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.


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.

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 rule of thumb, 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.

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.

Planetary Exploration Vessel
Scientific instruments, space ferries, airless landers
Cargo Vessel
Cargo holds
Extra propellant tanks (and a remarkably large mass ratio)
Space Tug
Ship grappling equipment, push plates, and an over-sized high-thrust propulsion bus.
Coast Guard vessel
rescue equipment, ship grappling gear, ship repair supplies space taxis, space pods, and a propulsion bus with extra delta V.
Orbital Guard vessel
Telescopes and other tracking gear, nuclear detonation detectors, asteroid redirecting equipment, weapons
Troop transports
Huge habitat modules
Blockade Runner
Stealth technology, small cargo hold, and a propulsion bus with high acceleration

You get the idea.

A warship's payload section can include anti-spacecraft weapons, orbital bombardment weapons (for revolt suppression type spacecraft as well), weapon mounts, weapon control stations, combat information center, armor, point defense, weapon heat radiators and heat sinks, and anything else that can be used to mission-kill enemy spacecraft.

Pirate ships and privateers might forgo defenses if they only expect to be engaging unarmed cargo ships. But they will regret this if they have the misfortune to encounter armed enemy convoy escort ships or are surprised by a Q-ship.

Spaceship Design 101

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)

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.
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 fuel tanks plus the mass of any fuel that is thrown out the exhaust nozzle. Does not include fuel that is retained after it is burnt, e.g., uranium fissioned inside a solid core reactor.
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.
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.
Propellant Fraction (Pf)
Percentage of wet mass that is propellant. 1 - ( 1 / MassRatio )

Propellant Mass Flow (mDot)

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.

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).

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) : Rule of thumb 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) : Rule of thumb 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) : Rule of thumb minimum to lift off from Terra's surface into LEO.

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.


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.

A Litte More Complicated Than That

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.

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.

Larry Niven Belter Singleship

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.

Waterskiing Spacecraft

This is a quite radical method to drastically reduce the structural mass of a spacecraft (and also dramatically increase the separation between a dangerously radioactive propulsion system and the crew). Please note this has never been tried, and warships with such a design would have their manoeuvring critically handicapped (or it's "crack-the-whip" time).

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.

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. This does reduce the effective thrust by an amount proportional to the cosine of the angle but for small angles it is acceptable.

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.

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 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 unobtanium 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)


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.

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.

Radial Symmetry

When laying out the floor plan, you want the spacecraft to balance. That is, if you draw a line straight through the exhaust bell (in the direction that thrust is applied), it had better pass through the spacecraft's center of gravity, and if the ship is intended for atmospheric flight, it should also go through the spacecraft's nose. Otherwise your ship is going to loop-the-loop or tumble like a cheap Fourth of July skyrocket (Heinlein calls this a rocket "falling off its tail").

This 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. 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. 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.

The Martian Way

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 rule of thumb, 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

If you are going to use aerobraking to land your rocket, Zubrin says mass of the heat shield and thermal structure will be about 15% of the total mass being braked. As a wild guess, aerobraking will be limited to killing a velocity of no more than 15 to 30 kilometers per second. The rule of thumb 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.

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.

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."

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.

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, 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.

Solar power arrays have an alpha ranging from 100 to 10 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.

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.

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. As a rule of thumb:

Es = 1366 * (1 / Ds2)


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

Remember that you divide distance in meters by 1.49e11 in order 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 = 1.53 astronomical units. So the available solar energy is:

  • Es = 1366 * (1 / Ds2)
  • Es = 1366 * (1 / 1.532)
  • Es = 1366 * (1 / 2.34)
  • Es = 1366 * 0.427
  • Es = 583 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).

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 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 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.

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.

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.

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.

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 with 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. 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.

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. Let me explain.

The same situation exists with respect to 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 that while there 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). 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.

This means that unless there exist "antimatter mines", antimatter is also an energy transport mechanism, not a fuel. In Star Trek, I believe they found drifts of antimatter in deep space. 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.

The Song of Distant Earth

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 Song of Distant Earth by Sir Arthur C. Clarke (1985)

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.

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.


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, 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, 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 specific energy of 2,700 Wh/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

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, a charge/discharge efficiency up to 70%, and a service life of up to 10,000 hours.

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.

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)
Perry Rhodan Schwarzschild Reactors

(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)
Perry Rhodan NUGAS-Ball Storage Tank

(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

RocketCat sez

You might think that the problem with surviving in the "zero degree cold of space" is keeping from freezing to death. Nope, the problem is Heat.

Human bodies are little furnaces, which you can discover if you wrap your limbs and torso in plastic wrap and see how little time it takes to pass out from heat prostration (note to jackasses THIS IS AN ILLUSTRATION, DO NOT ACTUALLY TRY TO DO THIS!). In space, it's not like you can open the window for a cooling breeze, either. Your cosy little habitat module will turn into an oven.

If you stayed awake during Physics 101 class you'll know that the blasted laws of thermodynamics say there are only three ways of getting rid of waste heat. But only one of them will work in space: radiation.

So you'll need heat radiators or the crew is going to die horribly while sweating bullets.

And I am quite sure that you are going to make things infinitely worse by insisting on your precious nuclear power reactors and megawatt laser cannons. Human bodies only make enough waste heat to kill everybody, reactors and lasers can make the entire freaking ship glow white-hot and vaporize.

Ever see those titanic curved towers around nuclear power plants? Yep, cooling towers. You'll need something a bit more high-tech if you do not want your spacecraft's aesthetics spoiled by a 40 meter cooling tower or two.

Laser cannon are much worse. Rick Robinson described them as observatory telescopes with a jet engine at the eyepiece. Ken Burnside said they were blast furnaces that produced coherent light as a byproduct. Whatever you call them they are hot enough to make your ship go from solid directly to Solar-surface hot ionized gas without passing through the molten metal stage first.

But of course heat radiators are one of the major things conspicuous by their absence in science fiction TV shows and movies. Concept artists don't want their ultra-futuristic spacecraft decked with 17th century billowed sails. They even over-ruled Arthur C. Clarke for cryin' out loud! The only exception that comes to mind is the ISV Venture Star from the movie Avatar.

Power plants and some propulsion systems are going to require heat radiators to avoid system meltdown. There are only three ways of getting rid of heat: convection, conduction, and radiation; and the first two do not work at all in the vacuum of space. So the ship designer is stuck with heat radiators. See Thermophotovoltaic Energy Conversion in Space Nuclear Reactor Power Systems and HIGH TRADER for details. Ken Burnside also noted that radiators are large, flimsy, and impossible to armor (except perhaps for the droplet radiator). A liability on a warship. As a matter of fact, Mr. Burnside has an entire essay about the problem of heat on combat spacecraft, entitled The Hot Equations: Thermodynamics and Military SF. It will repay careful study.

In the military the old bromide is that amateurs talk about battle tactics while professionals talk about logistics. In the real of spacecraft design, @AsteroidEnergy said "Amateurs discuss rockets, professionals discuss heat management."

If you want to calculate this for yourself use the Stefan-Boltzmann law:

P = A * ε * σ * T4

A = P / (ε * σ * T4)


  • P = the power of waste heat the radiator can get rid of (watts)
  • σ = 5.670373×10-8 = Stefan-Boltzmann constant (W m-2K-4)
  • ε = emissivity of radiator (theoretical maximum is 1.0 for a perfect black body, real world radiator will be less. Should be at least 0.8 or above to be worth-while)
  • A = area of radiator (m2)
  • T = temperature of radiator, this assumes temperature of space is zero degrees (degrees K)
  • x4 = raise x to the fourth power, i.e, x * x * x * x

Ken Burnside says that if one examine the equation carefully one will notice that the radiator effectiveness goes up at the fourth power of the heat of the radiator. The higher the temperature, the lower the surface area can be, which lowers the required mass of radiator fins. This is why most radiator designs use liquid sodium or lithium (or things more exotic, still). 1600K radiators mean that you need a lot less mass than 273 K radiators.

I had initially thought that the heat from the life-system could be simply dumped by the same radiator system dealing with the multi-gigawatt waste heat from the propulsion system. Richard Bell pointed out that I had not thought the problem through. Due to the difference in the temperatures of the waste heat from life-system and propulsion, unreasonably large amounts of energy will be required to get the low-level life-system heat into a radiator designed to handle high-level propulsion heat. The bottom line is that there will be two separate radiator systems.

Not only are you going to require two separate radiator systems, the one for the modest cooling required by the life-system is liable to have larger radiator surfaces than the one cooling the multi-gigawatt propulsion system. Radiator effectiveness goes up as the fourth power of the heat of the radiator, remember?

Propulsion systems like nuclear thermal rockets do not need heat radiators because the waste heat is carried away by the exhaust plume. In effect, the exhaust is their radiator (the technical term is "Open-Cycle Cooling"). Note this only works if the propulsion system has a high propellant mass flow (called "mdot"). Note that the lower the thrust the lower the mdot. Once the thrust gets too low there is not enough propellant in the exhaust plume for you to use the open-cycle cooling trick.

Electrical powered drives like ion drives will require radiators on their power plants. Fusion drives may or may not require radiators, depending upon whether the design can dump the waste heat into the exhaust or not.

My source (Matthew DeBell) says that if P = 150 gigawatts, ε = 0.94, and T = 3000 K, A would be 34,941 m2. Actually it could be half that if you have a two-sided radiator, which would make the radiator 17,470 m2 (a square 132 meters on a side). Which is still freaking huge.

For estimating the mass of the radiator array, go here.

Thermal Management in Space

It should be pointed out that in a vacuum environment, convection is no longer available and the only mechanism of rejecting heat is radiation. Radiation follows the Stefan-Boltzmann Law

E = σT4

E = the energy rejected
σ = the Stefan-Boltzmann constant, = 5.67 W m-2 K-4
T = the temperature at which the heat is radiated

That is, the total amount of heat radiated is proportional to the surface area of the radiator. And the lower the radiation temperature, the larger the radiator area (and thus the radiator mass, for a given design) must be.

The radiator can only reject heat when the temperature is higher than that of the environment. In space, the optimum radiation efficiency is gained by aiming the radiator at free space. Radiating toward an illuminated surface is less effective, and the radiator must be shielded from direct sunlight.

The rejection of heat at low temperatures, such as would be the case in environmental control and in the thermal management of a materials processing unit, is particularly difficult.

Space-Based Power Generating Systems

Solar photovoltaic systems have a generating capability of up to several hundred kilowatts. The power output range of solar thermal systems is expected to be one hundred to perhaps several hundred kilowatts. While in principle these power systems can be expanded into the megawatt region, the prohibitive demands for collection area and lift capacity would appear to rule out such expansion. Megawatt and multimegawatt nuclear power reactors adapted for the space environment appear to offer a logical alternative.

Solar photovoltaics themselves will not burden the power generating system with a direct heat rejection requirement, since the low energy density of the system requires such a great collection area that it allows rejection of waste radiant energy. However, if these systems are to be employed in low Earth orbit or on a nonterrestrial surface, then a large amount of energy storage equipment will be required to ensure a continuous supply of power (as the devices do not collect energy at night). And the round-trip inefficiencies of even the best energy storage system today will require that a large fraction—perhaps 25 percent—of the electrical power generated must be dissipated as waste heat and at low temperatures.

Solar thermal systems, which include a solar concentrator and a dynamic energy conversion system, are presumed to operate at relatively high temperatures (between 1000 and 2000 K). The efficiencies of the energy conversion system will lie in the range of 15 to perhaps 30 percent. Therefore we must consider rejecting between 70 and 85 percent of the energy collected. In general, the lower the thermal efficiency, the higher the rejection temperature and the smaller the radiating area required. As with solar photovoltaic systems, the inefficiencies of the energy storage system will have to be faced by the heat rejection system, unless high temperature thermal storage is elected.

The current concepts for nuclear power generating systems involve reactors working with relatively low-efficiency energy conversion systems which reject virtually all of the usable heat of the reactor but at a relatively high temperature. Despite the burdens that this low efficiency places on nuclear fuel use, the energy density of nuclear systems is so high that the fuel use factor is not expected to be significant.

In all of these systems the output power used by the production system in environmental control and manufacturing (except for a small fraction which might be stored as endothermic heat in the manufactured product) will have to be rejected at temperatures approaching 300 K.

As an example of the severity of this problem, let us examine the case of a simple nuclear power plant whose energy conversion efficiency from thermal to electric is approximately 10 percent. The plant is to generate 100 kW of useful electricity. The reactor operates at approximately 800 K, and a radiator with emissivity equal to 0.85 would weigh about 10 kg/m2. The thermal power to be dissipated from the reactor would be about 1 MW. From the Stefan Boltzmann Law, the area of the radiator would be about 50 m2 and the mass approximately 500 kg. This seems quite reasonable.

However, we must assume that the electricity generated by the power plant, which goes into life support systems and small-scale manufacturing, would eventually have to be dissipated also, but at a much lower temperature (around 300 K). Assuming an even better, aluminum radiator of about 5 kg/m2, with again an emissivity of 0.85, in this case we find that the area of the low temperature heat rejection component is 256 m2, with a mass approaching 1300 kg.

Using the Stefan-Boltzmann Law,

E1 = 5.67×10-8 W m-2 K-4 (800 K)4
E1 = 5.67×10-8 W m-2 K-4 × 4096×108 K4
E1 = 5.67 W m-2 × 4.10×103
E1 = 23.3 kW m-2

900 kW / 23.3 kW m-2 = 38.6 m2
and 38.6 m2 / 0.85 = 45.4 m2

E2 = 5.67×10-8 W m-2 K-4 (300 K)4
E2 = 5.67×10-8 W m-2 K-4 × 81×108 K4
E2 = 5.67 W m-2 × 81
E2 = 459 W m-2

100 kW / 459 W m-2 = 0.2179×103 m2 = 218 m2
and 218 m2 / 0.85 = 256 m2

Therefore, we can see that the dominant heat rejection problem is not that of the primary power plant but that of the energy that is used in life support and manufacturing, which must be rejected at low temperatures. Using the waste heat from the nuclear power plant for processing may be effective. But, ironically, doing so will in turn require more radiator surface to radiate the lower temperature waste heat.

Heat Rejection Systems

In this section I will deal with systems designed to meet the heat rejection requirements of power generation and utilization. These heat rejection systems may be broadly classified as passive or active, armored or unarmored. Each is expected to play a role in future space systems.

Heat pipes: The first of these, called the “heat pipe,” is conventionally considered the base system against which all others are judged. It has the significant advantage of being completely passive, with no moving parts, which makes it exceptionally suitable for use in the space environment.

For the convenience of the reader, I will briefly describe the operational mechanism of the basic heat pipe. (See figure 36.) The heat pipe is a thin, hollow tube filled with a fluid specific to the temperature range at which it is to operate. At the hot end, the fluid is in the vapor phase and attempts to fill the tube, passing through the tube toward the cold end, where it gradually condenses into the liquid phase. The walls of the tube, or appropriate channels grooved into the tube, are filled with a wick-like material which returns the fluid by surface tension to the hot end, where it is revaporized and recirculated.

Essentially the system is a small vapor cycle which uses the temperature difference between the hot and cold ends of the tube as a pump to transport heat, taking full advantage of the heat of vaporization of the particular fluid.

The fluid must be carefully selected to match the temperature range of operation. For example, at very high temperatures a metallic substance with a relatively high vaporization temperature, such as sodium or potassium, may be used. However, this choice puts a constraint on the low temperature end since, if the fluid freezes into a solid at the low temperature end, operation would cease until the relatively inefficient conduction of heat along the walls could melt it. At low temperatures a fluid with a low vaporization temperature, such as ammonia, might well be used, with similar constraints. The temperature may not be so high as to dissociate the ammonia at the hot end or so low as to freeze the ammonia at the cold end.

With proper design, heat pipes are an appropriate and convenient tool for thermal management in space systems. For example, at modest temperatures, the heat pipe could be made of aluminum, because of its relatively low density and high strength. Fins could be added to the heat pipe to increase its heat dissipation area. The aluminum, in order to be useful, must be thin enough to reduce the mass carried into space yet thick enough to offer reasonable resistance to meteoroid strikes.

A very carefully designed solid surface radiator made out of aluminum has the following capabilities in principle: The mass is approximately 5 kg/m2 with an emissivity of 0.85; the usable temperature range is limited by the softening point of aluminum (about 700 K). At higher temperatures, where refractory metals are needed, it would be necessary to multiply the mass of the radiator per square meter by at least a factor of 3. Nevertheless, from 700 K up to perhaps 900 K, the heat pipe radiator is still a very efficient method of rejecting heat.

A further advantage is that each heat pipe unit is a self-contained machine. Thus, the puncture of one unit does not constitute a single-point failure that would affect the performance of the whole system. Failures tend to be slow and graceful, provided sufficient redundancy.

Pump loop system: The pump loop system has many of the same advantages and is bounded by many of the same limitations associated with the heat pipe radiator. Here heat is collected through a system of fluid loops and pumped into a radiator system similar to conventional radiators used on Earth. It should be pointed out that in the Earth environment the radiator actually radiates very little heat; it is designed to convect its heat. The best known examples of the pump loop system currently used in space are the heat rejection radiators used in the Shuttle. These are the inner structure of the clamshell doors which are deployed when the doors are opened (fig. 37).

Pump loop systems have a unique advantage in that the thermal control system can easily be integrated into a spacecraft or space factory. The heat is picked up by conventional heat exchangers within the spacecraft, the carrier fluid is pumped through a complex system of pipes (extended by fins when deemed effective), and finally the carrier is returned in liquid phase through the spacecraft. In the case of the Shuttle, where the missions are short, additional thermal control is obtained by deliberately dumping fluid.

Since the system is designed to operate at low temperatures, a low density fluid, such as ammonia, may on occasion, depending on heat loading, undergo a phase change. Boiling heat transfer in a low gravity environment is a complex phenomenon, which is not well understood at the present time. Because the system is subjected to meteoroid impact, the basic primary pump loops must be strongly protected.

Despite these drawbacks, pump loop systems will probably be used in conjunction with heat pipe systems as thermal control engineers create a viable space environment. These armored (closed) systems are rather highly developed and amenable to engineering analysis. They have already found application on Earth and in space. A strong technology base has been built up, and there exists a rich literature for the scientist-engineer to draw on in deriving new concepts.

Advanced Radiator Concepts

The very nature of the problems just discussed has led to increased efforts on the part of the thermal management community to examine innovative approaches which offer the potential of increased performance and, in many cases, relative invulnerability to meteoroid strikes. Although I cannot discuss all of these new approaches, I will briefly describe some of the approaches under study as examples of the direction of current thinking.

Improved conventional approaches: The continuing search for ways to improve the performance of heat pipes has already shown that significant improvements in the heat pumping capacity of the heat pipe can be made by clever modifications to the return wick loop. Looking further downline at the problem of deployability, people are exploring flexible heat pipes and using innovative thinking. For example, a recent design has the heat pipes collapsing into a sheet as they are rolled up, the same way a toothpaste tube does. Thus, the whole ensemble may be rolled up into a relatively tight bundle for storing and deploying. However, because the thin-walled pipes are relatively fragile and easily punctured by meteoroids, more redundancy must be provided. The same principles, of course, can be applied to a pump loop system and may be of particular importance when storage limits must be considered. These are only examples of the various approaches taken, and we may confidently expect a steady improvement in the capability of conventional thermal management systems.

The liquid droplet radiator: The basic concept of the liquid droplet radiator is to replace a solid surface radiator by a controlled stream of droplets. The droplets are sprayed across a region in which they radiate their heat; then they are recycled to the hotter part of the system. (See figure 38.)

It was demonstrated some time ago that liquid droplets with very small diameters (about 100 micrometers) are easily manufactured and offer a power-to-mass advantage over solid surface radiators of between 10 and 100. In effect, large, very thin radiator sheets can be produced by the proper dispersion of the droplets. This system offers the potential of being developed into an ultralightweight radiator that, since the liquid can be stored in bulk, is also very compact.

The potential advantages of the liquid droplet radiator can be seen if we consider again the problem that was discussed at the end of the section on heat pipe radiators. We found that a very good aluminum radiator would require 256 m2 and have a mass of nearly 1300 kg to radiate the low temperature waste heat from lunar processing. Using the properties of a liquid droplet radiator and a low density, low vapor pressure fluid such as Dow-Corning 705, a common vacuum oil, we find that, for the same area (which implies the same emissivity), the mass of the radiating fluid is only 24 kg.

Even allowing a factor of 4 for the ancillary equipment required to operate this system, the mass of the radiator is still less than 100 kg.

To achieve efficiency, the designer is required to frame the radiator in a lightweight deployable structure and to provide a means of aiming the droplets precisely so that they can be captured and returned to the system. However, present indications are that the droplet accuracies required (milliradians) are easily met by available technology. Recently, successful droplet capture in simulated 0 g conditions has been adequately demonstrated. An advantage of a liquid droplet radiator is that even a relatively large sheet of such droplets is essentially invulnerable to micrometeoroids, since a striking micrometeoroid can remove at most only a few drops.

The reader may be concerned that the very large surface area of the liquid will lead to immediate evaporation. However, liquids have recently been found that in the range of 300 to 900 K have a vapor pressure so low that the evaporation loss during the normal lifetime of a space system (possibly as long as 30 years) will be only a small fraction of the total mass of the radiator.

Thus, the liquid droplet radiator appears promising, particularly as a low temperature system where a large radiator is required.

Liquid droplet radiators for applications other than 0 g have been suggested. For example, in the lunar environment fluids with low vapor pressures can be used effectively as large area heat dissipation systems for relatively large-scale power plants. We may well imagine that such a system will take on the appearance of a decorative fountain, in which the fluid is sprayed upward and outward to cover as large an area as possible. It would be collected by a simple pool beneath and returned to the system. Such a system would be of particular advantage in the lunar environment if low mass, low vapor pressure fluids could be obtained from indigenous materials. Droplet control and aiming would no longer be as critical as in the space environment; however, the system would need to be shaded from the Sun when it is in operation. While this system is far less developed than the systems previously discussed, its promise is so high that it warrants serious consideration for future use, particularly in response to our growing needs for improved power management.

Belt radiator concepts: The belt radiator concept is a modification of the liquid droplet concept in which an ultrathin solid surface is coated with a very low vapor pressure liquid (see fig. 39). While the surface-to-volume ratio is not limited in the same fashion as for a cylindrical heat pipe, it does not quite match that of the liquid droplet radiator. However, this system avoids the problem of droplet capture by carrying the liquid along a continuous belt by surface tension. The liquid plays a double role in this system by acting not only as the radiator but also as the thermal contact which picks up the heat directly from a heat transfer drum. Variations on this scheme, in which the belt is replaced by a thin rotating disk, are also feasible but have yet to be fully assessed.

From Thermal Management in Space by Abe Hertzberg Collected in Space Resources NASA SP-509 vol 2

The Glow

What color will the radiators glow? A practical one will only glow dull red. You can use the Blackbody Spectrum Viewer to see what temperature corresponds to what color. If it was glowing white hot, the temperature would be around 6000 Kelvin. This would be difficult for a solid radiator, since even diamond melts at 4300 degrees K.

Optimum Radiator Temperature

Here is some scary math about radiators from Dr. Tony Valle and Ray Robinson, along with some interesting conclusion. Remember that according to the radiator equation the hotter temperature the radiator is run at, the more waste heat it can dispose of.

RocketCat sez

Their "interesting conclusion" is that ya don't wanna design your heat radiator to run at 100% efficiency or the blasted thing will be huge, unwieldy, and bloated with penalty mass. Remember every gram counts!

For the sweet spot between maximum efficiency and minimum mass, design the radiator temperature to run between 3/5 and 3/4 of the power plant's hot end. But if you don't wanna take my word for it, feel free to dive into the following scary math.

It is surprising but there is an optimum temperature ratio at which to run a starship heat exchanger (or similar power source) to achieve maximum free power with a minimum of radiator area. The only assumptions necesary are that the power source obeys the laws of thermodynamics and that the starship may only get rid of waste heat by radiating.

Let us assume that we have a heat engine as a power source with a relative efficiency of η, and an absolute efficiency is η times the Carnot efficiency ε. We can write the available free power, F, as:

F = Qηε = Qη(1 - T1/T2)

where Q is the rate of heat flow into the exchanger and T1 and T2 are the temperatures of the cold and hot sides of the engine, respectively. The waste heat, H, released into the starship is Q - F, or:

H = Q(1 - η + ηT1/T2)
H = F (1 - η + ηT1/T2)/η(1 - T1/T2)

To simplify, we will measure temperature in units of T2 and let T1 be called just T. After dividing through by η the amount of waste heat associated with a given free power F is then:

H = F (η-1 - 1 + T) / (1 -T)

Now this waste heat must be radiated away from the ship. The power radiated by a black body at temperature T and with area A is given by the Stephan-Boltzmann Law:

P = AσT4

with σ a constant depending on the choice of units. Setting these equal to each other gives:

A = F (η-1 - 1 + T) / σ(T4 - T5)

Now we can ask what value of T will give the minimum radiator area. Taking the derivative of A with respect to T and setting it equal to zero gives:

(T4 - T5) - (4T3 - 5T4)(η-1 - 1 + T) = 0

Or, dividing by T3 and expanding:

T - T2 - 4η-1 - 4 + 4T + 5Tη-1 - 5T + 5T2 = 0

After collecting terms, we have:

4T2 + (5η-1 - 8)T + 4(η-1 - 1) = 0

or, dividing through by 4:

T2 + (5/4η-1 - 2)T + (η-1 - 1) = 0

We write η-1 as γ then the solution to the above quadratic can be written:

T = 1 - 5/8γ + 1/8 sqrt(25γ2 - 16γ)

In the special case where the exchanger runs at maximum theoretical efficiency, η = γ = 1 and the equation above gives T = 3/4. This means that the cold side of the heat engine is at 75% of the temperature of the hot side.

This is horribly inefficient as a Carnot heat engine goes, but if the radiator temperature drops, the surface area (and thus mass) must increase, because of the T4 behavior of the radiation law — colder radiators dump heat much less efficiently. This function is fairly flat — as η drops from 1 to a more plausible 0.1, T changes from 3/4 to 4/5

Dr. Tony Valle and Ray Robinson's commentary in Attack Vector: Tactical, "Science Behind The Rules: Power and Heat Generation" (2004)

Take a classic space opera warship. Onboard power is generated by one or more fusion reactors. If the overall power is 2 gigawatts, and the efficiency is 90% (a pretty generous estimate, since projections I've seen for MHD power generation are around 60%) then at full power, the reactors create 200MW of waste heat. At these sorts of power levels the waste heat of the crew, computers, coffee makers, etc. can be ignored. If there are energy weapons, assume they too are 90% efficient and use 500MW of power when fired, generating another 50MW waste heat. Lump in a lot of other minor systems and you get something like 300MW total waste heat that has to be gotten rid of at peak.

Where it gets complex, AFAIK, is the question of how hot you can allow the ship's interior to get. Let's assume that the environmental areas have heat pumps that allow them to stay a fair amount cooler than the engineering areas (since they're not generating the majority of the heat to begin with) and there are no low-temperature superconductors and so forth to worry about. If the engines and weapons can operate happily at 150 degrees Celsius, that's 423 degrees Kelvin. So that's our starting point — the coolant (probably liquid sodium or lithium at that temp) gets that hot before it's pumped through the radiators to cool off again, at which point:

Heat lost [watts] = area [m2] * emissivity * 5.67e-8[Stefan-Boltzmann constant] * T4 [degrees Kelvin].

(ed note: Stefan-Boltzmann law again)

If the radiators are perfectly black (emissivity of 1), and the coolant temperature is 423 degrees K, then in order to radiate away 275MW of heat, the radiator needs be about 150,000 square meters in area (of course it's double-sided, so the actual fin(s) only need to be 75,000 m2). That's a square 275 meters on a side, or roughly a large city block, simply to deal with the ship's own waste heat at full power. If the ship needs to radiate away additional heat due to taking in, say, 400MW of energy from an enemy ship's lasers, you'd probably have to double or triple that figure (and make darn sure to keep your fins edge-on to the enemy ship firing at you! :). Of course all this is very crude and assumes perfect efficiency of a number of things (some of which I'm probably unaware of :). In reality you might get 80% of that theoretical performance. Or perhaps less. And the first thing damaged in a battle would probably be the radiators (big, hard to protect).

The structural mass of a large radiator fin could be a substantial fraction of the entire ship's mass, and that slows down the acceleration of the ship, which needs more power for thrust, which gives off more waste heat, and so on... So the idea of using spray wands and droplet coolants is attractive.

OTOH, if you need to keep the whole ship at a comfy temperature like 20C, then it's almost hopeless. The radiating area required is so enormous that high acceleration isn't practical at all (something like half a million m2).

Another alternative is to design the ship to only radiate away normal, routine power levels, and to boil off propellant to deal with peak loads. But that goes through a lot of propellant pretty fast at high power levels. Dreadnaughts become like modern jet fighters — only good for a few minutes of intense combat before the fuel runs out. Once it's gone, you can't crash, but you have to surrender or be boiled...

(ed note: He is assuming that the radiator temperature is 423K. Some of the other estimates were for radiator temperatures of 1600K to 3000K, which would drastically lower the radiator surface area to 800 m2, if double sided it would be about 20 meters square.)

Radiator Types

Use the "Life Support" radiator data for life support and other low-waste-heat management. Use all the others for high-waste-heat management, such as fission/fusion reactors and weapons-grade lasers.

In each radiators Specific Area data table will be listed Heat Cap., Mass, and Op. Temp.

Heat Cap.: heat capacity in kWth/m2. This is how many kilowatts of waste heat each square meter of radiator can get rid of. Multiply the surface area of the entire radiator by the heat capacity to find the total amount of heat the radiator array can handle. kWth means "kilowatts of thermal energy" (i.e., waste heat) as opposed to kWe which means "kilowatts of electricity".

Mass: specific area mass of the radiator in kg/m2. This is the mass of each square meter of radiator in kilograms. Multiply the surface area of the entire radiator by the specific area mass to find the total mass of the radiator array.

Op. Temp.: the operating temperature of the radiator. You probably won't need this unless you want to fool around with the Stefan-Boltzmann equation. The higher the operating temperature, the higher the heat capacity. Which means the value listed for the heat capacity is only valid if the radiator operates at this temperature.

Use the "Specific Area" values in the tables to calculate the radiator mass.

  1. Decide how many kilowatts of waste heat the radiator will have to handle (from the engine, the power reactor, the laser cannon, etc.)
  2. Select which radiator type to use, and examine its Specific Area table.
  3. Divide the total waste head in kilowatts by the Heat Cap. entry of the table to get the square meters of radiator area required.
  4. Multiply the radiator area by the Mass entry to get the total mass of the radiator required.

or in other words:

radiatorMass = (wasteHeat / specificAreaHeat) * specificAreaMass


  • radiatorMass = mass of radiator array (kg)
  • wasteHeat = amount of waste heat to dispose of (kWth)
  • specificAreaHeat = Heat Cap. from radiator table (kWth/m2)
  • specificAreaMass = Mass from radiator table (kg/m2)

An Attack Vector: Tactical Medium Range Laser 2 has an input energy of 2 gigawatts (GW) and an efficiency of 12.5%. This means that 0.25 gigawatts become laser beam and 1.75 gigawatts turn into waste heat. About par for the course for lasers.

So the laser needs a radiator array that can deal with 1.75 gigawatts of thermal energy = 1,750,000 kWth. This is Step 1.

Looking at the list, by far the radiator with the largest heat capacity is the Molybdenum/Lithium Heat pipe: 469 kWth/m2. This is Step 2.

Divide the waste heat of the laser by the heat capacity of the Mo/Li Heat pipe and we get 1,750,000 / 469 = 3,731 square meters of radiator. This is Step 3.

The Mo/Li Heat pipe specific area mass is 150 kg/m2. Muliply the radiator area by the Mass entry and we get 3,731 * 150 = 559,650 kilograms = about 560 metric tons.

Which sounds like a lot, except if you used an ETHER Charged Dust radiator you'd need about 580 metric tons of radiator.

Note that Step 3 calculates the radiation surface of the radiator. If the radiator is layered flat on the ship's hull, the radiation surface is the same as the physical radiator size. However, if the radiator is attached edge on so it extends out as a fin or a wing, the physical radiator size will be one-half the radiation surface. This is because you can use both sides of the physical fin as radiator surface. Yes, even a liquid droplet radiator. This might not apply for some of the stranger radiator designs, but details are scarce.

Having said that, things are complicated for liquid drop radiators. The radiation surface is the surface area of the droplets. Figuring out the physical radiator size is compilcated, you can find the equations here. There is also Eric Rozier's online calculator.

Note, in the illustrations from the High Frontier game, it uses very strange game-specific terms. Each "mass unit" is equal to 40 tonnes, each thermometer is one "therm" and represents the radiator dealing with 120 megawatts of thermal waste heat (120,000 kWth). When a specific area value was missing I uesd the therm, mass points, and radiator area on the cards to calculate.

Here is a table of the various radiator types. Their area and mass has been calculated as if they were sized to handle 250 megawatts of waste heat.

The table is sorted by array mass, so the better ones are at the top. At least if you want the lowest mass radiator. If the radiation area was an issue you'd probably prefer a Mo/Li Heat Pipe instead.

The life support radiator was included even though it was not intended to handle waste heat over 100 kilowatts or so.

Radiator for 250,000 kilowatts waste heat
RadiatorSpecific area
(Heat Cap.)
Specific area
Marangoni Flow293.04 kWth/m224.4 kg/m2853 m220,816 kg
Electrostatic Membrane51.3 kWth/m24.275 kg/m24,873 m220,833 kg
Hula-Hoop300 kWth/m233 kg/m2833 m227,500 kg
Buckytube Filament293.03 kWth/m248.839 kg/m2853 m241,667 kg
Curie Point212.75 kWth/m235.459 kg/m21,175 m241,667 kg
Tin Droplet38.49 kWth/m26.4154 kg/m26,495 m241,669 kg
Flux-Pinned Superthermal76 kWth/m217 kg/m23,289 m255,921 kg
Attack Vector: Tactical357 kWth/m2100 kg/m2700 m270,028 kg
Bubble Membrane21.01 kWth/m27.00 kg/m211,899 m283,294 kg
Mo/Li Heat Pipe453.54 kWth/m2151.18 kg/m2551 m283,333 kg
Microtube Array102.6 kWth/m234.2 kg/m22,437 m283,333 kg
ETHER212.75 kWth/m270.92 kg/m21,175 m283,337 kg
Ti/K Heat Pipe150.22 kWth/m2100.14 kg/m21,664 m2166,656 kg
SS/NaK Pumped90.83 kWth/m260.554 kg/m22,752 m2166,669 kg
Salt-Cooled Reflux tube75 kWth/m275 kg/m23,333 m2250,000 kg
Life Support0.19 kWth/m23.1 kg/m21,315,789 m24,078,947 kg

Life Support

Specific Area
Heat Cap.~0.19 kWth/m2
Mass~3.1 kg/m2
Op. Temp.? K

Technically you also need radiators to keep the life-system habitable. Human bodies produce an amazing amount of heat. Even so, the life-system radiator should be small enough to be placed over part of the hull, since life-support waste heat is quite tiny compared to nuclear reactor or gigawatt laser waste heat.

Use this radiator type for life-support and other modest waste heat management. Use the other radiators for gigantic waste-heat producers.

The life-system radiators on the Space Shuttle are inside the cargo bay doors, which is why the doors are always open while the shuttle is in space.

Troy Campbell pointed me at a fascinating NASA report about spacecraft design. In the sample design given in the report, the spacecraft habitat module carried six crew members, and needed life-system heat radiators capable of collecting and rejecting 15 kilowatts of heat (15 kW is the power consumption for all the systems included in the example habitat module). The radiator was one-sided (basically layered over the hull). It required a radiating surface area of 78 m2, had a mass of 243.8 kg, and a volume of 1.742 m3. It used 34.4 kg of propylene glycol/water coolant as a working fluid. In addition to the radiator proper, there was the internal and external plumbing. The Internal Temperature Control System (coldplates, heat exchangers, and plumbing located inside the habitat module) had a mass of 111 kg and a volume of 0.158 m3. The External Temperature Control System had a mass of 131 kg, a volume of 0.129 m3, and consumes 1.109 kilowatts.

Simple math tells me the radiator has a density of about 140 kg/m3, a specific area of 3.1 kg/m2 and needs a radiating surface area of about 5.2 m2 per per kilowatt of heat handled (1/5.2 = 0.19 kWth/m2). The entire system requires about 35 kg per kilowatt of heat handled, and 0.13 m3 per kilowatt of heat. But treat these numbers with suspicion, I am making the assumption that these things scale linearly.

Liquid Droplet

Liquid Droplet Radiators use sprays of hot droplets instead of tubes filled with hot liquid in the radiator. This drastically reduces the mass of the radiator, which is always a good thing. A NASA report suggested that for 200 kW worth of waste heat you'd need a 3,500 kg heat pipe radiator, but you could manage the same thermal load with a smaller 500 kg liquid droplet radiator.

The droplet generator typically has 100,000 to 1,000,000 orifices with diameters of 50 to 20 μm. They are a bit more susceptible to damage than the components of more conventional radiators.

A drawback is that the spray is in free fall. This means if the radiator is operating and the ship starts accelerating, the spray will start missing the collector and precious radiator working fluid will be lost into space. Brookhaven National Laboratory has patented a way to magnetically focus the droplet stream. Using a large radiator it will allow the spacecraft to maneuver at acceleration of up to 0.001 g (0.00981 m/s2) which is barely an improvement. The acceleration can be increased but only if the single radiator is replaced by numerous smaller radiators. Which of course makes the sum of the radiators have a larger mass than the single large radiator. Oh, and Brookhaven's patent expired in 1994.

Many liquid droplet designs are well suited for warships, since they do not utilze large fragile panels vulnerable to hostile weapons fire. If a rail gun round or laser bolt passes through a spray of working fluid, it will just make a bit of fluid miss the collector. If weapons fire passes through a conventional panel it will wreck it.

temperature rangecoolant typeexample
250 K – 350 Ksilicone oils
370 K – 650 Kliquid metal eutectics
500 K – 1000 Kliquid tin
Rectangular LDR

Rectangular LDRs have collectors the same width at the droplet generator. The droplet density remains constant across the flight path. It is a simpler more robust design than a Triangular LDR, and has a larger radiating surface (twice the surface area). However the Triangular LDR is lighter (40% less massive) due to its smaller collector.

Triangular LDR

Triangular LDRs have a tiny collector a fraction of the width of the droplet generator. The droplet density increases across the flight path. It is 40% less massive compared to a comparable Rectangular LDR due to the smaller collector. However it is a more complicated design with more failure points, and it has only half the surface area of a same sized Rectangular LDR.

For reasons that have not been made clear to me, Triangular LDR is currently the focus of much of the research and development. NASA likes them better than Rectangular LDRs.

Eric Rozier has an online calculator for droplet radiators here, and for coolant systems in general here. He had this analysis:

Given that the main thing we want to determine is the surface area of the lithium droplets to calculate the heat it can radiate, I decided to build a model of the surface area.

Since no such radiator has been built we have to work with some plausible model data. To model the lithium drops themselves I dug into some meteorological data and found that raindrops typically range in size from 1mm to 3mm, sounds pretty reasonable. Assuming droplets are spherical (a reasonable assumption in zero gravity) then the surface area of any given droplet is of course 4*π*r2.

Working off the wedge based idea you cited here. We then model the full radiating body of the droplets as a triangle, reducing the emitter to a point source for simplification. I'm not sure how space out the droplets should be, but I figure if the distance between any two droplets is roughly twice the radius, the model is probably pretty conservative. Thus for an emitter with distance h from the emitter to the collector, and a collector plate of length h, we get the number of droplets suspended between them to be:

(0.5 * b * h)/(16r2)

We can then model the surface area of the lithium droplets as:

(0.5 * b * h)/(16r2) * 4*π*r2

If you want to modify the spacing of the drops, you can change the inter-droplet gap to q instead of r, rendering the following equation:

(0.5 * b * h)/(4r2 + 4r*q + q2) * 4*π*r2

Eric Rozier

So the equations are:

a = (0.5*b*h) / (16*r2) * 4*π*r2

a = (0.5*b*h) / (4*r2 + 4*r*q + q2) * 4*π*r2


  • a = surface area of lithium droplets in radiator surface
  • b = length of base of radiator triangle
  • h = length of height of radiator triangle
  • r = radius of indiviual droplet
  • q = inter-droplet gap

Liquid-droplet radiators are also a possibility. There do exist liquids which have extremely low vapor pressure at high temperatures — certain organics up to ~600K, liquid metals (esp. lithium) to ~1500K. Using a carefully-designed nozzle to create a fan-shaped spray of fine droplets towards a linear collector results in a very efficient radiator, with minimal weight per unit radiating surface, high temperature, and high throughput.

The radiator would be essentially triangular when "deployed", with the spray nozzle at one vertex and the collector along the opposite side. If the nozzle-vertes is adjacent to the ship body, the collector "arm" will have to extend outwards. Alternately, the collector can be run along the side of the ship, and the spray nozzle extended on a boom and aimed inwards. A series of closely-spaced, narrow-angle nozzles would approximate a rectangular array.

There is always some loss of coolant due to evaporation in vacuum, hence use of liquids with extremely low vapor pressure. You also lose coolant if such a radiator is run under acceleration, unless the collector is over-long and aligned parallel to the thrust axis, which imposes a constraint on system geometry. You also lose coolant if the radiator "panel" is hit by enemy weapons fire; on the other hand there is no mechanical damage unless the much smaller nozzle or collector arms are hit. Bottom line — you'll need a small surplus of coolant, unless you are running a warship, in which case you'll need a large surplus.

If liquid metal is used as the coolant, MHD pumping can be used at the collector arms, resulting in a simplified design with no moving parts. Indeed, in such a case the coolant could also be used as the working fluid in an MHD generator, resulting in a single-fluid, single-cycle power system from primary energy generation to waste heat radiation. Again, a simple, efficient design with no moving parts.

Specific Area
Heat Cap.~38 kWth/m2
Mass6.4 kg/m2
Op. Temp.1030 K

Tin droplet radiator

Atomization increases the surface area with which a fluid can lose heat. A hot working fluid sprayed into space as fine streams of sub-millimeter drops readily loses heat by radiation. The cooled droplets are recaptured and recycled back into the heat exchanger. If tin (Sn) is used as a working fluid, the kilos per power radiated is minimized, using a heat rejection temperature of 1030 K and a total power in the megawatt range (comparable to the game value of heat rejection of 120 MWth per therm). The low emissivity of liquid tin (0.043) is increased by mixing in carbon black, which distributes itself on the surface of the droplet. Evaporation losses are avoided by enclosing the radiator in a 1 μm plastic film, which transmits radiation in the 2 to 20 μm (IR) range. Such a film would continue to perform its function even if repeatedly punctured by micrometeoroids. The illustration shows a triangular liquid droplet geometry. The collector, located at the convergence point of the droplet sheet, employs centrifugal force to capture the droplets. The total specific area is 6.4 kg/m2.

K. Alan White, "Liquid Dropbt Radiator Devebpment Status," Lewis Research Center, 1987

From High Frontier by Philip Eklund
Spiral LDR
Specific Area
Heat Cap.~300 kWth/m2
Mass35 kg/m2
Op. Temp.1200 K

Curie point radiator

A ferromagnetic material heated above its Curie point loses its magnetism. If molten droplets of such a substance are slung into space, they radiate heat and solidify. Once below their Curie temperature, they regain their magnetic properties and can be shepherded by a magnetic field into a collector and returned to the heat exchanger. A 120 MW system operating at 1200 K includes a 13 tonne magnetic heat exchanger and a rotating dust recovery electromagnet on a 25-meter boom, plus 7 tonnes of dust spread in a spiraling disk 27-meters in diameter (35 kg/m2). The usual medium is iron dust, which has a Curie point of 1043 K and is easily scavenged by magnetic beneficiation from regolith.

M.D. Carelli, 1989

From High Frontier by Philip Eklund
Enclosed Disk LDR
Specific Area
Heat Cap.51 kWth/m2
Mass4.3 kg/m2
Op. Temp.1000 K

Electrostatic membrane radiator

This heat-rejection concept, also called a liquid-sheet radiator, encloses radiating liquid within a transparent envelope. It consists of a spinning membrane disk inflated by low gas pressure, with electrostatically-driven coolant circulating on its interior surfaces. The liquid coolant is only 300 μm thick and has an optical emissivity of 0.85 at a temperature of 1000 K. An electric field is used to lower the pressure under the film of coolant, so that leakage through a puncture in the membrane wall is avoided. The membrane has a specific area of 4.3 kg/m2 and 51 kWth/m2.

Shlomo Pfeiffer of Grumman, 1989

From High Frontier by Philip Eklund
Specific Area
Heat Cap.213 kWth/m2
Mass71 kg/m2
Op. Temp.1200 K

ETHER charged dust radiator

To avoid the evaporation losses suffered by radiators that use liquid droplets in space, dust radiators use solid dust particles instead. If the particles are electrostatically charged, as in an electrostatic thermal radiator (ETHER), they are confined by the field lines between a charged generator and its collector. If the spacecraft is charged opposite to the charge on the particles, they execute an elliptical orbit, radiating at 1200 K with a specific area of 71 kg/m2 and 213 kWth/m2. The dust particles are charged to 10-14 coulombs to inhibit neutralization from the solar wind.

Prenger 1982

From High Frontier by Philip Eklund

Heat Pipe

Mo/Li Heat Pipe
Specific Area
Heat Cap.~469 kWth/m2
Mass150 kg/m2
Op. Temp.1450 K

Mo/Li heat pipe radiator

A heat pipe quickly transfers heat from one point to another. Inside the sealed pipe, at the hot interface a two-phase working fluid turns to vapor and the gas naturally flows and condenses on the cold interface. The liquid is moved by capillary action through a wick back to the hot interface to evaporate again and repeat the cycle. For high temperature applications, the working fluid is often lithium, the soft silver-white element that is the lightest known metal. Molybdenum heat pipes containing lithium can operate at the white-hot temperatures of 1450 K, and transfer heat energy at 240,000 kWth/m2, almost four times that of the surface of the sun. The specific area is 150 kg/m2.

David Poston, Institute for Space and Nuclear Power Studies at the University of New Mexico, 2000

From High Frontier by Philip Eklund
Ti/K Heat Pipe
Specific Area
Heat Cap.~153 kWth/m2
Mass100 kg/m2
Op. Temp.1100 K

Ti/K heat pipe radiator

A Rankine evaporation-condensation cycle heat pipe uses metal vapor as the coolant, which is liquefied as it passes through a heat exchanger connected to the radiator. A liquid metal near the liquid/vapor transition is able to radiate heat at a nearly constant temperature. The pipe is made from SiC-reinforced titanium (Ti) or superalloy operating at up to 1100 K, and the working fluid is potassium (K). The pipe is covered with a lightweight thermally-conductive carbon foam, which protects the pipe from space debris and transfers heat to the radiating fins. The total specific area is 100 kg/m2.

From High Frontier by Philip Eklund

Bubble Membrane

Specific Area
Heat Cap.~21 kWth/m2
Mass7 kg/m2
Op. Temp.800 K

Bubble membrane radiator

This high-temperature concept uses a spinning bubble-shaped membrane to reject waste heat. A two-phase working fluid (hot liquid or gas) is centrifugally pumped and sprayed on the interior surface of the bubble. The fluid wets the inner surface of the sphere and is driven in the form of a liquid film by centrifugal force to the equatorial periphery of the sphere. As the liquid flows along the inner surface of the envelope it loses heat by thermal radiation from the outer surface of the balloon. The use of membranes woven from space-produced carbon nanotubes and cermet fabrics offers a specific area of 7 kg/m2, radiating from one side at 800 K. Liquid metal pumps return the liquid out of the sphere through rotated shaft seals to its source.

Koenig, 1985

From High Frontier by Philip Eklund

Buckytube Filament

Specific Area
Heat Cap.~300 kWth/m2
Mass~100 kg/m2
Op. Temp.1300 K

Buckytube filament radiator

Waste heat may be rejected by moving thousands of loops of thin (1 mm) flexible "Buckytubes" (carbon nanotubes), which radiate their thermal load prior to return to the heat exchanger. Cables constructed of Arm-chair type nanotubes are the strongest cables known, with design tensile strengths about 70% of the theoretical 100 GPa value. The moving filaments are heated by direct contact around a molybdenum drum filled with the heated working fluid, and then extended into space a distance of 70m by rotational inertia. Their speed is varied according to the temperature radiated (from 273 K to 1300 K). The loops are redundantly braided to prevent single point failures from micrometeoroids. Each element is heat treated at 3300 K to increase the thermal conductivity through graphitization to about 2500 W/mK.

Richard J, Flaherty, "Heat-transfer and Weight Aialysis Of a Moving-Belt Radiator System for Waste Rejection in Space", Lewis Research Center, Cleveland, Ohio, 1964.

From High Frontier by Philip Eklund

Flux-Pinned Superthermal

Specific Area
Heat Cap.76 kWth/m2
Mass17 kg/m2
Op. Temp.928 K

Flux-pinned superthermal radiator

Variable configuration radiators take advantage of the surprising physics of high-temperature flux-pinning superconductors. These materials resist being moved within magnetic fields, allowing stable formations of elements. No power or active feedback control is necessary. The radiating elements fly in a flux-pinned formation, not physically touching, but connected by superthermal ribbon. Superthermal compounds hypothetically conduct heat as effortlessly as superconducting materials conduct electricity. The radiating surfaces are graphite foams, which have both a high emissivity (0.9) and a high thermal conductivity (1950 W/m°K) if the heat conducts in a direction parallel to the crystal layers. Operating at 928K, the superthermal radiator has a specific area of 17 kg/m2 and 76 kWth/m2.

Dr. Mason Peck, 2005

From High Frontier by Philip Eklund


Specific Area
Heat Cap.300 kWth/m2
Mass33 kg/m2
Op. Temp.1300 K

Hula-Hoop radiator

By imparting heat to twin washer-shaped disks by direct conduction, the Hula-Hoop radiator avoids the diseconomies of scale that plague fluid radiators. Furthermore, they are robust against micrometeoroid strikes and hostile attack. The two hoop are 100-meters in diameter. They are made of braided cermets coated with graphite, and lubricated in a heat exchanger with tungsten disulfide (WS2). Radiating at 1300 K, each has a specific area of 33 kg/m2 and 300 kWth/m2.

This design is a Philip Eklund original, published here for the frst time.

From High Frontier by Philip Eklund

Marangoni Flow

Specific Area
Heat Cap.~300 kWth/m2
Mass24 kg/m2
Op. Temp.1300 K

Marangoni flow radiator

In zero-g, a surface tension gradient can create a heat pump with no moving parts, or drive micro-refining processes. This phenomena, called Marangoni flow, moves fluid from an area of high surface tension to one of low surface tension. Bubbles operating at 1300 K have a specific area of 24 kg/m2.

G. Harry Stine, "The Third Industrial Revolution," 1979

From High Frontier by Philip Eklund

Microtube Array

Specific Area
Heat Cap.~104 kWth/m2
Mass34 kg/m2
Op. Temp.1000 K

Microtube array radiator

Nanofacturing techniques can fabricate large, parallel arrays of microtubes for high performance radiators. The radiating surface comprises a heavily-oxidized, metal alloy with a 100 nm film of corrosion resistant, refractory platinum alloy deposited on it. The working fluid is hydrogen, which has low pumping losses and the highest specific heat of all materials. This fluid is circulated at 0.1 to 1 MPa through the microtubes, and the heat radiates through the thin (0.2 mm) walls. This allows a specific area of 34 kg/m2, including the hydrogen. The rejection temperature for titanium alloy tubes is from 200 K up to 1000 K, if a high temperature barrier against hydrogen diffusion is used. High speed leak detection capability and isolating valves under independent microprocessor control provide puncture survivability.

F. David Doty, Gregory Hosford and Jonathan B. Spitzmesser, "The Microtube-Strip Heat Exchanger," 1990

From High Frontier by Philip Eklund

Salt-Cooled Reflux

Specific Area
Heat Cap.~75 kWth/m2
Mass75 kg/m2
Op. Temp.1100 K

Salt-cooled reflux tube radiator

In contrast to a heat pipe, that uses capillary action to return the working fluid, a reflux tube uses centrifugal acceleration. This design is more survivable than heat pipes, especially when overwrapped with a high-temperature carbon-carbon composite fabric. Unlike metals, the strength of these composites increases up to temperatures of ~2300K. However, they degrade when subjected to high radiation levels. The working fluid is molten fluoride salts, the only coolant (other than noble gases) compatible with carbon-based materials. Radiating at 1100 K, this radiator has a specific area of 75 kg/m2.

Charles W, Forsberg, Oak Ridge National Laboratory, Proceedings of the Space Nuclear Conference 2005, San Diego, California, June 5-9, 2005.

From High Frontier by Philip Eklund

SS/NaK Pumped Loop

Specific Area
Heat Cap.~93 kWth/m2
Mass61 kg/m2
Op. Temp.970 K

SS/NaK pumped loop radiator

A Rankine evaporation-condensation cycle exchanges heat using a liquid metal as a coolant, which is vaporized as it passes through a heat exchanger connected to the radiator. A liquid metal near the liquid/vapor transition is able to radiate heat at a nearly constant temperature. The usual medium is sodium (Na) or sodium-potassium (NaK), which has a saturation temperature of nearly 1200 K at 1.05 atm. The plumbing is stainless steel (SS) tubes operating at up to 970 K with an emissivity of 0.9. The tube wall is half a millimeter thick to guard against meteoroid-puncture, and each pipe is an independent element so that a single puncture does not cause overall system failure. Molecular beam cameras on long struts scan for meteoroid leaks, which are plugged with pop rivets installed by a tube crawler. Radiating at 970 K from both sides, this radiator has a specific area of 61 kg/m2, including fluid and heat exchanger.

J. Ca/ogeras, NASA/LaRC, 1990.

From High Frontier by Philip Eklund

Attack Vector: Tactical

This fictional radiaor is from the tabletop wargame Attack Vector: Tactical, which is why the description talks about weird units like "power points" and "heat points."

  • One game turn segment is 16 seconds.
  • One power point is 1000 megajoules delivered in 1 segment.
  • So a starship reactor that outputs 1 power point produces at a rate of 1000 MJ / 16 seconds = 62.5 megawatts.
  • 1 heat point is 250 megawatts.
  • 1 hull space holds 20 metric tons.)
Science Behind The Rules: Radiators and Heat Sinks
Specific Area
Heat Cap.357 kWth/m2
Mass100 kg/m2
Op. Temp.1600 K
(1 - 0.1)

Knowing that our reactors produce 62.5 MW as a base power unit, and using the proof at right, we get an efficiency of 4 J of waste heat per J of power generated. This tells us that wee need to radiate ~250 MW per point of power. The Stefan-Boltzman law states that the surface emits power at a rate of (1-A) * 5.67×108 Wm2 K4 * T4 where A is the albedo, and T the absolute temperature in Kelvins. With an albedo of 0.1, a temperature of 1600K, and 250 MW of output, we need 700 square meters of radiating surface. Extending as a fin, radiating from both sides, this is roughly 18 meters square. At roughly 0.3m thick, and flexible enough to be retracted and extended, we get something that's reasonably 70 tons, or a bit shy of 3 hull spaces. For the sake of game play, one hull space of radiators dissipates 100 MW, or 0.4 heat points.

A civilian (starship) reactor has a built in 16 meter by 16 meter radiator that dissipates its waste heat; this radiator is built into the hull of the ship. This is why civilian reactors require part of the hull of the ship to be unarmored.

Storing the heat before radiation requires a heat sink. A sodium heat sink is ~21.5 cubic meters of sodium, with a density of 0.968 tons per cubic meter. Sodium has a thermal capacity of 28.2 J/mole/K. A mole of sodium weighs 22.98 grams. One gram of sodium absorbs 2.82/22.98 = 1.22 J per K of heat increase. A heat sink of sodium weighs 20.82 tons, raising that mass by 1 K absorbs 25.4 MJ. Sodium melts at 372 K and vaporizes at 1252 K. Pressurized, it remains liquid to 1600 K, our radiator temperature. Assuming a working range of 1300K (room temperature to 1600 K), each heat sink stores 1300 * 25.4 MJ = 33.02 GJ, which is one heat point, assuming other inefficiencies.

Lithium's thermal capacity of 24.8 J/mole/K and molar weight of 6.94 allows 1 gram to absorb 3.57 J per K of heat increase, or 2.92× the heat capacity of sodium. By using 22 tons of lithium, we get triple the capacity of the sodium heat sink.

Water's thermal capacity is 4.186 J/gram/K, 3.426 times that of sodium. Room temperature to boiling is ~85 K, which limits its usefulness. Raising 1 ton of water by 85 K takes 355.88 MJ. One heat point is 33 GJ, and the amount of water needed to store one heat point is 33 GJ/350.88 MJ = 93 tons. Including the extra mass for plumbing, that's 5 hull spaces all told.

Vaporizing water takes 2256 J/g, 6.3× the energy needed to raise it by 85 K. Because the vaporization is not quite perfect, we treat it as 6 heat points removed when the heat sink is vented. The liquid metal heat sinks aren't vented, as vaporized metal would deposit on the sensors of the ship.

From Attack Vector: Tactical Core Rulebook by Burnside, Finley, and Valle (2004)

Discovery XD-1

The spherical pressure hull formed the head of a flimsy, arrow-shaped structure more than a hundred yards long. Discovery, like all vehicles intended for deep space penetration, was too fragile and unstreamlined ever to enter an atmosphere, or to defy the full gravitational field of any planet. She had been assembled in orbit around the Earth, tested on a translunar maiden flight, and finally checked out in orbit above the Moon.

She was a creature of pure space - and she looked it. Immediately behind the pressure hull was grouped a cluster of four large liquid hydrogen tanks - and beyond them, forming a long, slender V, were the radiating fins that dissipated the waste heat of the nuclear reactor. Veined with a delicate tracery of pipes for the cooling fluid, they looked like the wings of some vast dragonfly, and from certain angles gave Discovery a fleeting resemblance to an old-time sailing ship,

At the very end of the V, three hundred feet from the crew-compartment, was the shielded inferno of the reactor, and the complex of focusing electrodes through which emerged the incandescent star-stuff of the plasma drive. This had done its work weeks ago, forcing Discovery out of her parking orbit round the Moon. Now the reactor was merely ticking over as it generated electrical power for the ship's services, and the great radiating fins, that would glow cherry red when Discovery was accelerating under maximum thrust, were dark and cool.

From 2001 A Space Odyssey by Sir Arthur C. Clarke (1969)

The final decision was made on the basis of aesthetics rather than technology; we wanted Discovery to look strange yet plausible, futuristic but not fantastic. Eventually we settled on the plasma drive, though I must confess that there was a little cheating. Any nuclear-powered vehicle must have large radiating surfaces to get rid of the excess heat generated by the reactors — but this would make Discovery look somewhat odd. Our audiences already had enough to puzzle about; we didn’t want them to spend half the picture wondering why spaceships should have wings. So the radiators came off.

From Lost Worlds of 2001 by Sir Arthur C. Clarke (1972)


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

The section of the spacecraft that the crew lives and works in is called the Habitat Module (Larry Niven calls it a "Lifesystem"). It is pressurized with a breathable atmosphere, and protects the crew from extremes of temperature and from radiation. Unlike spacecraft in TV and movies, most of a spacecraft is not pressurized. The vast majority of the ship is composed of the propellant tanks, rocket engine, and power plant. The habitat module is sort of tucked into some convenient corner.

Because every cubic meter of habitat module has to be pressurized and protected from the space environment, interior volume will be at a premium. Due to mass constraints, spacecraft designers will have no choice but to minimize the volume. Which will of course make them very cramped.


The TransHab concept was a NASA project to create an inflatable space station, which is not quite as insane as one would think. The walls include layers of Kevlar, and are probably harder to puncture than the metal walls of the International Space Station. The private company Bigelow Aerospace has purchased the rights to TransHab patents, and is in the process of developing a commercial space station. Bigelow already has launched two prototypes into orbit and they are working just fine.

The standard TransHab module had a mass of 34,050 kilograms (34 metric tons), an inflated volume of 350 cubic meters, an inflated diameter of 8.2 meters, four levels, and could support a crew of six for about eighteen months.

In the Mars Reference Mission, they had a bimodal nuclear thermal rocket on a Mars mission. The rocket could deliver the mission to Mars, come back to approach Earth but with dry propellant tanks. So the rocket would go sailing past Earth into the abyss while the crew bailed out to be rescued. Bye-bye rocket.

However, if you replaced the relatively massive hard-shell habitat module with a lightweight inflatable TransHab module, the increase in delta-V was enough so that the rocket would have enough extra delta-V to be able to brake into Earth orbit and be re-used.

There is an online calculator for TransHab modules here.

Troy Campbell pointed me at a fascinating NASA report about spacecraft design. The report shows how much easier it is to design a habitat module if it for a one gravity environment instead of free fall (surprise, surprise). It has the spacecraft separate into two parts connected by tethers, spinning for artificial gravity.

Some of the details of this design cannot be used with, say, a warship. You do not want to used an inflatable habitat module on a ship going into battle. But the lists of required equipment are very useful for your ship designs, as are their masses, volumes, and power requirements.

For its habitat module, the report take a TransHab inflatable habitat module, and modifies it for one gravity. TransHab modules are low mass since the walls are made of woven Kevlar instead of metal. For the report design, interior suspension cables are added to support the decks (since the basic TransHab is designed for free fall), and an anti-radiation storm cellar added to the core. The other main reason for using a TransHab is because the proposed launch vehicles used to boost the module into orbit had severe payload size limits. The TransHab could fit into the limits while collapsed, then inflated to full size when in space. For your design, you probably will not have such payload size limits, so you will not need to use an inflatable habitat.

To cool off the module, a small heat radiator is wrapped around the exterior. This radiator can only collect and reject 15 kilowatts of heat, since it is only for life support. The propulsion system and power system will require a much larger radiator (read the report for more details).

The report gave a sample set of deck plans. The first floor is the lowest, at the 1.03g level. For some odd reason the first floor deck plan is rotated 45 degrees counterclockwise with respect to the other two deck plans, as you can see if you try to match up the ladder and pass throughs on the three plans.

Note how all the crew beds are inside the storm cellar.

The module is designed to house a crew of six for eighteen months. According to the report, the bare minimum internal volume for a crew of six is 101 cubic meters (about 17 m3 per crewperson). This design has more than that. The TransHab has 350 cubic meters of internal volume, and of that 193 is habitable (about 32 m3 per crewperson). Please note that this is the total habitable volume, the crew's personal volume is much smaller (basically their bunk and their desk).

The module has an exterior surface area of 233 m2. Just the cylindrical exterior surface has an area of 153 m2.

Again remember that this is for a crew of six and an endurance of eighteen months. The values for mass and volume of all the components will have to be scaled up or down with the size of the crew and the amount of endurance.

SystemMass (kg)Stowed Vol. (m3)
Power System150517.98
Battery System4850.44
Power Management and Distribution6251.05
Voice Peripherals40.01
Attitude Initialization60.01
Displays & Controls140.01
Environmental Control & Life Support503031.50
Atmosphere Control11334.67
Atmosphere Revitalization10213.25
Temperature and Humidity Control1136.32
Fire Detection and Suppression130.05
Water Recovery and Management21996.02
Waste Management55011.19
Thermal Control System5762.43
Internal Thermal Control System1350.34
External Thermal Control System1670.13
Crew Accommodations1198991.03
Galley and Food System806331.35
Waste Collection System3278.83
Personal Hygeine2835.00
Recreational Equipment and Personal Stowage1503.00
Operational Supplies and Restraints1200.01
Sleep Accommodations1202.82
EVA Systems161316.29
Space Suits6904.15
Vehicle Support for EVA2910.40
EVA Translation Aids1233.36
EVA Tools1320.20
Structure and Mechanism1294184.51
Fixed Elements50682.55
Deployed Elements787381.96
Med Ops10486.17
Human Research Facility2892.50
Crew Health Care Systems7593.67

From the report (which goes into this in much greater detail):

Power System

Mass (kg)Stowed Vol. (m3)Quantity
Secondary Power
Fiber Li-Ion Battery0.173351
Battery Charge/Discharge Unit0.09503
Main Bus Cable0.847.53
Jumper Cables0.424.524
Secondary Power Distribution
Wiring Harness Secondary
Support Structure
Power Management and Distribution
Galaxy Inverter Boxes0.04283
Custom Built 400 Hz, 115 Vac
Kilovac Relays0.001245
Unitron PS-95-448-1 400 Hz
to 60 Hz Frequency Converter
Vikor AC/DC Rectifiers0.000729

The primary power system for the spacecraft is a pair of nuclear reactors on the other end of the boom. Since they are external to the habitat module, their mass and volume are not included here.

The secondary power system is internal to the module. It consists of three main subsystems:

  1. Secondary Power
  2. Wiring
  3. Power Management and Distribution

These three subsystems can be further broken down to the component level as shown in the table to the right.

The assumption was made that the power entering the habitat would be 115 Vac, delivered at 400 Hz. A final assumption that was made was that the habitat would nominally use 15 kW of power. The final subsystem that needed to be sized for this habitat was the secondary power source. Upon analyzing the architecture and the type of primary power sources, a decision was made to supply 24 hours of emergency power to the habitat that will accommodate 50% of the nominal load (180 kW-h).


Includes a communication system; a guidance, navigation and control system; a crew interface system; and an integrated vehicle management system. It has a peak power consumption of 864 watts. It provides for the command, control, communications, and computation required for the carrying out the mission including insertion into transit orbits. This involves provisions for crew displays; data, voice, and video communications home base, other orbital assets, and EVA crewmembers; an integrated health management system for onboard and ground monitoring of all systems; and a full flight system capability for Guidance, Navigation, and Control. The flight system must also integrate requirements for data communication and computational support for remote commanding of the spacecraft during any uncrewed phase as well as ground commanding during crewed phases. The crew interface must be integrated with data communications and computational support for remote commanding of visiting vehicles.

Environmental Control and Life Support System

The Air Management Subsystem is characterized by a 4-Bed Molecular Sieve (217.7 kg, 0.6 m3, 733.9 W), a Sabatier CO2 Reduction Unit (26 kg, 0.01 m3, 227.4 W), an Oxygen Generation Subsystem (501 kg, 2.36 m3, 4,003 W), and high-pressure storage tanks for O2 (20.4 kg, 0.78 m3, 6 W) and N2 (94.4 kg, 3.6 m3, 6 W). The Water Management Subsystem uses a Vapor Phase Catalytic Ammonia Removal system (1,119 kg, 5.5 m3, 6,090.7 W) and potable water storage tanks (145.9 kg, 0.54 m3, 5 W). The Waste Management Subsystem uses a Warm Air Dryer (527.2 kg, 11.2 m3, 2,043.7 W).

Thermal Control System

Fluid mass (kg)Dry mass (kg)Volume (m3)Power (kw)
Internal TCS0.0111.00.1580.000
External TCS34.4131.00.1291.109

The TCS system concept makes use of flexible lightweight body mounted radiators, which are attached to the outer surface. The TCS system has been sized to collect and reject 15.0 kW of heat. Mass, power, and volume are listed below. ITCS refers to coldplates, heat exchangers, and plumbing located inside Transhab, while ETCS refers to similar equipment mounted on the outside. Radiators are listed separately.

A propylene glycol/water coolant is circulated inside the module to collect heat from heat exchangers and coldplates and this heat is rejected to space through the body mounted radiators mounted on the outer shell of the module. Radiator size was determined for the warmest case (0.5 A.U. orbit). The results indicate a required area of 78 m2. This represents 51% of the available area of the cylindrical portion of the shell.

Two other sizing exercises were also conducted for the module. The first determined the radiator area needed to reject twice the average load of 15 kW. Assuming the warmest environment temperature at 0.5 A.U., the analysis indicated approximately 157 m2 was required. This is just slightly over the total cylindrical area of the shell of 153 m2, therefore rejecting just under 30 kw on average is the maximum amount of heat rejection possible without adding something like a heat pump to raise the radiator temperature.

Another sizing exercise determined the heat rejection given the following scenario: The module is in Mars orbit and the crew has left the module for the Martian surface leaving the AG module uninhabited. If the heat loads are reduced and the TCS fluid is allowed to approach its freezing temperature of -50°C, the question becomes how much heat can be rejected. The analysis indicated that the radiators could still reject up to 11 kW of heat with the TCS fluid just above its freezing temperature. This is in part due to the much colder environment at the low Mars orbit assumed. At the 0.5 A.U. orbit location heat rejection would be approximately zero because the radiator and sink temperature would be identical for this scenario.

Propylene glycol was selected for the working fluid. The relevant options are water or 60% propylene glycol with 40% water or some other working fluid. While water is non-toxic and has greatest thermal capacity per mass of working fluid, it also freezes at 273.2 K and thus may not allow sufficient radiator availability for some mission phases. 60% propylene glycol with 40% water is also non-toxic but, compared to water, it is a less desirable thermal working fluid. However, 60% propylene glycol with 40% water freezes at roughly 223 K, a significant advantage over water. Thus, tentatively the working fluid for the thermal control fluid loops is 60% propylene glycol with 40% water. As above, complete resolution of this issue also requires in-depth thermal environment modeling focusing on radiant rejection from the habitat.


This provide crew accommodations systems and layout to make an 18-month mission habitable for six crewmembers. Functions covered include the following: crew support (meal preparation, eating, meal clean-up, full-body cleansing, hand/face cleansing, personal hygiene, human waste disposal, training, sleep, private recreation and leisure, small-group recreation and leisure, dressing/undressing, clothing maintenance), and operations (facilities for meetings and teleconferences, planning and scheduling, general housekeeping). It is also responsible for configuring work and personal stations such that traffic congestion are minimized. Work efficiency, space use, crew comfort, and convenience should be maximized.

EVA Systems

The EVA system is designed to be used for three planned, two person EVA days per mission. The airlock will transfer two crewmembers per cycle. If full crew transfer is required in LEO, this system assumes all three EVAs are used to transfer crew out of the habitat. EVA days are sized to be 8 hrs, and are accomplished with a personal life support system (PLSS) that is sized for eight hours. The system includes a single flexible airlock with umbilical support and PLSS recharge system; no gas reclamation is planned due to the minimal number of EVAs (3). Two EVA tools boxes are provided. Translation aids are provided to aid crew transportation about the vehicle. EVA system spares are also provided.

Included in the airlock arrangement is a single flexible airlock that allows two persons to egress the AGH at one time. A staging area by the inside airlock door is included in the concept. This area provides volume to store all space suits as well as space suit spares and expendables. Provisions for donning, suit expendables recharge, and checkout are included as well. An unpressurized area by the outside airlock doors is included in the concept. It provides a place for EVA tool storage and allows handling of large objects.

EVA tools provided consist of two toolboxes containing mechanical, electrical, and storage/tie downs. The tools are stowed in the unpressurized area just outside the airlock. EVA system spares as needed to support the six suits and airlock suit recharge provisions are stowed in the AGH in the EVA staging area and remain stored there until needed.

Structure and Mechanism

ElementMass (kg)
Unpressurized End cone650
Pressurized End cone800
Internal fixed structure2,120
Internal deployable structure1,870
Outer Shell6,000
Crew Quarters Radiation Insulation1,500

The structure and shell are to provide a safe habitat for the crew and the necessary space to store supplies and equipment to sustain them for the duration of the entire mission. The inflatable module design was chosen because it is the best means to effectively increase the habitable volume of a spacecraft while keeping the diameter of the core within acceptable payload size limits set by current launch vehicles. The airlock system is to provide the crew with the capability to perform extravehicular activities. It is to be located atop the habitat module, so as to allow the fully suited EVA astronauts to take advantage of a slightly lower gravitational pull.

Medical Ops

The medical operation capabilities onboard the artificial gravity habitat during transit will provide medical contingencies to promote successful mission completion, crew health, safety, and optimal crew performance.

The potential medical contingencies that are to be addressed include those currently required for International Space Station and additional procedures unique to a continuously rotating spacecraft. Following the convention for classification of medical contingencies onboard ISS, the artificial gravy habitat will enable the practice of emergency medicine, environmental medicine, countermeasures or preventive medicine, rehabilitation, and dentistry. Emergency medical procedures will provide for Advanced Cardiac Life Support (ACLS), Basic Cardiac Life Support (BCLS), and trauma. Additionally, emergency medical contingencies may include shock, behavioral, compromised airway or breathing, drug overdose, and smoke inhalation. Environmental medicine will enable treatment for exposure to toxic and hazardous materials. Countermeasures/Preventive Medicine and Rehabilitation will enable countermeasures to prevent neurovestibular dysfunction resulting from the Coriolis effect induced by the rate of rotation of the spacecraft. Coriolis effects induced by rotation of the spacecraft develop within the neurovestibular system and impacts motor performance, behavior, and motion sickness. Exposure to partial gravity, 0.38G, may greatly impact musculoskeletal and cardiopulmonary systems. Dentistry onboard the artificial gravity habitat will enable basic cleaning, crown replacement and treatment of exposed pulp.

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.

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)

Keep in mind that every gram of equipment or supplies takes several grams of propellant. Try to make every gram do double duty.


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

Rocketeers would tend to be short and wiry. In Sir Arthur C. Clarke's classic THE OTHER SIDE OF THE SKY, space station construction crews got a pay bonus if they kept their weight below 150 pounds. Note that this would also be a good argument for rocketeers being:

  1. Oriental
  2. Female OR
  3. Both

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 female cadet shipboard uniforms will not have skirts. The first reason is the impossibility of keeping a skirt 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

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