes of hydrogen which we can cascade back up, again stopping wherever we like below that magic number, iron fifty-six. A ladderdown economy sees value not only in what a substance is, but also in what it can become, and uranium, alone among the stable elements, can become anything." (all the elements with an atomic number over 82 {uranium} only have isotopes that are known to decompose through radioactive decay.)

Many people are surprised to learn that lead's energy potential is only twenty-five percent less than uranium's, but the thing to remember is that lead has ten fewer transmutation targets—eighty-one versus ninety-one—which translates into a factor of a thousand reduction in its value (Lead has 82 protons and uranium has 92 protons, so lead has 10 fewer transmutation targets). Gold, three rungs lower still, is worth about a five-thousandth as much as uranium (Gold has 79 proton, 13 fewer transmutation targets than uranium, 3 fewer than lead). It has beautiful mechanical and electrical properties, but really, the major cost of paving the streets with it is the labor.

The energy density of antihydrogen is about 250 times what we can achieve with ladderdown, and the production and storage are difficult. Wonderful fuel, the best, but the last time I checked, a gram of it cost over eighty thousand g.u.

Turns out we'll be paying for our food and clothing purchases after all, using, of all things, our shoes. No kidding! Our guide pointed out some bracelets, and though they were fashioned of plain gold he assured us they were very expensive. From the labor that went into them, I assumed, for they were handmade, but no, it turns out the fingernail-sized "dollars" that have been spent on our behalf are also made of gold, and derive their value from their own intrinsic worth as metal. As if we Munies walked around trading actual grams of uranium back and forth. This is what comes of not using ladderdown!

The Gladholders think our "duck shoes" are frightfully amusing anyway, and when they found out what the sole weights were made of, I thought they'd never stop laughing. and when they offered to replace those same blocks with equivalent masses of lead, which of course is five times more valuable back home, I thought we'd never stop laughing.

The biophysicist's voice came back careful, almost embarrassed. "It's never been a secret that nuclear energy presents … certain dangers. We think of ladderdown as a 'clean' technology, which in a radiation sense it certainly is."


"But. The quantum spatial distortion is normally induced and focused within a shielded reactor, where its effects can be controlled to within a few Planck radii. How else to tunnel out only the desired nucleons, yes? But if we invert the distortion function along the B-axis, essentially turning it inside out in three-dimensional space, the same ladderdown tunneling can be induced stochastically in a much larger spherical shell, centered about the inductor. Shielding irrelevant, because it's inside the affected region, you see? Considered too hazardous for use in bloom cauterization, the phenomenon has no industrial applications. Look it up under Things Not to Try."

Most of that went right over my head, but the gist seemed clear enough: he was talking about releasing energy, lots of it, in an uncontrolled manner. He was talking about a bomb.

From BLOOM by Wil McCarthy (1998)

Mass Converters

Mass Converters are fringe science. You see them in novels like Heinlein's Farmer in the Sky, James P. Hogan's Voyage from Yesteryear, and Vonda McIntyre's Star Trek II: The Wrath of Khan. You load the hopper with anything made of matter (rocks, raw sewage, dead bodies, toxic waste, old AOL CD-ROMS, belly-button lint, etc.) and electricity comes out the other end. In the appendix to the current edition of Farmer in the Sky Dr. Jim Woosley is of the opinion that the closest scientific theory that would allow such a thing is Preon theory.

Preon theory was all the rage back in the 1980's, but it seems to have fallen into disfavor nowadays (due to the unfortunate fact that the Standard Model gives better predictions, and absolutely no evidence of preons has ever been observed). Current nuclear physics holds that all subatomic particles are either leptons or composed of groups of quarks. The developers of Preon theory thought that two classes of elementary particles does not sound very elementary at all. So they theorized that both leptons and quarks are themselves composed of smaller particles, pre-quarks or "preons". This would have many advantages.

One of the most complete Preon theory was Dr. Haim Harari's Rishon model (1979). The point of interest for our purposes is that the sub-components of electrons, neutrons, protons, and electron anti-neutrinos contain precisely enough rishon-antirishon pairs to completely annihilate. All matter is composed of electrons, neutrons, and protons. Thus it is theoretically possible in some yet as undiscovered way to cause these rishons and antirishons to mutually annihilate and thus convert matter into energy.

Both James P. Hogan and Vonda McIntyre new a good thing when they saw it, and quickly incorporated it into their novels.

Back about the same time, when I was a young man, I thought I had come up with a theoretical way to make a mass converter. Unsurprisingly it wouldn't work. My idea was to use a portion of antimatter as a catalyst. You load in the matter, and from the antimatter reserve you inject enough antimatter to convert all the matter into energy. Then feed half (or a bit more than half depending upon efficiency) into your patented Antimatter-Makertm and replenish the antimatter reserve. The end result was you fed in matter, the energy of said matter comes out, and the antimatter enables the reaction but comes out unchanged (i.e., the definition of a "catalyst").

Problem #1 was that pesky Law of Baryon Number Conservation, which would force the Antimatter-Maker to produce equal amounts of matter and antimatter. Which would mean that either your antimatter reserve would gradually be consumed or there would be no remaining energy to be output, thus ruining the entire idea. Drat!

Problem #2 is that while electron-positron annihilation produces 100% of the energy in the form of gamma-rays, proton-antiproton annihilation produces 70% as energy and 30% as worthless muons and neutrinos.

Pity, it was such a nice idea too. If you were hard up for input matter, you could divert energy away from the Antimatter-maker and towards the output. Your antimatter reserve would diminish, but if you found more matter later you could run the mass converter and divert more energy into the Antimatter-maker. This would replenish your reserve. And if you somehow totally ran out of antimatter, if another friendly ship came by it could "jump-start" you by connecting its mass converter energy output directly to your Antimatter-maker and run it until you had a good reserve.


Basically this is when a ship lands at the spaceport, hooks up to the port's electrical umbilical cable, pays the service charge, and powers down the ship's internal nuclear reactor. This reduces the ship's consumption of reactor fuel. There might be port anti-idling laws requiring the use of shorepower if the ship's internal power source gives off air pollution, radiation, or whatever.


Shore power or shore supply is the provision of shoreside electrical power to a ship at berth while its main and auxiliary engines are shut down. While the term denotes shore as opposed to off-shore, it is sometimes applied to aircraft or land-based vehicles (such as campers, heavy trucks with sleeping compartments and tour buses), which may plug into grid power when parked for idle reduction.

The source for land-based power may be grid power from an electric utility company, but also possibly an external remote generator. These generators may be powered by diesel or renewable energy sources such as wind or solar.

Shore power saves consumption of fuel that would otherwise be used to power vessels while in port, and eliminates the air pollution associated with consumption of that fuel. A port city may have anti-idling laws that require ships to use shore power. Use of shore power may facilitate maintenance of the ship's engines and generators, and reduces noise.

Oceangoing ships

"Cold ironing" is specifically a shipping industry term that came into use when all ships had coal-fired engines. When a ship tied up at port, there was no need to continue to feed the fire and the iron engines would literally cool down, eventually going completely cold – hence the term "cold ironing". If commercial ships can use shore-supplied power for services such as cargo handling, pumping, ventilation and lighting while in port, they need not run their own diesel engines, reducing air pollution emissions.

(ed note: maybe a rocketpunk ship will do "cold uraniuming" or "cold u-ing" ?)

From the Wikipedia entry for SHOREPOWER

      Mitsuko Tamura welcomed the bulk of the machinery around her, and the illusion of privacy it afforded. Sweat beaded on her forehead beneath the heavy face shield and trickled down her temples as she slipped the tip of the soldering gun toward the broken feeder. She took comfort in the concentration such precise work demanded, directing the long tool with slender, supple fingers; it helped to push the mutterings further back in her awareness, to mute, briefly, the thousand tiny invasions of her every waking moment (Mitsuko is a telepath, and cannot turn the volume down).

     Otherwise she might have yielded to her constant urge simply to draw her legs up, slam the maintenance hatch shut behind her, and hide there until they all went away…

     She had a sudden flashing vision of the Wild Goose resting gracelessly in her concrete and girder berth, an unnatural nest for a mechanical evocation of bird-soul, coupled with a rush of affection she certainly didn’t feel for the balky, aging hardware she did battle with daily. Moses was returning to his ship.

     He was still halfway across the field. That wasn’t unusual. This was his ship, imbued with enough of his presence to sensitize her to him to begin with. Beyond that, there was nothing subtle about Moses Callahan—he thought as clearly and loudly as some people spoke, announcing his passions and preoccupations of the moment with innocent vigor, and his sleep was a bright procession of vivid dreams that seldom lingered into wakefulness.

     He was happy now, or at least happier than she'd felt him to be in the two weeks they’d spent on Hybreasil. Mitsuko wondered how long that would last. He hadn’t noticed the missing cable…

     The Wild Goose was a blunt, gunmetal-gray wedge that seemed to crouch within her berth, as though waiting for a chance to spring clear of the tracery of catwalks and loading cranes surrounding her and leap back into the skies. But the tall intake vents for her atmospheric fans were slatted shut on her topsides, while the narrow ports that looked in on her underslung flight deck were empty and dark. She needed him, Moses thought, to put the breath back in her and restore the light of life and purpose to her eyes, and the prospect of a ship alive under his feet again was a glad thing.

     The gladness broke against a quick spark of irritation when the passenger hatch ignored his key, remaining obstinately closed. Moses cursed and slid back the cover plate to the manual override.

     “Spooky!” (the Captain's nickname for Mitsuko is "Spooky", since she is always skittish. Captain doesn't know this is because Mitsuko is a telepath, since psionic people are illegal)

     The narrow passenger deck corridor was empty and dark, lit only by the sunlight admitted by the open airlock and the scattered glows of the emergency lanterns.

     “Spooky!” Moses called again. “What the hell have you got the power off for?” Cursing, he turned and levered the airlock through its cycle again, cutting off the daylight. He stood blinking in the scarlet glow for a moment, then tumed and started aft.

     Mitsuko nearly landed on top of him as she dropped down through the drive-room ladderway. She didn’t pause, but started forward toward the flight deck, with Callahan lumbering after her like a bear trying to follow a marten down its burrow.

     “Dammit, Spooky, what the hell have you got the power down for?”

     “I haven’t got the power down, the port’s got it down, Moses.” She stopped at the bulkhead before them, to throw her full forty-five kilos’ weight on the manual hatch lever. Callahan leaned past her and shoved it into place. The hatch slid open and she ducked in. “The port pulled our umbilical for nonpayment (the captain had fallen on hard times and was trying to economize). When I went to cut in the on-board power, the converter blew out. I told you that was going to happen.”

From THE SHATTERED STARS by Richard S. McEnroe (1984)

Ship to Shore

Electricity can flow both ways.

On December 17 1929, the city of Tacoma Washington was suffering from a drought. The city's hydroelectric dams did not have enough water to generate electricity. Tacoma was about to go dark. They begged President Herbert Hoover to help.

It just so happened that the aircraft carrier USS Lexington (CV 2) was being refurbished at the Puget Sound Navy Yard, right near Tacoma. The Lexington was dispatched to Tacoma’s Baker Dock, was hooked up to the city's power grid, and used steam turbines to generate power. The ship stayed at Backer Dock from December 17, 1929 to January 16, 1930; feeding the city 4 million kilowatt-hours of electricity. By mid January enough snow had melted to power the hydroelectric dams and the Lexington could disconnect. The city was saved.

This is called "Ship to Shore Power for Humanitarian Purposes".

In a RocketPunk future, any spacecraft with an onboard power source that does not depend upon the engines thrusting can do the same trick. This can come in handy if the planetary spaceport got hit by a hurricane, a Belter asteroid colony suffers a failure of their nuclear reactor, or if a settlement suffering from the Long Night does not have the spare part or the repair skills to fix their power plant. A visiting spacecraft can save the day. This would be a useful capability to build into a Long Night Insurance Ship.

A spacecraft with a Bimodal Nuclear Engine is especially suited to do Ship to Shore.


This paper was originally intended to be a follow on my experiences as an engineer aboard commercial tankers. The original intent was to provide a description of World War II-built turboelectric Destroyer Escorts and to illustrate the commonality they shared with commercial T-2 Tanker power plants. In the process of preparing this post, it became apparent that it would be desirable to expand it’s scope to include a discussion of the experiences that the U.S. Navy had in the delivery of ship to shore electrical power for humanitarian assistance.

In general, the amount of ship to shore power that can be delivered by US naval vessels is limited by a number of factors, including installed generating plant capacity and the availability of topside shore power connections. By far, the majority of current naval vessel electrical distribution systems are three phase, 450 VAC, 60 Hz. Exceptions are nuclear carriers, the newest LHA and LHD types, and the DDG 1000 Class ships which have (or will have) 4160 VAC distribution systems. The new T-AKE 1 Class ships operated by MSC have 6600 VAC integrated diesel-electric power plants. As a general rule of thumb, it becomes necessary to go to higher voltages aboard ships with generating plants with a capacity of 10,000 kW or greater because of circuit breaker interrupting capacity and cable limitations.

The DDG 51 Flight IIA Class will be used as an example to illustrate existing limitations in generating plant capacity. Each of these ships is fitted with three gas turbine driven ship service generators (SSGTG), rated at 2500 kW 450 VAC, 60 Hz (3000 kW on DDG 91 and follow). Using these ships as an example, this would appear to provide a total generating plant capacity of at least 7500 kW. However, there are several additional limitations that must be taken into account.

  1. Generators must never be intentionally loaded to more than 90% capacity.
  2. Due to circuit breaker limitations, only two sets may be operated continuously in parallel. The third set serves as a standby unit.

Given these limitations, the usable generating plant capacity aboard these ships is approximately 4500 kW (5400 kW on the later ships). In addition, ships must supply their own in-port services that may be as much as 2500 kW or more. This only leaves a margin of about 2000 to 3000 kW of available excess energy. This is a bit misleading because the ships only had two topside shore power connections, each consisting of four cables; each rated at 400 amperes giving a total of 3200 amperes through eight cables. Assuming a power factor of .8 and no more than 90% loading, this results in a total delivery capability of about 1800 kW. Coupled with the fact that many of the countries that could require humanitarian relief have 50 Hz distribution systems, these factors impose severe limitations on the ability of modern surface combatant ships to deliver shore power. Destroyer tenders (AD) and submarine tenders (AS) could deliver as much as 7000 kW at 450 VAC to ships alongside them. Only two submarine tenders remain in service as of 2014, USS Emory S. Land (AS 39), based in Diego Garcia, and USS Frank Cable (AS 40), based in Guam. Modern ships with integrated electric drive plants have high generating plant capacities. However, significant alterations would be required to make them capable of delivering large amounts of shore power.

The above limitations did not exist aboard older ships with turboelectric and diesel-electric plants because these ships had separate propulsion and ship service generating plants. It was possible to divert the bulk of the power from the main propulsion generators to shore at either 50 or 60 Hz provided that adequate cable reels were available. Some examples are discussed in the following paragraphs.

Approximately 440 Destroyer Escorts (DE) was built between 1943 and 1944. Ninety-five of them were converted to high-speed transports, and another seventy-eight was delivered to the United Kingdom under the Lend Lease agreement where they served as Captain Class frigates. The ships were divided into six classes and had four different propulsion plants including geared steam turbine, turboelectric, geared diesel, and diesel-electric systems.

One hundred and two ships of the Buckley (DE 51) and an additional twenty-two ships of the Rudderow (DE 224) had twin-screw turboelectric (TE) propulsion plants rated at 12,000 SHP. Maximum sustained speed was approximately twenty-four knots. A major reason for the use of turboelectric propulsion systems was limitations in reduction gear manufacturing capabilities during the war. Priority had to be given to manufacturing the double reduction gears required on destroyers, which had propulsion plants rated at 60,000 SHP. General Electric and Westinghouse manufactured the systems. They had many commonalities with the propulsion plants aboard the T-2 tankers described in a previous post. The machinery arrangement was similar to that aboard navy destroyers with alternating fire rooms and engine rooms. Each fire room contained a single D type boiler which produced superheated steam at a pressure of 450 PSI and a temperature of 750° F. Each engine room contained one main propulsion generator rated at 4600 kW, 2700 VAC, 93.3 Hz, 5400 RPM, one ship service turbo generator rated at 300 kW at 450 VAC/40 kW DC, and a 6000 SHP, 400 RPM main propulsion motor. The main propulsion control consoles were very similar in appearance to those on T-2 tankers. The ships had the capability of operating both main motors on a single main generator.

During World War II, a total of five ships of the Buckley Class and two British Captain Class frigates were converted into floating power stations for the purpose of supplying electrical power to shore in the event of a power outage. It is understood that a number of other ships of the class were recycled as floating power stations for coastal cities in Latin America under a program sponsored by the World Bank. However, no additional information is readily available concerning this program. A discussion of the services provided by the five Buckley Class ships is contained in the following paragraphs.

A major part of the conversion process consisted of the removal of torpedo tubes and installation of large cable reels located on the O1 Deck, as shown in the following illustrations:

The floating power plants had a total generating plant capacity of approximately 8000 kW (estimated), 2300 VAC, 60 Hz, .8 Power Factor. This equates to a usable generating plant capacity of approximately 7200 kW taking into account the 90% load factor. 50 Hz power could be easily provided to locations where necessary. The only action required was to slow the main generators down from 3600 to 3000 RPM by the use of the governor control levers. This capability does not exist in any vessels currently in service.

USS Donnell (DE 56) was converted into a power barge in England in 1944 after a torpedo struck it during convoy duty. Damage was fairly extensive and propulsion power could not be readily restored. The ship was then towed to Cherbourg, France, where it supplied power for a period of time. This experiment was considered to be very successful. It resulted in the decision to convert the other vessels on this list into floating power plants.

USS Foss (DE 59) provided power to the city of Portland, Maine, in 1947-1948 during a severe drought and a number of forest fires. At the time, it was assigned to operational development duties along with its sister ship, the USS Maloy (DE 791). There is no record of Maloy ever being converted into a floating power plant. Foss later supplied shore power to various ports in Korea in 1950-1951.

USS Whitehurst (DE 634) and USS Wiseman (DE 667) supplied power to the city of Manila for several months in 1945. During that period, Wiseman also provided drinking water to Army facilities in the harbor area. Wiseman later supplied power to the city of Masan, South Korea in 1950. USS Marsh (DE 699) supplied power to the island of Kwajelin from May until September in 1946. It later supplied power to the cities of Masan and Pusan in 1950 during the Korean War.

USS Lexington (CV-2) and USS Saratoga (CV-3) entered service in 1928. Both ships were ahead of their time. They were fitted with turbo-electric propulsion systems rated at 180,000 SHP. The ships had four steam turbine driven main propulsion generators each rated at 35,200 kW, 5000 VAC. Unlike more modern installations, the plants were not integrated and ship service power was DC supplied by 6 separate generators, each rated at 750 kW, 240 Volts DC. Up until the early 1930s, the only use the U.S. Navy had made of AC was in the propulsion systems aboard the USS Langley (CV-1) and six battleships that entered service in the 1920s.

In 1929, Washington State suffered a drought that resulted in a loss of hydroelectric power to the city of Tacoma. The U.S. Navy sent Lexington, which had been in the shipyard at Bremerton to Tacoma to provide power to the city. A considerable amount of coordination was required between the city and the ship in order to allow Lexington to provide power. The hookup consisted of twelve cables connected to circuit breakers and a bank of transformers located on the dock with a total rating of 20,000 kVA. The ship then provided a total of 4,520,960 kilowatt hours from one main propulsion generator between 17 December, 1929 until 16 January, 1930, at an average rating of 13,000 kW until melting snow and rain brought the local reservoirs up to a level where normal power could be restored.

The US Army also had a Nuclear Power Program in the 1960s. As part of this program they converted an existing Liberty ship into the Sturgis (MH-1A), a floating nuclear power station. This involved the removal of the existing propulsion plant and installation of a pressurized water reactor in a 350-ton containment vessel. After several months of testing Sturgis was towed to the Panama Canal Zone where it supplied 10,000 kW of power to operate the locks from 1968 through 1976 because of a water shortage which had an impact due to the loss of hydroelectric power. Unfortunately, the cost of operation proved to be very high and Sturgis was retired in 1976 after the Army Reactor Program was discontinued. Sturgis was then defueled and placed into the James River Reserve Fleet.


  1. NAVPERS 10864-C – Shipboard Electrical Systems, 1969
  2. Paper – Ship to Shore Power, US Navy Humanitarian Relief, Scott, 2006
  3. Transactions, SNAME, 1929
  4. NAVSEA Ship Information Book, AS39/40
  5. DDG 51 Flight IIA Electrical Plant Load Analysis
  7. USS Lexington (CV-2) report following supplying power to the City of Tacoma for a month, 1930.


While the author was training to become a US Navy Enlisted Reactor Operator, qualified operators repeatedly stated, “This sub could power a small city.” In a similar vein, it was proposed that US Navy ships should provide electrical power during the response to Hurricane Katrina in New Orleans. These off the cuff assessments prompted a more realistic assessment: is it feasible to power facilities ashore from a ship?


During World War II, there were seven destroyer escorts converted into Turbo-Electric Generators (TEG) specifically for the purpose of providing electrical power to shore facilities. They were the Donnell (DE-56), Foss (DE-59), Whitehurst (DE-634), Wiseman (DE-667), Marsh (DE-699), and two British lend-lease ships; Spragge (K-572, ex-DE-563) and Hotham (K-583 ex-DE-574). Data for these ships are sparse in general.

Consider the Wiseman, for which more data is available. This ship had oil fired boilers producing steam to turn turbine generators which in turn powered electric propulsion motors. This electric ship configuration is optimal for providing electric power ashore since all the power in the ship is already being converted to electric. The Wiseman had transformers and cable reels topside to deliver power at high voltages over relatively long distances. Wiseman powered the city of Manila during WWII and the port of Mason during the Korean War. Wiseman delivered 5,806,000 kWh to Manila over five and a half months, giving an average generation capability greater than 1.4 MW.

The US Army also used ship to shore power to power remote stations. One notable case is that of the Sturgis/MH-1A, A WWII era Liberty ship equipped with a nuclear power plant used to provide power to the Panama Canal Zone from 1968 to 19753. The MH-1A power plant on the Sturgis generated 10MW electrical power which allowed the canal locks to be operated more frequently.

Thus history shows that ships can provide power to the shore, if only in limited amounts, and using specialized ships.


There are currently no US Navy ships designed specifically to provide power to the shore. They are however designed to be powered from the shore and this capability could be used to act as a power source. For example, the author’s ship, USS Key West (SSN-722), a Los Angeles class nuclear powered fast attack submarine, once received ‘shore power’ from a destroyer while moored alongside the destroyer anchored off Monaco. This allowed the labor intensive nuclear reactor plant on the submarine to be shutdown. The gas turbine generators on the destroyer require fewer watchstanders and had to run to power the destroyer’s own loads. This anecdotal evidence shows that power can be made to flow from at least one US Navy ship and conceivably could flow from most.

The capability to provide power can be evaluated by considering the ship as a load and assuming that whatever power it can draw, it can deliver. For USN ships smaller than carriers and amphibious ships, the unit of measure is the single shore power cable. These cables are rated to 400A at 450V 3 phase or 0.312MW assuming a unity power factor4. Submarines and surface combatant ships typically can connect up to eight cables, yielding a total of 2.5MW. For a carrier, the shore power supply must deliver 21MVA at 4160V5. Amphibious ships are presumably between these values. Without significant changes, current Navy ships could theoretically supply 2.5 to 21MW of electrical power to the shore. This again assumes generation capacity to match the ship as a load and also assumes this capacity is above that required to power the ship and its power plant.

What if more power is needed? More ships could be used, but there is also more power onboard each ship. This other power is the power for propulsion. Remembering the Wiseman, it was an ideal ship for supplying power because all the power of the boilerswas first converted to electricity by turbine generators. Today’s Navy ships are not ‘all electric’ and so a significant portion of the power onboard is dedicated to propulsion and is often coupled directly to the propeller shafts. Steam plants fired by oil or nuclear reactors offer a sort of middle ground. While the propulsion turbines are coupled to the shafts, the steam can be diverted upstream. In this scheme, high pressure steam would be piped out of the ship and used to drive a larger turbine generator. The spent low energy steam and condensate would then be piped back into the ship and into the condensate system, closing the loop. Piping is not as forgiving or flexible as cabling, this would not be a trivial set up and is probably impossible for a submarine.

Considering the publicly available shaft horsepower ratings for the ship as the electrical power available, it is clear that much more power is in the hulls than is available through the shore power connections.

Table 1
Power Available From Steam Plant Ships
Ship typeTotal
Fast Attack Submarine35,00026
Large Deck Amphib70,00052

Note that most surface combatants are driven by gas turbine or diesel engines and their propulsion power cannot feasibly be extracted from the ship.


The Navy is driving toward all electric ships in a case of history repeating. This is driven by the desire to access propulsion power to supply combat systems. As stated previously, all electric ships are ideal for providing power to shore since all their power is first converted to electricity. The future destroyer DD(X) is being designed as an all electric ship with two Rolls-Royce MT-30 gas turbine generators producing a total of 78MW of electrical power.

The future carrier CVN(X) will be nuclear powered and have a steam plant but will also have increased electrical generation (104MVA) to support launching planes using electrical power.

Neither DD(X) nor CVN(X) is designed to deliver power outside the hull, but it would be easier to export it as electrical current than as steam. Loads

To investigate the claim of powering a small city, a ‘rough order of magnitude’ (ROM) calculation was performed. The author’s most recent electrical utility bill was used to determine the average power of a house, and then this number was used to determine how many houses could be powered. The bill was for 1203kWh over a 29 day period giving an average load of 1.7kW. Again, this is a ROM calculation and does not incorporate seasonal variations in power use nor the likelihood of reduced use in an emergency situation.

Using the existing shore to ship power capability, the submarine can power 1,500 nominal homes: more of a town than a small city. The carrier can power 12,000 houses and that is a small city.

Table 2
Powering Houses with
Existing Ships Equipment
Ship typeShore

If the steam plant of an amphib or a carrier were modified to increase electrical generation to match propulsion power, many more houses could be powered, equivalent to a medium city based on population only.

Table 3
Powering Houses with Steam Plants
Ship typeSteam

Lastly, if all the generation capability of future ship classes could be made available external to the hull, a large population could be supplied.

Table 4
Powering Houses with Future Ships
Ship typeGeneration

The US Navy is not chartered to act as a power utility, they are not likely to power the shore except at forward military or disaster locations. In these cases, residential housing is not likely to be the first load supplied. Instead, hospitals and other vital infrastructure are likely to receive priority. This prioritization is important since a single hospital can be a significant load. Based on one report discussing emergency generation installation, a value of 2MW per hospital was determined.

Using shore power, a submarine or surface combatant can power one hospital with a small surplus. This undermines the claim for a small city since few loads will be powered after the hospital. A carrier can power ten and a half hospitals, likely allowing some residential power after the vital infrastructure is supplied.


MH-1A was the first floating nuclear power station. Named Sturgis after General Samuel D. Sturgis, Jr., this pressurized water reactor built in a converted Liberty ship was part of a series of reactors in the US Army Nuclear Power Program, which aimed to develop small nuclear reactors to generate electrical and space-heating energy primarily at remote, relatively inaccessible sites. Its designation stood for mobile, high power. After its first criticality in 1967, MH-1A was towed to the Panama Canal Zone that it supplied with 10 MW of electricity from October 1968 to 1975. Its dismantling began in 2014 and was completed in March 2019.


The MH-1A was designed as a towed craft because it was expected to stay anchored for most of its life, making it uneconomical to keep the ship's own propulsion system.

It contained a single-loop pressurized water reactor, in a 350-ton containment vessel, using low enriched uranium (4% to 7% 235U) as fuel.

The MH-1A had an elaborate analog-computer-powered simulator installed at Fort Belvoir. The MH-1A simulator was obtained by Memphis State University Center for Nuclear Studies in the early 1980s, but was never restored or returned to operational service. Its fate is unknown after the Center for Nuclear Studies closed.

Panama Canal Zone, 1968–1976

After testing at Fort Belvoir for five months starting in January 1967, Sturgis was towed to the Panama Canal Zone. The reactor supplied 10 MW (13,000 hp) electricity to the Panama Canal Zone from October 1968 to 1975.

A water shortage in early 1968 jeopardized both the efficient operation of the Panama Canal locks and the production of hydroelectric power for the Canal Zone. Vast amounts of water were required to operate the locks and the water level on Gatun Lake fell drastically during the December-to-May dry season, which necessitated curtailment of operations at Gatun Hydroelectric Station

The ship was moored in Gatun Lake, between the Gatun Locks and the Chagres dam spillway. Beginning in October 1968 the 10 MW electrical power produced by the MH-1A plant aboard the Sturgis allowed it to replace the power from the Gatun Hydroelectric Station, which freed the lake water for navigation use. To help out further, the Andrew J. Weber, a diesel-fueled power barge of 20 MW capacity, was deployed to the Canal Zone in November 1968. These two barges not only contributed to meeting the Canal Zone’s power requirements, but also made possible the saving of vast quantities of water that otherwise would have been needed to operate the hydroelectric power station. The Corps of Engineers estimates that over one trillion gallons were saved (or, rather, freed up) between October 1968 and October 1972 – enough to permit fifteen additional ships to pass through the locks of the canal each day.

After one year of operations in the Canal Zone, the MH-1A reactor had to be refueled, a process which took one week (17–25 October 1969), according to a 1969 Corps of Engineers report. According to a 2001 report by the Federation of American Scientists, the MH-1A reactor had a total of five cores during its operational life. It used low-enriched uranium (LEU) in the range of 4 to 7 %, with a total amount of uranium-235 supplied being 541.4 kilograms (for the five cores).

The Sturgis was eventually replaced by two 21 MW Hitachi turbines, one on the Pacific side of the isthmus and one on the Atlantic side.

From the Wikipedia entry for MH-1A

Power Storage

Often the power plant generates more power than is currently needed. Spacecraft cannot afford to throw the excess power away, it has to be stored for later use. This is analogous to Terran solar power plants, they don't work at night so you have to store some power by day.

Energy Transport Mechanism

There are a couple of instances where people make the mistake of labeling something a "power source" when actually it is an "energy transport mechanism." The most common example is hydrogen. Let me explain.

In the so-called "hydrogen economy", proponents point out how hydrogen is a "green" fuel, unlike nasty petroleum or gasoline. Burn gasoline and in addition to energy you also produce toxic air pollution. Burn hydrogen and the only additional product is pure water.

The problem is they are calling the hydrogen a fuel, which it isn't.

While there do exist petroleum wells, there ain't no such thing as a hydrogen well. You can't find hydrogen just lying around somewhere, the stuff is far too reactive. Hydrogen has to be generated by some other process, which consumes energy (such as electrolysing water using electricity generated by a coal-fired power plant). Not to mention the energy cost of compressing the hydrogen into liquid, transporting the liquid hydrogen in a power-hungry cryogenically cooled tank, and the power required to burn it and harvest electricity.

This is why hydrogen is not a fuel, it is an energy transport mechanism. It is basically being used to transport the energy from the coal-fired power plant into the hydrogen burning automobile. Or part of the energy, since these things are never 100% efficient.

In essence, the hydrogen is fulling much the same role as the copper power lines leading from a power plant to a residential home. It is transporting the energy from the plant to the home. Or you can look at the hydrogen as sort of a rechargable battery, for example as used in a regenerative fuel cell. But one with rather poor efficiency.

The main example from science fiction is antimatter "fuel." Unless the science fiction universe contains antimatter mines, it is an energy transport mechanism with a truly ugly efficency.


Buck Kendall has invented a sort of super-battery that will store huge amounts of electricity with incredible efficiency. It stores the power in pools of mercury.

"That's it, Tom. I wanted to show you first what we have, and why I wanted all that mercury. Within three weeks, every man, woman and child in the system will be clamoring for mercury metal. That's the perfect accumulator." Quickly he demonstrated the machine, charging it, and then discharging it. It was better than 99.95% efficient on the charge, and was 100% efficient on the discharge.

"Physically, any metal will do. Technically, mercury is best for a number of reasons. It's a liquid. I can, and do it in this, charge a certain quantity, and then move it up to the storage tank. Charge another pool, and move it up. In discharge, I can let a stream flow in continuously if I required a steady, terrific drain of power without interruption. If I wanted it for more normal service, I'd discharge a pool, drain it, refill the receiver, and discharge a second pool. Thus, mercury is the metal to use.

"Do you see why I wanted all that metal?"

"I do, Buck — Lord, I do," gasped Faragaut. "That is the perfect power supply."

"No, confound it, it isn't. It's a secondary source. It isn't primary. We're just as limited in the supply of power as ever — only we have increased our distribution of power."

From THE ULTIMATE WEAPON by John W. Campbell, jr. (1966)


What is needed are so-called "secondary" batteries, commonly known as "rechargable" batteries. If the batteries are not rechargable then they are worthless for power storage. As you probably already figured out, "primary" batteries are the non-rechargable kind; like the ones you use in your flashlight until they go dead, then throw into the garbage.

Current rechargable batteries are heavy, bulky, vulnerable to the space environment, and have a risk of bursting into flame. Just ask anybody who had their laptop computer unexpectedly do an impression of an incindiary grenade.

Nickle-Cadmium and Nickle-Hydrogen rechargables have a specific energy of 24 to 35 Wh/kg (0.086 to 0.13 MJ/kg), an energy density of 0.01 to 0.08 Wh/m3, and an operating temperature range of -5 to 30°C. They have a service life of more than 50,000 recharge cylces, and a mission life of more than 10 years. Their drawbacks are being heavy, bulky, and a limited operationg temperature range.

Lithium-Ion rechargables have a specfic energy of 100 Wh/kg (0.36 MJ/kg), an energy density of 0.25 Wh/m3, and an operating temperature range of -20 to 30°C. They have a service life of about 400 recharge cylces, and a mission life of about 2 years. Their drawbacks are the pathetic service and mission life.


A flywheel is a rotating mechanical device that is used to store rotational energy. In a clever "two-functions for the mass-price of one" bargain a flyweel can also be used a a momentum wheel for attitude control. NASA adores these bargains because every gram counts.

Flywheels have a theoretical maximum specific energy of 2,700 Wh/kg (9.7 MJ/kg). They can quickly deliver their energy, can be fully discharged repetedly without harm, and have the lowest self-discharge rate of any known electrical storage system. NASA is not currently using flywheels, though they did have a prototype for the ISS that had a specific energy of 30 Wh/kg (0.11 MJ/kg).


Previously I have tweeted on super-strong carbon nanotube fibres for use in energy storing flywheels, based on this News story.

First, a review of the underlying physics.

From basic rotational physics, we can describe the flywheel rotor as a solid cylinder of even composition and constant density ρ. For any rotating object the important figures of merit are the Moment of Inertia I and the angular velocity ω. For a cylinder of mass m, length l and circular radius r the Moment of Inertia is:

Rotation Kinetic Energy is:

Material strength limits the flywheel rotor’s performance. Stress in the flywheel’s material is from the centrifugal reaction force that is acting to explode the rotor. The rotor material’s molecular structure must counter that with its tensile strength, the force that the material exerts on itself to keep it together. In the case of the purported nanotube fibre it’s internal strength is measured as upwards of 80 billion pascals or 80 gigapascals (GPa). Steels typically have 0.25 GPa tensile strength, so the nanotube material is 320 times stronger.

In a spinning rotor the stress to be countered by material strength at its maximum radius is:

The maximum the rotor can safely spin is when that stress equals its tensile strength. Past that point and the material will eventually ‘fail’, pulling itself apart violently due to all its kinetic energy, likely vapourising it in the process. To operate safely the rotor should be run at a maximum of some fraction of that limit. A factor of 50% is considered reasonable, allowing wiggle room for fluctuations. Thus the maximum operating stress should be about 2/3 of its maximum – in this example 2/3 x 80 GPa = 54 GPa.

Notice that the stress and the rotational kinetic energy look very similar. In fact their relationship is simply:

This allows the energy Figure of Merit, the Specific Energy Density or stored energy per unit mass – to be derived as:

For the carbon nanotube material, with a density of about 1,300 kg/m3, and an operating maximum stress of 54 GPa, that means a specific energy density of 10 MJ/kg.

Consider the power storage needs of the Starshot interstellar sail, which masses 2 grams and cruises to Alpha Centauri at 0.25 c. About 60 terajoules per Starshot is needed, expended over about 20 minutes. Assuming near perfect conversion from rotational energy to laser power, the mass of spinning rotors needed per shot is about 6,000 tonnes. This can be an arrangement of multiple flywheels, hooked up to a massive solar array farm or a high efficiency nuclear reactor, that can be powered up over several hours.


Regenerative Fuel Cells

A "regenerative" or "reverse" fuel cell is one that saves the water output, and uses a secondary power source (such as a solar power array) to run an electrolysers to split the water back into oxygen and hydrogen. This is only worth while if the mass of the secondary power source is low compared to the mass of the water. But it is attractive since most life support systems are already going to include electrolysers anyway.

In essence the secondary power source is creating fuel-cell fuel as a kind of battery to store power. It is just that a fuel cell is required to extract the power from the "battery."

Currently there exist no regenerative fuel cells that are space-rated. The current goal is for such a cell with a specific energy of up to 1,500 Wh/kg (5.4 MJ/kg), a charge/discharge efficiency up to 70%, and a service life of up to 10,000 hours.

Superconducting magnetic energy storage

Superconducting Magnetic
Energy Storage
Specific energy4–40 kJ/kg
(0.004–0.04 MJ/kg)
(1–11 Wh/kg)
Energy densityless than 40 kJ / L
Specific power10–100,000 kW/kg
Self-discharge rate>0% at 4 K
100% at 140 K
Cycle durabilityUnlimited cycles

Superconducting Magnetic Energy Storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil which has been cryogenically cooled to a temperature below its superconducting critical temperature.

A typical SMES system includes three parts: superconducting coil, power conditioning system and cryogenically cooled refrigerator. Once the superconducting coil is charged, the current will not decay and the magnetic energy can be stored indefinitely.

The stored energy can be released back to the network by discharging the coil. The power conditioning system uses an inverter/rectifier to transform alternating current (AC) power to direct current or convert DC back to AC power. The inverter/rectifier accounts for about 2–3% energy loss in each direction. SMES loses the least amount of electricity in the energy storage process compared to other methods of storing energy. SMES systems are highly efficient; the round-trip efficiency is greater than 95%.

Due to the energy requirements of refrigeration and the high cost of superconducting wire, SMES is currently used for short duration energy storage. Therefore, SMES is most commonly devoted to improving power quality.

Low-temperature versus high-temperature superconductors

Under steady state conditions and in the superconducting state, the coil resistance is negligible. However, the refrigerator necessary to keep the superconductor cool requires electric power and this refrigeration energy must be considered when evaluating the efficiency of SMES as an energy storage device.

Although the high-temperature superconductor (HTSC) has higher critical temperature, flux lattice melting takes place in moderate magnetic fields around a temperature lower than this critical temperature. The heat loads that must be removed by the cooling system include conduction through the support system, radiation from warmer to colder surfaces, AC losses in the conductor (during charge and discharge), and losses from the cold–to-warm power leads that connect the cold coil to the power conditioning system. Conduction and radiation losses are minimized by proper design of thermal surfaces. Lead losses can be minimized by good design of the leads. AC losses depend on the design of the conductor, the duty cycle of the device and the power rating.

The refrigeration requirements for HTSC and low-temperature superconductor (LTSC) toroidal coils for the baseline temperatures of 77 K, 20 K, and 4.2 K, increases in that order. The refrigeration requirements here is defined as electrical power to operate the refrigeration system. As the stored energy increases by a factor of 100, refrigeration cost only goes up by a factor of 20. Also, the savings in refrigeration for an HTSC system is larger (by 60% to 70%) than for an LTSC systems.

From the Wikipedia entry for

There are two significant limits.

First is the force trying to make the superconductor explode.

You can consider that the energy of a persistent supercurrent circulating through a superconductor is stored in the magnetic field it produces. The best design is thus to wrap your superconducting wire into a solenoid (or inductor, or electromagnet). To avoid annoying effects from the extremely strong field leaking out of the end, wrap the ends of the solenoid around so they join, giving a torroidal (or doughnut shaped) configuration. Now the field will act to maintain the current that produces it, and can induce strong "voltages" (technically an electromotive force, or EMF, but for practical purposes you can treat it as a voltage from a battery) to drive the current through any load you apply to it.

But now you have a problem. The current produces the field, and you need the field to maintain the current. But the field also exerts a force on the current, pushing the current-carrying loops apart and trying to expand them. For high currents and strong field (what you get when you are storing lots of energy), these forces can be high enough to rip matter apart and make your superconductive "battery" explode.

The way to avoid this is to support the superconductive wire with a very strong backing material that holds it in place. The upper limit on the energy storage per unit weight comes down, ultimately, to the strength of the chemical bonds that hold your backing material together. The best you can do here is use some strongly-bound light element. The carbon-carbon chemical bond is going to be ideal. So a carbon super-material like carbon nanotubes or graphene will give you the best energy per weight. The theoretical upper limit is around 40 to 50 MJ/kg (11,000 to 14,000 Wh/kg). Of course, a power storage unit energized up to this limit will be on the verge of failure, and failure means exploding with ten times its weight of TNT (4.2 MJ/kg). So throw in some engineering safety factors of 2 or 3 (25 to 17 MJ/kg or 7,000 to 5,000 Wh/kg).

The other limit is that high enough magnetic fields will shut down a superconductor. This is called the critical field, here the substance goes back to being a normal conductor instead of a superconductor (with the subsequent loss of all of your energy to resistive heating and probably exploding, again). This puts an upper limit on your energy per volume rather than energy per mass. I'm not aware of any theoretical upper limit on what the critical field could be, so you can probably adjust this to whatever you need. Note that you need a safety factor here, too, since the critical field decreases as temperature increases. You don't want your power supply to turn into a bomb just because the air conditioning starts acting up.

From a Google Plus thread entry Luke Campbell (2017)

Kerr-Newman black hole

The popular conception of a black hole is that it sucks everything in, and nothing gets out. However, it is theoretically possible to extract energy from a black hole, for certain values of "from."

And by the way, there appears to be no truth to the rumor that Russian astrophysicists use a different term, since "black hole" in the Russian language has a scatological meaning. It's an urban legend, I don't care what you read in Dragon's Egg.

For an incredibly dense object with an escape velocity higher than the speed of light which warps the very fabric of space around them, black holes are simple objects. Due to their very nature they only have three characteristics: mass, spin (angular momentum), and electric charge. All the other characteristics got crushed away (well, technically they also have magnetic moment, but that is uniquely determined by the other three). All black holes have mass, but some have zero spin and others have zero charge.

There are four types of black holes. If it only has mass, it is a Schwarzschild black hole. If it has mass and charge but no spin, it is a Reissner-Nordström black hole. If it has mass and spin but no charge it is a Kerr black hole. And if it has mass, charge and spin it is a Kerr-Newman black hole. Since practically all natural astronomical objects have spin but no charge, all naturally occurring black holes are Kerr black holes, the others do not exist naturally. In theory one can turn a Kerr black hole into a Kerr-Newman black hole by shooting charged particles into it for a few months, say from an ion drive or a particle accelerator.

From the standpoint of extracting energy, the Kerr-Newman black hole is the best kind, since it has both spin and charge. In his The MacAndrews Chronicles, Charles Sheffield calls them "Kernels" actually "Ker-N-el", which is shorthand for Kerr-Newman black hole.

The spin acts as a super-duper flywheel. You can add or subtract spin energy to the Kerr-Newman black hole by using the Penrose process. Just don't extract all the spin, or the blasted thing turns into Reissner-Nordström black hole and becomes worthless. The attractive feature is that this process is far more efficient than nuclear fission or thermonuclear fusion. And the stored energy doesn't leak away either.

The electric charge is so you can hold the thing in place using electromagnetic fields. Otherwise there is no way to prevent it from wandering thorough your ship and gobbling it up.

The assumption is that Kerr-Newman black holes of manageable size can be found naturally in space, already spun up and full of energy. If not, they can serve as a fantastically efficient energy transport mechanism.

Primordial black holes

Alert readers will have noticed the term "manageable size" above. It is impractical to use a black hole with a mass comparable to the Sun. Your ship would need an engine capable of moving something as massive as the Sun, and the gravitational attraction of the black hole would wreck the solar system. So you just use a smaller mass black hole, right? Naturally occurring small black holes are called "Primordial black holes."

Well, there is a problem with that. In 1975 legendary physicist Stephen Hawking discovered the shocking truth that black holes are not black (well, actually the initial suggestion was from Dr. Jacob Bekenstein). They emit Hawking radiation, for complicated reasons that are so complicated I'm not going to even try and explain them to you (go ask Google). The bottom line is that the smaller the mass of the black hole, the more deadly radiation it emits. The radiation will be the same as a "black body" with a temperature of:

6 × 10-8 / M kelvins

where "M" is the mass of the black hole where the mass of the Sun equals one. The Sun has a mass of about 1.9891 × 1030 kilograms.

Jim Wisniewski created an online Hawking Radiation Calculator to do the math for you.

In The McAndrew Chronicles Charles Sheffield hand-waved an imaginary force field that somehow contained all the deadly radiation. One also wonders if there is some way to utilze the radiation to generate power.

In the table:

  • R is the black hole's radius in attometers (units of one-quintillionth or 10-18 of a meter). A proton has a diameter of 1000 attometers.
  • M is the mass in millions of metric tons. One million metric tons is about the mass of three Empire State buildings.
  • kT is the Hawking temperature in GeV (units of one-billion Electron Volts).
  • P is the estimated total radiation output power in petawatts (units of one-quadrillion watts). 1—100 petawatts is the estimated total power output of a Kardashev type 1 civilization.
  • P/c2 is the estimated mass-leakage rate in grams per second.
  • L is the estimated life expectancy of the black hole in years. 0.04 years is about 15 days. 0.12 years is about 44 days.

Table is from Are Black Hole Starships Possible?, thanks to magic9mushroom for this link.

"I think Earth's worst problems are caused by the power shortage," he said. "That affects everything else. Why doesn't Earth use the kernels for power, the way that the USF does?"

"Too afraid of an accident," replied McAndrew. His irritation evaporated immediately at the mention of his specialty. "If the shields ever failed, you would have a Kerr-Newman black hole sitting there, pumping out a thousand megawatts—mostly as high-energy radiation and fast particles. Worse than that, it would pull in free charge and become electrically neutral. As soon as that happened, there'd be no way to hold it electromagnetically. It would sink down and orbit inside the Earth. We couldn't afford to have that happen."

"But couldn't we use smaller kernels on Earth?" asked Yifter. "They would be less dangerous."

McAndrew shook his head. "It doesn't work that way. The smaller the black hole, the higher the effective temperature and the faster it radiates. You'd be better off with a much more massive black hole. But then you've got the problem of supporting it against Earth's gravity. Even with the best electromagnetic control, anything that massive would sink down into the Earth."

"I suppose it wouldn't help to use a nonrotating, uncharged hole, either," said Yifter. "That might be easier to work with."

"A Schwarzschild hole?" McAndrew looked at him in disgust. "Now, Mr. Yifter, you know better than that." He grew eloquent. "A Schwarzschild hole gives you no control at all. You can't get a hold of it electromagnetically. It just sits there, spewing out energy all over the spectrum, and there's nothing you can do to change it—unless you want to charge it and spin it up, and make it into a kernel. With the kernels, now, you have control."

I tried to interrupt, but McAndrew was just getting warmed up. "A Schwarzschild hole is like a naked flame," he went on. "A caveman's device. A kernel is refined, it's controllable. You can spin it up and store energy, or you can use the ergosphere to pull energy out and spin it down. You can use the charge on it to move it about as you want. It's a real working instrument—not a bit of crudity from the Dark Ages."

from THE McANDREW CHRONICLES by Charles Sheffield (1983)

In this model of the interaction of a miniature black hole with the vacuum, the black hole emits radiation and particles, as though it had a temperature. The temperature would be inversely proportional to the mass of the black hole. A Sun-sized black hole is very cold, with a temperature of about a millionth of a degree above absolute zero. When the mass of the black hole is about a hundred billion tons (the mass of a large asteroid), the temperature is about a billion degrees.

(ed note: one hundred billion tons is 100,000 million tons or 5 × 10-17 solar masses. 6 × 10-8 / 5 × 10-17 = 1,200,000,000 Kelvin)

According to Donald Page, who carried out lengthy calculations on the subject, such a hole should emit radiation that consists of approximately 81% neutrinos, 17% photons, and 2% gravitons. When the mass becomes significantly less than a hundred billion tons, the temperature increases until the black hole is hot enough to emit electrons and positrons as well as radiation. When the mass becomes less than a billion tons (a one kilometer diameter asteroid), the temperature now approaches a trillion degrees and heavier particle pairs, like protons and neutrons are emitted. The size of a black hole with a mass of a billion tons is a little smaller than the nucleus of an atom. The black hole is now emitting 6000 megawatts of energy, the output of a large power plant. It is losing mass at such a prodigious rate that its lifetime is very short and it essentially "explodes" in a final burst of radiation and particles.

(ed note: one billon tons is 1000 million tons. An atomic nucleus is about 1750 to 15,000 attometers in diameter.)

If it turns out that small black holes really do exist, then I propose that we go out to the asteroid belt and mine the asteroids for the black holes that may be trapped in them. If a small black hole was in orbit around the Sun in the asteroid belt region, and it had the mass of an asteroid, it would be about the diameter of an atom. Despite its small size, the gravity field of the miniature black hole would be just as strong as the gravity field of an asteroid and if the miniature black hole came near another asteroid, the two would attract each other. Instead of colliding and fragmenting as asteroids do, however, the miniature black hole would just penetrate the surface of the regular asteroid and pass through to the other side. In the process of passing through, the miniature black hole would absorb a number of rock atoms, increasing its weight and slowing down slightly. An even more drastic slowing mechanism would be the tides from the miniature black hole. They would cause stresses in the rock around the line of penetration and fragment the rock out to a few micrometers away from its path through the asteroid. This would cause further slowing.

After bouncing back and forth through the normal matter asteroid many times, the miniature black hole would finally come to rest at the center of the asteroid. Now that it is not moving so rapidly past them, the miniature black hole could take time to absorb one atom after another into its atom-sized body until it had dug itself a tiny cavity at the center of the asteroid. With no more food available, it would stop eating, and sit there and glow warmly for a few million years. After years of glowing its substance away, it would get smaller. As it got smaller it would get hotter since the temperature rises as the mass decreases. Finally, the miniature black hole would get hot enough to melt the rock around it. Drops of melted rock would be pulled into the miniature black hole, adding to its mass. As the mass of the black hole increased, the temperature would decrease. The black hole would stop radiating, the melted rock inside the cavity would solidify, and the process would repeat itself many centuries later. Thus, although a miniature black hole left to itself has a lifetime that is less than the time since the Big Bang, there could be miniature black holes with the mass of an asteroid, being kept alive in the asteroid belt by a symbiotic interaction with an asteroid made of normal matter.

To find those asteroids that contain miniature black holes, you want to look for asteroids that have anomalously high temperatures, lots of recent fracture zones, and anomalously high density. Those with a suspiciously high average density have something very dense inside. To obtain a measure of the density, you need to measure the volume and the mass. It is easy enough to get an estimate of the volume of the host asteroid with three pictures taken from three different directions. It is difficult to measure the mass of an object in free fall. One way is to go up to it with a calibrated rocket engine and push it. Another is to land on it with a sensitive gravity meter. There is, however, a way to measure the mass of an object at a distance without going through the hazard of a rendezvous. To do this, you need to use a mass detector or gravity gradiometer.

Once you have found a suspiciously warm asteroid that seems awfully massive for its size, then to extract the miniature black hole, you give the surface of the asteroid a strong shove and push the asteroid out of the way. The asteroid will shift to a different orbit, and where the center of the asteroid used to be, you will find the miniature black hole. The black hole will be too small to see, but if you put an acoustic detector on the asteroid you will hear the asteroid complaining as the black hole comes to the surface. Once the black hole has left the surface you can monitor its position and determine its mass with a mass detector.

The next step in corralling the invisible black maverick is to put some electric charge on it. This means bombarding the position of the miniature black hole with a focused beam of ionized particles until the black hole has captured enough of them to have a significant charge to mass ratio. The upper limit will depend upon the energy of the ions. After the first ion is absorbed, the black hole will have a charge and will have a tendency to repel the next ion. Another upper limit to the amount of charge you can place on a black hole is the rate at which the charged black hole pulls opposite charges out of the surrounding space. You can keep these losses low, however, by surrounding the black hole with a metal shield.

Once a black hole is charged, you can apply forces to it with electric fields. If the charged black hole happens to be rotating, you are in luck, for then it will also have a magnetic field and you can also use magnetic fields to apply forces and torques. The coupling of the electric charge to the black hole is very strong—the black hole will not let go. You can now use strong electric or magnetic fields to pull on the black hole and take it anywhere you want to go.

from INDISTINGUISHABLE FROM MAGIC by Robert L. Forward (1995)

(ed note: for you Ugly Americans who have never heard of Perry Rhodan, this is a science fictional device)

Schwarzschild reactors have power output ten thousands time higher than a fusion reactor.

The reactor create a artificial pulsating micro black hole in size of one hundred nanometers. It shifts between being a black hole with event horizon and space time warp with no event horizon.

The black hole is fed with particle beam of ultra-catalyzed deuterium. Approximately 50% of deuterium is transformed into gamma-rays, the rays are collected by "super solar cells" and transformed into usable energy with an efficiency of 80%.

The other 50% of the deuterium is transformed into antimatter, swallowed by black hole (in space time warp mode) where it vanishes into the depths of hyperspace.

Michel Van (2015)

(ed note: for you Ugly Americans who have never heard of Perry Rhodan, this is a science fictional device)

Humans found the Schwarzschild reactors performance to be disappointing. Only 50% deuterium into gamma-rays could be improved upon. Human scientists developed the NUGAS-Schwarzschild Reactors.

The principle remain almost the same.

However instead of the antimatter being discharged into hyperspace, it is directed into the path of a particle beam for mutual annihilation. Thus 100% of the deuterium is converted into gamma rays.

Due the higher pulse rate and antimatter annihilation, ultra catalyzed deuterium was unsuitable as fuel. Instead ionized hydrogen nucleons (protons) were subsituted. They are conpressed to a density of 3.5×107 kilograms per cubic meter to form the Nucleon Gas (NUGAS) fuel ball. The NUGAS fuel ball has a mass of 200,000 metric tons. It is surrounded by containment generators forming a reactor with a diameter of 12 meters.

NUGAS is also used as fuel for starship Puls proton beam engines, the successor to the older impuls engines.

Of course NUGAS is dangerous, but it gave the 1970s Perry Rhodan authors interesting plot complications (such as a NUG-ball in danger of losing its containment field). The technological levels in the Perry Rhodan universe eventually became too unbelievable, so in 2003 the authors "reset" it to tone everything down. Now NUGAS only compress to a density of 8.75×106 kg/m3, and have a mass of only 50,000 metric tons.

The idea of the Schwarzschild Reactor and the NUG version came from German science fiction author Kurt Mahn. He was a real life physcist who worked for Pratt & Whitney, Martin Marietta, and Harris Electronics. He wrote for Perry Rhodan from 1962 to 1969 and later from 1972 to 1993.

Michel Van (2015)

Heat Radiators

This section has been moved here


Typically the percentage of spacecraft dry mass that is propulsion is 3.7% for NASA vessels.

For a list of various spacecraft propulsion systems, go to the engine list.

Habitat Module

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This section has been moved here

Conserving Payload Mass

Penalty Weight

As you are beginning to discover, mass is limited on a spacecraft. Many Heinlein novels have passengers given strict limits on their combined body+luggage mass. Officials would look disapprovingly at the passenger's waistlines and wonder out loud how they can stand to carry around all that "penalty weight". There are quite a few scenes in various Heinlein novels of the agony of packing for a rocket flight, throwing away stuff left and right in a desperate attempt to get the mass of your luggage below your mass allowance.

Keep in mind that every gram of equipment or supplies takes several grams of propellant. Try to make every gram do double duty.

Tex hauled out his luggage and hefted it. "It's a problem. I've got about fifty pounds here. Do you suppose if I rolled it up real small I could get it down to twenty pounds?"

"An interesting theory," Matt said. "Let's have a look at it -- you've got to eliminate thirty pounds of penalty-weight."

Jarman spread his stuff out on the floor. "Well," Matt said at once, "you don't need all those photographs." He pointed to a dozen large stereos, each weighing a pound or more.

Tex looked horrified. "Leave my harem behind?" He picked up one. "There is the sweetest redhead in the entire Rio Grande Valley." He picked up another. "And Smitty -- I couldn't get along without Smitty. She thinks I'm wonderful."...

...Matt studied the pile. "You know what I'd suggest? Keep that harmonica -- I like harmonica music. Have those photos copied in micro. Feed the rest to the cat."

"That's easy for you to say."

"I've got the same problem." He went to his room. The class had the day free, for the purpose of getting ready to leave Earth. Matt spread his possessions out to look them over. His civilian clothes he would ship home, of course, and his telephone as well, since it was limited by its short range to the neighborhood of an earth-side relay office...

..He called home, spoke with his parents and kid brother, and then put the telephone with things to be shipped. He was scratching his head over what remained when Burke came in. He grinned. "Trying to swallow your penalty-weight?"

"I'll figure it out."

"You don't have to leave that junk behind, you know."


"Ship it up to Terra Station, rent a locker, and store it. Then, when you go on liberty to the Station, you can bring back what you want. Sneak it aboard, if it's that sort of thing." Matt made no comment; Burke went on, "What's the matter, Galahad? Shocked at the notion of running contraband?"

"No. But I don't have a locker at Terra Station."

"Well, if you're too cheap to rent one, you can ship the stuff to mine. You scratch me and I'll scratch you."

"No, thanks." He thought about expressing some things to the Terra Station post office, then discarded the idea -- the rates were too high. He went on sorting. He would keep his camera, but his micro kit would have to go, and his chessmen. Presently he had cut the list to what he hoped was twenty pounds; he took the stuff away to weigh it.

From SPACE CADET by Robert Heinlein (1948)

Long as he had been earthbound he approached packing with a true spaceman's spirit. He knew that his passage would entitle him to only fifty pounds of free lift; he started discarding right and left. Shortly he had two piles, a very small one on his own bed -- indispensable clothing, a few capsules of microfilm, his slide rule, a stylus, and a vreetha, a flutelike Martian instrument which he had not played in a long time as his schoolmates had objected. On his roommate's bed was a much larger pile of discards.

He picked up the vreetha, tried a couple of runs, and put it on the larger pile. Taking a Martian product to Mars was coal to Newcastle.

From BETWEEN PLANETS by Robert Heinlein (1951)

“Is all your cargo aboard? How much did they let you take?”

“A hundred kilos. It’s in the airlock.”

“A hundred kilos?” Norden managed to repress his amazement. The fellow must be emigrating—taking all his family heirlooms with him. Norden had the true astronaut’s horror of surplus mass, and did not doubt that Gibson was carrying a lot of unnecessary rubbish. However, if the Corporation had O.K.‘d it, and the authorised load wasn’t exceeded, he had nothing to complain about.

From THE SANDS OF MARS by Sir Arthur C. Clarke (1951)


In Frank Herbert's DUNE, spacemen had books the size of a thumb-tip, with a tiny magnifying glass.

"If it's economically feasible," Yueh said. "Arrakis has many costly perils." He smoothed his drooping mustache. "Your father will be here soon. Before I go, I've a gift for you, something I came across in packing." He put an object on the table between them-black, oblong, no larger than the end of Paul's thumb.

Paul looked at it. Yueh noted how the boy did not reach for it, and thought: How cautious he is.

"It's a very old Orange Catholic Bible made for space travelers. Not a filmbook, but actually printed on filament paper. It has its own magnifier and electrostatic charge system." He picked it up, demonstrated. "The book is held closed by the charge, which forces against spring-locked covers. You press the edge-thus, and the pages you've selected repel each other and the book opens."

"It's so small."

"But it has eighteen hundred pages. You press the edge-thus, and so . . . and the charge moves ahead one page at a time as you read. Never touch the actual pages with your fingers. The filament tissue is too delicate." He closed the book, handed it to Paul. "Try it."

From DUNE by Frank Herbert

Ruthless Optimization

Other innovations are possible. Perhaps boxes of food where the boxes are edible as well. The corridor floors will probably be metal gratings to save mass (This is the second reason why cadet shipboard uniforms will not have skirts or kilts. Looking up at the ceiling grating will give you a peekaboo up-skirt glimpse of whoever is in the next deck up. No panchira allowed. The first reason is the impossibility of keeping a skirt or kilt in a modest position while in free-fall.) In Lester Del Rey's Step to the Stars all documents, blueprints, and mail are printed on stuff about as thick as tissue paper (have you ever tried to lift a box full of books?).

With regards to low mass floors, the lady known as Akima had an interesting idea:

Unless the deck is also a pressure bulkhead, how about omitting deck plates and beams entirely, and making the floor a metal-mesh version of the trampoline decks used on sailing catamarans? That way, "weights" bearing on the decks would be transmitted into the tubular structure of the hull as an inward tension.

David Chiasson expands upon Akima's idea. There is an outfit called Metal Textiles which produces knitted wire mesh.

The meshes are knitted, as opposed to woven like a screen door. They are manufactured in densities (% metal by volume) from 10% to 70%. There are a wide variety of materials that the mesh can be made from, including aluminum, steels, Teflon, Nylon, even tungsten. Unfortunately, titanium is not on that list, I can only suspect that it must be difficult to get into a wire form suitable for making a knitted mesh.

Direct quote from site's main page: "In compressed form, knitted metal can handle shock loadings up to the yield strength of the material itself. The load may be applied from any direction-up, down or in from all sides."

I can speculate that with some kind of structural forming breakthrough, the mesh could be heated over a (ceramic?) mold to a near-melting point and simply pressed into place, compressing the mesh into a solid.

David Chiasson

Michael Garrels begs to differ:

I need to point out some issues with the idea of mesh floors.

First off there's the idea that bulkheads have to be bulky. In nautical settings, bulkheads have to be bulky to withstand the large pressure of water, to mount things like hatches on, and to provide overall rigidity to the ship during turning and impact. Most partitions in a spaceship would be a thin pressure membrane sandwiched between a mesh to avoid punctures. The skin on the Apollo lander module was thinner than common aluminum foil. If all you're trying to do is partition, pull up pictures of Skylab - you'll see curtains and isogrid all over the place.

Next is your distinction between floors and walls. Unless there is spin or thrust, there will be no such distinction.

Which brings us to the most important point - the floor that you're currently standing on isn't made out of mesh for a reason. Remember that classic description of a gravity well with a weight on a rubber sheet? Many building codes don't limit the weight allowed on floors but instead the amount of deflection allowed. Floors have to be bulky with occasional beams - otherwise you'll never be able to wheel a torpedo or a gurney, and debris will roll toward where you're standing. It might work in a hallway, or as on your boat for stowage of light items, but not for spans more than a couple meters at 1 g using real materials - especially if you want to mount something like a chair and a console in the center of the cabin.

Michael Garrels


If you are dealing with a conventional spacecraft ruled by the iron law of Every Gram Counts, a stowaway is a disaster. If they had not jettisoned a payload mass equal to their mass, there will not be enough propellant to perform the vital maneuvers. The ship will run out, and go sailing off into the Big Dark and a lonely death for everybody on board.

Even if the stowaway jettisions enough mass, there probably won't be enough breathing mix and food aboard for the additional person. Everybody will suffocate and/or starve.

As Dr. Feynman observed about the Challenger disaster, "Nature cannot be fooled." If the equations say that your spacecraft does not have enough fuel, they don't mean "maybe."

For survival's sake, the crew will have little choice but to immediately throw the stowaway out the nearest airlock.

This was highlighted in a famous story called The Cold Equations by Tom Godwin. The story is chilling abet scientifically accurate, but it still caused an uproar when it first came out.

But if the ship is a torchship or uber-powerful faster-than-light starship, things are a little less tense. Since they are not actually threatening the lives of the crew, stowaways will be treated more like their terrestrial counterparts if discovered on a sea-going vessel.


(ed note: Homir Munn has set out in his one-man sports-cruiser hyperspace starship on a desperate mission to fight the Second Foundation. His 14 year old "niece" Arcadia Darrel stows away on board, determined to help.)

     In the luggage compartment, Arcadia found herself, in the first place, aided by experience, and in the second, hampered by the reverse.
     Thus, she met the initial acceleration with equanimity and the more subtle nausea that accompanied the inside-outness of the first jump through hyperspace with stoicism. Both had been experienced on space hops before, and she was tensed for them. She knew also that luggage compartments were included in the ship’s ventilation-system and that they could even be bathed in wall-light. This last, however, she excluded as being too unconscionably unromantic. She remained in the dark, as a conspirator should, breathing very softly, and listening to the little miscellany of noises that surrounded Homir Munn…
     …Yet, eventually, it was the lack of experience that caught up with Arcadia. In the book films and on the videos, the stowaway seemed to have such an infinite capacity for obscurity. Of course, there was always the danger of dislodging something which would fall with a crash, or of sneezing — in videos you were almost sure to sneeze; it was an accepted matter. She knew all this, and was careful. There was also the realization that thirst and hunger might be encountered. For this, she was prepared with ration cans out of the pantry. But yet things remained that the films never mentioned, and it dawned upon Arcadia with a shock that, despite the best intentions in the world, she could stay hidden in the closet for only a limited time.
     And on a one-man sports-cruiser, such as the Unimara, living space consisted, essentially, of a single room, so that there wasn’t even the risky possibility of sneaking out of the compartment while Munn was engaged elsewhere…
     …She tried to poke her eyes outside the door without moving her head and failed. The head followed the eyes.
     Homir Munn was awake, of course — reading in bed, bathed in the soft, unspreading bed light, staring into the darkness with wide eyes, and groping one hand stealthily under the pillow.
     Arcadia’s head moved sharply back of itself. Then, the light went out entirely and Munn’s voice said with shaky sharpness, “I’ve got a blaster, and I’m shooting, by the Galaxy—”
     And Arcadia wailed, “It’s only me. Don’t shoot.”…
     …After a wild moment in which he almost jumped out of bed, but remembered, and instead yanked the sheet up to his shoulders, Munn gargled, “W … wha … what—”
     Arcadia said meekly, “Would you excuse me for a minute? I’ve got to wash my hands.” She knew the geography of the vessel, and slipped away quickly.

From SECOND FOUNDATION by Isaac Asimov (1953)

Stowaways in Space

     When I sat down to write this it was with misgivings. The idea of anyone hiding on board a ship in space seems a little farfetched to me. When Traveller was released in the ’70’s and people began SF roleplaying there was not an inkling that one day everything would have processing capacity and our wallpaper would have micro sensors in it.
     If you wanted to find a stowaway you had to, you know, look for the little sneak yourself.

     Perusing the rules for the anti-hijack program only indicates it will lock people out of control systems. 
     The classic Starship Operators’ Handbook from Digest Group went into great detail about how a ship’s computer could in fact scan a person for weapons, emotional state and then turn snitch on them long before they’d get to anyplace sensitive. However, we must also realize your computer is already pretty heavily tasked and internal sensors and AI cost credits. Not every ship will have them, certainly not many heavily mortgaged tramp fusers plodding along and plying their trade. So let’s go over some ways to stow away.

     Stowing away is defined for my purposes as obtaining passage onboard a ship through nonviolent but illegal means. If you’re holding the captain at gunpoint till he breaks orbit congratulations — you’re a hijacker.
     The easiest way of getting free passage is if you are a powerful telepath (“Aha! Over … wait I was wrong. Just a potted plant in here.”) Let the computer tag you for a drifter all it wants. Who cares if the men sent to grab you don’t seem to see or remember you? The Traveller race, Dronyne, with their cloaking ability would excel at this. S&S Andromedans used to have a molting period where they were invisible and surely an exceptional psi could be found in any race desired. The Dralasites from the Star Frontiers game weren’t invisible but darned near boneless and could easily hide in nooks and spaces no human could reach.
     The problem with all those species is when people begin noticing the missing oxygen, water, and food or see your little footprints on the deck using a thermograph or hear the head flushing. After that it’s a simple matter to order the crew into spacesuits and start evacuating sections of the hull. Note that to get the really satisfying WHOOSH and suck people off their feet we’re talking opening the hangar bay doors.

     Another use for psionics that doesn’t involve making like a Jedi (I am not the stowaway you are looking for …” “Doesn’t matter, you’ll do!”) is to use a power that lets you slow your body’s metabolism down to nothing. Mail yourself wherever in an airtight crate with an alarm clock.
     Medical Fast Drug can serve the same purpose. Make sure you have enough. It also sucks when people do find you as you can’t put up much of a fight (the ship may land before your first punch does.)

     Some stowaways will attempt to ride out their trip in a cargo container keeping a very low profile indeed. These crates are often elaborate affairs with thermal and sonic shielding, cryogenic devices and heat sinks to store and eliminate any thermal traces. This is not too odd when you realize money is not the only reason people stowaway. There are many reasons to let people think you are still on planet. Just be sure you take everything you need. Remember the stowaway who hid in a crated ATV swaddled in chill cans and thermal insulating blankets with water and food bars to spare. They caught him when he emerged after three days to use the bathroom. Yes zero sediment food bars are a thing. Buy some. For added safety bring your own breathing equipment in case they decide to evacuate the cargo bay for whatever reason.
     Of course there are people who convert cargo containers into stowaway modules. Some of them are better than the accommodations you pay for legally.

     There is another way to stow away that few people outside the well travelled circles. We’ve seen how institutions like CT’s Travellers’ Aid Society that gives high passage tickets as dividends.
     Now again back in the ’70’s we figured they were printed tickets (on very nice cardstock) and the recipients lugged them around in wheelbarrows or some such. Actually this works fairly well if you think about it. Give the person a ticket on physical media at each aid station or whatever dated and with a ledger (electronic or physical) that shows the receipt of said tickets. He presents himself, gets a ticket and gets his book stamped all very legal. Note that duplicating a high passage may mean presenting yourself as the recipient or forging a paper trail to prove its sale to you.
     I think those tickets would be very hard to duplicate. Hard but not impossible. counterfeit a high passage and suddenly you are indistinguishable from a paying customer. You can sit around your individual stateroom wondering what the poor stowaways are doing this time of year.
     For my part if I could forge high passage tickets I’d trade them in for the credits then spend them on a middle passage and have a thousand credits to spend on my trip. But then you blur the lines between scam artists and stowaways.

     Another way to blur the lines is to introduce squatters. When you’re hiding aboard a ship you’re a stowaway. When you hide aboard a station you’re a squatter. Being a squatter is an order easier (of course you do need to get to the station first.) Stations are bigger than most ships providing more hiding options. A station with a lot of personnel passing through might not notice the discrepancy in life support and since a station doesn’t go anywhere your mass will not throw it off course. Large space colonies are the ultimate in squatters and may have dozens or hundreds of people living off the grid … in spaaaaace.
     Sadly the results of being found remain the same: possible spacing and most likely arrest and deportation. In the event of deportation they will probably give you a space suit and re-entry kit (used).
     Squatters may even be tolerated on some stations. They provide a steady supply of dayworkers/scabs (or even thugs) if necessary. They could be engaged in all manner of illegal sales and services. If their uses outweighs the oxygen and water loss the station authorities may turn a blind eye to them for years or even decades.

     Of course they can cobble together a stowaway pod. Why do you ask?

From STOWAWAY THE EASY WAY by Rob Garitta (2017)

(ed note: the spacecraft uses some sort of technobabble antigravity device called the "Field Compensation Drive Generator")

The main field went on, and weight ebbed from the Centaurus. There were protesting groans from the ship's hull and structure as the strains redistributed themselves. The curved arms of the landing cradle were carrying no load now; the slightest breath of wind would carry the freighter away into the sky.

Control called from the tower: ‘Your weight now zero: check calibration.’

Saunders looked at his meters. The upthrust of the field would now exactly equal the weight of the ship, and the meter readings should agree with the totals on the loading schedules. In at least one instance this check had revealed the presence of a stowaway on board a spaceship — the gauges were as sensitive as that.

’One million, five hundred and sixty thousand, four hundred and twenty kilograms,’ Saunders read off from the thrust indicators. ‘Pretty good — it checks to within fifteen kilos. The first time I’ve been underweight, though. You could have taken on some more candy for that plump girl friend of yours in Port Lowell, Mitch.’…

…There was no sense of motion, but the Centaurus was now falling up into the summer sky as her weight was not only neutralised but reversed. To the watchers below, she would be a swiftly mounting star, a silver globule climbing through and beyond the clouds. Around her, the blue of the atmosphere was deepening into the eternal darkness of space. Like a bead moving along an invisible wire, the freighter was following the pattern of radio waves that would lead her from world to world.…

…With the silence of limitless power, the ship shook itself free from the last bonds of Earth. To an outside observer, the only sign of the energies it was expending would have been the dull red glow from the radiation fins around the vessel’s equator, as the heat loss from the mass-converters was dissipated into space.…

…An hour after take-off, according to the hallowed ritual, Chambers left the course computer to its own devices and produced the three glasses that lived beneath the chart table. As he drank the traditional toast to Newton, Oberth, and Einstein, Saunders wondered how this little ceremony had originated. Space crews had certainly been doing it for at least sixty years: perhaps it could be traced back to the legendary rocket engineer who made the remark, ’I’ve burned more alcohol in sixty seconds than you've ever sold across this lousy bar.’

Two hours later, the last course correction that the tracking stations on Earth could give them had been fed into the computer. From now on, until Mars came sweeping up ahead, they were on their own. It was a lonely thought, yet a curiously exhilarating one. Saunders savoured it in his mind. There were just the three of them here — and no one else within a million miles.

In the circumstances, the detonation of an atomic bomb could hardly have been more shattering than the modest knock on the cabin door…

…A stowaway was simply impossible. The danger had been so obvious, right from the beginning of commercial space flight, that the most stringent precautions had been taken against it. One of his officers, Saunders knew, would always have been on duty during loading; no one could possibly have crept in unobserved. Then there had been the detailed preflight inspection, carried out by both Mitchell and Chambers. Finally, there was the weight check at the moment before take-off; that was conclusive. No, a stowaway was totally…

(ed note: turns out the stowaway is Henry IX, crown prince of England. He had been trying to travel into space for years but the stodgy prime minister wouldn't hear of it. Henry was smuggled aboard with the aid of the two British crewmen Mitchell and Chambers.)

Saunders swallowed hard. Then, as the pieces of the jigsaw fell into place, he looked first at Mitchell, then at Chambers. Both of his officers stared guilelessly back at him with expressions of ineffable innocence. ‘So that's it,’ he said bitterly. There was no need for any explanations: everything was perfectly clear. It was easy to picture the complicated negotiations, the midnight meetings, the falsification of records, the off-loading of nonessential cargoes that his trusted colleagues had been conducting behind his back. He was sure it was a most interesting story, but he didn't want to hear about it now. He was too busy wondering what the Manual of Space Law would have to say about a situation like this, though he was already gloomily certain that it would be of no use to him at all.

It was too late to turn back, of course: the conspirators wouldn't have made an elementary miscalculation like that. He would just have to make the best of what looked to be the trickiest voyage in his career.

From THIS EARTH OF MAJESTY by Arthur C. Clarke (1955)

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