Inspired By Reality

These are some spacecraft designs that are based on reality. So they appear quite outlandish and undramatic looking. In the next page will appear designs that are fictional, but much more breathtaking. Obviously the spacecraft on this page are all NASA style exploration vehicles, they are not very suited for interplanetary combat (well, most of them at least).

For slower-than-light star ships, go here.

Many of these spacecraft have a table of parameters. You can find the meaning of many of them here. Numbers in black are from the documents. Numbers in yellow have been calculated by me using the document numbers, these might be incorrect.

I'm toying with the idea of making some spacecraft "trading cards."

Asteroid Mining Crew Transport

Asteroid Mining
Crew Transport
ΔV51,000 m/s
Exhaust Velocity100,000 m/s
Specific Impulse10,000 s
Propellant Mass Flow0.96 kg/s
Thrust96,000 N
PropulsionD-D Fusion
Propulsion Bus2,000,000 kg
Propellant mass4,000,000 kg
Payload Section
1250 person habitat
+1250 persons (85,000 kg)
+ consumables
+ priority cargo (150,000 kg)
4,000,000 kg
Wet Mass10,000,000 kg
Dry mass6,000,000 kg
Mass Ratio1.67
Single Fusion powerplant6 GW
Number powerplantsx2
Total power12 GW
Thrust power4.8 GW
Waste Heat2.8 GW
Waste Neutrons4.4 GW
Length480 m
Diameter400 m
Centrifuge radius150 m
Radiator surface area19,200 m2

This little gem is from Aerospace Projects Review Blog. Scott Lowther discovered it in a 1981 Boeing report Controlled Ecological Life Support System: Transportation Analysis.

The spacecraft was a fusion powered rocket designed to transport miners to the asteroid belt. 1,250 miners per trip. And a cargo of 150 metric tons.

In case it is not clear, the nose of the ship is in the upper left corner, with the tanks on girders. The tail of the ship is in the lower right corner, with the engine.

In the first picture, note the fly-like object in the upper left corner. That looks as if it was supposed to be a 37 meter long Space Shuttle. The pods on the ends of the centrifuge arms may or may not be shuttle external tanks, 47 meters long and 9 meters in diameter.

This thing is freaking ginourmous.

I made a quick and dirty 3D model in Blender and scaled it to with the assumption that the fly in the corner is indeed a 37 meter Space Shuttle. Hard to do since the drawing of the "shuttle" is just a smudge. Reading the dimensions off my model it says the ship is roughly 480 meters long, has a diameter of 400 meters at the tips of the heat radiators, and the centrifuge has a 150 meter radius. Treet these numbers with grave suspicion, they could be off by an order of magnitude plus or minus.

The report Scott found only mentioned the ship in passing, he has filed a FOIA request for another report that might go into more details. I hope so, it would be nice to have some solid figures to work with instead of all this conjecture and assuming.

For the Terra-asteroid run, the vehicle would boost for 11 days, coast for 226 days and brake for 13 days to rendezvous. Adam Crowl calculates if the jet-power is 4.8 GW and the mass-ratio is 5/3 for a return to Earth mission, then an exhaust velocity of ~100 km/s and a total delta-vee of 51 km/s. That means a mass-flow rate of 0.96 kg/s.

There are absolutely huge heat radiators because the engine has to get rid of 2.8 freaking Gigawatts of waste heat.

The heat radiators are triangular, so that they can stay inside the shadow cast by the anti-radiation shadow shield. This is for three reasons:

  1. parts of the radiator extending out of the shadow could scatter deadly radiation onto the passengers
  2. parts of the radiator extending out of the shadow will suffer neutron activation
  3. parts of the radiator extending out of the shadow will suffer neutron embrittlement.

Dr. Luke Campbell points out that the engine is going to need one heck of a shadow shield, because distance attenuation ain't gonna do diddly-squat. Not against 4.4 gigawatts of neutron radiation it isn't. At a measly distance of 480 meters the 4.4 GW of neutron radiation will still be strong enough to give everybody on board the ship a lethal dose of radiation in about 1/5 of a second.

(Doing my own calculation, assuming a person with a body mass of 68 kilograms, a cross section of 0.445 m2, who is 480 meters away from 4.4 GW of neutrons, I figure that with no shadow shield they will be exposed to about 10 grays per second, or a LD35 lethal dose of 2 grays in 1/5 second)

He goes on to say that even with a perfect shadow shield enough neutron radiation would scatter around it if a heat radiator, another spacecraft, space station, or asteroid was outside of the shadow and close to the ship. This can be dangerous over the 12 or so days of continuous thrust (the 226 days between thrust events would probably allow any acute radiation injury to heal — but the chronic stuff will accumulate from different burns).

Dr. Campbell goes on to say that a good shadow shield would probably have an interaction length on the order of a few centimeters at the 2.2 MeV energy of D-D fusion, so a few meters of neutron shielding would reduce the dose by something like 40 orders of magnitude. By way of comparison, the shadow shield on an old NERVA nuclear rocket was only about 0.25 meters thick.

Assuming that the centrifuge arm is indeed 150 meters long, it can spin at a safe no-nausea 2.5 RPM and produce a full gravity of acceleration.

If those pods are indeed Space Shuttle external tanks, they will have an internal volume of a bit more than 2050 cubic meters (the sum of the LH2 and LOX tank). If all of that is passenger volume (meaning the consumables, cargo, life support systems, and everything else is at the ship's spine), then each of the 1,250 passengers will have about 14.8 m3 to call their own for the 8.3 month journey. This is less than the 17 m3 NASA figures the crew needs for missions longer than six months or so. On the other hand, NASA is talking about crew members, not passengers. They are not actually running the ship, so as long as they don't actually start foaming at the mouth and go berserk, 14.8 m3 is probably good enough. Spartan but managable.

Let's fly further into unsubstantiated fantasy, piling shaky assumptions upon shaky assumptions. Do not take these next figures seriously.

Assuming my hasty 3D model based on a crude sketch is anywhere near accurate, my modeling package says that one of the triangular radiators has a surface area of about 6,400 square meters (counting both sides). This means the entire radiator array has a total radiating surface area of about 19,200 square meters.

This has to cope with the 2.8 gigawatts of heat.

Say that the heat radiators are titanium-potassium heat pipes. These have a specific area heat of 150.22 kWth/m2, so to handle 2.8 GW it will take about 18,640 m2. This is less than the model's radiator area of 19,200 m2 so we are in good shape.

Ti/K heat pipes have a specific area mass of 100.14 kg/m2 so 18,640 m2 worth would have a mass of 1,866,600 kg. This is less than the propulsion bus mass of 2,000,000 kg so we are in good shape.

This means the propulsion bus has 133,400 kg left for the engine and structure. Sounds reasonable to me. But again all of this is fantasy, done for amusement value.

In the data block, the figures in black are from the report, the yellow figures are deduced. Treat the yellow figures with some skepticism.

4.5 Asteroid Base

The mission assumes an asteroid mining operation with a 5000 person habitat. The complex transportation scenario for this advanced mission involves four different vehicles and three separate space bases (refs. 86 and 91).

c. The GEO base serves as the final assembly area for the large fusion rocket system used to propel payloads out to the asteroids. Cargo and propellant are unloaded from electric-powered transfer vehicles sent up from the LEO base. The enlarged OTV used to transfer personnel and priority cargo is designed to transport 441,000 lb (200,000 kg) from LEO to GEO. The complex fusion propulsion system is assembled at the base with the fusion power core, propellant tanks, large thermal radiators, and the personnnel and priority cargo modules. The resulting vehicle, shown in figure 4-11, can transport 1250 passengers and 150 metric tons of priority cargo to the asteroids.

The gross start mass for the resupply mission would be 10,000 metric tons, of which power plant comprises 2000 tons; hydrogen propellant, 4000 tons; and payload, 4000 tons (1250-person habitat plus consumables and priority cargo). The power plant consists of two 6 GW fusion reactors utilizing the deuterium-deuterium fusion reaction. The total power plan provides 4.8 GW of thrust power while radiating almost 2.8 GW of waste heat and 4.4 GW of high energy neutrons.

d. There are two methods the fusion rocket will use to propel vehicles to the asteroid base: fast transfer for personnel and priority cargo, and slow transfer for nonpriority cargo. The manned resupply mission is a fast hyperbolic transfer orbit consisting of an 11-day thrust period to achieve hyperbolic velocity, followed by a 226-day coasting, and a 13-day deceleration to match velocity with the asteroid base. The return mission leaves the asteroid approximately 113 days later for a reverse of the ascent mission.

The second method is used to accelerate unmanned cargo pods on a slow elliptical (Hohmann) transfer orbit out to the asteroid base. Figure 4-12 illustrates the different trajectories. The slower trip takes 130 days longer but costs less than half of what the fast, hyperbolic trip costs. All nonpriority cargo is brought to the asteroid facility in this manner. Empty cargo pods are not returned to Earth, they may be discarded or used in a variety of ways as storage modules or Closed Ecological Life Support System (CELSS) modules.

e. A fleet of two fusion rockets is envisioned. They each make one round trip per asteroid orbit (synodic cycle) to the asteroid mining facility and leave a few days apart. Because of the synodic cycle, the fusion rocket vehicles are delayed at the asteroid base for approximately 113 days, at the GEO location they are delayed approximately 288 days. During these delays the fusion rockets are used to decelerate unmanned cargo pods at the asteroid base and to accelerate the pods at GEO. Cargo pod launches are timed to arrive at the asteroid base shortly after the manned resupply vehicles so that the fusion rockets can decelerate the cargo pods. The rendezvous opportunity (synodic cycle) repeats itself every 928 days. This transportation system allows half of the total crew to be rotated each cycle.

86. Advanced Propulsion Systems Concept for Orbital Transfer. 1981. NAS8-33935.

91. Technology Requirements for Future Earth-to-Geosynchronous Orbit Transportation Systems. June 1980. NAS1-15301.

Atomic V-2 Rocket

Atomic V-2
ΔV8,120 m/s
Specific Power277 kW/kg
Thrust Power4.7 gigawatts
EngineSolid-core NTR
Specific Impulse915 s
Exhaust velocity8,980 m/s
Initial Thrust850,000 N
Maximum Thrust1,050,000 N
Wet Mass42,000 kg
Propellant Mass25,000 kg
Dry Mass17,000 kg
Payload3,600 kg
Inert Mass13,400 kg
Mass Ratio2.47
Turbopump Mass1,800 kg
Engine Mass
(including reactor)
4,200 kg
Reactor Mass1,600 kg
Height~60 m

The German V-2 rocket was an ultra-scientific weapon back in World War 2, in 1944. Unfortunately it only had a payload size of 1,000 kilograms. This is adequate for a small chemical warhead, but too small for a worth-while 1945 era nuclear warheads. If you want to invent an ICBM, the V-2 is just too weak.

Scott Lowther found an interesting 1947 report by North American Aviation (details in Aerospace Project Review vol 2, no.2, page 110). It had a simple yet audacious solution: take a V-2 design and swap out the chemical engine with a freaking nuclear engine! Atomic powered ICBMs, what a concept!

Anti-nuclear activists reading this are now howling with dismay over their narrow escape, but the NERVA will give the rocket a whopping 3600 kilograms worth of payload. That is large enough for a useful sized ICBM warhead.

But the US military managed to design two-stage chemical ICBMs, and the atomic V-2 became another forgotten footnote to history. But if you are an author writing an alternate history novel, you might consider how differently WW2 would have turned out if Germany had developed this monster.

Bimodal NTR

Bimodal NTR
PropulsionSolid core NTR
Ternary Carbide
Number of engines3
Fuel Volume11.5 L
Core Power
5 MW/Liters
Number Reactor
Number Safety
Reactor Vessel
0.65 m
Reactor Fueled
0.55 m
Engine Mass2,224 kg
Total Engine
4.3 m
Nozzle Exit
Engine (Thrust Mode)
Thrust per engine67,000 N
Total Thrust200,000 N
Exhaust Velocity9,370 m/s
Specific Impulse955 s
Mass Flow
7.24 kg/s
Full Power
Engine Lifetime
4.5 hours
Reactor Power335 MWthermal
Engine (Power Mode)
Reactor Power110 kWthermal
Brayton Power
per reactor
25 kWelectricity
Brayton Power
(2 reactors)
50 kWelectricit

This is from a NASA study TM-1998-208834-REV1. The idea was to take NASA's Mars Design Reference Mission (DRM) and update it. Specifically a throwaway stage with a nuclear thermal rocket (NTR) was to be replaced with a reusable stage using an NTR with the bimodal option.

Three 200 kilonewton NTR can easily generate enough delta V to put the spacecraft through the Mars DRM. It's just that it consumes a measly 10 grams of Uranium-235 out of the 33,000 grams of 235U in each engine. It would be insane to throw away the remaining 32,990 grams of expensive 235U (per engine) as the rocket stages when leaving LEO, as per the DRM.

That's where the bimodal part comes it. Instead of using the rocket for about an hour total then either throwing it away or letting it sit idle for the rest of the 4.2 year long mission, put that sluggard to work! You throttle each engine from 335 megawatts down to 110 kilowatts and use it to run a Brayton electricity generator (about 25 kilowatts of electricity per reactor). A maximum of two reactors can be run simultaneously for generating electricity. The electricity will come in real handy to keep the fifty-odd tons of liquid hydrogen refrigerated instead of rupturing the propellant tanks. This will also remove the need for heavy fuel cells for power. And it will make the stage reusable.

Common Core Bimodal Stage
Structure2.5 mTon
Propellant Tank5.98 mTon
Propellant Tank7.4m I.D. × 19.0m
LH2 Refrigeration
System (@~75 Wt)
0.30 mTon
1.29 mTon
Avionics and Power1.47 mTon
Reaction Control
System (RCS)
0.45 to 0.48 mTon
NTR engines (x3)6.67 mTon
Shadow Shields (x3)0 or 2.82 mTon
Brayton Power
System (@ 50 kWe)
1.35 mTon
Propellant feed,
TVC, etc.
0.47 mTon
Contingency (15%)3.07 to 3.50 mTon
Total Dry mass23.55 to 26.83 mTon
LH2 Propellant51.0 mTon
RCS Propellant
1.62 to 2.19 mTon
Total Wet mass76.2 to 80.0 mTon

For this study they designed a common core stage, and made a family of designs by putting different payload modules on top of the core. The core has three bimodal NTR with power generation (50 kW total) and heat radiators, a propellant tank with a capacity of 50 or so tons of liquid hydrogen, and a propellant refrigeration system.

For manned missions each of the three NTR is fitted with an anti-radiation shadow shield to protect the crew. If there this is an unmanned mission the shadow shields are left off, which reduces the stage's dry mass by 3.2 metric tons. The unmanned cargo is relatively immune to radiation.

The integral liquid hydrogen tank is cylindrical with √2/2 ellipsoidal domes. It has a 7.4 meter internal diameter and a length of 19 meters. It has a maximum propellant capacity of 51 metric tons with a 3% ullage factor.

The forwards cylindrical adaptor contains avionics, storable RCS, docking systems, and a turbo-Brayton refrigeration system to prevent the liquid hydrogen propellant from boiling off over the 4.2 year mission. The highest level of solar heat for the Mars mission is when the spacecraft is in LEO, about 75 watts of solar heat penetrates the 5 centimeter Multi-layer insulation (MLI) blanketing the propellant tank (the stuff that looks like gold foil). The refrigeration system requires about 15 kWe to deal with the 75 watts of heat.

At the aft end, the conical extension of the thrust structure supports the heat radiator, about 71 square meters of radiator. Inside the cone is the closed Brayton cycle (CBC) power conversion system. It has three 25 kWe Brayton rotating units, one for each bimodal reactors. Only a maximum of two of the three units can be operated simultaneously. The CBC's specific mass is ~27 kg/kWe.

The payload is held on a "saddle truss" spine that is open on one side. This allows supplemental propellant tanks and contingency crew consumables to be carried and easily jettisoned when empty. The saddle truss would also be handy for a cargo carrying spacecraft who wants the ability to load and unload cargo in a hurry.

Bono Mars Glider

This is from "A Conceptual Design for a Manned Mars Vehicle" by Philip Bono, in Advances in the Astronautical Sciences, Vol. 7, pp. 25-42 (1960). Actually since I have yet to locate a copy of the paper, this is mostly from David Portree's article in his always worth reading Beyond Apollo blog.

In 1960 the Boeing Airplane Company was working on the X-20A Dyna-Soar orbital glider for the US Air Force. This inspired Philip Bono to envision a huge version for a Mars mission. Just like the Widmer Mars Mission, it was optimistically scheduled to depart in 1971, to take advantage of the next Hohmann launch window. Oh, isn't it just precious how idealistic we were back in the 1970's?

The Dyna-Soar was only 10.77 meters long and 6.34 meters wide at the tips of its delta wings, carrying a single person. Bono's glider was a monstrous 38 meters long and 29 meters along the wing, carrying a crew of eight. The glider is split into two stages, as part of the strategy to blast off from Mars.

Bono's Mars mission stack had the glider perched on a habitat module, which was in turn perched on a short booster rocket. This is the core. Six full sized booster rockets would be clustered around the core. Stack would be 76 meters tall and have a wet mass of about 3,800 metric tons.

The habitat module is 13.7 meters tall and 5.5 meters in diameter. Internal breathing mix is 40% oxygen + 60% helium, so it's going to be Donald Duck time for the next thirty months. Module has an inflatable 15 meter radio dish to communicate with Terra. It also has a Pratt & Whitney Centaur engine with 89 kiloNewtons of thrust.

Electricity is supplied by a small nuclear reactor located in the glider's nose. Which is why the crew will be spending most of the time living in the habitat module, as far away from the reactor as they can possibly get.

The boosters use plug nozzles instead of conventional bell nozzles to reduce engine mass and cooling requirements. This is why the boosters in the pictures have pointed ends instead of the usual bell-shaped exhaust. The boosters would have a combined thrust of about 40,000 kiloNewtons.

After lift-off, at an altitude of about 60 kilometers, four of the outer boosters would be jettisoned. The stack would continue with just the core and two outer boosters. At 100 kilometers the two remaining outer boosters would be jettisoned. The short core booster continues to burn until the stack enters the trans-Mars trajectory, then it is jettisoned.

If at any point a booster fails, the upper stage of the glider will perform an emergency detachment and do its darnest to land the crew back on Terra.

The stack is oriented with the glider nose aimed at the Sun, to protect the habitat module and its rocket engine from solar heating. The eight crew members leave the glider, crawling through a tunnel to enter the habitat module.

Transit time from Terra to Mars is 259 days. I trust they brought along a poker deck.

Upon arrival at Mars, the habitat module would eject a 9 metric ton capsule containing 256 days worth of eight astronaut's sewage. This would eventually impact Mars' surface, prompting every exobiologist on Terra to howl for Philip Bono's head (now they will never ever be sure if a newly-discovered Martian bacterium is an alien life form or an e. coli fugitive from some astronaut poop).

The eight crew members exit the habitat module and enter the glider. The glider separates from the habitat module and heads for a Mars landing. Meanwhile the habitat moduel uses the Centaur engine for Mars orbit insertion, under automatic control. This means the glider is in for a hot time as it has to aerobrake not only the orbital velocity but also the transfer velocity. But it saves on Centaur fuel. Remember: every gram counts.

The glider enters the Martian atmosphere, slows with a drag parachute, and glides to the landing site. At an altitude of 600 meters it uses three landing engines to hover and gently set down. The glider sits on landing skids with its nose pointed 15° above horizontal (angled for the future blast-off).

(Unfortunately for Bono's design, it was crafted with the assumption that Martian surface air pressure was 8% of Terra. We now know that it is less than 1%. Neither the parachute nor the glider wings would function at all in such a tenuous atmosphere. Oops.)

The crew would remove the reactor from the glider's nose and relocate it about a kilometer away, so the radiation doesn't kill them. It supplies electricity to the camp via cables that are, you guessed it, about a kilometer long. A six meter living dome is inflated, and a two metric ton Mars rover is unpacked.

The crew will live on Mars for the next 479 days, doing scientific research, until the next Mars-Terra Hohmann launch window arrives. Curse those long synodic periods.

On the eve of the launch window, the nuclear reactor is re-mounted on the glider's nose, and the landing rockets are moved so they can serve as ascent engines.

(as a side note, I use the above image as inspiration when I designed the scoutships for an illustration of the tabletop boardgame Stellar Conquest.)

The upper stage of the glider blasts off into orbit, using the lower stage as a launch rail.

In orbit, the glider rendezvouses with the habitat module. The crew perform an EVA to manually dock the glider to the habitat module, and to jettison the empty Centaur engine fuel tank. This torus shaped tank surrounds the fuel tank for the return trip. The empty was retained until now to protect the inner full tank from meteor strikes. But now it has to go because (chorus) every gram counts.

The Centaur engine does a burn to enter a Mars-Terra Hohmann trajectory. Transit time is about 120 days. Time to break out a fresh deck of poker cards.

It is unclear to me from the description if the stack does a further Centaur burn to enter Terra orbit, or if it uses aerobraking. Seeing the strategy of the rest of the mission, my money is on aerobraking. In any event, after the crew enter the glider, they jettison both the habitat module and nuclear reactor (and presumably 120 days worth of sewage). These burn up in the atmosphere, with the reactor causing screams of outrage from the anti-nuclear community.

The glider lands on its skids at a NASA landing site in the desert. The crew open the doors and can now stop talking like Donald Duck. The news reporters take lots of photos as the crew is stuffed into a quarantine unit. True if there were any lethal Martian plague germs the incubation period would probably be less than 120 days, but you can never be too careful with possible Martian versions of The Andromeda Strain.

Basic Solid Core NTR


RocketCat sez

Now this is design to pay attention to. Dr. Crouch did this one to a queen's taste, with plenty of delicious detail. Even if he did have some outrageous ideas, like detaching the freaking atomic reactor for splashdown and recovery in the Pacific Ocean!

This is from NUCLEAR SPACE PROPULSION by Holmes F. Crouch (1965).

Please note that this is a strict orbit-to-orbit ship. It cannot land on a planet.

The Command Capsule contains the payload, the habitat module for the crew, the ship controls, life-support, navigation equipment, and everything else that is not part of the propellant or propulsion system. It is designed to detach from the ship proper along the "Payload Separation Plane."

The Rocket Reactor is the actual nuclear thermal rocket propulsion system. It too is designed to detach from the ship proper along the "Reactor Separation Plane." This allows such abilities as to jettison the reactor if a criticality accident is immanent, to swap an engine for an undamange or newer model engine, or to return the engine Earth via splashdown.

The book had most of a chapter about returning an engine to various locations in the Pacific ocean where international condemnation was low enough and the problems of designing an ocean-going recovery vessel that can fish the reactor out of the water without exposing the crew to radiation. What an innocent age the 1960's were, that sort of thing would never be allowed nowadays. The illustrations above are provided for their entertainment value.

The propellant tank contains the liquid hydrogen propellant. The payload interstage and the propulsion interstage are integral parts of the propellant tank, and contains hardware items of lesser value than the payload and the reactor. The propulsion interstage also contains the attitude jets. As with all rockets, the propellant and its tank dominate the mass of the spacecraft. A larger propellant tank or smaller strap-on tanks can be added to increase the mass ratio. Note that the main propellant tank is load-bearing, it has to support the thrust from the engine. But the strap-on tanks are not load-bearing, they can be made lightweight and flimsy.

ItemMass (kg)Average Diameter (m)Overall Length (m)
Engine6,8001.52 to 3.056.10
Tank (empty)22,7007.3238.1
Tank (full)90,700--

Sample specifications : wet mass: 112,500 kg, maximum thrust 445 kN, specfic impulse 800 seconds. That implies a thrust-to-weight ratio of 0.4, which is its acceleration in gs when the propellant tank is full. The figures below imply a mass ratio of 1.5, and a ΔV capability of 3,200 meters per second. The spacecraft's specific power is 23 kilowatts per kilogram

The book implied that a solid core engine could be devloped up to a specific impulse of 1000 seconds, with a max of 12,000 seconds (but at max you'll be spewing molten reactor bits in your exhaust). A later design in the book had a specific impulse of 1000 seconds and a ΔV capability of 15,000 m/s (which implies a mass ratio of about 4.6, which is a bit over the rule-of-thumb maximum of 4.0). Please note that the dimensions below were originally in feet and pounds in the book, that's why they are such odd numbers (e.g., 1.52 meters is 5 feet).

Rescue Ship

This is a variant on the basic NTR rocket: the nuclear rescue ship. This is for use by the outer-space version of the Coast Guard.

Note the "Neutron isolation shield" between the two reactors. Nuclear reactors are throttled by carefully controlling the amount of available neutrons within the reactor. A second reactor randomly spraying extra neutrons into the first reactor is therefore a Bad Thing. "Neutronically isolated" is a fancy way of saying "preventing uninvited neutrons from crashing the party."


The propulsion interstage is the non-nuclear part of the propulsion subsystem. It contains the propellant plumbing, the turbopump, and the attitude control system.

The nuclear part of the propulsion system is the rocket reactor. This is basically the reactor, the exhaust nozzle, and the radiation shadow shield.

The rocket reactor is designed to be detachable from the rest of the spacecraft.

Shadow Shield

The shadow shield casts a protective shadow free from deadly radiation. Care has to be taken or other objects can scatter radiation into the rest of the ship. Any side tanks will have to be truncated so they do not emerge from the shadow. Otherwise they will be subject to neutron embrittlement, and they will also scatter radiation. The reason the reactor does not have shielding all around it is because the shielding very dense and savagely cuts into payload mass allowance. The shadow shield typically casts a 10 degree half-angle shadow.

Note that shadow shields will more or less force the docking port on the ship to be in the nose, or the other ship will be outside of the shadow and exposed to reactor radiation.

When the reactor is idling, the shadow shield does not have to be as thick. In order to widen the area of shadow (for adding side tanks or whatever), the secondary shadow shield could extrude segments as extendable side shields.

Plug Nozzle

For nuclear thermal rockets, the exhaust bell tends to be about twice the size of a corresponding chemical rocket nozzle. A small concern is meteors. While very rare, the shape of the bell will funnel any meteors into a direct strike on the base of the reactor. This can be avoided by replacing the bell nozzle with a Plug Nozzle.

The basic design uses a bell nozzle, and powers the attitude jets from the reactor. This might not be the best solution. Compared to a chemical rocket, the moment of inertia of a nuclear rocket is about ten to thirty times as large (diagram omitted). This is due to the larger mass of the engine (because of the reactor) and due to the more elongated shape of the nuclear rocket (because of the shadow cast by the shadow shield, and designers taking advantage of radiation's inverse square law). Taking into account the relative moment arms, the attitude jets will have to be four to twelve times as powerful. Conventional attitude jets might not be adequate.

Also note that with this design, the attitude jets cannot be used during a main engine burn. Further: attitude jets are pulse reaction devices (maximum change in the minimum time). Also there is a mandatory delay time between reaction pulses to permit the nozzles to cool off and to allow propellant feed oscillations to dampen out. None of these limits work well with nuclear thermal rockets.

Mr. Crouch suggests that the basic problem is that bell nozzles are not the optimal solution for nuclear engines. He suggests that plug nozzles (aka "annual throat nozzle") can solve the problems. Plug nozzles have problems with chemical rockets, but have advantages with nuclear rockets. Mr. Crouch mentions that wide design flexibity arises from the fact that the outer boundary radius (rβ) and cowl lip angle (β) can be varied. Translation: you can design a hinge into the shroud that will allow the cowl lip to wiggle back and forth. This will allow thrust vectoring.

Mr. Crouch also likes how a plug nozzle can be structurally integrated into the reactor, unlike a conventional bell nozzle. It is also nice that the subsonic setion of the nozzle requires structural support in the very region where the core exit needs support. What a happy coincidence! The support grid, the plenum chamber, the plug body, and the plug supports could be integrated into one common structure. You will, however, have to ensure that the hot propellant passes through the plug body support, not across it.

Note the reversed curvature of the propellant flow. This allows placement of neutron reflection material to prevent neutrons going to waste out the tail pipe. The propellant can move in curves, but neutrons have to move in straight lines. This will create a vast improvement in the neutronics of the reactor.

Of course there are problems. The biggest one is burnout of the cowl lips. The lip is thin and the exhaust is very hot. The lip will be burnt away unless special cooling techiques are invented (Here Mr. Crouch waves his hands and states that such cooling will only be invented if there is a compelling need, and the desire for a nuclear plug nozzle is such a need. Which is almost a circular argument). Some form of regenerative cooling will probably be used, where liquid hydrogen propellant flows through pipes embedded in the lips as coolant.

Thrust Vectoring

The plug nozzle lends itself well to thrust vectoring, thrust throttling, and nozzle close-off. This is because of the short shroud and the configuration of the cowl lip. Unlike a conventional bell nozzle there is no fixed outer boundary. While the cowl lip defines the outer periphery of the annular throat, there isn't an outer boundary. So all you have to do is alter the cowl lip angle to adjust the throat area, which will vector the thrust (that's what Mr. Crouch meant when he was talking about varying rβ and β).

In the diagram above, variable throat segments A, B, C, and D are sections of the cowl which are hinged (so as to allow one to alter the lip angle). This will allow Yaw and Pitch rotations.

If the pilot wanted to pitch the ship's nose up, they would decrease the mass flow through segment A while simultaneously increasing the mass flow through segment C. Segment A would have its lip angle increased which would choke off the throat along its edge, while Segment C's lip angle would be decreased to open up its throat section. The increased thrust in segment C would force the ship to pitch upwards.

It is important to alter the two segments such that the total thrust emitted remains the same (i.e., so that segment A's thrust lost is exactly balanced by segment C's gain). Otherwise some of the thrust will squirt out among the other segments and reduce the amount of yaw or pitch thrust. With this arrangement, it is also possible to do yaw and pitch simultaneously.

The moment arm of thrust vectoring via a plug nozzle is greater than that of thrust vectoring from a conventional bell nozzle. This is because the thrust on a bell nozzle acts like it is coming from the center, along the thrust axis. But with a plug nozzle, the thrust is coming from parts of the annular throat, which is at some distance from the center. This increases the leverage.

Nozzle close-off means when thrusting is over, you can shut the annular throat totally closed. This keeps meteors, solar proton storms, and hostile weapons fire out of your reactor.

Pivoting each section of cowl lips is a problem, because as you pivot inwards you are reducing the effective diameter of the circle that defines the edge of the lips. The trouble is that the lip is not made of rubber. The solution used in jet fighter design is called "turkey feathers" (see images above). It allows the engine exhaust to dialate open and close without exposing gaps in the metal petals.

Cascade Vanes

With chemical rockets, retrothrust is achieved by flipping the ship until the thrust axis is opposite to the direction of motion, then thrusting. This is problematic with a nuclear rocket, since it might move another object out of the shadow of the shadow shield and into the radiation zone. For example, the other object might be the space station you were approaching for docking. Ideally you'd want to be able to perform retrothrust without changing the ship's orientation. What you want to do is redirect the primary thrust stream.

Jet aircraft use "thrust reversers." These are of two type: clam shell and cascade vanes. For complicated reasons clam shell reversers are unsuited for nuclear thermal rockets so Mr. Crouch focused on cascade vanes reversers. The main thing is that the actuators for cascade vanes are simpler than clam shell, and unlike clam shells a cascade vane reverser surface is segmented. There are five to ten vanes in each surface.

Note that the maximum reverse thrust is about 50% of the forwards thrust.

Each vane is a miniature partial nozzle. It takes its portion of the propellant flow and bends it backwards almost 180°. In the "cascade reverser end view" in the right diagram above, there are eight reversers, the wedge shaped surfaces labeled A, A', B, B', C, C', D, and D'. Each reverser is normally retracted out of the propellant stream, so their rear-most edge is flush with the tip of the cowl lip. When reversal is desired, one or more reversers are slid into the propellant stream. At maxmimum extension, the rear-most edge makes contact with the plug body.

Vane segmentation of the reverser surface eases the problem of center-of-pressure changes as the reverser's position is varied in the propellant stream.

Inserting all eight reversers causes retrothrust (see "Full Reverse" in below left diagram). Inserting some but not all reversers causes thrust vectoring. You'd expect that there would be a total of four reversers instead of eight (due to the four rotations Yaw+, Yaw-, Pitch+, Pitch-), but each of the four were split in two for reasons of mechanical alignment and the desirablity of shorter arc lengths of the vanes. This means the reversers are moved in pairs: to pitch upward you'd insert reverser A and A' (see "Thrust Vectoring" in below left diagram).

I am unsure if using reversers means that it is unnecessary to use the variable throat segments for yaw and pitch rotations, Mr. Crouch is a little vague on that. And the engineering of reversers that can withstand being inserted into a nuclear rocket exhaust is left as an exercise for the reader. There will be temperature issues, supersonic vibration issues, and edge erosion issues for starters. These are desgined for a solid-core NTR, where the propellant temperatures are kept down so the reactor core remains solid. This is not the case in a gas-core NTR, where the propellant temperatures are so high that the "reactor core" is actually a ball of hot vapor. The point is that a gas core rocket might have exhaust so hot that no possible material cascade vane could survive. There is a possibility that MHD magnetic fields could be utilized instead.

But the most powerful feature of cascade vanes is their ability to perform "thrust neutralization". When all the reversers are totally out of the propellant stream, there is 100% ahead thrust. When all the reversers are totally in the propellant stream, there is 50% reverse thrust. But in the process of inserting the reversers fully in the propellant stream, the thrust smoothly varies from 100% ahead, to 75% ahead, to 50% ahead, to 25% ahead, to 25% reverse, and finally to 50% reverse.

The important point is that at a specific point, the thrust is 0%! The propellant is still blasting strong as ever, it is just spraying in all directions, creating a net thrust of zero.

Why is this important? Well, ordinarily one would vary the strength of the thrust while doing maneuvers. Including stopping thrust entirely. Trouble is, nuclear thermal rocket reactors and turbopumps don't like having their strength settings changed. They lag behind your setting changes, and the changes put stress on the components.

But with the magic of thrust neutralization, you don't have to change the settings. You put it at a convenient value, then leave it alone. The cascade vanes can throttle the thrust to any value from 100% rear, to zero, to 50% fore. And do thrust vectoring as well.

Mr. Crouch also notes that while using thrust vectoring for maneuver, the rocket will have to be designed to use special auxiliary propellant tanks. The standard tanks are optimized to feed propellant while acceleration is directed towards the nose of the ship. This will not be true while manuevering, so special "positive-expulsion" tanks will be needed. These small tanks will have a piston or bladder inside, with propellant on the output tube side of the piston and some neutral pressurized gas on the othe side of the piston.

I was having difficulty visualizing the cascade reversers from the diagrams. I used a 3D modeling program called Blender to try and visualize them.

Blue Max Studio Liberty Bell

Liberty Bell
ParamLaunchLunar Run
6.18 MN2.2 MN
10.5 (4.2)*1.56
Accel10 m/s24.58 m/s2
1,132.4 kg/s1,000 kg/s
5,457 m/s2,200 m/s
556.2 s452.3 s
770 s105 s
≈2 hrs5.56 days
Δv7,842 m/s4,947 m/s

The Liberty Bell is a tramp freighter created by Ray McVay for his Black Desert universe

The Liberty Bell proper is a command module with a dry mass of 50 tons, and 50 tons of propellant. It has a power plant, life support, and thrusters. It can carry a crew of five plus up to 20 passengers from the surface into LEO.

On the nose is an airlock with an androgynous docking port and a maneuvering unit.

On the tail there are four couplers, each of which can hold one cargo container. The containers are cylinders 9.5 meters long and 5 meters in diameter. They are rated to carry a maximum of 62.5 tons of cargo each.

There are four remote manipulator arms used to handle cargo containers. The arms are not permanently attached, they can move like a giant inchworm over the spacecraft's surface just like the Canadarm 2 on the International Space Station.

The Liberty Bell is boosted into orbit with an L-Drive assembly. This is a laser launch system. At the spaceport, the launch pad has a huge stationary laser built into it. The L-Drive assembly is attached to the bottom of the Liberty Bell. The L-Drive is an air-breather, it scoops up atmosphere and sprays it into the mirrored dish-with-a-spike. The laser beam from the launch pad heats the air, creating the thrust to boost the spacecraft into orbit. The laser beam tracks the L-Drive as it climbs into the sky. When the L-Drive reaches an altitude where the air is too thin, it switches to its internal propellant tanks.

Typically the L-Drives are owned and maintained by the spaceport, they cost $1,250,000 Black Desert dollars. The spaceport will rent an L-Drive, laser boost time, plus fees and taxes to the captain of the Liberty Bell. This will cost the captain $100,000 total to boost the Liberty Bell into LEO.

Upon reaching LEO, a Liberty Bell generally makes a rendezvous with an orbital transport nexus, unloads its four cargo containers (250 tons of cargo total) and 20 passengers, loads new cargo and new passengers to be delivered to Terra's surface, pays the spaceport for laser landing services (including fresh propellant for the L-Drive), and rides the laser beam back down to the spaceport.

However, our Liberty Bell is heading to Luna.

The Liberty Bell jettisons the L-Drive, delivering the rental vehicle back into the hands of spaceport personnel (the orbital representatives). The captain knows that when they make the return trip, the spaceport will be more than happy to reserve them an L-Drive for the trip down.

On this trip, instead of carrying four cargo containers, the Liberty Bell only has two containers (125 tons), a translunar rocket engine (20 tons, thrust equivalent to a SSME), and a small cobbled together weapons package (105 tons). The total payload tonnage is 250 tons, same as four cargo containers.

The weapons package contains two Kinetic Kill Vehicles (KKV) at 40 tons each, two Caltrop space mines at ten tons each, and a laser turret with power supply at five tons.

The Liberty Bell then moves into a higher orbit, to make a rendezvous with a transfer space station. In the Black Desert universe, the orbits are patrolled by the astromilitaries of various nations, all looking for trouble and whatever they can get away with. This is the main reason for the Liberty Bell's weapons package.

At the transfer station, the Liberty Bell outfits itself for the Lunar trip. It leases four propellant tanks to feed the translunar rocket engine. It also leases or purchases a cupola.

Using the remote manipulator arms, the translunar rocket engine and the airlock/docking ring swap positions. The rocket engine is mounted on the nose and the four propellant tanks are attached. The docking ring is mounted next to the other cargo, and a cupola installed on top. For the rest of the trip, the cupola will serve as the Liberty Bell's cockpit.

As it turns out, one of the captain's business partners had three cargo containers waiting at the transfer station to be delivered to Luna. The remote manipulator arms install these as well.

The Liberty Bell is ready for the trip to Luna. The command module now faces opposite the direction of thrust it had at launch, with the cupola and the weapons package aimed at the new forwards that used to be backwards. It is carrying three hundred tons of cargo.

It has enough life support and consumables to haul five crew and twenty passengers on the five and a half day trip to Luna or one of the La Grange stations.

Discovery II

RocketCat sez

This should happen more often. A team of rocket scientist at the Glenn Research Center were inspired by the Discovery from the movie 2001. So they designed one with modern technology that would actually work!

Discovery II
ΔV223,000 m/s
Specific Power3.5 kW/kg (3,540 W/kg)
Thrust Power3.1 gigaWatts
Specific Impulse35,435 s
Exhaust Velocity347,000 m/s
Wet Mass1,690,000 kg
Dry Mass883,000 kg
Mass Ratio1.9
Mass Flow0.080 kg/s
Thrust18,000 newtons
Initial Acceleration1.68 milli-g
Payload172,000 kg
Length240 m
Diameter60 m wide

This design for a fusion propulsion spacecraft is from the NASA report TM-2005-213559 by Craig H. Williams, Leonard A. Dudzinski, Stanley K. Borowski, and Albert J. Juhasz of the Glenn Research Center (2005). The goal was to produce a modern design for the spacecraft Discovery from the movie 2001 A Space Odyssey. The report has all sorts of interesting details about where the movie spacecraft design was correct, and the spots where things were altered in the name of cinematography. The movie ship had no heat radiators, and the diameter of the centrifuge was too small. Arthur C. Clark was well aware of this, but was overruled by the movie people.

Enzmann Starship

This section has been moved into the Slower Than Light page.

Exacting Class Starfighter

Exacting Class Starfighter
ΔV7,000,000 m/s
Specific Power450 MW/kg
(450,000,000 W/kg)
Thrust power9 terawatts
PropulsionICF Fusion
Thrust3,000,000 newtons
Exhaust velocity6,000,000 m/s
Dry Mass20 metric tons
Wet Mass40 to 65 metric tons
depending upon fuel
Length60 meters
(Whipple shield)
5 meters
(Internal hull)
4 meters
Heat radiator
30 meters
Heat radiator
5 meters
Power plant50 MW Braydon-cycle
w/argon working fluid
ArmamentUV laser (3 turrets)
Spinal coilgun (2)
Exhaust plume

This is a design by Artist Zach Hajj (a.k.a. Zerraspace), which I found astonishingly good. Personally I cannot find anything scientifically inaccurate with it. The artist mentioned that he used this website as a resource, and I'd say he did his homework.

The structural components of the spacecraft are composed of high-emissivity graffold (folded graphene) scaffolding. The skin is armored with low absorptivity + high emissivity alloy for anti-laser armor, and a Whipple shield to defend against kinetic attacks. For sensors it has frontal and rear IR batteries and several antennae incorporated into the skin.

This is a departure from my regular work towards something more speculative; a true “spacefighter”, a small vessel capable of operating both in space and in an atmosphere. Each one of these is an enormously demanding task on its own, hence the hybrid craft must make a number of compromises to fully operate.

A military vessel first and foremost, the starfighter is not a comfortable ride. Variable thrust and gravity will send the pilot rocking, which the gyroscopic cockpit can only do so much to accommodate, and it has no life support, so he must remain in his space suit at all times. The craft is not meant to hold him for more than a few hours — ideally it is only operated from carriers or planetary bases during skirmishes. That being said, the frontal module can be ejected in case of an emergency, at which solar panels unfurl to provide enough power to operate antenna and coordinate a rescue mission. The starfighter’s minimal design lends to easy conversion to a drone or smart-ship, as this only requires putting a decent computer in place of the cockpit.

In battle, the starfighter serves chiefly as an interceptor or assault craft. As an interceptor, it shoots down incoming missiles and directs fire away from more vital ships. When on the attack, the onboard lasers might not be powerful enough to do significant damage to larger ships, but equipped nukes allow it take down a limited number of opponents of any size. Some militaries even prefer them to capital ships, as many can be built for the same cost as a larger vessel and each loss is less of a hit to the fleet, yet in their numbers they are harder to take down.

Firstly, I set up the ship so as to handle a continuous 24 hours of 5 G acceleration with a mass ratio of 2 when exhaust velocity is 2% c (maximum delta-v 1.5% c), which is probably well beyond how it'll generally operate. I figured it could moderate exhaust velocity by only partially igniting the fuel, letting it get that incredible thrust when needed. If helium-3 fuel is used at maximum exhaust velocity, delta-v is tripled, but then power limits acceleration to 1.5 G at most.

Second, the radiators are deliberately shaped to form a delta-wing when fully unfolded, as NASA pages I found on the matter suggested this was best suited for supersonic flight, and the flaps and slats on the side are used to increase lift at lower speeds where this configuration doesn't work so well. The radiators are also used as an aerobrake while landing, and directly dispose of much of the heat built up.

Lastly, the length required for the coilguns made me think they would have to be stationary mounts, making them somewhat less useful than the ship's missile and laser retinues, so they'd likely more often be used for accelerating missiles than as weapons in their own right. To help compensate, they're open on both ends. When the coilguns are to fire backwards, the projectile is moved to the front end, then accelerated back along the full length, letting it be used both ways.

From Exacting Class Starfighter by Zerraspace

First Men to the Moon

This design is from a book called First Men to the Moon (1958) written by a certain Wernher von Braun, aka "The Father of Rocket Science" and the first director of NASA. The book came out shortly after the Sputnik Crisis.

Gasdynamic Mirror

Gasdynamic Mirror
PropulsionDT Fusion
Specific Impulse200,000 s
Exhaust Velocity1,960,000 m/s
Wet Mass? kg
Dry Mass? kg
Mass RatioWet/Dry
ΔV1,960,000 * ln(MassRatio) m/s
Mass Flow0.0240 kg/s
Thrust47,000 newtons
Initial Acceleration(47,000/wet)/9.81 g
Payload? kg
Length? m
Diameter? m wide

There are problems with attempting to confine ionized plasma in a reaction chamber long enough for most of it to undergo nuclear fusion. In the Gasdynamic Mirror propulsion system, they attempt to avoid that by making the reaction chamber a long and skinny tube, so the plasma just travels in a straight line. The trouble is that it has to be really long.

GCNR Liberty Ship

RocketCat sez

Ho, ho! This brute kicks butt and takes names! You want to boost massive amounts of payload into orbit? Freaking monster rocket has eight times the payload of a Saturn V rocket. It can haul three entire International Space Stations into LEO all at once!

But to do this it packs seven honest-to-Heinlein nuclear lightbulb engines! The only rocket that could come close to this beast is a full blown Orion drive rising on a stream of nuclear explosions at about one Hertz.

Liberty Ship
ΔV15,000 m/s
Specific Power350 kW/kg
(350,430 W/kg)
Thrust Power560 gigawatts
Specific Impulse3060 s
Exhaust Velocity30,000 m/s
Wet Mass2,700,000 kg
Dry Mass1,600,000 kg
Mass Ratio1.6875
Mass Flow1246 kg/s
Thrust37,380,000 newtons
Initial Acceleration1.4 g
Payload900,000 kg
Length105 m
Diameter20 m wide

Anthony Tate has an interesting solution to the heavy lift problem, lofting massive payloads from the surface of Terra into low Earth orbit. In his essay, he says that if we can grow up and stop panicking when we hear the N-word a reusable closed-cycle gas-core nuclear thermal rocket can boost huge amounts of payload into orbit. He calls it a "Liberty Ship." His design has a cluster of seven nuclear engines, with 1,200,000 pounds of thrust (5,340,000 newtons) each, from a thermal output of approximately 80 gigawatts. Exhaust velocity of 30,000 meters per second, which is a specific impulse of about 3060 seconds. Thrust to weight ratio of 10. Engine with safety systems, fuel storage, etc. masses 120,000 pounds or 60 short tons (54 metric tons ).

Using a Saturn V rocket as a template, the Liberty Ship has a wet mass of six million pounds (2,700,000 kilograms). Mr. Tate designs a delta V of 15 km/s, so it can has powered descent. It can take off and land. This implies a propellant mass of 2,400,000 pounds (1,100,000 kilograms). Using liquid hydrogen as propellant, this will make the propellant volume 15,200 cubic meters, since hydrogen is inconveniently non-dense. Say 20 meters in diameter and 55 meters long. It will be plump compared to a Saturn V.

Design height of 105 meters: 15 meters to the engines, 55 meters for the hydrogen tank, 5 meters for shielding and crew space, and a modular cargo area which is 30 meters high and 20 meters in diameter (enough cargo space for a good sized office building).

A Saturn V has a dry mass of 414,000 pounds (188,000 kilograms).

The Liberty Ship has seven engines at 120,000 pounds each, for a total of 840,000 pounds. Mr. Tate splurges and gives it a structural mass of 760,000 pounds, so it has plenty of surplus strength and redundancy. Add 2,400,000 pounds for reaction mass, and the Liberty Ship has a non-payload wet mass of 4,000,000 pounds.

Since it is scaled as a Saturn V, it is intended to have a total mass of 6,000,000 pounds. Subtract the 4,000,000 pound non-payload wet mass, and we discover that this brute can boost into low earth orbit a payload of Two Million Pounds. Great galloping galaxies! That's about 1000 metric tons, or eight times the boost of the Saturn V.

The Space Shuttle can only boost about 25 metric tons into LEO. The Liberty Ship could carry three International Space Stations into orbit in one trip.

Having said all this, it is important to keep in mind that a closed-cycle gas-core nuclear thermal rocket is a hideously difficult engineering feat, and we are nowhere near possessing the abilty to make one. An open-cycle gas-core rocket is much easier, but there is no way it would be allowed as a surface to orbit vehicle. Spray charges of fissioning radioactive plutonium death out the exhaust nozzle at fifty kilometers per second? That's not a lift off rocket, that's a weapon of mass destruction.

There is an interesting analysis of the Liberty Ship on Next Big Future.


HELIOS Stage One
Thrust12,000,000 newtons
Wet Mass700 metric tons
not including
Stage 2
Dry Mass32 metric tons
Body Diameter6 meters
Wingspan27 meters
HELIOS Stage Two
ΔV21,000 m/s
Specific Power57 MW/kg
(566,100 W/kg)
Thrust Power3.8 gigawatts
PropulsionSolid Core NTR
Thrust981,000 newtons
Exhaust Velocity7,800 m/s
Reactor Power2,600 MW
Wet Mass100 metric tons
Payload6.8 metric tons

HELIOS stands for Heteropowered Earth-Launched Inter-Orbital Spacecraft. Unfortunately "HELIOS" became a catch-all term for quite a few post-Saturn studies around 1963. This entry is about the 1959 version from Krafft Ehricke at Convair.

As you should recall, when dealing with a radioactive propulsion system the three anti-radiation protection methods are Time, Distance, and Shielding. A rocket cannot shorten the time, a burn for specific amount of delta V takes as long as it takes. Most designs use shielding, even though the regrettable density of shielding savagely cuts into payload mass.

But some designers wondered if distance could be substituted. The advantage is that distance has no mass. The disadvantage is it makes the spacecraft design quite unwieldy. You'd have to either put the propulsion system far behind the habitat module on a long boom, or more alarmingly have the propulsion system in front with the habitat module trailing on a cable. In theory the exhaust plume is not radioactive, so in theory the habitat module can survive being hosed like that.

There is no way this design would work as a warship. It would be like trying to run through a maze while carrying a ladder.

The break-even point is where the mass of the boom or cable is equal to the mass of the shadow shield.

This is the Waterskiing school of spacecraft design.

Dr. Ehricke design was two-staged. It has a liftoff mass of 800 metric tons, a diameter of 6 meters (omitting the delta wings) and a length of 60 meters.

The first stage was chemical powered since even in 1959 they knew nobody was going to allow a nuclear propulsion system to lift off from the ground. The lower stage has a delta wing, and will glide back to base after stage separation to be reused on future missions. The lower stage has a diameter of 6 meters, and a wingspan of 27 meters. Wet mass of 700 metric tons, dry mass of 32 metric tons, twin chemical engines with a combined thrust of 12,000,000 newtons. The first stage pilot rides in a little red break-away rocket in case the first stage has an accident.

The first stage separates from the second at an altitude of about 50 kilometers when the velocity reaches 4.5 km/s. The corrugated coupler that held the two stages together falls away.

The second stage will use retrorockets to lower the habitat module on cables about 300 meters below the nuclear stage, then let'er rip. The second stage has a wet mass of 100 metric tons, the nuclear reactor has a power of 2,600 Megawatts, and a thrust of 981,000 newtons. Initial acceleration is 1 g.

When it comes to Lunar landing, the habitat module touches down, then the nuclear stage move down and sideways so it stays 300 meters away as it lands. HELIOS can deliver about 6.8 metric tons of payload to the Lunar surface, and stil carry enough propellant to make it back to LEO.

Dr. Ehricke does not give details above the return trip, but it would need to involve some sort of ferry rocket to retrieve the crew from Terra orbit. There is no way anybody would allow that radioactive doom rocket to actually land. Even if it could carry enough propellant. Dr. Ehricke Convair Space Shuttle would do nicely to retrieve the crew.

Nowadays most experts agree that a 300 meter separation from a 2,600 MW reactor is totally inadequate to protect the astronauts from a horrible radioactive death. I've heard estimates of a minimum 1,000 meter separation from a 1 MW reactor. For 2,600 MW you'd want a separation more like 14,000 meters, which probably has more mass than a conventional radiation shadow shield.

Hermes from The Martian

The Martian movie is based on the novel of the same name. Both have the Atomic Rockets Seal of Approval. Enough said.

Warning: this section contains spoilers for the novel and the movie.

Hermes (Novel)
PropulsionIon drive
Acceleration0.002 m/s2
Gravity0.4 g
Gravity TypeBola Spin

Hermes in the Novel

Author Andy Weir based the original mission on Robert Zubrin's Mars Direct proposal. Weir updated Zubrin's chemical rocket to a nuclear-reactor-powered ion drive using argon propellant. You see, a puny chemical rocket has to use Hohmann transfer orbits which have launch windows tied to the synodic period of Mars. That mission would have had a required stay time on Mars of a little over a year. For dramatic reasons, Weir needed the mission capable of being aborted at any time with a return to Terra. The ion drive allowed this.

In Andy Weir's original conception, the Hermes is cone-shaped so it can aerocapture at Mars and at Terra, saving precious propellant mass.

The Hermes has an acceleration of 0.002 m/s2 (2 millimeters per second, per second). Andy Weir said that the delta V budget for the return trip was about 5,000 m/s.

The spacecraft is split down the middle parallel to the long axis. This allows the two halves to separate, attached with cables, so they can spin like a bola to provide artificial gravity. The halves are called "Semicone-A" and "Semicone-B".

The main airlock/docking port is located in the customary place, on the nose.

Andy Weir mentioned that the movie version of the Hermes has quite a different design. But he also noted it was "way cooler-looking than the version I imagined."

flight plan
Terra to Mars124 days
Surface Ops31 days
Mars to Terra241 days
Sol-6 Abort
flight plan
Terra to Mars124 days
Surface Ops6 days
Mars to Terra236 days
flight plan
Terra to Mars124 days
Surface Ops6 days
Mars to Terra236 days
Terra Slingshot0 days
Terra to Mars Flyby322 days
Mars to Terra211 days
The Martian: A Technical Commentary

An Ares mission begins with 14 uncrewed launches (probably with an Atlas or similar sized booster) dropping airbag-cushioned payloads on Mars. These would each weigh about 1000kg on launch, with up to 600kg of payload to the surface. This includes parts of the Hab and supplies.

The crewed part of the mission is mediated by the Hermes, a large vehicle for deep space with a nuclear powered ion drive designed to fly between Earth and Mars and back. The Hermes is used by every mission and was assembled in Earth orbit at (no doubt) astronomical expense.

The six astronauts of Ares 3, together with their supplies are launched from Earth to Hermes. The Mars Descent Vehicle is launched separately towards Mars at about this time.

Hermes and the MDV travel to Mars, parking in Mars orbit after 124 days in deep space. Hermes remains in orbit, uncrewed, while the MDV flies the astronauts to the surface.

On the surface, the astronauts build their Hab from airbag cushioned cargo drops and perform their mission. After 30 days on the surface, they climb into the Mars Ascent Vehicle and fly back up to orbit, where they meet the Hermes and fly back to Earth, taking 208 days to return.

Hermes parks in Earth orbit and the crew return to Earth in some re-entry vehicle like Orion or Dragon.

The MAV was launched years before, made its return fuel on Mars using electricity and ambient atmosphere, then was used for about 6 hours to get back to the Hermes.

This mission architecture is very credible, given a nuclear powered ion drive, which is technically possible but politically problematic. IMO, the architecture is inefficient given most of the hardware is used only once, Hermes is not self sufficient, and the astronauts spend only 30 days on the surface.

Mars Ascent Vehicle (MAV) is quite large. It had to be soft landed, but even empty weighs much more than all 14 presupply missions combined. Given that NASA has (in the story) developed soft-landing capability for tens of tonnes, it's not clear why stuff as mission critical as the Hab is landed relatively inaccurately in lots of parts. It could be that this simply reduces mission cost and complexity, or that there was no practical way to land something as bulky as the hab (even disassembled) in one piece.

The MAV employs In-Situ Resource Utilization (or ISRU) to make fuel and oxidizer for the return flight. Two (Earth) years of power from a 100W Radioisotope Thermoelectric Generator (RTG) is enough to make 13kg of fuel (methane) and oxidizer (oxygen) from every 1kg of hydrogen (H2) precursor brought from Earth, for a total of nearly 20T of fuel.

At various points of the novel, Weir describes the MAV as weighing 32 metric tons when fully fueled, and standing 27m tall. This implies that it is very long and skinny, which is unnecessary in the thin Martian atmosphere. Not only that, this means a lot of rocket mass relative to the amount of fuel it can carry (spherical rockets are vastly more efficient, absent significant atmosphere). Needless to say that's a bad thing. By comparison, the Falcon 9, a long and skinny rocket by usual standards, is about 70m tall and weighs about 600T on launch. The MAV could easily be a conical shape perhaps 5m wide and 10m tall.

Weir states that it has two stages, though one stage is perfectly adequate for the relatively low delta-V required to reach Low Mars Orbit (4.1km/s). Nevertheless, with a 325s Isp methane-oxygen engine, a two stage system would have a 16T first stage, a 8T second stage, and a 8T orbital module, with an implied mass fraction of 81% fuel vs 19% metal in each stage.

Towards the end of the novel, engineers at JPL describe the MAV as having an unrealistically low launch weight of 12,600kg (12.6T) — similar to a fully-loaded Dragon capsule. So we'll assume this is the dry mass. Let's assume, then, that the orbital module is 8T, the first stage is 3T, and the second stage is 1.6T, empty. The 19.397T of fuel is distributed accordingly, implying an engine Isp of 405s in order to reach 4.1km/s of Low Mars Orbit. This is low for H2/O2 engines, but extremely high for a methane-oxygen engine. Even SpaceX's planned monster Raptor engine has a notional vacuum Isp of 380s.

In order to get to 5.8km/s and intercept the Hermes, the mass of the orbital module needs to be reduced from 8000kg to 4280kg, a reduction of 3720kg. This takes into account adding 780kg of fuel, removing 500kg from the first stage (pulling off an engine), and so on. The accuracy of the numbers indicate that Andy Weir did the math, but it's not clear on what metrics he designed the MAV and its launch system.

More generally, given that the total delta-V needed to get from Mars to Earth is *only* 7.8km/s, a MAV that flies all the way back to Earth is completely possible, though it would probably need to be bigger than the MAV presented here to have adequate life support. But given that the fuel/delta-V is most easily obtained on the surface of Mars, rather than brought from Earth, a direct ascent architecture actually makes a lot of sense.

On Sol 68, Watney points out that NASA never used large RTGs on crewed missions before Ares, but during the Apollo program RTGs were deployed by astronauts to power lunar seismometers. On Sol 69, Watney states that Lewis had buried the RTG for safety reasons. A RTG stashed somewhere on the surface, however, is much less likely to overheat.

Mars Descent Vehicle (MDV). In the Sol 7 log entry, Watney mentions that the Mars Descent Vehicle (MDV) is useless to him for escaping, since its thrusters cannot even lift its own weight. This, of course, refers to its weight when fully fueled. Before landing, much of its fuel has burned off and it can achieve neutral thrust for a hovered landing. Nevertheless, it lacks (by far) the fuel capacity, thrust, and delta-V necessary to fly anything back to orbit!

In Chapter 8, Bruce and Teddy discuss potential MDV modifications. It is strongly implied, though not stated, that the design would not admit the addition of more engine clusters, and they don't have the time to invent a bigger engine. It is likely that this is a narrative device.

Orbital Mechanics

On the first page, Watney states that he was six days into the best two months of his life. Evidently he was confused, as later it's made clear that the surface operations of the mission were only 30 days long.

30 days on the surface is the extent of surface stay permitted under an "opposition" class mission, wherein the astronauts fly by Venus on either the outbound or inbound leg. While shortening the overall mission, opposition class missions significantly lengthen the time spent in space, and also bring the spacecraft much closer to the Sun, increasing the crew's exposure to radiation.

The alternative mission design is the "conjunction" class mission, wherein the crew takes a relatively short 4-6 month Hohmann transfer flight either way, with a ~560 day stay on the Mars surface in between launch windows. Obviously, if Watney had been stranded on a conjunction mission, he would have had no shortage of snacks!

One additional detail is that Weir's spaceship, the Hermes, employs ion thrusters throughout the mission, enabling a wider class of missions and trajectories than the traditional point-and-shoot orbital mechanics described in the previous paragraphs.

In Chapter 16, the Purnell maneuver is discussed, by which the crew can return to Mars fewer days than the 404 it would take Iris to get there. It's probably worth noting that there is a very similar delta-V requirement for Iris to get to Mars vs a resupply probe to reach the Hermes. The advantage of the Hermes approach is that Iris had to be able to do entry, descent, and landing. If this is the case, Iris could also get there faster by borrowing a basic ion thruster package from, say, the Asteroid Redirect Mission (ARM) spacecraft. It's also not clear why all the crew need to return to Mars (aside from narrative reasons) - most or all could return to Earth in the entry vehicle while Hermes takes the Purnell maneuver to Mars to pick up Watney before he starves. Although the remaining crew would then depend on a new entry vehicle being sent up to meet them on their eventual return to Earth.

In Chapter 20, Annie somewhat incongruously asks why Hermes can't wait at Mars for Watney to get there, when it seems he'll be slowed by the dust storm. Venkat points out that Hermes is on a fly by and can't slow down enough to be captured into orbit, but this is not entirely true. On Sol 505, Bruce says to Venkat that Hermes is flying by Mars at 5.8km/s. Mars escape velocity is only 5.5km/s (the Earth's is 11.2km/s by comparison) meaning that a delta-V of only 300m/s is needed to capture into orbit. Given that Hermes can accelerate at 2mm/s/s, a two day burn would be sufficient to capture into a big elliptical orbit, drastically increasing their margin of error. Perhaps, if Hermes slows enough for an orbital capture, its launch window to return to Earth will close too quickly to be useful.

Of course, the MAV was designed to reach Low Mars Orbit, with a delta-V of 4.1km/s. Getting to 5.8km/s is highly non-trivial, as discussed in the previous section describing the MAV. Of course the unmodified MAV has life support, so Watney could wait while Hermes spirals down to 4.1km/s to pick him up (~30 days, because Hermes can't exploit the Oberth effect), while in the modified MAV he gets close to 5.8km/s, making it much easier for Hermes to rendezvous. A MAV that got to, say, 5.2km/s would split the difference nicely. Either way, the most likely explanation is that maneuvering Hermes to do this would make them miss the Earth launch window.

On Sol 543, Beck mentions that the modified MAV will hit 12gs during launch. While they have lightened the primary payload by about 16%, Watney also removed a spare engine, suggesting that the unmodified MAV would hit at least 10gs during launch, which is unlikely for a rocket designed to fly humans! Later, Johanssen reads out a velocity of 741m/s at an altitude of 1350m, which is staggeringly fast, implying an acceleration of 20.7gs. Perhaps she dropped a zero?

When Johanssen and Vogel talk about getting Watney to orbit, what they mean is solar orbit, since Watney will have to escape Mars entirely in order to be intercepted by Hermes.

During the intercept procedure, Ares 3 crew have to think fast to find additional sources of delta-V to move the Hermes close enough to catch Watney as he flies by. The distances and velocities mentioned during this passage in Chapter 26 are correctly calculated and almost entirely realistic.

Watney suggests making a small breach in his suit and using the stored gas as a rocket to close the velocity mismatch of 42m/s. Assuming he has 5kg of gas on board (including reserve tanks) and an exhaust velocity of 400m/s (unlike rocket exhaust, it's not hot) this confers about 17m/s of delta-V, which is just not enough. This idea is transferred to the Hermes, which will spit out its atmosphere to slow down. Assuming Hermes weighs 100T, it would have to lose about 5T of air to make up the required 29m/s of delta-V. At sea-level atmospheric conditions, this implies that Hermes has a volume of 4000 cubic meters, or a floor area of 1300 square meters, or 13,000 square feet, which is the same as a very large house. Perhaps Hermes has large pressurized volumes that aren't used much for habitation? Martinez estimates that the air will take 4 seconds to leave, which implies a relatively small hole, since the shockwave would take about 0.1s to cross a Hermes-sized volume of air. A realistic concept is that Hermes is composed of two large Bigelow inflatable modules each with a diameter of about 12m, such as the BA 2100 habitats. Also worth mentioning that the process of blowing the "Vehicular Airlock" (VAL) will send lots of airlock fragments into space, hopefully missing Watney.

From The Martian: A Technical Commentary by Handmer, Jermyn, Paragano, Lommen, Nosanov (2015)

Hermes (francisdrakex)
Inert Mass60,000 kg
Crew+Payload Mass7,000 kg
RCS Propellant6,000 kg
Argon Propellant29,000 kg
Dry Mass67,000 kg
Wet Mass102,000 kg
Mass Ratio1.52
ΔV24,000 m/s
EngineIon drive
Exhaust Velocity50,000 m/s
Single Engine Thrust5 N
Engines in Arrayx40
Total Thrust200 N
Acceleration0.002 m/s
Length85 m
Span22 m
Reactor Power10 MWth
Reactor Fuel600 kg 239Pu
Gravity0.4 g
Gravity TypeTumbling Pigeon
Gravity Spin3 rpm
Gravity Radius40 m

francisdrakex's version of Hermes

francisdrakex is a talented space artist who took a stab at designing the Hermes. He did an outstanding job if I say so myself, and not just because he was assisted by some data from this website.

The entire spacecraft was designed to fit inside a 5 m payload fairing for easy boosting into LEO.

His design used the "tumbling pigeon" method of artificial gravity, which is always a good choice to reduce the spin rate below nausea levels.

The ion engine array is mounted at the spin center, a classic technique from Stuhlinger's Ion Rocket.

As per standard best practices, the dangerously radioactive nuclear reactor is mounted as far as possible from the habitat module and the crew. The reactor has a set of heat radiators to reject waste heat. The radiators are in a triangular pattern, to keep them inside the shadow cast by the anti-radiation shadow shield.

The habitat module is a standard TransHab inflatable module.

Hermes (Movie)
Length83 m ?
120 m ?
Gravity0.4 g
Gravity TypeCentrifuge
Gravity Spin4.97 to 6.30 rpm
Gravity Radius9.0 to 14.5 m

Movie version of Hermes

The movie Hermes has the engines mounted aft the reactor, and a centrifuge to provide artificial gravity.

Ship's power is apparently from a set of 12 solar cell arrays, straight off of the International Space Station (the brown elongated rectangles). Which seems a bit redundant if you already have a nuclear reactor.

Rhett Allain did some calculations about the gravity centrifuge on the movie version of Hermes, and not surprisingly discovered that there was a bit of artistic license involved.

The novel states that the artificial gravity is 0.4 g. Mr. Allain did some measurements from the movie and figured the centrifuge is spinning at about 1.08 rotations per minute (0.109 radians per second). Unfortunately to produce 0.4 g the radius of the centrifuge would have to be an outrageous 329 meters! According to one of the graphics in the flight center, the Hermes is only 80 meters long.

Mr. Allain made some further measurements from the movie and concluded the centrifuge was about 9.0 to 14.5 meters in radius. To produce 0.4 g it would have to spin at an angular speed of 4.97 to 6.30 rotations per minute (0.52 to 0.66 rad/second). Which is right at the nausea limit.

Anyway 1.08 rpm is six times slower than 6.30 rpm, which is where the artistic license comes in.

Using the movie figures of 14.5 meters in radius and a spin rate of 1.08 rpm, the artifical gravity would be a pathetic 0.02 g, not 0.4.

Another difficulty is that the spacecraft is supposed to slow down at Mars and Terra by using aerobraking. This will require something like the ballute from 2010 The Year We Make Contact. Two of them, one for each braking.

Hyde Fusion Rocket

Hyde Fusion Rocket
PropulsionInertial Confinement
Thrust40,000 N
Exhaust Velocity2,650,000 m/s
Thrust Power54 gigawatts
Engine Specific
110 kW/kg
Ignition Rate
100 Hz
Magnetic Nozzle
Engine Mass
Magnetic Nozzle
coil and matrix8.7 metric tons
anti-burst structure8.5 metric tons
coolant coil8.1 metric tons
neutron shield44.4 metric tons
gamma-ray shield56.3 metric tons
lithium coolant27 metric tons
heat radiators13 metric tons
sub total166 metric tons
Driver module
0.520 metric tons
Driver module
0.435 metric tons
Driver module
0.955 metric tons
x200 Driver modules191 metric tons
Driver system
12 metric tons
Driver system
optical system
6 metric tons
Driver system
11 metric tons
Power transmission
5 metric tons
Compulsator12 metric tons
Capacitor banks25 metric tons
Driver system
sub total
262 metric tons
Total428 metric tons
Misc. Mass
Cargo Payload
(VIP Payload)
1,458 metric tons
(50 metric tons)
Cargo Fuel
(VIP Fuel)
650 metric tons
(2,058 metric tons)
Fuel Tank16 metric tons
Thrust Truss20 metric tons
Shadow Shield
17 metric tons
Auxiliary reactor5 metric tons
Total Mass
Wet Mass2,594 metric tons
Cargo Mass Ratio
(VIP Mass Ratio)

This is from A Laser-Fusion Rocket for Interplanetary Propulsion by Roderick A. Hyde (1983). I apologize for any mistakes but the document appears to be scanned from a poor photocopy of a pre-print that was almost unreadable. As you can see from the diagrams below.

Pellets of fusion fuel (with a coating of propellant) are injected into the reaction point at a rate of 100 pellets per second. There they are imploded by the Driver using a 2 megajoule? pulse of laser radiation from a krypton fluoride laser (which is only 6% efficient). The laser pulse is divided into 8 laser beams which are reflected by mirrors to converge at the reaction point from all directions. The laser pulse compresses the pellet, igniting the fusion reaction. Two krypton fluoride lasers will be used at 50 Hz, alternating pulses to make an effective pulse rate of 100 Hz. The Magnetic nozzle directs as much as it can of the exploding pellet's plasma energy into producing rocket thrust, and prevents as much as it can of the plasma energy from frying engine components.

Dr. Hyde estimates that this engine can carry 1,500 metric tons of payload, with an average trip-time of 6 weeks to Mars, 3 months to Jupiter, and 1 year to Pluto.


For the fusion fuel inside the pellet, you want as much of the energy to be in hot plasma as possible. Any neutrons and x-rays produced are wasted energy, since they contribute nothing to the thrust and cause damage to the ship and crew. The report has a long section discussion the relative merits of various fusion fuels, but Dr. Hyde settles on Deuterium-Deuterium fusion. The pellets contain 15 milligrams of deuterium salted with 36 micromoles of tritium.

About 30% to 50% of the deuterium fuel will burn, the rest will be wasted. Deuterium has a specific energy of 345 megajoules per milligram. The engine is designed for 2000 megajoules per pulse, so for deuterium at 40% burnup each pellet will require 15 milligrams of deuterium. The pellet of deuterium will be coated with propellant to increase thrust (increasing the propellant mass flow mdot) so that the total pellet mass is 150 milligrams. Pretty much any element can be used for propellant, Dr. Hyde used spare deuterium. This means that by varying the amount of extra propellant the engine can shift gears, i.e., trade exhaust velocity for thrust and vice versa. The on-board pellet factory can change this on the fly.

1280 megajoules will be in the form of charged particle fusion energy for thrust, about 710 megajoules will be wasted in the form of x-ray and neutron radiation (330 MJ of x-rays, 380 MJ of neutrons).

Magnetic Nozzle

The primary task of the thrust chamber's magnetic nozzle is to convert the exploding plasma into thrust. The secondary task is to generate the power required to energize the lasers in the driver. The tertiary task is to breed tritium for the fusion pellets, since the blasted stuff has an unstable half-life of barely 12 years.

A superconducting coil creates a magnetic field that reflects the exploding plasma. The coil is encased in neutron radiation shields, and has a heat radiator to keep the radiation shields from melting. The radiators are on the part of the coil farthest from the ignition point.

In a latter design described in a document I have as yet failed to lay my hands on, there are two coils instead of one: the S-coil and the B-coil. The S-coil is above the ignition point and the B-coil is below. The S-coil is smaller with a denser magnetic field, to encourage the plasma blast to exit out the weaker B-coil. The coils resemble slices of a cone

Since there is not much that can be done to stop the harmful x-rays and neutrons, the idea is to make the magnetic nozzle as "transparent" as possible. It is an open framework where all the components occupy a small fraction of the solid angle as seen from the fusion pellet explosion (just see the diagram below). You want to make all the components "edge on" to the explosion, which is why the coils look like conic sections. In other words, they are blade shields.

As the exploding plasma expands against the magnetic field of the thrust chamber, the field is moved. Induction coils (next to the nozzle coils) harvest this motion to generate electricity for the driver. 33 megajoules of electricity is generated, and stored in a compulsator flywheel for the laser driver to use for the next laser pulse.

Other engine designs try to turn all the plasma into electricity and use that to run an ion drive or something. This adds penalty mass in the form of the generator, and reduces the power available by the inefficiency of the generator. Dr. Hyde thinks it is better to just use the plasma directly as thrust.

The tritium breeder has to produce a minimum of 36 micromoles of tritium per pellet explosion. Otherwise the tritium supply will be operating at a loss, and will eventually run out. This is done with a tube full of liquid lithium-6 and lithium-7 with a loop near the detonation point. The lithium converts the neutrons from each blast into tritium. The trouble is that with the design of the lithium blanket there was a pathetic breeding ratio of only 54%, which mandates a neutron interception fraction of 0.055 in order to make 36 micromoles of tritium. The bottom line is that the liquid lithium will be heated at a hideous rate of 4.2 gigawatts, needing a huge heat radiator to prevent the rocket from vaporizing.

This means that the tritium breeding places a floor on vehicle heating; pellets should have a little tritium as possible. Dr. Hyde set it at 36 micromoles for reasons too complicated for me to understand.

The lithium (Li) loop tritium breeder is placed on the inner side of the magnetic nozzle coils, so they can soak up the neutrons and partially shield the coils. The loop and the coil are wrapped by a blade-shaped metal skin, which acts as an eddy current shield.

The magnetic nozzle is a high-energy type, unlike the low energy nozzle on a Daedalus starship. This means the magnetic field contains about five times the energy of the exploding pellet (radius of 6.5 meters with a current of 22 MA), where the Daedalus magnetic field is weaker than the pellet. High-energy types are more efficient at converting pellet explosion energy into thrust.

This is an axially-symmetric nozzle which means a jet of hot plasma will escape along the long axis, i.e., right into the ship's backside. A smaller magnet is used to deflect this jet so it misses the ship's derrière

The magnetic nozzle is 65% efficient at converting the exploding plasma into thrust. Coupled with the 1280 megajoules of plasma energy per pellet detonation means that the entire engine converts about 42% of the total pellet energy into thrust.

The coil will have to be a superconductor, if for no other reason because 22 MA of current will vaporize a conventional coil. Dr. Hyde specified a vanadium-gallium (V3Ga) superconductor with a 15.8 Tesla peak coil field at 4.8 K temperature will have a current density of 270 kA cm-2. The coil will be embedded in a matrix of vanadium and aluminum. The coil and matrix will have a radius of 6.5 meters and a mass of 8.7 metric tons.

However, remember that the same charge of magnetic field repel each other. 15.8 freaking Teslas will be doing their darndest to expand the coil (read: make the coil violently explode in all directions). The technical term is "magnetic bursting force." This will be resisted by 8.5 metric tons of structural composite. So the total coil+structure mass is 17.2 metric tons.

Keeping the coil cooled down to 4.8 K when it is being exposed to 2 gigawatts of neutron and x-ray energy is somewhat of a challenge. The lithium loop will remove the heat created by neutron radiation in addition to breeding tritium. Dr. Hyde figures it can handle the 2000 watt heat load.

Even worse, some of the neutrons that enter the coil's lithium hydride (LiH) radiation shield will scatter right back out, thus they can hit the coil from its unprotected rear side. The radiation shield will have to cover one side of the coil that is not in direct line of sight of the detonation point (the side where radiation can be reflected off the payload's radiation shield right back at the coil).

And then there is x-rays and gamma-rays, requiring a lead coating on the radiation shield.

So the lithium loop will require an 8.1 metric ton refrigerator to reject the heat plus 10 tons of liquid lithium, the lithium hydride neutron radiation shield is about 44.4 metric tons, and the gamma-ray radiation shield is about 56.3 metric tons.

Total magnetic nozzle mass: 126 metric tons.

As previously mentioned the magnetic nozzle system has to cope with 4.2 gigawatts of waste heat, from x-rays hitting the lead shield and neutrons hitting the lithium loop. The lithium will be the thermal working fluid to move the heat to the heat radiators (then it will have its tritium harvested). It will be pumped by a 20 megawatt MHD pump utilizing the thrust chamber's magnetic field. At a given time there is 10 metric tons of liquid lithium inside the thrust chamber sopping up neutrons, but the total system has 27 metric tons. This includes the liquid lithium in the long pipes leading to the heat radiators, and inside the radiators themselves.

The heat radiators are an array of 7,800 heat pipes, each 11 meters long and using lithium at 1500 K. The array mass is 40 metric tons.

Driver system

The pellets are imploded by 2 megajoules worth of laser beam applied in 10 nanoseconds. Dr. Hyde considered electron beams, but they are hard to focus on a tiny pellet and also cause nasty bremsstrahlung radiation. Proton beams have no bremsstrahlung, but since the detonation point is going to be ten to twenty meters away the proton beam will bloom due to electrostatic repulsion and be impossible to focus on the pellet.

Dr. Hyde considers free electron lasers (FEL), carbon dioxide (CO2) lasers, neodymium-doped glass (Nd:glass) lasers, and krypton fluoride (KrF) lasers. After a long discussion he figures the krypton fluoride laser is the least bad option.

Due to problems with heat rejection speed, Dr. Hyde decided to go with two laser systems alternatively firing at 50 Hz instead of one laser system firing at 100 Hz.

Each system will require 100 laser amplifiers (see diagram above). Each amplifier has a rotating cylinder with its lasers and a non-rotating heat pipe radiator. There are five lasers in the cylinder, firing at a rate of 10 Hz. Actually the cylinder is more like gas filled tube with five laser "buckets" on the rim. Between pulses the hot laser gas is exchanged with cool gas in the core, and the heat is rejected by liquid sodium heat pipes. The heat pipes radiate at a temperature of 900 K.

A module has a mass of 955 kilograms, of which 520 kilograms is laser and 435 kilograms is heat radiator. 100 modules per laser system and 2 laser systems means 200 modules are needed. Total mass is 191 metric tons.

In addition, the laser systems will need 12 metric tons of connecting trusses, 6 metric tons of optical system to combine and plus-stack the beams, and 11 metric tons for the oscillator system.

The laser driver will require 33 megajoules of energy per pulse. As described above, energy will be harvested from the engine and stored in a compulsator. 33 MJ will be extracted from the compulsator and placed in a capacitor bank, much like a camera strobe.

The power transmission lines connecting the engine and the compulsator have a mass of 5 metric tons, the compulsator has a mass of 12 metric tons, and the capacitor banks mass 25 metric tons.

Miscellaneous Components

There will be an neutron+gamma ray radiation shadow shield located 20 meters from the detonation point to protect the payload region. It will subtend a 3° half-angle, plus thin fins to shadow the thrust chamber heat radiators. Unfortunately the magnetic nozzle coil will scatter some radiation over the edge of the shield. Right behind the shield will be a magnet to deflect the naughty plasma jet.

The fusion pellets have to travel from the rear of the shadow shield to the detonation point during inter-pulse time. This means they have to have a speed of 2 kilometers per second. They will be propelled by a magnetic accelerator or laser ablation. Extreme precision will be required. In practice a pellet might be deliberately delivered slightly off axis from the detonation point in order to do thrust vectoring.

Behind the payload region is the pellet factory. It will take deuterium, tritium from the tritium breeder, and propellant and manufacture them into pellets. Dr. Hyde did not bother designing this but said he doubted it would be massive.

For cargo missions, Dr. Hyde figures the spacecraft will require 650 metric tons of deuterium fuel. If the acceleration is always below 0.1 g then the mass of the fuel tank would be about 16 metric tons.

The engine will be off in between missions and during coasting, so the engine will generate no power. An auxiliary nuclear fission reactor will be provided for housekeeping power and to restart the propulsion system. A 1 megawatt reactor with a mass of 5 metric tons will do.

There will be a truss to transmit thrust from its origin at the magnetic nozzle coil up to the payload. It has a mass of 20 metric tons. It has 8 primary thrust-bearing members. As with the lithium pipes (heat transfer and tritium breeding) the thrust-bearing members will be shadowed by the magnetic nozzle coil shield until the members reach the inner edge of the thrust chamber radiator (50 meters from the detonation point). At that location the members are laterally tied together by a structural ring, then fan out towards the laser driver radiating array. Upon reaching this site, the truss no longer has circular symmetry but is instead biased towards the radiation plane of the rocket. The thrust bearing members are hollow pipes tied together by a lateral truss to avoid buckling. It is rated for a maximum of 5,000 kilonewtons thrust.

Vehicle Performance

The important performance numbers to look at are the ratio of maximum exhaust power to engine mass (power-to-mass ratio) and the exhaust velocity.

The main limit on the power-to-mass ratio is the heat rejection capacity of the laser driver and thrust heat radiators (if you run the engine at a higher rate than the radiators can cope with the ship will melt or vaporize). The secondary limit is the actual mass of the engine.

Exhaust velocity is limited to a maximum of about 2.6×106 m/s due to the energy limits of deuterium fusion. In practice it will be further limited by the energy wasted producing neutron and gamma-rays, inefficiency of magnetic nozzle converting plasma energy into thrust, and most importantly the fraction of the pellet mass used to implode the fuel.

The engine is pretty lousy for interstellar propulsion at least 20% c due to the the exhaust velocity (20% c is greater than 2.6×106 m/s so mass ratio will be ugly).

However the engine will be marvelous for interplanetary travel. In that role the main limit is the power-to-mass ratio. Given a good ratio, the engine can be optimized to increase the thrust a little bit at the expense of the exhaust velocity (shifting gears). As a rule of thumb you want the thrust and wet mass of the spacecraft to be such that it can crank out a minimum of 5 milligees (0.05 m/s2) of acceleration. Otherwise the spacecraft will take years to change orbit. It is worth it to up the thrust enough to allow this even if you are robbing the exhaust velocity.

w = ƒτ * sqrt( (2 * Ek) / mp )

P = (mp * ν * w2) / 2

P = ν * Ek * ƒτ2

P = (F * w) / 2

αp = Pjet / Me

F = mp * ν * w

mDot = mp * ν


w = exhaust velocity (m/s) {2,650,000 m/s} which elsewhere in this site is symbolize by Ve
P = jet power, thrust power (W) {54,100,000,000 W = 54.1 GW} which elsewhere in this site is symbolize by Fp
αp = power-to-mass ratio or specific power (W/kg) {110,000 W/kg = 110 kW/kg}
F = thrust (N) {40,000 N}
ƒτ = efficiency of magnetic nozzle in converting charged particle energy into jet energy {0.65}
Ek = charged particle fusion energy (J) {1,280,000,000 J}
mp = pellet mass (kg) {0.00015 kg}
ν = pellet repetition rate (Hz) {100 Hz}
Me = mass of engine (kg) {486,000 kg}
mDot = propellant mass flow (kg/s)
sqrt(x) = square root of x
x2 = x squared

In the following tables, a "VIP Mission" is one with the shortest possible trip time, but with a microscopic payload. A "Cargo Mission" is one with a longer trip time in exchange for a reasonable cargo. In the cargo mission, given the total starting mass of the spacecraft (2,592 metric tons), 4/16ths is fuel/propellant mass (648 metric tons), 9/16th is payload mass (1,458 metric tons), and 3/16ths is engine mass (486 metric tons). The VIP mission has the same total starting mass and engine mass. The difference is that the payload mass is reduced and the fuel mass is increased.

Over a given mission the exhaust velocity and thrust is varied by changing the pellet mass.

Note that an acceleration of 1 g is 981 cm/s/s. 5 milligees is about 5 cm/s/s

VIP Missions
Table 4
Distance (AU)
Transit Time (dys)9.439.8153.9
Speed Max (km/s)165339667
Acceleration Max
Exhaust Vel
start (km/s)
Exhaust Vel
end (km/s)
Pellet Mass
start (gm)
Pellet Mass
end (gm)
Cargo Missions
Table 5
Distance (AU)
Transit Time (dys)22.293.6362
Speed Max (km/s)70144284
Acceleration Max
Exhaust Vel
start (km/s)
Exhaust Vel
end (km/s)
Pellet Mass
start (gm)
Pellet Mass
end (gm)

The tables above were calculated by with the following equations, whose implications I have not fully digested.

For interplanetary travel, the capability of an Inertial-confinement Fusion Rocket (IFR) is limited more by its power-to-mass ratio, than bi its exhaust velocity. The limitation on exhaust power translates into a bound on the product of exhaust speed "w" and acceleration "a", i.e., on aw. While a large value of w will eventually enable the rocket to reach a high speed, the low a means that doing so takes up a lot of time and distance. When the goal is to travel a given distance "D" as quickly as possible, the optimum technique involves accepting a lower w value in exchange for higher acceleration. This dialing of w can be accomplished in an IFR by placing excess propellant mass outside of the fusion pellet. The extra material lowers the exhaust velocity of the pelet, while increasing its impulse. Pellet-nozzle expansion calculations have been performed for different overall pellet masses, and shown no change in nozzle efficiency.

We have used three models, of increasing sophistication and opacity, to analyze the performance of this IFR. The first is the classic power-limited model, in which gravity and exhaust velocity limits are neglected. This case is easy to solve, and indicates the the pertinent scaling and operational modes. Next the w constraint is included. The resulting zero-gee motion can also be analytically solved, but in less useful form. This solution is then used as the starting point when numerically solving the 2D problem in which solar graviyt is included along with the w and aw constraints.

By neglecting solar gravity and vehicle exhaust velocity limits, we gain a simple insight into the performance capabilities of an IFR. Suppose one wishes to travel a distance "D" in time "T", starting and stopping at rest. The rocket has an initial mass of "M0", of which the powerplanet accounts for a fraction "β" and is characterized by a power-to-mass ratio of "η". The optimum tradeoff between a and w occurs for the time dependent acceleration:

The payload fraction "λ" can be shown to be given by

Note that D, T, and η appear only in the dimensionless parameter α. The trip time is seen to vary with the 2/3 power of distance, and with the inverse cube root of the power-to-mass ratio. There are two interesting operational modes suggested by Eq. 2. The "VIP" mode yields the shortest possible trip time, but a vanishing payload. For this mode:

For economical operation, one is willing to accept a longer trip time in exchange for a large payload fraction λ. The "Cargo" mode results from maximizing the payload throughput λ / T by optimizing over T and the choice of β. This optimum occurs at α = 1/16 and β = 3/16, resulting in a payload fraction λ = 9/16 and a fuel fraction of 1/4 (4/16). For this mode:

In Table 4 we demonstrate the VIP mode capabilities of this rocket, and show Cargo performance in Table 5. For purposes of comparison, the VIP mode numbers assume the same powerplant fraction, β = 3/16, which is optimal for cargo carrying; so the payload of the Cargo mission is swapped for more fuel in the VIP mission. The rocket exhaust power and exhaust velocity are given by:

(ed note: those two equations were already presented above)

For the current design, the nozzle efficiency ƒτ is 0.65, so the power P is 54.1 GW. Using the powerplant of Table 3, we find a power-to-mass of 110 W gm-1. The examples illustrated in Tables 4 and 5 span the range of solar system mission; Martian close approach to show high acceleration capability, Pluto transit to show the opposite extreme, and an average Jupiter mission. The Tables list the distance, trip time, maximum speed, maximum acceleration, the exhaust speed at the beginning and end, and the overall pellet mass at beginning and end.

The potential of this IFR for solar system propulsion is graphically illustrated by the trip times shown in Tables 4 and 5. A quick trip to Mars can be made in 9 days, while even in Cargo mode, Pluto can be reached in a year. In Cargo mode, the rocket can deliver 1,500 metric tons per mission; while the VIP method still permits delivery of ≈ 50 metric ton payloads.

While enlightening, the above analysis is incomplete. The acceleration profile of Eq. 1 requires a zero acceleration and infinite exhaust velocity at the midpoint of the trajectory. Hence, the exhaust speed constraint will be violated during these trajectories. This will certainly occur in the middle of the trips, and for longer missions can occur throughout the journey. When a limitation on w is imosed on the trajectory optimization problem, its solution is no longer as transparent as Eq. 2; but analytic results can still be derived.

(ed note: for more details see the report)


Human Outer Planet Exploration (HOPE) is from the NASA report TM-2003-212349 by Melissa McGuire, Stanley Borowski, Lee Mason, and James Gilland (2003). Revolutionary Concepts for Human Outer Planet Exploration (HOPE) . Revolutionary Concepts for Human Outer Planet Exploration (HOPE) { slide show }.

This is for a hypothetical mission to the Jovian moon Callisto. There are three spacecraft: a one-way tanker, a one-way cargo ship, and a round-trip manned ship. Note the manned ship uses an inflatable TransHab for the habitat module.

The Jovian moon Calllisto was choosen as a destination because it is outside of Jupiter's radiation belts, and it has water ice on the surface for propellant production. The purpose of the mission was to establish an outpost and propellant production facility near the Asgard impact site on Callisto. The In-situ Resource Utilization (ISRU) propellant processing plant will turn water ice into oxygen and hydrogen fuel for the lander. It and the surface habitat will be powered by a 250 kW nuclear reactor. The plant can produce enough fuel for one lander sortie mission between the base and the orbiting ship every 30 days.

The cargo vehicle is unmanned. It transports to Callisto a reusable crew lander, a surface habitat, and the ISRU propellant processing plant.The tanker is unmanned. It transports to Callisto orbit propellant tanks full of propellant the manned ship will need for the trip back to Terra. Both will be dispatched on a slow low-energy trajectory to Calliso.

Only after the unmanned vessels arrive at Callisto (especially the tanker) will the Piloted Callisto Transfer Vehicle (PCTV) be dispatched. It will arrive with most of its propellant expended. It will replenish its propellant from the tanker. The crew will explore Callisto for 120 days, then depart back home to Terra.

Several spacecraft were designed for the mission, each around a different propulsion system for comparision purposes.


ΔV138,000 m/s
Specific Power38 kW/kg
(37,700 W/kg)
Thrust Power111 megawatts
PropulsionFission Fragment
Payload60,000 kg
Wet Mass303,000 kg
Dry Mass295,000 kg
Propellant Mass4,000 kg
Length120 m
Span62 m
Radiator area6,076 m2
Total Power1 GW
Thrust43 N
Isp527,000 s
Exhaust Velocity5,170,000 m/s

Final Report: Concept Assessment of a Fission Fragment Rocket Engine (FFRE) Propelled Spacecraft . This HOPE spacecraft was designed using a Fission Fragment Rocket Engine.


This HOPE spacecraft was designed using Magnetoplasmadynamic (MPD) Nuclear Electric Propulsion (NEP). The habitat module is surrounded by tanks for radiation shielding. The tail radiators are cut in a triangular shape, and the outer heat radiators are arc shaped to keep them inside the shadow shield's radiation free zone, to prevent them from scattering radiation into the ship.

HOPE Cargo vehicle

HOPE Cargo vehicle
ΔV20,600 m/s
Specific Power2 W/kg
Thrust Power430 kW
PropulsionMPD thrusters
Specific Impulse8,000 s
Exhaust Velocity78,500 m/s
Wet Mass242,000 kg
Dry Mass182,000 kg
Mass Ratio1.3
Mass Flow1.4 x 10-4 kg/s
Thrust11 n
Initial Acceleration4.6 x 10-6 g
Payload120,000 kg
Length130 m
Diameter55 m

HOPE Tanker

HOPE Tanker
ΔV20,600 m/s
Specific Power2 W/kg
Thrust Power430 kW
PropulsionMPD thrusters
Specific Impulse8,000 s
Exhaust Velocity78,500 m/s
Wet Mass244,000 kg
Dry Mass184,000 kg
Mass Ratio1.3
Mass Flow1.4 x 10-4 kg/s
Thrust11 n
Initial Acceleration4.6 x 10-6 g
Payload103,000 kg
Length135 m

HOPE Crew vehicle

Piloted Callisto Transfer Vehicle
ΔV26,400 m/s
Specific Power6 W/kg
Thrust Power1.5 MW
PropulsionMPD thrusters
Specific Impulse8,000 s
Exhaust Velocity78,500 m/s
Wet Mass262,000 kg
Dry Mass188,000 kg
Mass Ratio1.4
Mass Flow3.6 x 10-4 kg/s
Thrust28 n
Initial Acceleration1.1 x 10-5 g
Payload79,000 kg
Length117 m
Diameter52 m

Stuhlinger Ion Rocket

Stuhlinger Ion Rocket
Length150 m
Wet mass360 metric tons
Dry massLander ship: 240 metric tons
Cargo ship:170 metric tons
Lander ship: 120 metric tons
Cargo ship: 190 metric tons
Mass of
Mars Lander
70 metric ton
Storm cellar
50 metric tons
Storm cellar
1.9 m
Storm cellar
Rotation rate1.3 rpm
0.14 g
115 MWt
40 MWe
4,300 m2
75 MWt
thrust98 N

Note the similarity to this 1962 Ernst Stuhlinger design for a Mars ion-drive rocket. In the mission plan, the expedition would have three spacecraft carrying a Mars lander, and two without. The astronauts would live in the storm cellars for the 20 days it would take to pass through the Van Allen radiation belts. Earth-to-Mars transfer would span mission days 57 through 204. On day 130 the thrust would be changed 180°, brachistochrone style.


Revolutionary Concepts for Human Outer Planet Exploration (HOPE) .

The third option utilizes Variable Specific Impulse Magnetoplasma Rocket (VASIMR) propulsion for all vehicles. VASIMR systems heat hydrogen plasma by RF energy to exhaust velocities up to 300 km/s producing low thrust with a specific impulse ranging from 3,000 to 30,000 seconds.

There is significant debate in the advanced propulsion community with respect to the complexity of the engineering challenges associated with the VASIMR system and hence for the purposes of the HOPE study, VASIMR was viewed at a lower state of TRL than MPD thrusters.

VASIMR performance potential was utilized in this option to improve upon the previous option. A single VASIMR propelled vehicle is used to transport the surface systems and return propellant to Callisto as opposed to two. As in the previous scenarios, the tanked/cargo vehicle remains in orbit around Callisto to be used a future propellant depot.

The piloted VASIMR vehicle was fitted with a second TransHab and configured with its main tanks clustered around the rotation axis. The two TransHabs balance each other and are connected by a pressurized tunnel so that the crew can move between them. Like the previous option, there are hydrogen tanks protecting the crew but they do not begin to empty till the last few months of the return mission. The resulting configuration reduces risk by having two crew habitats, the ability to generate artificial gravity throughout the entire mission plus significantly improved radiation protection.

The down side is that the payload masses have gone up due to combining the cargo and tanker vehicles and the piloted vehicle enhancements. The 10 MW that was used for the MPD option is not enough power for the VASIMR option to meet mission requirements. The VASIMR option does close assuming 30 MW on each vehicle resulting in a piloted mission round trip time of around 4.9 years with 32 days at Callisto. The total mission mass is between the previous two options with the benefits of increased safety and robustness.

From the report

HOPE (Z-Pinch Fusion)

Z-Pinch HOPE ship
Specific Power6.6 kW/kg
(6,553 W/kg)
Thrust Power3.6 gigawatts
PropulsionZ-Pinch fusion
Specific Impulse19,346 s
Exhaust Velocity189,780 m/s
Mass Flow0.2 kg/s
Thrust38,120 N
Dry Mass552,000 kg
Payload150,000 m
Length126 m
Diameter47 m
Mars 90 day mission
Wet Mass635,227 kg
Mass Ratio1.15
Total ΔV27,500 m/s
Mars 30 day mission
Wet Mass887,300 kg
Mass Ratio1.61
Total ΔV93,200 m/s
Jupiter mission
Wet Mass663,100 kg
Mass Ratio1.20
Total ΔV36,100 m/s
550 AU mission
Wet Mass738,400 kg
Mass Ratio1.34
Total ΔV57,200 m/s

Z-Pinch Pulsed Plasma Propulsion Technology Development Final Report. Like all the others, they started with the HOPE study, replaced the HOPE Magnetized Target Fusion engine with their own engine, and compared the two.


ΔV100,000 m/s
Specific Power34 kW/kg
(34,400 W/kg)
Thrust Power11.9 gigawatts
PropulsionAntimatter Catalyzed
Specific Impulse13,500 s
Exhaust Velocity132,000 m/s
Wet Mass707,000 kg
Dry Mass345,000 kg
Mass Ratio2
Mass Flow1.36 kg/s
Thrust180,000 newtons
Initial Acceleration0.255 g
Payload82,000 kg
Length72 m
Diameter190 m wide

This design for an antiproton-catalyzed microfission/fusion propulsion spacecraft is from the University of Pennsylvania.

Fuel pellets have 3.0 grams of nuclear fuel (molar ratio of 9:1 of Deuterium:Uranium 235) coated with a spherical shell of 200 grams of lead. The lead shell is to convert the high energy radiation into a form more suited to be absorbed by the propellant. Each pellet produces 302 gigajoules of energy (about 72 tons of TNT) and are fired off at a rate of 1 Hz (one per second). The pellet explodes when it is struck by a beam containing about 1×1011 antiprotons.

A sector of a spherical shell of 4 meters radius is centered on the pellet detonation point. The shell is the solid propellant, silicon carbide (SiC), ablative propellant. The missing part of the shell constitutes the exhaust nozzle. Each fuel pellet detonation vaporizes 0.8 kilograms of propellant from the interior of the shell, which shoots out the exhaust port at 132,000 meters per second. This produces a thrust of 106,000 newtons.

The Penn State ICAN-II spacecraft was to have an ACMF engine, a delta-V capacity of 100,000 m/s, and a dry mass of 345 metric tons. The delta-V and exhaust velocity implied a mass ratio of 2.05. The dry mass and the mass ratio implied that the silicon carbide propellant shell has a mass of 362 metric tons. The wet mass and the thrust implied an acceleration of 0.15 m/s2 or about 0.015g. It can boost to a velocity of 25 km/sec in about three days. At 0.8 kilograms propellant ablated per fuel pellet, it would require about 453,000 pellets to ablat the entire propellant shell.

It carries 65 nanograms of antiprotons in the storage ring. At about 7×1014 antiprotons per nanogram, and 1×1011 antiprotons needed to ignite one fuel pellet, that's enough to ignite about 453,000 fuel pellets.

Kuck Mosquito

RocketCat sez

This thing looks really stupid, but it could be the key to opening up the entire freaking solar system. Orbital propellant depots will make space travel affordable, and these water Mosquitos are just the thing to keep the depots topped off.

Kuck Mosquito
ΔV5,600 m/s
Specific Power4.8 kW/kg
(4,840 W/kg)
Thrust Power484 megawatts
PropulsionH2-O2 Chemical
Specific Impulse450 s
Exhaust Velocity4,400 m/s
Wet Mass350,000 kg
Dry Mass100,000 kg
Mass Ratio3.5
Mass Flow49 kg/s
Thrust220,000 newtons
Initial Acceleration0.06 g
Payload100,000 kg
Length12.4 m
Diameter12.4 m

Kuck Mosquitoes were invented by David Kuck. They are robot mining/tanker vehicles designed to mine valuable water from icy dormant comets or D-type asteroids and deliver it to an orbital propellant depot.

They arrive at the target body and use thermal lances to anchor themselves. They drill through the rocky outer layer, inject steam to melt the ice, and suck out the water. The drill can cope with rocky layers of 20 meters or less of thickness.

When the 1,000 cubic meter collection bag is full, some of the water is electrolyzed into hydrogen and oxygen fuel for the rocket engine (in an ideal world the bag would only have to be 350 cubic meters, but the water is going to have lots of mud, cuttings, and other non-water debris).

The 5,600 m/s delta-V is enough to travel between the surface of Deimos and LEO in 270 days, either way. 250 metric tons of H2-O2 fuel, 100 metric tons of water payload, about 0.3 metric tons of drills and pumping equipment, and an unknown amount of mass for the chemical motor and power source (probably solar cells or an RTG).

100 metric tons of water in LEO is like money in the bank. Water is one of the most useful substance in space. And even though it is coming 227,000,000 kilometers from Deimo instead of 160 kilometers from Terra, it is a heck of a lot cheaper.

Naturally pressuring the interior of an asteroid with live steam runs the risk of catastrophic fracture or explosion, but that's why this is being done by a robot instead of by human beings.

In the first image, ignore the "40 tonne water bag" label. That image is from a wargame where 40 metric tons was the arbitrary modular tank size.

There are more details here.


Normal Growth LCOTV
PropulsionIon Drive
Specific Impulse8,000 s
Exhaust Velocity78,480 m/s
Input power
per Engine
46 kW @
beam voltage
Engine Efficiency82%
Engine Life6,000 hours @
beam current
16 amps
per Engine
20 kg
Thrust per Engine0.7 N
Number of Engines206
Total Thrust145 N
Thrust to Weight5×10-5
Trip Time180 days
Payload Bay227 metric tons
at 100 kg/m3
Power PlantSolar Cell
Solar Cell Area54,416 m2
Structural Mass4,057 kg
Power Plant Mass26,831 kg
System Mass
11,671 kg
System Mass
2,217 kg
Thermal Control377 kg
Avionics520 kg
Growth Margin9,339 kg
Inert Mass55,012 kg
Payload Mass227,000 kg
Dry Mass282,012 kg
Propellant Mass29,744 kg
Propellant Reserves892 kg
Wet Mass312,648 kg
Mass Ratio1.11
ΔV8,190 m/s
Accelerated Technology LCOTV
PropulsionIon Drive
Specific Impulse8,000 s
Exhaust Velocity78,480 m/s
Thrust per Engine5.2 N
Number of Engines26
Total Thrust135 N
Thrust to Weight4.76×10-5 g's
Payload Bay227 metric tons
at 100 kg/m3
Power PlantSolar Cell
Solar Cell Area41,495 m2
Structural Mass2,880 kg
Power Plant Mass22,212 kg
System Mass
1,979 kg
System Mass
2,005 kg
Thermal Control68 kg
Avionics520 kg
Growth Margin5,075 kg
Inert Mass34,739 kg
Payload Mass227,000 kg
Dry Mass261,739 kg
Propellant Mass26,901 kg
Propellant Reserves784 kg
Wet Mass289,424 kg
Mass Ratio1.11
ΔV8,190 m/s

This is from Technology Requirements For Future Earth-To-Geosynchronous Orbit Transport Systems (1979). The unmanned Large Cargo Orbital Transfer Vehicle (LCOTV) transports cargo from Low Earth Orbit to Geosynchronous Orbit. The report also described a chemically powered LEO to GEO transport for priority cargo.

The report was trying to put a price tag on an transport system capable of handling the construction of a large solar power station.

According to a later report, for various reasons, the report concluded the project would require a fleet of 13 LCOTVs. Not all of the fleet would be on line, some would be undergoing maintenance. The ships in the fleet would be allocated such that the SPS project would receive a total of 56 flights, transport a total payload of 29,860 metric tons, make from 1 to 13 flights a year, and transport from 33 to 2010 metric tons per year.

Two designs were considered. The "Normal Growth" design used conservative extrapolations of the state-of-the-art, the "Accelerated Technology" assumed additional money was invested to increase the state of the art.

Lewis Research Center Ion Rocket

Lewis Ion Rocket
Propulsionion drive
Exhaust velocity20,000 to
100,000 m/s
Return payload
(cabin, etc.)
23,000 kg
Crew supplies
(4.5 kg/day/man)
18,000 kg
Exploration Rocket
(Mars lander)
18,000 kg
Powerplant41,000 kg
Propellant104,000 kg
Wet Mass204,000 kg
Dry Mass100,000 kg
Mass Ratio2.04
ΔV14,000 to
71,000 m/s
Mars mission
500 days
Length120 m

This is from a 1965 study by the Lewis Research Center entitled Space Flight Beyond The Moon.

This is a standard ion rocket powered by a nuclear reactor. The reactor is at the nose, behind a shadow shield and separated from the crew compartment by a 120 meter long boom in order to use distance as extra radiation shielding. The heat radiators are trimmed in order to not extend outside of the radiation shadow, their narrow aspect indicates the shadow shield is minimal in order to save on mass.

Higher ion drive exhaust velocities come at the expense of higher power requirements, which means higher power plant mass, which means lower acceleration. The report mentions that one can calculate the optimal exhaust velocity/power plant mass, but does not go into details. As a rule of thumb a rocket's acceleration should not be below 0.05 m/s2 (5 milligee) or it will take years to change orbits.

Some of the designs depict the crew cabin as two modules, which probably means they spin around the ship's spine for artificial gravity.

Luna from Destination Moon

PropulsionLiquid Core NTR
Specific Impulse1,050 s
Exhaust Velocity10,300 m/s
Wet Mass226,000 kg
Dry Mass45,000 kg
Mass Ratio5.0
ΔV16,600 m/s
Thrust11,000,000 N
Thrust Power56 gigawatts
Initial Acceleration5g
49 m/s2
Engine Mass9,000 kg
Structure Mass27,000 kg
Payload Mass9,000 kg
Propellant Mass181,000 kg
Dry Volume70 m3
Total Volume100 m3
Length46 m
Ladder Length25 m
Body Diameter5.6 m
Wing Span21 m

Inspired by a post by Retro Rockets I took a look at the classic spaceship Luna from the movie Destination Moon (1950). With Robert Heinlien as technical consultant, this movie was the most scientifically acurate one since Frau im Mond (1929). It held the throne for 18 years, until it was supplanted by the movie 2001: A Space Odyssey (1968).

For the specifications I used data from Spaceship Handbook and the Retro Rockets article. I then massaged the figures until they were internally consistent.

Spaceship Handbook calculated that a round trip mission to the surface of Luna would take about 16,480 m/s of delta V. So that's our performance limit for the mission. In addition, it will have to have a thrust-to-weight ratio greater than 1.0, since it has to lift off from Terra's surface. The movie specifies 5 gs, which translates to 11,000,000 newtons.

The movie specified that the reaction mass was water, not liquid hydrogen. While this does simplify the tankage, it does cut the exhaust velocity/specific impulse in half.

A solid-core nuclear thermal rocket engine is not going to be able to crank out enough delta V, not at the specifed mass-ratio it ain't. But the liquid-core Liquid Annular Reactor System (LARS) will do nicely. It can jet out liquid-hydrogen propellant at 20,000 m/s or better, so it can probably manage to hurl water at 10,300 m/s. That will give the Luna a delta-V of 16,600 m/s, just a tad larger than the required 16,480 m/s for the Lunar mission. More than enough, assuming you don't waste a lot of delta V during the landing.

The movie says the structural mass is 27 metric tons, which makes it 60% of dry mass. Nowadays NASA vessels typically have a structural mass of 21.7% of structual mass. 60% is a bit extravagant but believable with 1950's technology. If you made the structure NASA-light, you could add about 17 metric tons to the payload. The payload is the crew, equipment, life support, acceleration couches, and controls.

Lighter and Tanker

Specific Impulse450 s
Exhaust Velocity4,410 m/s
Wet Mass56,300 kg
Dry Mass25,898 kg
Inert Mass898 kg
Payload25,000 kg
Mass Ratio2.17
ΔV3,410 m/s
Mass Flow31.8 kg/s
Thrust140 kiloNewtons
Initial Acceleration0.25 g
Length18.3 m+engine
Diameter≈4.57 m
gas core NTR
Specific Impulse3,600 s
Exhaust Velocity35,000 m/s
Wet Mass433,000 kg
Dry Mass268,000 kg
Mass Ratio1.61
ΔV16,730 m/s
Inert Mass
(dry mass - payload)
108,000 kg
Payload Mass total160,000 kg
Payload Mass Hydrogen
(less tankage)
139,000 kg
Mass Flow100 kg/s
Thrust3,500 kiloNewtons
Initial Acceleration0.6 g
Length37 m+engine
Diameter≈18 m

These two designs are from The Resources of the Solar System by Dr. R. C. Parkinson (Spaceflight, 17, p.124 (1975)). The Lighter ferries tanks of liquid hydrogen from an electrolyzing station on Callisto into orbit where waits the Tanker. Once the Tanker has a full load of tanks it transports them to LEO. All the ships are drones or robot controlled, there are no humans aboard. The paper makes a good case that shipping hydrogen from Callisto to LEO would eventually be more economically effective than shipping from the surface of Terra to LEO, with the break-even point occurring at 7.8 years. Please note that this study was done in 1975, before the Lunar polar ice was discovered, and probably before the ice of Deimos was suspected.

Warning: most of the figures in the table are my extrapolations from the scanty data in the report. Figures in yellow are sort of in the report. Use at your own risk.

The tanker uses a freaking open-cycle gas-core nuclear thermal rocket. This is an incredibly powerful true atomic rocket, but it is only fractionally more environmentally safe that an Orion nuclear bomb rocket. The report says it should be possible to design it so the amount of deadly fissioning uranium escaping out the exhaust is kept down to as low as one part per 350 of the propellant flow (about 300 grams per second), but I'll believe it when I see it. Since it is used only in deep space we can allow it, this time. The report gives it an exhaust velocity of 35,000 m/s, which is about midway to the theoretical maximum.

The lighter can get by with a more conventional hydrogen-oxygen chemical rocket. It will need an acceleration greater than Callisto's surface gravity of 1.235 m/s2, for safety make it 1.5x the surface gravity, or about 1.9 m/s (0.6g).

The four major Galilean moons are within Jupiter's lethal radiation belt, except for Callisto. The black monolith from 2010 The Year We Make Contact only told us puny humans to stay away from Europa, so Callisto is allowed. If you want ice that isn't radioactive, you've come to the right place. It is almost 50% ice, and remember this is a moon the size of planet Mercury. That's enough ice to supply propellant to the rest of the solar system for the next million years or so. Europa has more, but it is so deep in the radiation belt it glows blue. Callisto is also conveniently positioned for a gravitational sling shot maneuver around Jupiter to reduce the delta-V required for the return trip to Terra.

The report says that the requirements for an economically exploitable resource are:

  1. It is not available in the Terra-Luna system
  2. It must provide more of it than the mass originally required to be assembled in Terra orbit at the outset of the expedition
  3. It must be done within a reasonably short time (the break-even time)

Hydrogen fits [1], or at least it did until the Lunar ice was discovered. [2] and [3] depend upon the performance of the vehicle.

There are three parts. First is the Tanker, which is an orbit-to-orbit spacecraft to transport the hydrogen back to LEO and brings the expedition to Callisto in the first place. Next is an electrolysis plant capable of mining ice, melting it into water, cracking it into oxygen and hydrogen, and liquefying the hydrogen. Last is a Lighter which is an airless lander that ferries liquid hydrogen from the plant on Callisto to the orbiting Tanker.

The report decided to use modular cryogenic hydrogen tanks that would fit in the Space Shuttle's cargo bay. They would have to be about 18.3 meters x 4.57 meters, about 300 cubic meters capacity. The report has a filled tank massing at 26,000 kg, with 22,000 kg being liquid hydrogen and 4,000 kg being tank structural mass. Examining the drawing of the tanker, the front cluster is composed of four tanks while the rear has nine, for a total of thirteen. The tanker will have a length of two tanks plus the length of the rocket engine, 37 meters plus rocket. The rear has tanks arranged in a triangular array about four tanks high. So a diameter roughly 18 meters or so.

The lighter carries a single tank, so it is roughly one tank in diameter, and one tank long plus the fuel tanks+engine length. It will need a large enough liquid hydrogen/liquid oxygen chemical fuel capacity to lift off from Callisto to the tanker and land back on Callisto.

The report figures that the electrolysis plant can produce hydrogen for about 39 kW-h/kg, that is, each kilogram of hydrogen in the plant requires 39 kilowatt-hours. Figure it needs more electricity to liquefy the hydrogen, and more to produce the liquid oxygen needed by the lighter, for a total cost of 50 kW-hr/kg for liquid hydrogen delivered to the orbiting tanker. So a 2 megawatt nuclear reactor could produce 350 metric tons of hydrogen per year. Launch windows back to Terra occur every 398.9 days.

Once the lighter has made enough trip to fully load the tanker, the tanker departs for LEO. It will use some of the hydrogen for propellant, some will be the payload off-loaded at LEO, and enough will be left to return the tanker to Callisto. The amount of payload is specified to have a mass equal to 37% of the fully loaded mass of the tanker. It also specifies that the inert mass fraction of the tanker is 25% of the tankers fully loaded mass.

The report had an esoteric equation that calculated the mass of the lighter and electrolysis plant as a percentage of the tanker mass in order to be economically viable. It turns out to be 13% of the fully loaded mass of the tanker. When the expedition is launched the tanker will carry the lighter, the electrolysis plant, and enough propellant so that the total mass is 52.9% of the fully loaded mass (i.e., it departs half empty). The lighter will have its tanks full.

Five years later, upon arrival at Callisto, the lighter lands the electrolysis plant on a prime patch of ice. It then starts the cycle: patiently waiting for the plant to fill the payload tank and the fuel tanks, boost the payload to the tanker, then land back at the plant to start again.

In context: this was one of a series or articles I wrote for Spaceflight at the encouragement of the then editor, Ken Gatland, triggered off in the dark days following abandonment of the Apollo programme by a discussion at the BIS as to what would be needed to make spaceflight self-supporting. The first article was published in Spaceflight 1974 p.322 under the title "Take-Off Point for a Lunar Colony." There was then a second on "The Colonization of Space" (S/F 1975 p.88) and a couple of subsequent ones on Lunar Colonies (S/F 1977 p.42/103). Later, when they invented the first spreadsheets, I did some speculation on how the economics of everything might fit together economically in a big input-output model which got published as "The Space Economy of 2050 AD" in JBIS v.44, p.111 (1991) which also appeared in my book Citizens of the Sky (1989) later. It is unlikely that I was consistent through all of this — my opinions develop with time — and by the 1991 period I was heavily in to the economics or reusable launchers and what would happen if the models were pushed to very high flight rates.

Going back to "The Resources of the Solar System", I'm not sure how much detail I managed at the time. I remember that there were a couple of things influencing me at the time. One was the concept of a gas core nuclear engine (GCR) which might have a specific impulse of about 3500 sec (35 km/s). To really move around the Solar System you need a high thrust-to-mass engine with this sort of specific impulse, and GCR had the interesting property of using hydrogen as propellant. (Ion motors can meet the specific impulse, but to do a similar job would require a power-mass ratio several orders better than anything we could consider then or even today — VASIMIR suffers the same problem). Nowadays I might put my money more on a pulsed-fusion system (see “Using Daedalus for Local Transport,” JBIS, 62, p. 422-426 (2009)) — note NOT using helium-3, which would change the model significantly.

The second thing at the time that influenced me — at a time when the Space Shuttle was still a paper vehicle — was that the Space Shuttle payload bay was just about the right size (15 ft × 60 ft) to carry a full liquid hydrogen tank (there are reasons now why it wouldn't which led to the abandonment of design work using Centaur as an upper stage) — so my modular design was based around using that as a standard tank. For use in long duration space missions the tank would have to have some sort of active cooling system to keep the hydrogen from boiling away, but given that you could then ship LH2 around the Solar System on slow, economical trajectories like modern oil tankers on Earth. Once you have rerfuelling stations at either end interplanetary flight becomes a lot easier and you can think of using higher speed trajectories for special cargo like human beings.

From personal email from Dr. Parkinson (2014)

All the other figures in the table are ones I've extrapolated from the few figures given in the report.

A plausible figure for nuclear power generation is 0.12 Megawatts per ton of generator. This would make the electrolysis 2 MW power reactor have a mass of 16,000. This is close to the 25,000 kg mass of a payload tank. So to simplify, assume the electrolysis rig with liquefaction gear and all masses a total of 25,000. This also ensures that the lighter is capable of landing it.

The tanker's inert mass fraction is 25%, and hydrogen payload is 37%. This means the dry mass is 62%, which means the mass ratio is 1.61. With an exhaust velocity of 35,000 m/s, this yields a total delta-V of 16,730 m/s. I am unsure if this is enough for a Callisto orbit-LEO mission followed by a LEO-Callisto orbit mission. Not without a heck of a gravitational sling-shot it isn't. Or I could have made a mistake in math.

Note both the payload and the propellant is hydrogen, stored in the same array of tanks. If the inert mass fraction is 25%, then the payload+propellant mass fraction is 75%. If there are 13 tanks each of 25,000 kg, then the total is 325,000 kg. If this is 75% of the wet mass, the actual wet mass is 433,000 kg. If the payload is 37% of the wet mass, it is 160,000 kg. If a hydrogen tank is 87% hydrogen and 13% tankage, the amount of hydrogen payload is 139,000 kg.

On the initial trip, the tanker carries the electrolysis plant and the lighter (with no payload, but with full fuel tanks). This is 13% of the wet mass or 56,300 kg. If the electrolysis plant is 25,000 kg, the lighter (with no payload) must be 31,290 kg. The lighter payload is one payload tank at 25,000 kg. So the lighter wet mass is 56,290 kg.

The lighter needs a delta-V of 3,414 m/s (Callisto-surface-to-orbit + orbit-to-Callisto-surface). Chemical fuel has exhaust velocity of 4,410 m/s. This means the mass ratio has to be 2.17. This implies the dry mass is 25,898 kg. Subtract the 25,000 kg payload, and there is 898 kg for the structure and the engine. Seems a little flimsy to me, perhaps 25,000 kg is a bit to generous for the payload tank.

Tank is scaled to fit in Space Shuttle cargo bay. At least the the proposed size of the bay in 1975 when the report was written, it was later reduced in size.

  • 4.55 m wide × 18.2 m long
  • Mass 26 metric tons
  • LH2 Mass 22 metric tons

Notes, from left to right:

  1. Docking Ring
  2. Limited amount of pressurization equipment round head end, also radar transponder
  3. Strong ring with attachment points
  4. Recirculating and pressurization pipes
  5. Strong ring with attachment points
  6. Docking Ring
  7. Fill valve connect. Associated propellant management equipment
  8. Radiator

Mars Expedition Spacecraft

This is from a NASA Manned Spaceflight Center (renamed the Johnson Space Center in 1973). The study was done in 1963. I have not been able to find lots of hard details, but there is some information in David Portree's monograph Humans to Mars on pages 15 to 18, available here.

It travels in a Hohmann transfer to Mars, separated into two parts spinning like a bola for artificial gravity. In Mars orbit, the heat shield, laboratory, and rendezvous ship separate and land. After a forty day stay, the astronauts use the rendezvous ship to climb back into orbit and travel to the mother ship. After the journey back to Terra, the astronauts land via the re-entry module.

Mars Umbrella Ship

RocketCat sez

There is just something about this surreal design that gets to you. People who briefly saw the deep space umbrella in 1957 still remember it. Totally unlike any other spacecraft you've ever seen. That is, except for science fiction ships from artist who also were haunted by the blasted thing.

Not a bad ship either. Except that pathetic one-lung ion drive is so weak that it takes a third of a year to reach orbit halfway between Terra and Luna. I'm sure we can do better than that today. Swap it out for a VASIMR or something and you'll have a ship that can go places and do things!

Stuhlinger Umbrella Ship
Total Travel
6.30×107 sec2 years
Payload1.50×108 g150 tons
Ionic Mass
2.20×10-22 g
Specific Power5.00×104 erg/sec g0.95 kW/kg
6.70×10-2 cm/sec26.7×10-5 G
Initial Mass
7.30×108 g730 tons
3.65×108 g367 tons
Power Planet
2.15×108 g215 tons
Total Power
1.15×1015 erg/sec114.5 MW
Total Electrical
2.29×1014 erg/sec22.9 MW
in Jet
2.06×1014 erg/sec20.9 MW
Driving Voltage4.88×103 volts
Total Ion
4.22×103 amperes
1.15×104 cm115 metres
Total Length8.50×103 cm85 metres
8.40×106 cm84 km/s
Thrust4.85×107 dyne48.5 kgf
*dm/dt5.77×100 g/sec0.00577 kg/s
*delta-vee5.82×106 cm/sec58.2 km/s
Values from paper. * values derived by Adam Crowl

Unusual spacecraft designed by Ernst Stuhlinger in 1957, based on a US Army Ballistic Missile Agency study. It made an appearance in a Walt Disney presentation "Mars and Beyond". 4 December 1957. David S. F. Portree, noted space history researcher and author of Wired's Beyond Apollo blog, managed to uncover the identity of Dr. Stuhlinger's report for me, it is NASA TMX-57089 Electrical Propulsion System for Space Ships with Nuclear Power Source by Ernst Stuhlinger, 1 July 1955. Thanks, David!

Detailed blueprints of this spacecraft can be found in the indispensable Spaceship Handbook by Jack Hagerty and Jon C. Rogers, or are available separately.

The spacecraft resembled a huge umbrella, with the parasol part being an enormous heat radiator.

At the very bottom is a 100 megawatt (thermal power) fast neutron nuclear reactor, mounted on a 100 meter boom to reduce the radiation impact on the crew habitat. A fast neutron reactor design was chosen because they can be built will a smaller mass and smaller size (reducing the size of the shadow shield). The reactor is capped with a shadow shield broad enough to cast a shadow over the entire heat radiator array. The part of the shadow shield closest to the reactor is 1.8 meters of beryllium. This stops most of the gamma rays, and slows down the neutrons enough that they can be stopped by an outer layer of boron. The shadow shield has a mass of 30 metric tons, and coupled with the boom distance it reduces the radiation flux at the habitat ring to 10 fast neutrons per second per cm2 and 100 gamma rays per second per cm2.

The liquid sodium will be carried in pipes constructed of molybdenum. The reactor will have a specific power around 0.1 kW per kg. It contains 0.6 cubic meters of uranium enriched 1.7%, and has a mass of 12 metric tons. No moderator or reflector is required. "Cool" liquid sodium (500° C) enters the reactor and leaves the reactor hot (800° C) at the rate of 300 kg/sec. The reactor contains 600 molybdenum pipes with an inner diameter of 1.8 cm and a length of 1 meter. Electromagnetic pumps move the liquid sodium, since it is metallic. Such pumps are used since the only way to make pumps that will operate continuously for over a year with high reliability is to have no moving parts. The pumps will consume about 100 kW.

The hot sodium enters the heat exchanger, where it heats up the cool silicon oil working fluid. The now cool liquid sodium goes back to the reactor to complete the cycle. The heat exchanger is used because silicon oil is more convenient as a working fluid, and because the liquid sodium becomes more radioactive with each pass through the reactor. The heat exchanger contains 3000 tubes for liquid sodium, with a total length of 1,800 meters and an inner diameter of 1.3 cm. The silicon oil is boiled into a vapor at 500° C under 20 atmospheres of pressure.

The hot oil vapor travels up the boom to a point just below the umbrella. There it runs a turbine which runs a generator creating electricity. The turbine is a low-pressure, multi-stage turbine with a high expansion ratio. Silicon oil was selected since it can carry heat and simultaneously lubricate the turbine, since this has to run continuously for over a year. Silicon oil is also liquid at 10° C, allowing the power plant to be started in space with no preheating equipment. The oil has a specific heat of about 0.4 cal per g per degree C, a heat of vaporization of 100 cal per g, a density of 1 g per cm3. If the umbrella heat radiator is at a temperature of 280° C, this implies that about 100 kg/sec must flow through the turbine. The feed pumps will consume about 200 kW. The total mass of the working fluid in the entire system will be about 8 metric tons.

Newton's third law in the turbine causes the section of the spacecraft from turbine upwards to rotate, including the ring habitat module and the umbrella heat radiator. The spin rate is about 1.5 rotations per minute. The generator is cooled by small square heat radiators mounted on the habitat ring.

The boom below the turbine is counter-rotated so it remains stationary. This is because the boom has the ion engine. If the boom was not counter-rotated, the ion engine would also rotate. The result would be a stationary ship behaving like a merry-go-round, spinning in place while spraying ions everywhere like an electric Catherine wheel.

The hot silicon oil vapor is injected into the central part of the rotating umbrella heat radiator (the radiator feed), and centrifugal force draws it through the radiator. The cooled oil is collected at the rim of the radiator, and pumped back to the reactor to complete the cycle. The rotation of the ring habitat module provides artificial gravity for the crew. The habitat ring is in the central part of the umbrella.

The umbrella heat radiator will have a temperature of 280° C. The silicon oil vapor will be reduced to the low pressure of 0.1 atmosphere, to reduce the required mass of heat radiator. The ship will be oriented so that the umbrella is always edge on to the Sun, for efficiency. The diameter of the umbrella will be about 100 meters, constructed of titanium. The wall thickness is 0.5 mm, the thickness of the disk is 6 cm near the center and 1 cm near the rim. The umbrella is composed of sectors, each with an inlet valve near the center and an outlet valve at the rim. If any sector is punctured by a meteorite, the valve will automatically shut until repairs can be made. The other sectors will have to take up the slack.

The electricity runs an ion drive, mounted on the lower boom at the ship's center of gravity. The ion drive uses cesium as propellant since that element is very easy to ionize. Cesium jets have a purplish-blue color. The umbrella section and the reactor have about the same mass, since the reactor is composed of uranium. The habitat ring has a bit more mass, this is why the ion drive is a bit above the midpoint of the boom.

Cesium has a density of 1930 kilograms per cubic meter. The spacecraft carries 332,000 kilograms of cesium reaction mass. This works out to 172 cubic meters of reaction mass, which would fit in a cube 5.6 meter on a side. Which is about the size of the block in between the ion drive and the landing boat, the one with the boom stuck through it. (ah, as it turns out my deduction was correct, now that I have the original report to read)

However, cesium propellant is now considered obsolete, nowadays ion thrusters instead use inert gases like xenon. Cesium and related propellants are admittedly easy to ionize, but they have a nasty habit of eroding away the ion drive accelerating electrodes. Xenon is inert and far less erosive, it is now the propellant of choice for ion drives.

Mounted opposite the ion drive is the Mars landing boat. It is attached so its center of gravity is along the thrust axis. This ensures that the umbrella ship's center of gravity does not change when the landing boat detaches. The landing boat uses a combination of rockets and parachutes to reach the surface of Mars. The upper half lift off to return to the orbiting umbrella ship.

The habitat ring has an outer radius of 19.5 meters, an inner radius of 15 meters, and a height of 6 meters (according to the blueprints). If I am doing my math properly, this implies an internal volume of 2,900 cubic meters, less the thickness of the walls. At a spin rate of 1.5 rotation per minute, that would give an artificial gravity of about 0.05g.

Above the umbrella and habitat ring is an airlock module containing two "bottle suit" space pods. Above that is a rack of four sounding rockets with instruments to probe the Martian atmosphere. At the top is the large rectangular antenna array.

The spacecraft is much lighter than an equivalent ship using chemical propulsion, and has a jaw-droppingly good mass ratio of 2.0, instead of 5.0 or more. However, the spacecraft's minuscule acceleration is close to making the ship unusable. It takes almost 100 days to reach an orbit only halfway between Terra and Luna. At day 124 it finally breaks free of Terra's gravity and enters Mars transfer orbit. It does not reach Mars capture orbit until day 367, but it takes an additional 45 to lower its orbit enough so that the landing boat can reach Mars. All in all, the umbrella ship takes about 142 days longer than a chemical ship for a Mars mission, due to the low acceleration. Which is bad news if you are trying to minimize the crew's exposure to cosmic radiation and solar proton storms.

The design might be improved by replacing the ion drive with an ion drive with more thrust, or with a magnetoplasmadynamic, VASIMR or other similar drive invented since 1957.

In his paper, Dr. Stuhlinger proposed that the Mars expedition be composed of a fleet of several ships. The Mars exploration equipment would be shared among all the ships. In addition, there would be some "cargo" ships. These would only carry enough propellant for a one-way trip, so they could transport a payload of 300 metric tons instead of 150. They would be manned by a skeleton crew, who would ride back to Terra on other ships.

Master artist Nick Stevens has recreated the umbrella ship in a series of images. Click to enlarge.

I am not quite the artist that Nick Stevens is, but I had to try my hand at it. Click to enlarge.

Michael Nuclear Pulse Battleship

RocketCat sez

Oooooh, Yeah!!! The Orion-drive Michael Battleship is the biggest meanest son-of-a-spacer in the cosmos! Well, maybe second to the Project Orion Battleship.

Just look at that bad boy! Can't you just see that unstoppable titan blazing into orbit on a pillar of multiple nuclear explosions, ready to kick that alien bussard ramjet's buns up between its shoulder blades? The drawback to Orion-drive is that it don't scale down worth a darn. So they didn't even try. No "every gram counts" worries here, they freaking chopped the main guns off the freaking Battleship New Jersey and welded them on!

Any casaba howitzer weapons? Naw, spears of nuclear flame are too feeble. They are using full-blown freaking Excalibur bomb-pumped x-ray lasers! Not infrared, not visible light, not even ultraviolet. X-rays. Just like Teller intended.

What's that you ask? What about the pumping bomb? Well, this is an Orion-drive, moron. That's whats driving the ship. Spit out a few Excaliburs, they aim their hundreds of laser rods on their targets, then the next pulse unit simultaneously thrusts the ship and energizes the graser beams. Another jumbo-sized order of crispy-fried elephant, coming right up!

Still have megatons of payload allowance left over? Well, how about carrying a small fleet of gunships with nuclear missiles? And all four space shuttles?

The look on the elephant's faces was priceless! Michael is coming. And is he pissed!

Battleship Michael
PropulsionOrion Drive
Height226 meters
Diameter113 meters
Massbetween 35,000
and 50,000
metric tons

Warning: spoilers for the book Footfall by Larry Niven and Jerry Pournelle to follow. On the other hand, the novel came out decades ago in 1985. I mean, in the novel the U.S.S.R. still exists. It takes place in the far flung future year 1995.

Footfall is arguably the best "alien invasion" novel ever written. Just like The Mote in God's Eye is arguably the best "first contact" novel ever written. But I digress.

Aliens (called "Fithp") who look like baby elephants arrive from Alpha Centauri in a Bussard ramjet starship (hybrid Sleeper ship and Generation ship). The starship is named "Message Bearer." They immediately ditch the Bussard drive module into the Sun, destroying it. If the Fithp are defeated, the humans can jolly well build their own Bussard drive from scratch to travel to Alpha C and attack the Fithp homeworld.

The Fithp evolved from herd animals, unlike humans. They have a very alien idea of conflict resolution. When two herds meet, they fight until it was obvious which one was superior. Then everybody immediately stops fighting, and the inferior herd is peacefully incorporated into the superior tribe as second-class citizens. Fithp do not comprehend the concept of "diplomacy".

They make the unwise assumption that human beings operate the same way. Big mistake!

The Fithp have somewhat superior technology compared to humans. They attack and seize the Russian space station (the ISS was not started until 13 years after the novel was written), annihilate military sites and important infrastructure with rods from God, then invade Kansas. The Fithp think "Look, humans. We are obviously superior. Now is the time to stop fighting and be peacefully incorporated into our herd." The Fithp calmly wait for the human surrender.

Humans don't work that way (and they have no idea that the Fithp have such a bizarre way of interacting). They savagely counterattack with the National Guard and three US armored divisions. The Fithp are taken aback, and beat off the counterattack with orbital lasers and more rods from God. The humans respond with a combined Russian and US nuclear strike on Kansas, obliterating the Fithp invasion force and most of the Kansas heartland.

The Fithp start panicking. What is it going to take to make these crazy humans surrender?

Finally the Fithp decide to forgo all half-measures. They drop a small "dinosaur killer" asteroid on Terra. The asteroid is called "The Foot." This causes global environmental damage, and more or less kills everybody living in India. Surely this will make the humans surrender!

The Fithp obviously don't know humans very well.

The humans have their backs to the wall, since surrender is not in their nature. The US president has a tiger team of advisers, who were drawn from the ranks of science fiction authors. After all, they are the only experts on alien invasions (in the novel, the various advisers are thinly disguised versions of actual real-world authors. Nat Reynolds is Larry Niven, Wade Curtis is Jerry Pournelle, and Bob Anson is Robert Anson Heinlein). They have got to find a way to carry the battle to the enemy: the orbiting starship and the fleet of "digit" ships. But how do you get thousands of tons of military hardware into orbit quickly enough not to be shot down while in flight using only technology they can develop in a dozen months?

There is only one answer. Project Orion. Old boom-boom. And to heck with the limited nuclear test-ban treaty that killed the project in 1963.

Orion has already been developed. Orion is mass-insensitive, it doesn't care if you are boosting tens of thousands of metric tons. This also means you can use quick and dirty engineering, since you are not stopping every five minutes trying to shave off a few grams of excess mass. You don't have to spend a decade trying to engineer featherweight kinetic energy weapons, just go tear the gun turrets off the Battleship New Jersey and weld 'em on. You can also carry a fleet of gunboats. And all four space shuttles.

The gunboats are going to be quick and dirty as well. Spaceships built around a main gun off a Navy ship, firing nuclear shells. Yes, a spinal mounted weapon

What about the Orion drive battleship's main weapon? Heh. Another cancelled project rises from the grave.

Back in the days of the Strategic Defense Initiative, Edward Teller et al came up with Project Excalibur. What was that? No less than bomb-pumped x-ray lasers. But wait, what about the bomb you need to pump the laser? Well, Orion is an nuclear-bomb-powered drive, remember? Make the propulsive bombs do double duty.

The weapons are called "spurt bombs." Dispensers on the pusher plate eject a flight of the little darlings. The spurt bombs unfurl their 100 laser rods apiece and aim them at Fithp ships. The next nuclear pulse unit is positioned, then detonated. This simultaneously gives thrust to the spacecraft, and pumps all of the spurts bombs. The Fithp ships are sliced and diced by a hail of x-ray laser beams. Spurt bombs look like fasces, "bundles of tubes around an axis made up of attitude jets and cameras and a computer."

Note that the nuclear pulse units will have to be specially designed. Standard Orion pulse units are nuclear shaped charges, designed to channel 80% of the x-rays upwards into the pusher plate (well, to create a jet of plasma directed at the plate but I digress). The battleship's pulse units need to be designed to also direct x-rays at the spurt bombs.

What is the battleship's name? Michael of course. The Biblical Archangel who cast Lucifer out of heaven.

The Michael launches through a cloud of Fithp digit ships, cutting them to pieces but suffering serious damage. The Fithp defecate in their pants and frantically rip the starship out of orbit and start running away. Their superior acceleration make escape possible, up until the point where the crew of the Space Shuttle Atlantis commits suicide and rams the starship. The main drive is damaged, and their acceleration is no longer higher than the Michael. Who catches up and starts pounding the living snot out of the starship.

There is something breathtaking about the Michael that captures the imagination of science fiction fans. On pretty much every single online forum about spacecraft combat, it isn't long until somebody brings it up. There have been many examples of fans trying to make blueprints, illustrations, or even scratch-build models of the battleship.

The original Michael diagram was made by Aldo Spadoni, president of Aerospace Imagineering. Mr. Spadoni is an MIT educated mechanical/aerospace engineer with over 30 years of experience designing and developing advanced aerospace vehicle and weapon system concepts (with most of the more advanced work being classified). He is also a personal friend of Larry Niven and Jerry Pournelle.

Mr. Spadoni did the Michael diagrams around 1997, working directly with Niven and Pournelle. They went through several iterations to arrive at the resulting diagrams.

Aldo Spadoni's Michael

However, this does bring up a good point that Scott alluded to. Footfall is a novel of course, not an engineering proposal for a space battleship. You glean details regarding the various Footfall spacecraft from the conversations of characters in the story, many of which are not experts wrt what they are describing. As Scott also pointed out, there are inconsistencies in the descriptions that are either intentional or simply mistakes on the part of the authors. Thus, the design of the Footfall spacecraft are open to interpretation.

As an engineer and concept designer, I particularly like the way Larry and Jerry write their stories. They provide enough big picture detail to determine the general design direction for their concepts, but leave the smaller details and the system integration issues to anyone willing to take a crack at envisioning their concepts. Fun stuff! So, I think my overall design captures what the authors intended, but many of the details are open to different interpretations, as some of you have done here.

As I move into discussing some of Michael’s details, I want to note that my primary design goal was to be true to the novel and the authors’ intentions as I understood them. I have my own vision of what a space battleship might look like, as I’m sure many of you have. But that’s not the subject of this design exercise.

As did Scott (Lowther), I struggled to determine Michael’s overall dimensions, given the novel’s inconsistencies. Whatever they wrote, Larry and Jerry envisioned the most compact possible vehicle that would get the job done. Note that Scott is showing an older version of my drawing that shows Michael with the shock absorber array fully compressed along with incorrect dimensions. The final dimensions I came up with are somewhat larger, on the same order as those Scott mentions in a separate post.

Regarding the comment that this is a slick ILM Hollywood design, I think this is reading quite a lot into a hemisphere, a rectangular prism and a shallow cone! Perhaps the commenter is confusing vehicle configuration design with render quality. These drawings were never intended to portray Michael’s actual exterior finish, surface markings, etc. These drawings were created way back when using an ancient vector-based illustration software application called MacDraw Pro. They look pretty awful and it’s certainly not the way I would render Michael today. In hindsight, I should have left them as line drawings and avoided the use of MacDraw Pro’s primitive shading tools.

Regarding the battleship-derived gun turrets, I agree with Scott’s assertion that the text of the book is vague in this area. But based on my discussions with Larry and Jerry, the authors definitely intended for Michael to include two of the full-up 16-inch Iowa class turrets, as well as some smaller gun turrets, not the guns alone.

Regarding the Shock absorbers array configuration, I disagree with you guys. Thinking that Michael is a straight extrapolation of the conventional Orion design configurations is incorrect. The primary purpose of the shock absorber array is, of course, to smooth out the “ride” for the payload/passenger portion of the vehicle. Most of the Orion designs were configured for non-military applications, whereas Michael is a maneuvering warship with massive nuclear pumped steam attitude thruster arrays. In addition to primary Orion thrusting, Michael will be subjected to multi-axial mechanical loads that are NOT along the longitudinal axis of the ship. Also consider that Michael’s design incorporates a pusher “shell” that is far more massive as a fraction of total vehicle mass than the typical Orion pusher plate design. When Michael is thrusting under primary propulsion while engaging in combat maneuvers, an angled shock absorber array design is a good choice for handling the inevitable side loads and for stabilizing the shell wrt the passenger/payload “brick”. Consider a high performance off road vehicle, which must provide chassis stability while the wheels and suspension are being subjected to loads from many directions. You don’t see any parallel straight up and down shock absorbers in the suspension system, do you?

If you look carefully at me design, you can see that that central shock absorber is longitudinal and more massive than the rest. This one is primarily responsible for handling the Orion propulsive loads. Perhaps it should be a bit beefier than I’ve depicted it in the original drawing. The remaining angled shock absorbers handle some of that propulsive load while also providing multi-axial stability. Admittedly, these 2D drawings don’t convey the Shock absorber array configuration that I have envisioned very well.

Since the time these drawing were created, I’ve discussed Michael with Larry and Jerry on a number of occasions. I’ve reconsidered and refined many of Michael’s technical aspects and I’ve designed a more detailed and representative configuration, including an updated shock absorber array. I’m also involved in creating my own high fidelity 3D model of Michael with a few fellow conspirators. I’m looking forward to sharing that with everyone at some point.

(ed note: one of those "few fellow conspirators" was me. Another was Andrew Presby, who is featured on one or two pages of this website.)

From a comment on the Unwanted Blog by Aldo Spadoni (2012)

Around 2010 Andrew Presby and I were commissioned by Aldo Spadoni to turn his Michael blueprints into 3D renders. Click for larger images.

Scott Lowther, author of Aerospace Projects Review is working on a book about nuclear space propulsion. Of course he wouldn't dream about leaving out the coolest Orion Drive spacecraft of all.

Now, strictly by the novel, the Michael is a mile high, which is ludicrous. The protagonists would have to have built a mile-high dome to cover it, which the aliens might have found a bit suspicious. In the diagrams below, Mr. Lowther shows the "large" Michael (one mile) and the more reasonable "small" Michael (1/8th mile).

Master artist William Black also had to turn his formidable talents on the Michael.

William Black's Michael

Nuclear pulse propulsion battleship Michael from the novel Footfall by Larry Niven & Jerry Pournelle.

“Michaels nose was a thick shield … armored in layers: steel armor, fiberglass matting, more steel armor, layer after layer of hard and nonresiliant soft.” —from Footfall, pg. 446 and 472

“Two great towers stood on the curve of the hemispherical shell, with cannon showing beneath the lip, aimed inward. Four smaller towers flanked them. A brick-shaped structure rose above them. The Brick was much less massive than the Shell, but its sides were covered with spacecraft: tiny gunships, and four Shuttles with tanks but no boosters. The bricks massive roof ran beyond the flanks to shield the Shuttles and gunships.”  —from Footfall, pg. 432

Michael is one of the Orion based concepts I knew I would have to take a run at sooner or later. I referenced the novel, extensively, and Scott Lowther condensed all the design bits he gleaned from Footfall into an Excel spreadsheet, available here, for a project he set aside. The spreadsheet is an excellent guide to all the passages describing Message Bearer, the digit ships, Michael, the stovepipes and Shuttles, and it proved invaluable in my effort.

Most people are probably familiar with Aldo Spadoni's visualization of the iconic warship from Niven and Pournelle’s novel, but for those who are not, Aldo’s drawings are available here.

What I’ve done is meet the Aldo Spadoni design half-way with my own interpretations. My intent was to complement Aldo’s design-thought without entirely rewriting it, keeping in mind what Aldo had to say about the process. One point Aldo raised in conversation on Scott Lowther’s blog is in regards to who is providing description in various scenes from the novel.

Aldo Spadoni: “Footfall is a novel of course, not an engineering proposal for a space battleship. You glean details regarding the various Footfall spacecraft from the conversations of characters in the story, many of which are not experts [with regard to] what they are describing. As Scott also pointed out, there are inconsistencies in the descriptions that are either intentional or simply mistakes on the part of the authors. Thus, the design of the Footfall spacecraft are open to interpretation.”

Aldo makes a good case for the distinctive angled shock absorbers of his design, and I’ll provide his commentary below, the sticking point for me, however, is the parabolic pusher plate Niven and Pournelle describe—early design work on Orion solidly ruled out a parabolic pusher. With shaped-charge nuclear pulse units the parabolic plate will only heat up while offering almost no thrust advantage. Heating and impact stress on the pusher would be of no small concern, the bombs necessary to loft something the scale and mass of Michael would not be the tame little devices used to propel a dinky NASA/USAF 10-meter Orion. Heating is the cost of even partially containing the ionized plasma resulting from nuclear detonation.

Orion works because the plasma is dynamically shaped (as the explosion happens) by the specially designed shaped charge nuclear explosive, X-rays are channeled by the radiation case in the instant before the weapon is vaporized, these exit a single aperture, striking and heating up a beryllium oxide channel filler and propellant disk (tungsten), resulting in a narrow conical jet of ionized tungsten plasma, traveling at high velocity (in excess of 1.5 × 10⁵ meters per second). This crashes into the pusher plate, accelerating the spacecraft. The jet is not physically contained by the pusher, and contact with the pusher is infinitesimally brief, so the pusher is not subject to extreme heating during thrust maneuvers. So, while offering very little performance difference compared to a flat pusher design, the parabolic plate would need regenerative cooling in the bargain, adding weight and complexity to the system. Engineering such a pusher plate would be fraught with difficulties, and conditions under which Michael is built, in my opinion, rule out any eccentric messing with the baseline system. A legion of Ted Taylors would already be kept busy night and day with the mere task of readying a conventional Orion designed under such circumstances—for delivery under a one year drop-rocks-from-orbit-dead deadline.

As Aldo points out, the text of Footfall leaves room for different interpretations and here is where I took some of Aldo’s design-thought and creatively merged it with my own toward the end of addressing the design as presented in the novel. (No, not the army of Ted Taylor clones inhabiting a maze of cubicles in some deep bunker somewhere—that’s just me.)

It occurred to me that what Aldo had done (following Niven and Pournelle’s description), was move the functions of the Orion standard propulsion module down, mounting them directly on the top of the plate, so really it’s a built up intermediate platform/propulsion module. What I’ve done is run with that thought: I chose to treat the entire pusher plate as an early large Orion: a dome sitting on flat pusher plate, concentric rows of toroidal shock absorbers surrounding a core array of gas-piston shock absorbers. There is no central hole-and-bomb-placement-gun-protection-tube in my design (but there is an anti-ablation oil spray system). Instead, pulse units are shot by bomb placement guns mounted to fire around the edge; exactly as in Aldo’s design (the early large Orion had rocket assisted bombs riding tracks on the exterior of the spacecraft—imagine the show that would make). The body of the “dome” in my design is stowage for tanked pressurization gas (for the shock absorbers), anti-ablation oil, and perhaps a reserve number of pulse units.

I’ve retained the scheme of duel pulse unit magazines. Niven & Pournelle called them “thrust bomb” towers. Four “spurt bomb” towers are also mounted to the base—the “spurt bomb” Niven and Pournelle describe is a type of bomb-pumped laser using gamma-radiation rather than X-rays. All of my towers are a good deal beefier than those on Aldo’s design. Narrative in the novel describes the “thrust bomb” towers as doing double duty, providing an extra layer of armor and shielding for the CIC/control room, the nerve center of the spacecraft, which is located in the lower portion of the Brick, wedged between two large water tanks (and two nuclear reactor containment vessels). The water tanks are frozen at lift-off, providing Michael with an ample heat-sink.  

As I mentioned above, Aldo makes an excellent case for the angled shock absorbers on his design, his description below:

Aldo Spadoni: “Most of the Orion designs were configured for non-military applications, whereas Michael is a maneuvering warship with massive nuclear pumped steam attitude thruster arrays. In addition to primary Orion thrusting, Michael will be subjected to multi-axial mechanical loads that are NOT along the longitudinal axis of the ship. … When Michael is thrusting under primary propulsion while engaging in combat maneuvers, an angled shock absorber array design is a good choice for handling the inevitable side loads and for stabilizing the shell [with regard to] the passenger/payload “brick.” Consider a high performance off road vehicle, which must provide chassis stability while the wheels and suspension are being subjected to loads from many directions. You don’t see any parallel straight up and down shock absorbers in the suspension system, do you?

If you look carefully at my design, you can see that that central shock absorber is longitudinal and more massive than the rest. This one is primarily responsible for handling the Orion propulsive loads. … The remaining angled shock absorbers handle some of that propulsive load while also providing multi-axial stability.”

Scott Lowther (of Aerospace Projects Review) offers this insight in regards to angled shock absorbers:

Scott Lowther: “I remain unconvinced at the off-axis "angled" shock absorbers, but they seem to be the popular approach. However, if you do go that route, you have to deal with the central piston in the same way... ball joints fore and aft. *All* the pistons must be free to swing from side to side. If one, even the central one, is locked, then either the pusher assembly cannot move sideways *thus negating the value of the angled shocks), or it'll simply get ripped off its mounts the first time there's an off-axis blast.

Given that the ship is clearly described as having nuclear steam rockets for attitude control, I don't see the value in off-axis blasts for steering. But... shrug.”

I spent a good deal of time reproducing Aldo’s shock absorber array because frankly I think it is brilliant, going back and forth between Aldo’s drawings and my file … in the end the detail would be invisible, so I created a cutaway render with two of the “spurt bomb” towers removed to reveal the system.

True to the novel Michael’s main guns are the 16"/50 caliber Mark 7 gun and turret taken directly off the New Jersey. There is a good deal of discussion (on Scott’s blog and elsewhere) on the suitability of the guns and turrets—the mounting is rotated ninety degrees to vertical relative to the orientation turret, guns, and loading mechanisms were designed for—however, Aldo is quite clear that mounting the full turrets “as is” reflects the author’s intention, and so I’ve kept to their vision in this regard.

In the novel the guns are described firing a nuclear artillery round, this would be a modern version of the W23 15-20 kiloton nuclear round. The Mark 23 was a further development of the Army's Mk-9 & Mk-19 280mm artillery shell. This was a 15-20 kiloton nuclear warhead adapted to a 16 in naval shell used on the 4 Iowa Class Battleships1. 50 of these weapons were produced starting in 1956 but shortly after their introduction the four Iowa's were mothballed. The weapon stayed in the nuclear inventory until October 1962. Presumably under war conditions a new production run would produce the numbers necessary for Michael’s assault on Message Bearer.

Secondary batteries: a generic turret roughly based on the secondary turrets of the Iowa class.

Missile launchers based on the MK-41 Vertical Launching System (VLS).

The “Battle Management Array” is a set of phased-array radars and tight-beam communications antenna for passing targeting information to Michaels secondary spacecraft, all mounted to a pair of shock-isolated cab, each riding its own set of shock absorbers, one mounted atop each “thrust bomb” tower.  A fall-back set of communications antenna and radar are mounted beneath the overhang of the forward shield atop the Brick.

I’ve gone with the dimensions Scott arrived at, which Aldo confirmed in his comments on Scott’s blog: Length:742’ Diameter: 371’.

Different opinions have been offered in regards to Michael’s mass, between 35,000 and 50,000 tons have been opinioned on Scott Lowther’s blog. Pournelle was quoted as saying 2 million tons on one occasion, and 7 million tons on another.

Michaels launch, in the novel, is shortened for reasons of narrative brevity; one character wonders if there were perhaps 30 or more nuclear detonations. Putting Michael in orbit would require 8 minutes of powered flight and about 480 bombs lit off at one bomb per second.

The novel is clear that Michael carries four Space Shuttles mounted to their external tanks sans their SRBs. The number of Gunships is less clear. Nine Gunships are described as destroyed in combat, an unspecified number survive to confront Message Bearer in the final scene. Designing the most compact spacecraft necessary to fill the role, my Gunship measures 100 feet in length, 25 feet in diameter. At these dimensions, 14 Gunships total can be comfortably mounted to Michaels flanks.

For detail on my Gunship design see my following post, Gunship.

1 W23

From Michael by William Black (2015)
William Black's Gunship

Gunship from Larry Niven and Jerry Pournelle's novel Footfall. See my related post Michael for additional detail.

“They take one of the main guns off a Navy ship. Wrap a spaceship around it. Not a lot of ship, just enough to steer it. Add an automatic loader and nuclear weapons for shells. Steer it with TV.” —from Footfall, pg. 354

In the novel these Gunships are referred to as “Stovepipe’s.” I was far less concerned with designing to match that narrative description than I was with designing the most compact spacecraft possible capable of the mission described. Michaels construction (including all its auxiliary spacecraft and subsystems) takes place in secret under wartime conditions, perhaps the moniker is derived from a code name picked randomly (that’s how the 1958 Project Orion was named), or perhaps dockworkers handling the vehicle sections, packed in featureless cylindrical shipping containers strapped to pallets, named the craft, and it stuck. See Aldo Spadoni’s commentary on character-delivered descriptions on my Michael post.    

I built my Gunship around the 5"/54 caliber Mark 45 gun.

Nuclear Round

The nuclear round fired by the Gunship would be something akin to the UCLR1 Swift, a 622 mm long, 127 mm diameter nuclear shell, weighing in at 43.5 kg.

In 1958 a fusion warhead was developed and tested. At its test it yielded only 190 tons; it failed to achieve fusion and only the initial fission explosion worked correctly. There are unconfirmed reports that work on similar concepts continued into the 1970s and resulted in a one-kiloton warhead design for 5-inch (127 mm) naval gun rounds, these, however, were never deployed as operational weapons. See paragraph 9 (not counting the bulleted list) under United States Nuclear Artillery.

Gunship Crew & Crew Module  

The text of the novel is unclear on the number of crew manning the Gunships, but my opinion is no more than 2 would be required, and dialogue in the novel tends to back this up. The loading mechanism is automated, so only targeting and piloting skills are involved. Considering urgency involved in readying Michael, I doubt an entirely new capsule, man-rated for spaceflight, would be considered. Michaels designers would fall back on tried and tested designs and modify them as required. In this case a stripped down Gemini spacecraft and its Equipment Module fits the bill nicely. The life support system matches the mission requirements. Leave off the heat shield (these are one-way missions), and reaction control system—the capsule never operates separate from the Gunship rig. Mount targeting and firing controls for the gun. Probably a single hatch rather than Gemini’s double hatch, and internal flat-screen displays rather than viewports—looking on this battle with naked eyes would leave the astronaut seared, radiation burned, and blinded.

“The exhausts of the gunboats were bright and yellow: solid fuel rockets.” —from Footfall, pg. 454

Eight SRBs akin to the GEM-40 allow options: they could be fired in pairs, allowing four separate burns, or two burns of 4, or a single burn of all eight – needs depending. The SRBs are strapped around a ten foot diameter 40 foot long core containing ample tank stowage for hypergolic reaction control propellants, pressurization gas, and nitrogen for clearing the breech and gun barrel. The reaction control system is used to aim the gun; propellant expenditure would be prodigious.

1UCRL - University of California Radiation Laboratory

From Gunship by William Black (2015)

Mini-Mag Orion

Mini-Mag Orion
PropulsionMini-Mag Orion
Thrust1,870,000 n
Exhaust Velocity157,000 m/s
Thrust Power147 GW
Pulse Unit Energy340 GJ
Nozzle Efficiency87.1%
Nozzle Mass199.6 metric tons

Data is from Mini-Mag Orion Program Document: Final Report from Ralph Ewig's website.

The nuclear pulse Orion drive propulsion system had both reasonably high exhaust velocity coupled with incredible amounts of thrust, a rare and valuable combination. A pity it was driven by sequential detonation of hundreds of nuclear bombs, and required two stages of huge shock absorbers to prevent the spacecraft from being kicked to pieces.

Andrews Space & Technology tried to design a variant on the nuclear Orion that would reduce the drawbacks but keep the advantages. The result was the Mini-Mag Orion.

First off, they crafted the explosive pulses so each was more 50 to 500 gigajoules each, instead of the 20,000 gigajoules typically found in the nuclear Orion. Secondly they made the explosions triggered by the explosive charge being squeezed into critical mass using an external power source instead of each charge being a self-contained easily-weaponized device. Thirdly they made the blast thrust against the magnetic field of a series of superconducting rings (Magnetic Nozzle) instead of the nuclear Orion's flat metal pusher plate.

In the standard nuclear pulse Orion, the pulse units are totally self-contained, that is, they are bombs. Since this makes it too easy to use the pulse units as impromptu weapons (which alarms the people in charge of funding such a spacecraft) a non-weaponizable pulse unit was designed. The Mini-Mag Orion pulse unit has the fissionable curium-245 nuclear explosive, an inexpensive Z-pinch coil to detonate it, but no power supply for the coil. The Z-pinch power comes from huge capacitor pulse power banks mounted on the spacecraft, i.e., the pulse unit ain't anywhere near being "self contained". The banks have a mass of a little over seven metric tons, far too large to use in a weapon (especially one that explodes with a pathetic 0.03 kilotons of yield). The Z-pinch coil should be inexpensive since it will be destroyed in the blast.

For a 50 gigajoule yield (with a burn fraction of 10%), the nuclear explosive is 42.9 grams of curium-245 in the form of a hollow sphere 1.27 centimeters radius (yes, I know the diagram above says the compression target is 0.47 centimeters radius. I think they mean the compressed size). This is coated with 15.2 grams of beryllium to act as a neutron reflector. According to the table below, a 120.7 gigajoule yield uses 21 grams of curium, which does not make sense to me. Usually you need more nuclear explosive to make a bigger burst. I guess the pulse units in the table have a larger burn fraction. The Z-pinch will squeeze the curium sphere from a radius of 1.27 centimeter down to 0.468 centimeters, leading to a chain reaction and nuclear explosion. Since curium-245 has a low spontaneous fission rate, the pulse unit will need a deuterium/tritium diode to provide the triggering neutrons. The pulse units will be detonated about one per second (1 Hz).

The Z-pinch needs 70 megaAmps of electricity. This is 70 million amps, which is a freaking lot of amps. The trouble is that you cannot lay big thick cables to the Z-pinch coil in the pulse unit. The cable will be vaporized by the nuclear explosion, which is OK. But a vaporized massive cable composed of heavy elements will drastically lower the exhaust velocity. This is very not OK. Remember that one of the selling points of the Mini-Mag Orion is the high exhaust velocity. Reduce the exhaust velocity and Mini-Mag Orion becomes much less attractive.

So instead of heavy cables the pulse unit uses gossamer thin sheets of Mylar (20 μ thick). I know that Mylar is usually considered an insulator, but 70 megaAmps does not care if it is an insulator or not. The report calls these Mylar cables Low Mass Transmission Lines (LMTL). They have a total mass of only 2 kilograms, which is good news for the exhaust velocity.

The 70 megaAmps go from the pulse power banks to permanent electrodes mounted on the magnetic nozzle. These take the form of five meter diameter metal rings. Two rings, positive and negative, just like the two slots in an electrical wall socket. The pulse unit proper is a minimum of 0.0244 meters diameter (double the 1.27 centimeter radius). So the LMTL has to stretch from the permanent electrodes to the pulse unit. This makes a five meter diameter disk of Mylar with with the grape sized pulse unit in the center. Actually two stacked Mylar disks (positive and negative) separated by about 2 centimeters of space (g0 in diagram above) so they won't short circuit. Ordinarily you'd use an insulator to prevent a short, something like, for instance, Mylar. Unfortunately here you are using Mylar as the conductor so instead you need a gap. The edge of each Mylar disk has an aluminum rim, each making contact with one of the magnetic nozzle's two permanent electrodes.

To place the pulse unit in the proper detonation point inside the magnetic nozzle, the pulse unit has to be five meters lower than the permanent electrodes in the nozzle. This forces the Mylar LMTL to be an upside down cone instead of a flat disk.

The pulse unit, Mylar LMTL and the aluminum rims are all vaporized during detonation. The magnetic nozzle with its permanent electrodes remain.

There are two power supplies: the steady-state reactor and the pulse power banks.

The reactor is the "charger." It charges up the superconducting magnetic nozzle, and gives the pulse power banks their initial charge. Finally it supplies power to the payload (including the habitat module). In the reference designs below, it outputs 103 kilowatts, has a mass of 9 metric tons, and is expected to supply 50 kilowatts to the payload. It takes 1 hour to give the pulse power banks (main and backup) their initial charge, and takes 39 hours to charge up the superconducting magnetic nozzle. Since the nozzle uses superconductors, its charge will last a long time before it leaks out.

The reactor has to supply 192 megajoules over one hour to charge up the main and backup pulse power banks. The reactor has to supply 7,446 megajoules over 39 hours to charge up the superconducting nozzle.

The tiny bombs need 70 megaAmps in 1.2 microseconds in order to detonate, but the reactor can only produce that many amps in one hour. The standard solution is to use capacitors, which can be gradually filled up but can dump all their stored energy almost instantly. This is the pulse power banks, a Marx bank of capacitors.

The reactor takes half an hour to charge up one pulse power bank, one hour to charge up the bank and the backup bank. The bank discharges all that energy into the pulse unit to detonate it. A separate system in the magnetic nozzle converts about 1 percent of the explosion into electricity and totally recharges the pulse power bank. For subsequent detonations, the reactor is not needed, the detonating bombs supply the power.

In the reference design, the pulse power banks hold 96 megaJoules per bank, there is a main bank and a backup bank for a total of 192 megaJoules, each bank has a mass of 3.5 metric tons, main and backup bank have a combined mass of 7.1 metric tons. The banks have to sustain a pulse unit detonation rate of 1 per second (1 Hz).

The backup bank is in case of a misfire, resulting in a lack of a recharge for the main bank. The still-full backup bank takes over energizing the pulse detonations while the reactor starts slowly re-charging the main bank.

Since the electrical system will be operating at megawatt levels, it will need a sizable set of heat radiators (Thermal Management System). By "sizable" we mean "up to 30% of the spacecraft's dry mass." In the first reference mission, the radiators have to handle 2,576 kW of waste heat, with the radiators having a mass of 15,456 kg and a surface area of 7,728 square meters.

The heat radiators are tapered in order to keep them inside the shadow cast by the radiation shadow shield. This keeps the radiators relatively free of neutron activation and neutron embrittlement. It also prevents the radiators from backscattering deadly nuclear radiation into the crew compartment.

The engine core and feed mechanism will have to inject the pulse units into the detonation point at rates of up to 1 Hz. It too will need redundancy and a minimum of moving parts.

In the second diagram above:

  1. Cycle begins. A pulse unit is at the detonation point with its LMTL contact rings touching the magnetic nozzle's permanent electrodes. Both blast doors are closed. The nozzle is fully extended.
  2. 70 megaAmps detonates the pulse unit. The explosion transmits force into the magnetic nozzle, producing thrust. 1% of the blast energy is converted into electricity which re-charges the pulse power bank. The nozzle moves upward along the feed system as part of the compression cycle. Meanwhile, the upper blast door opens to allow the next pulse unit to enter the feed system.
  3. The explosion plasma dissipates. The nozzle continues to move upward. As the next pulse unit enters the feed system, the upper blast door closes.
  4. The lower blast door opens. The nozzle reaches its highest position. The fresh pulse unit is injected into nozzle at the detonation point with a velocity matching the nozzle, LMTL contact rings of pulse unit touching nozzle's permanent electrodes. The lower blast door closes as the nozzle starts to travel downward along the feed system. When the nozzle reaches it lowest point, a new cycle begins.

The report had three sample "Design Reference Missions", and created optimal spacecraft using MiniMag Orion propulsion. As it turns out, the spacecraft for mission 1 and mission 2 were practically identical, so they only showed the two ship designs.

Design Reference Missions

DRM-1: Crewed Mars Mission:
50 kWe, 100 km/s Δv, 100 ton payload, 90 to 100 days one way trip time.
DRM-2: Crewed Jupiter Mission:
50 kWe, 100 km/s Δv, 100 ton payload, 2 years one way trip time.
DRM-3: Robot Pluto Sample Return:
50 kWe, 150 km/s Δv, 5 ton payload, 8 years one way trip time.

DRM-1/DRM-2 Spacecraft

DRM-1 Mass Budget
Mission delta-v100 km/s
Specific Power347 kW/kg
(347,400 W/kg)
Thrust Power87 gigawatts
Payload Mass100,000 kg
Specific Impulse9,500 sec
Exhaust Velocity93,000 m/s
Power System Mass (Charge)9,038 kg
Power System Mass (Pulse Banks)7,115 kg
Heat Radiators15,456 kg
Magnetic Nozzle Mass102,893 kg
Propellant Mass481,625 kg
Dry Mass (no remass, no payload)150,300 kg
Burnout Mass (no remass)250,300 kg
Ignition (Wet) Mass731,924 kg
Payload Fraction0.137
Propellant Fraction0.66
Dry Mass Fraction0.21
Power System - Pulse Banks
Peak Compression Current89 MA
Capacitor Voltage170 kV
Energy per Bank96 MJ
Capacitor Energy Density54 kJ/kg
Capactior Mass (one bank)1,779 kg
Pulse Bank Mass (total for 2 banks)7115 kg
Power System - Charge Power
Pulse Bank Charge Time60 minutes
Nozzle Charge Time39 hours
Pulse Banks Energy Content192 MJ
Nozzle Energy Content7,446 MJ
Payload Power Requirement50 kW
Power Output Electric103 kW
System Power Density11.4 W/kg
Thermal to Electric Efficiency0.04 fraction
Total Mass9,038 kg
Heat Radiators
Waste Heat Load2,576 kW
Area per Watt3 m2/kW
Mass per Area2 kg/m2
Radiator Area7,728 m2
Radiator Mass15,456 kg
Engine Performance
Specific Impulse9,500 sec
Exhaust Velocity93,164 m/s
Nozzle Efficiency0.45 fraction
Coupling Efficiency0.55 fraction
Pulse Yield120.7 GJ
Pulse Unit Mass6.9 kg
Standoff Distance5.9 m
Fission Assembly Mass21 g
Firing Rate1 Hz
Mass Flow6.9 kg/s
Thrust642 kN
Power29,894 MW
Gain563,631 ratio
Alpha (specific power)222,251 W/kg
Maximum Acceleration0.26 g's
Minimum Acceleration0.09 g's

DRM-3 Spacecraft

DRM-3 Mass Budget
Mission delta-v150 km/s
Specific Power551 kW/kg
(551,300 W/kg)
Thrust Power87 gigawatts
Payload Mass5,000 kg
Specific Impulse9,500 sec
Exhaust Velocity93,000 m/s
Power System Mass (Charge)9,067 kg
Power System Mass (Pulse Banks)7,115 kg
Heat Radiators15,505 kg
Magnetic Nozzle Mass102,895 kg
Propellant Mass630,963 kg
Dry Mass (no remass, no payload)152,723 kg
Burnout Mass (no remass)157,723 kg
Ignition (Wet) Mass788,686 kg
Payload Fraction0.006
Propellant Fraction0.8
Dry Mass Fraction0.19
Power System - Pulse Banks
Peak Compression Current89 MA
Capacitor Voltage170 kV
Energy per Bank96 MJ
Capacitor Energy Density54 kJ/kg
Capactior Mass (one bank)1,779 kg
Pulse Bank Mass (total for 2 banks)7115 kg
Power System - Charge Power
Pulse Bank Charge Time60 minutes
Nozzle Charge Time39 hours
Pulse Banks Energy Content192 MJ
Nozzle Energy Content7,447 MJ
Payload Power Requirement50 kW
Power Output Electric103 kW
System Power Density11.4 W/kg
Thermal to Electric Efficiency0.04 fraction
Total Mass9,038 kg
Heat Radiators
Waste Heat Load2,576 kW
Area per Watt3 m2/kW
Mass per Area2 kg/m2
Radiator Area7,752 m2
Radiator Mass15,505 kg
Engine Performance
Specific Impulse9,500 sec
Exhaust Velocity93,164 m/s
Nozzle Efficiency0.45 fraction
Coupling Efficiency0.55 fraction
Pulse Yield120.7 GJ
Pulse Unit Mass6.89 kg
Standoff Distance5.9 m
Fission Assembly Mass21 g
Firing Rate1 Hz
Mass Flow6.89 kg/s
Thrust642 kN
Power29,894 MW
Gain560,167 ratio
Alpha (specific power)222,122 W/kg
Maximum Acceleration0.41 g's
Minimum Acceleration0.08 g's

NASA Space Tug

RocketCat sez

This is a spiffy design for giant robot fans. Those titanic mecha arms will immediately grab the attention anybody who adores Jaegers.

NASA Space Tug
ΔV3,800 m/s
Specific Power3.5 kW/kg
(3,520 W/kg)
Thrust Power49.3 megawatts
Specific Impulse450 s
Exhaust Velocity4,400 m/s
Wet Mass32,000 kg
Dry Mass14,000 kg
Mass Ratio2.3
Mass Flow5 kg/s
Thrust22,400 n
Initial Acceleration0.17 g (lunar g)
Payload5,900 kg

This is a 1970's era NASA concept for a modular space tug. The "waldo" arms on the crew module are interesting. The Grumman image said that the space tug had a wet mass of 70,000 pounds, 40,000 pounds of fuel (oxygen-hydrogen), and 13,000 pounds of payload (presumably implying 17,000 pounds of structural mass). This implies a mass ratio of about 2.3, and 3,800 m/s of ΔV. You need about 2,370 m/s to land on Luna.

Images courtesy of NASA. Magazine cover by David Hardy. Two images following magazine cover by Robert McCall.


Nuclear rocket using
Indigenous Martian Fuel
PropulsionSolid core
Pebble Bed
Reactor Power2,513 MWth
Thrust1,942,000 N
Engine Mass
(with shield)
15,430 kg
Dry Mass
(without cargo)
44,717 kg
Wet Mass
346,417 kg
Mass Ratio
ΔV at 1400 K3,254 m/s
ΔV at 2800 K5,685 m/s
Isub>sp at 1400 K162 s
Isp at 2800 K283 s
Ve at 1400 K1,589 m/s
Ve at 2800 K2,776 m/s
Propellant Mass
301,700 kg
Propellant Tank
4 m
Width10.92 m
Height24.79 m
Height with
landing gear
29.62 m

Nuclear rocket using Indigenous Martian Fuel (NIMF) is from a 1980's Martin Marietta study by Robert Zubrin. The basic idea was to attempt to avoid the tyranny of Every Gram Counts by using in-situ resource utilization for the propellant. So instead of lugging miserly limited amounts of high-performance propellant all the way from Terra, the rocket would make do with unlimited amounts of low-performance propellants available locally on Mars.

There were several vehicles designed: including a supersonic winged craft and a ballistic "hopper". The latter is pictured here. They all featured a single solid core nuclear thermal rocket engine fed by a single propellant tank.

Ideal Specific Impulses of Martian Propellants
1400 K162222460162110
2800 K283370606253165
3000 K310393625264172
3200 K337418644274178
3500 K381458671289187

The easiest propellant to manufacture is liquid carbon dioxide. It can be produced from the Martian atmosphere using just high pressure (690 kPa) with no cryogenic cooling needed (a 30 horsepower pump will do, requiring 25 kW, or 80 kilowatt hours per metric ton). The Martian atmosphere is about 95% CO2 so it is not like there is any shortage of the stuff.

Other propellants have superior performance but are much harder to manufacture. Carbon dioxide has enough specific impulse to boost the NIMF from the surface of Mars into low Mars orbit, so the designers figured it was good enough. It also has enough Isp to hop the vehicle from point A to any other point on Mars.

As I mentioned each metric ton of propellant sucked out of the atmosphere takes 80 kilowatt-hours, and the propellant tank holds about 302 metric tons total. About 24,160 kilowatt-hours to fill it. How long it takes to fill the tank depends upon how many kilowatts the power source can feed the pump.

  • Power can come from the nuclear reactor in the rocket engine, if you make it bi-modal. It would produce about 100 kilowatts of electricity (kWe).
    Advantage: The report says this will fill the tank in 12 days (my slide rule says it will take 10 days at 100 kWe for 24,160 kWH). It would also require zero mass for the power supply, since the rocket engine has already been accounted for.
    Disadvantage: is that operating the reactor while the ship is landed with spray deadly radiation all over the landing site. This makes it difficult for the crew to do things like disembark, embark, and linger near the ship.

  • Power can come from a solar cell array. It can produce about 25 kWe (averaged around the clock to take account of nighttime)
    Advantage: no radiation
    Disadvantage: The report does not mention how long it will take to fill the tank but presumably 1.2 as much time as the RTG: 60 days (my slide rule says it will take 40 days at 25 kWe for 24,160 kWH. Why 1.2? 30 kWe / 25 kWe = 1.2). The array is about 3,500 m2 and has a penalty mass of 8.8 metric tons. It also takes three crew members about 2 days to set up and break down, making the total delay about 44 days between flights.

  • Power can come from an RTG. It can produce about 30 kWe.
    Advantage: practically no radiation. It has a penalty mass of 4 metric tons, about half of the solar cell array. Unlike the solar cell array it requires no setup or breakdown time. The report says this will fill the tank in 50 days (my slide rule says it will take 34 days at 30 kWe for 24,160 kWH), which is less than the solar cell array.
    Disadvantage: It has a penalty mass of 4 metric tons as compared to the bi-modal engine. It will take many more days than the bimodal engine.

The paper decided the RTG was the optimal solution.

A reactor temperature of 2800 K (Isp 283) is required to boost the vehicle from the surface of Mars into high orbits (ΔV 5,685 m/s).

But only 1400 K (Isp 162) would be needed for Mars to Mars hops (ΔV 3,254 m/s).

From Top to Bottom:

  • Storage Dome
  • Flight Deck
  • Habitation Deck (crew living quarters, supports a crew of three for more than one year)
  • Mechanical Deck (machinery to liquifly atmospheric carbon dioxide)
  • Propellant Tank
  • Nuclear Engine

The nuclear engine has a shadow shield on top composed of steel, boron, and lithium hydride to protect the crew from radiation when the reactor is operating. A secondary toroidal propellant tank surrounds the reactor to protect crew walking on the surface when the reactor is idling. There is a second radiation shield right under the crew, to protect them from backscattered radiation reflected off the ground during landing.

The reactor elements will need special cladding, since when carbon dioxide is heated to high temperatures inside the reactor it will oxidize the heck out of everything it touches. Reactor elements designed for liquid hydrogen propellent won't work, they will rapidly erode and spray powdered glowing radioactive death while the reactor stops producing power and the ship plummets out of the sky.

North American Rockwell MEM

North American
Mars Excursion Module
Diameter9.6 m
Height8.8 m
Deorbit Stage
3,357 kg
Descent Stage
29,207 kg
Ascent Stage
16,874 kg
Total Mass
large mission
49,437 kg
Mission Parameters
500 km
300 km by
66,900 km
e = 0.9
Small Mission2-crew/4-day
Large Mission4-crew/30-day
Deorbit Stage
(beryllium solid)
Isp325 s
Thrust133,500 N to
204,600 N
Burn Time48 seconds
Total Mass3,357 kg
Deorbit ΔV200 m/s
Deorbit T/W0.4
Descent Stage
Isp383 s
1,070 m/s
(mom circular) or
1,450 m/s
(mom elliptical)
623,000 kg
1.5 to 0.15
Jettisoned Structure2,109
Retained Structure2,880
Lab Structure612
Guidance &
Landing Gear1,256
Science Payload1,905
Tanks & System1,179
Ascent Stage
Isp383 s
Ascent ΔV4,880 m/s
(mom circular) or
6,200 m/s
(mom elliptical)
Ascent Thrust156,000 kg
Ascent T/W1.0
100 m/s
Structure445 kg
EPS227 kg
Communication95 kg
Guidance &
102 kg
ECLSS608 kg
RCS240 kg
Return Payload
(Geological samples)
136 kg
(90 percentile)
318 kg
Contingency215 kg
STAGE II(4,277 kg)
Tanks & System313 kg
Engine222 kg
Propellant3,742 kg
STAGE I10,210 kg
Tanks & System730 kg
Propellant(9,480 kg)
16,874 kg

The Mars Excursion Module is from a 1966 study by North American Rockwell. This was the first Mars lander designed after the bombshell from Mariner 4 that astronomers had drastically over-estimated how dense the Martian atmosphere was. They had figured it was a useful 85 hectopascals (hPa), in reality it was an almost worthless 6 hPa (just slightly better than a vacuum). By way of comparison Terra's atmospheric pressure at sea level is 1013 hPa.

The low atmospheric blow Mariner 4 dealt to the scientist was just the cherry on top of the sundae. Much more serious was the photographs. The scientists knew there could be no chance of images of scantily-clad Barsoomian princesses, but they were hopefull there would be some lakes and maybe even a canal or two. But nothing but a bunch of freaking craters? The scientists got a sinking feeling in their stomachs, as they could almost see the Mars exploration program go swirling down the toilet right before their very eyes. Once the taxpayers saw these photos the NASA tax dollars would dry up. Mars looks like the freaking moon, for cryin' out loud! And NASA has already been to the moon. Been there, done that, got the T-shirt. No need to go to Moon part deux.

But NASA put a brave face on things, and proceeded to plan for a Mars mission anyway. Sadly, they were right. As I write this it is fifty years later and the movie The Martian is still science fiction, not a documentary. That furry "whumph" noise you hear is RocketCat doing a facepalm.

Given the pathetic whisp of Martian atmosphere, NAR went with a classic gum-drop shape much like the Apollo command module for aerobraking purposes. For one thing all the expertise obtained from Apollo could be leveraged. Plus there was Terra's atmosphere conveniently located for heat shield test purposes.

Though I did read a recent report suggesting that even with the gum-drop design the Martian atmosphere is not up to the task of aerobraking the MEM before it splats into the ground at hypersonic velocities. The report suggested that entirely new technologies are needed.

In a genius move, NAR made the design modular. If you needed a lean and mean mission, you could remove some internal compartments, ascent propellant, and surface supplies to get the total lander mass down to 30 metric tons. Or you could max it out. Or anything in between.

The price of a low mass lander is that it could only support two crew for four days, and the mothership had to be in a low circular Mars orbit for both departure and return to the mother. The high mass lander needed lots more delta V from the mothership, but it could support four crew for thirty days, and the mothership could be in a high elliptical Martian orbit.

You can also make a lander with no ascent stage at all. This can be used to land supplemental equipment, such as an extended-stay shelter, nuclear power module, or a huge Mars mobile lab with fuel supply.

  1. MEM has a mass of 49,437 kg when it separates from the mothership. The deorbit motors fire for about 200 m/s ΔV. Deorbit motor has a thrust-to-weight-earth of 0.4. The MEM starts falling out of orbit, and the deorbit motors are jettisoned. The MEM now has a mass of 46,078 kg.

  2. The MEM enters the Martian atmosphere at an angle of attack of 147°. It starts aerobraking, subjecting the crew to about 7 g's. When it slows to a mere Mach 3.5, it pops a hypersonic drogue chute to stabilize then inflates a 18 meter diameter ballute. This will slow the MEM down to Mach 1.5.

  3. Once the MEM lowers to 3 kilometers of the surface, it jettisons the ballute. The plug in the heat shield over the descent engine is jettisoned. The descent engine is canted about 13° off center, because the MEM center of gravity is off center, because due to design consideration the MEM is not radially symmetric. Mostly because of that pesky crew quarters and laboratory.

  4. The conical section of the heat shield is jettisoned. The descent engine ignites and burns for 1,070 m/s to 1,450 m/s ΔV, depending upon whether the mothership was in a circular or elliptical orbit when the MEM detached. At this point the engine will have a thrust-to-weight-earth of 1.5 to 0.15.

  5. The design managed to squeeze in enough extra propellant for about two minutes of hovering (about 457 m/s of ΔV). Which could be a life-saver if the landing site unexpectedly turned out to be full of jagged boulders or something. Instead of hoving, the extra propellant can move the MEM laterally about 6.7 kilometers to an alternate landing site. The design had six landing legs. In concert with the incredibly stable gum-drop shape, they could manage a ground slope of up to 15°. Actually the shape is similar to a no-spill coffee mug, and for the same reason.

  6. The crew then frantically does as much Martian scientific research as they can cram into 30 days. The pressurized volume is 21.6 m3 (14.4 is laboratory/living quarters, 7.2 is ascent capsule). 20% is taken up by equipment, leaving barely 4.3 m3 per crewperson (right at the ragged limit before claustrophobia strikes).

  7. When it is time for departure, the descent stage becomes the launch pad (which stays behind on Mars), and the center becomes the ascent stage. It brings the crew and 136 kilograms of Martian geological samples back to the orbiting mothership. It launches as Ascent Stage I.

  8. When the Stage I tanks run dry, they are jettisoned. The ascent stage continues as Ascent Stage II. The two stages have a combined ΔV of 6,200 meters per second. The ascent has five components.
    1. Initial burn to 19 kilometer altitude (mothership circular: 4,206 m/s ΔV, mothership elliptical: 4,286 m/s ΔV)
    2. Coast to 185 kilometer altitude
    3. Burn to circularize orbit (23 m/s ΔV)
    4. At appropriate time, burn to ascend for rendezvous (mothership circular: 168 m/s ΔV, mothership elliptical: 1,327 m/s ΔV)
    5. Rendezvous with mothership at apoapsis
    6. Total ΔV: mothership circular 4397 m/s, mothership elliptical 5,635 m/s

  9. The ascent stage docks with the mothership using its Reaction Control System (RCS). It has 100 meters per second of ΔV left for the docking at this point. The rest was burnt during the descent and ascent phases.

The deorbit stage uses a beryllium solid rocket fuel with a specfic impulse of 300 to 325 seconds, a thrust of 133,500 to 204,600 Newtons, and a burn time of 48 seconds.

The reaction control system was supposed to use Chlorine pentafluoride (ClF5) oxidizer with Mixed Hydrazine Fuel-5 (MHF-5). The latter is a devil's brew of monomethylhydrazine, unsymmetrical dimethylhydrazine, diethyline triamine, acetonitrile, and hydrazine nitrate. Which is just as vile as it sounds. It has a specific impulse of 336 seconds.

The space shuttle used a more modern mix of monomethylhydrazine fuel with nitrogen tetroxide oxidizer. Still toxic but nowhere near as bad. It also has a specific impulse of 336 seconds.

The one joker in the deck was the specified fuel for the descent and ascent stages. It seems they couldn't quite get the ΔV they needed out of conventional liquid oxygen (LOX) and liquid hydrogen (LH2). With the mass ratio the design had (i.e., the tiny fuel tanks), LOX/LH2 could not even manage the 4,880 m/s ΔV required to reach the mothership in a low circular orbit, much less the 6,200 m/s ΔV required if it was in a high elliptical orbit. The problem was that LH2 takes up a lot of room, but the MEM's fuel tanks are cramped. There wasn't room for enough LH2 even if the entire area was converted into one gigantic tank.

So they used FLOX and liquid methane (CH4) instead. That can do 6,200 m/s ΔV easy because liquid methane is more than six times as dense as liquid hydrogen. So you can cram six times as much liquid methane mass into the same sized tanks. Using FLOX instead of LOX makes up for the lower energy in methane. FLOX/CH4 has a specific impulse of 383 seconds, compared to LOX/LH2's specific impulse of 449 seconds. LOX/CH4 is lucky to get a pathetic 299 seconds.

What is FLOX I hear you ask? Why, just a simple mixture of liquid oxygen, and Liquid Fluorine.

Fluorine is beyond insanely dangerous. It is incredibly toxic, and will corrode almost anything (some explosively). They don't call it "The Gas of Lucifer" for nothing. The pious hope of the MEM designers was to contain the FLOX in tanks lined with nickel or something similar that would form a passivation layer.

The FLOX mix is 82.5% fluorine and 17.5% oxygen. Mixing liquid fluorine and liquid oxygen is actually relatively safe. For some odd reason those two will not chemically combine without some coaxing. If they do combine, however, you get the dreaded compound Dioxygen Difluoride. This is the compound with the chemical formula FOOF, which coincidentally is the sound your laboratory will make as it blows up. This is the most famous compound in Derek Lowe's hysterical list of Things I Won't Work With (take a minute to read it, the article is a scream).

Another concern is that in a tank the fluorine and oxygen might separate. Then the engine would periodically be sucking pure fluorine, which certainly will not be doing the engine any good.

Carrying entire tankfuls of ultra-corrosive flaming explosive death to Mars seems to be a questionable decision, to say the least. If the MEM lands a trifle hard and the tanks rupture, you won't have the basis for a re-make of The Martian. More like a large melted crater with a few odd pieces of corroded metal and polished skeleton bits at the bottom.

At least the MEM designers saved mass on the ignition system. You don't need any. FLOX/CH4 is hypergolic (because fluorine is hypergolic with almost anything). This is also a help when the ascent stage does staging, you can easily re-start the engines in mid-flight.

I'm doing more research, but apparently the MEM design is so popular, that it was later redesigned just a bit to remove the need for liquid fluorine oxidizer. This would involve removing equipment and increasing the size of the fuel tanks.

North American Rockwell PEM

These are from Technological Requirements Common to Manned Planetary Missions: Appendix D by the space division of North American Rockwell (1965). They detail a s family of Planetary Excursion Modules (PEM).

Here I will focus on the retrobraking PEMs, that is, the ones that use retrorockets to land because the target planets have no atmosphere to allow aerobraking. These particular PEMs were intended for landing on Ceres, Vesta, Ganymede and Mercury. They are based on the Apollo Lunar Excursion Module with an Ascent Stage on top of a Descent Stage. They are designed for a 30 day stay on the planetary surface, before returning to orbit.

They do, however, have Rockwell's fixation on using the insanely dangerous FLOX as an oxidizer, and the only somewhat dangerous Monomethylhydrazine (MMH) as fuel. The reason is that liquid hydrogen requires preposterously huge tanks, but if you use anything else the specific impulse goes way down. Unless you take the mad-scientist step of using FLOX oxidizer to make up for it.

In the diagrams below, the Ascent Stage is pink, the Descent Stage is green, and the mission crew quarters is gold.

3-Crew Ceres-Vesta PEM

North American
3-Crew Ceres-Vesta PEM
Wet Mass
14,036 kg
Descent Stage
Dry Mass
9,000 kg
Wet Mass
11,350 kg
Propellant Mass
2,350 kg
Propulsion Isp333 s?
Descent Engine
8000 N
(835 kg-force)
Descent Engine
1.85 kg
0.28 m/s2
Descent Start
2.0 Ceres
Ascent Stage
Dry Mass
2,280 kg
Wet Mass
2,686 kg
Propellant Mass
406 kg
Ascent Engine
3,780 N
(384 kg-force)
Ascent Engine
0.85 kg
Ascent Cabin
6.3 m3
Ascent Cabin
2.44 m
Ascent Cabin
1.83 m
Crew Quarters
14.0 m3
Crew Quarters
2.44 m
Crew Quarters
3.96 m
1.1 m3
0.915 m3
Crew Quarters
(inclu. airlock)
15.1 m3
Total Crew
21.2 m3
(30 day, 3 crew)
MMH54.4 lbs/ft3
FLOX90.0 lbs/ft3
Nitrogen12.30 ft3
295 kg
Oxygen10.00 ft3
325 kg
Water8.86 ft3
244 kg
Food5.63 ft3
175 kg

The mission crew quarters is merged with the ascent stage. Note the Descent Stage Dry Mass does not include the mass of the ascent stage.

10-Crew Ceres-Vesta PEM

North American
10-Crew Ceres-Vesta PEM
Descent Stage
Dry Mass
18,100 kg
Wet Mass
Propellant Mass
4,700 kg
Propulsion Isp333 s?
Descent Engine
(1,675 kg-force)
Descent Engine
3.7 kg
0.28 m/s2
Descent Start
Crew Quarters
Crew Quarters
Crew Quarters
2.14 m
1.41 m3
0.915 m3
Crew Quarters
(inclu. airlock)
66.0 m3
North American
10-Crew Ceres-Vesta PEM
Wet Mass
27,800 kg
Ascent Stage
Dry Mass
4,530 kg
Wet Mass
Propellant Mass
815 kg
Ascent Engine
(760 kg-force)
Ascent Engine
1.68 kg
Ascent Cabin
11.2 m3
Ascent Cabin
3.06 m
Ascent Cabin
1.94 m
Total Crew
North American
10-Crew Ceres-Vesta PEM
(30 day, 10 crew)
MMH54.4 lbs/ft3
FLOX90.0 lbs/ft3
Nitrogen980 kg
Oxygen1,080 kg
Water815 kg
Food590 kg

A page appears to be missing from my copy of the document

3-Crew Ganymede PEM

North American
3-Crew Ganymede PEM
Wet Mass17,300 kg
Descent Stage
Dry Mass
9,500 kg
Wet Mass
Propellant Mass
3,740 kg
Propulsion Isp333 s?
Descent Engine
27,000 N
(2,800 kg-force)
Descent Engine
6.15 kg
1.428 m/s2
Descent Start
North American
3-Crew Ganymede PEM
Ascent Stage
Dry Mass
2,280 kg
Wet Mass
Propellant Mass
3,010 kg
Ascent Engine
13,000 N
(1,340 kg-force)
Ascent Engine
2.95 kg
Ascent Cabin
6.3 m3
Ascent Cabin
2.44 m
Ascent Cabin
1.83 m
Crew Quarters
14.93 m3
Crew Quarters
2.44 m
Crew Quarters
3.96 m
1.55 m3
0.915 m3
Crew Quarters
(inclu. airlock)
Total Crew
21.2 m3
North American
3-Crew Ganymede PEM
(30 day, 3 crew)
MMH54.4 lbs/ft3
FLOX90.0 lbs/ft3
Nitrogen12.30 ft3
295 kg
Oxygen10.00 ft3
325 kg
Water8.86 ft3
244 kg
Food5.63 ft3
175 kg

The mission crew quarters is merged with the ascent stage.

10-Crew Ganymede PEM

North American
10-Crew Ganymede PEM
Descent Stage
Dry Mass
28,100 kg
Wet Mass
Propellant Mass
7,500 kg
Propulsion Isp333 s?
Descent Engine
55,000 N
(5,550 kg-force)
Descent Engine
12.3 kg
1.428 m/s2
Descent Start
Crew Quarters
66 m3
Crew Quarters
6.1 m
Crew Quarters
2.14 m
0.915 m3
Crew Quarters
(inclu. airlock)
North American
10-Crew Ganymede PEM
Wet Mass
45,800 kg
Ascent Stage
Dry Mass
4,536 kg
Wet Mass
Propellant Mass
6,050 kg
Ascent Engine
26,000 N
(2,670 kg-force)
Ascent Engine
5.9 kg
Ascent Cabin
11.2 m3
Ascent Cabin
3.3 m
Ascent Cabin
1.98 m
Total Crew
North American
10-Crew Ganymede PEM
(30 day, 10 crew)
MMH54.4 lbs/ft3
FLOX90.0 lbs/ft3
Nitrogen985 kg
Oxygen1,090 kg
Water815 kg
Food590 kg

The mission quarters is merged with the descent stage, instead of the ascent stage as is the case with the 3-crew vehicles.

Nuclear DC-X

Nuclear DC-X
Propulsionpebble-bed NTR
NTR Specific Impulse1000 s
LANTR Specific Impulse600 s
NTR Exhaust Velocity9,810 m/s
LANTR Exhaust Velocity5,900 m/s
Wet Mass460,000 kg?
Dry Mass? kg
Mass Ratio?
ΔV? m/s
Mass Flow? kg/s
NTR Thrust per engine1,112,000 n
LANTR Thrust per engine3,336,000 n
NTR Thrust total5,560,000 n
LANTR Thrust total16,680,000 n
NTR Acceleration12 g?
LANTR Acceleration38 g?
Payload100,000 kg
Length103 m
Diameter10 m

This is from a report called AFRL-PR-ED-TR-2004-0024 Advanced Propulsion Study (2004). It is a single stage to orbit vehicle using a LANTR for propulsion. They figure it can put about 100 metric tons into orbit at a cost of $150 per kilogram. You can read the details in the report.

Nuclear OTV Light Commercial Transport

Light Freighter for intraorbital service between space colonies and industrial platforms, designed for the System States Era of my Orion’s Arm future history setting.

A timeline for my future history is to be found here: Timeline

In the System States Era asteroid mining operations thrive throughout the asteroid belt and among the moons of Jupiter and Saturn the Martian terraforming program has left legacy: a sprawling archipelago of island stations and industrialized moons, Bernal Sphere's and O'Neill Cylinders, Spindle and Wheel cities, and a population of humanity growing into the millions. Space colonies are independent city-states and trade is their lifeblood. Entire generations are born and live their lives in spinning cylinders, bubbles, and torus shaped habitats, harvesting, mining, and fabricating all they need from the environment of the outer solar system.

Orion and Medusa style nuclear pulse freighters haul payloads of raw materials across interplanetary distances, while nuclear orbital transfer vehicles (OTV’s) provide light freight and passenger service between space habitats in Jupiter and Saturn orbit.

For a table of Delta V required for travel using Hohmann orbits among the moons of Saturn see
Why Saturn on Winchell Chung’s Atomic Rockets site. Scroll a little further down the page and you will find a Synodic Periods and Transit Times for Hohmann Travel table for Moons of Saturn.

Nuclear propulsion Systems: Operational Constraints

The abundance of various chemical ices for use as reaction mass among the moons of the outer system gas giants makes NERVA an excellent option for commercial application. Nuclear thermal rockets provide excellent efficiency; they also impose certain operational restrictions. The engine emits significant levels of radiation while firing and even after shut-down, and while passengers and crew are protected by the engines shadow-shield and hydrogen tanks, you wouldn’t want to point the engine at other spacecraft or space platforms. During the U.S. nuclear thermal rocket engine development program NFSD contractors had recommended that no piloted spacecraft approach to within 100 miles behind or to the sides of an operating NERVA I engine. The only safe approach to a spacecraft with a NERVA engine is through the conical “safe-zone” within the radiation shadow created by its shadow-shield and hydrogen tanks. Docking NERVA propelled spacecraft to a space station or habitat is problematic because structures protruding outside the conical safe-zone can reflect radiation back at the spacecraft, irradiating the passengers and crew.

These facts impose a set of mandatory operational parameters and flight rules for nuclear operation. An exclusion zone for nuclear propulsion (60 kilometers minimum) is imposed around every orbital platform. Orbital Guard units would hold broad discretionary powers—violate an exclusion-zone or disregard traffic-control and the local guard will cheerfully vaporize your spacecraft. No warning shots, no second chances. A crew that violates flight rules doesn’t live long enough to worry about fines or attorney fees, and the public’s time and funds are not wasted with trials of incompetent captains and crew.

Nuclear Freighters “park” propulsion modules in station-keeping orbit with their destination, and the freight/passenger module undocks, separating from its nuclear propulsion module, proceeding to birthing under thrust of a chemical maneuvering unit.

Because the nuclear propulsion modules are valuable, and are potentially deadly missiles if mishandled — codes to access the autonomous flight computer and possession of the nuclear propulsion module are temporarily handed over to the local orbital-guard for safe keeping.

For a good example of Space traffic control see the entry on Winchell Chung’s Atomic Rockets site here and scroll down to quote from Manna by Lee Correy.

At this point in my future history, 750 years post Martian colonization, spacecraft are essentially stacks of common modules which can be swapped out to suit application.

Independent Operators, like today’s truckers, might “own” only the CMOD (Command Module) with other units being leased per flight. The Freight Carrying Structural Spine, essentially a rigid frame with mountings for cargo modules, might be leased by the shipper and loaded with cargo (but owned by a separate freight transport supplier) and since different payloads mass differently it might be the responsibility of the shipper to lease suitable nuclear and chemical propulsion modules rated to the task. Passenger transport services might likewise lease passenger modules of varying capacity and Transport Brokerage firms would coordinate freight and passenger payloads assigned to same destinations and offer these in an open-bid market.

Propulsion Modules

Different payload masses require different propulsion module configurations, the light freighter detailed here requires only a single Solid-Core nuclear thermal rocket. A heavy payload freighter might use clusters of solid-core, or Open-Cycle Gas-Core, nuclear thermal rockets.


3D models are my own conception based on various real-world proposals.

As research for the passenger/crew module I studied the POTV (Personnel Orbital Transfer Vehicle) pages 86-96 from NASA Technical Memorandum 58238 Satellite Power System: Concept Development and Evaluation Program Volume VI1 -Space Transportation available: here.

Propulsion for my light freighter is a Solid-Core NERVA Derivative, details available here.

In conversation Winchell Chung suggested the modification Cascade-Vanes: details available here.

Related Image: Nuclear OTV Commercial Transport

Nuclear Ferry

Nuclear Ferry
PropulsionNTR Solid Core
Propellant159,000 kg
Dry Mass
nuclear stage
32,500 kg
Wet Mass
nuclear stage
191,500 kg
Wet Mass
271,600 kg
Engine Thrust213,000 N
Number Engines4
Total Thrust852,000 N
Length59.5 m
Diameter10 m
Lunar Shuttle
PropulsionChemical LOX/LH2
Engine Thrust89,000 N
Number Engines3
Total Thrust267,000 N
Cargo Mass16,400 kg
Gross Mass57,100 kg

This is from a 1964 Ling-Temco-Vought, Inc. study for NASA.

The single ferry tank is sized to contain 349,828 pounds of liquid hydrogen and has attached to its after-end, a thrust structure and four fast spectrum metallic-core nuclear reactor engines. Each of the four engines will produce 47,900 pounds of thrust so that with one engine inoperative, the initial thrust-to-weight ratio is 0.24. The dry weight of the nuclear stage is 71,658 pounds, which results in a stage mass fraction of 0.830.

The lunar shuttle, shown in docked position, employs the techique of receiving its propellant by the assembly of prepackaged modules in lunar orbit with the empty module being jettisoned on the lunar surface. The lunar shuttle propulsion system uses LOX-LH2 propellants and is designed around a cluster of three 20,000-pound thrust engines. The lunar shuttle personnel module is of the segmented sphere configuration sized to accommodate a total of 30 men and having all expendables and time-dependent subsystems sufficient for a two-day mission duration.

From Ling-Temco-Vought, Inc. study (1964)

One spaceship that could make the trip is now under study by Ling-Temco-Vought, Inc. for NASA and is show here as it debarks its 28 passengers into a landing craft for the descent to the moon's surface.

The tapered passenger-carrying nose of this ship (below, left) had departed from Earth three days earlier atop a booster. Once in orbit around the Earth, it had then hooked onto a 125-foot-long nuclear-powered ferry (the section carrying the American flag) that carried into the region of the moon.

Having now attained a lunar orbit, the passenger section has swung down 17 feet on long tubular links, alowing a small spherical craft to hook on and load up the passengers for the 100-mile trip down to the moon. In the background at right a similar nuclear ferry also has lowered its passenger section and is about to receive a moon shuttle. Scientists say the passenger section could be used over and over again—for as many as 50 round trips. Because of this reusability, space scientists estimate the eventual cost of maintaining a moon station at only $1 million per man per year.

From LIFE Magazine October 2, 1964

PARTS Plasma Accelerated Reusable Transport System

P.A.R.T.S. is a 2002 study by the Embry-Riddle Aeronautical University for a reusable Earth-Mars cargo spacecraft utilizing a VASIMR propulsion system powered by an on-board nuclear reactor. The report has lots of juicy details, especially about the reactor. Thanks go out to William Seney for bringing this study to my attention.

RMBLR (Rotating Multi-Megawatt Boiling Liquid-Metal Reactor) "Rambler" System. Fuel: Blocks with coolant channels UN+Moly alloy with Rhenium & hafnium, Primary coolant : Potassium, Reactor outlet temperature:1440K, power conversion: Direct Rankine, Specific Mass: 1-2kg/kWe @ 20 MWe assuming a bubble membrane radiator.

Pilgrim Observer

RocketCat sez

Another blast from the past! Rocket fans who built this plastic model back in the 1970's agree it makes the needle of their Nostalgia-meter slam over and wrap itself around the end peg. But what is more astonishing is the real-world roots of the blasted thing. Sure it has a couple of design problems, but it makes far more scientific sense than 99% of the other plastic models.

Pilgrim Observer
PropulsionNERVA 2b
PropulsionUprated J2 chemical
NERVA Specific Impulse850 s
J2 Specific Impulse~450 s
NERVA Exhaust Velocity8,300 m/s
J2 Exhaust Velocity4,400 m/s
Wet Mass? kg
Dry Mass? kg
Mass Ratio?
ΔV? m/s
NERVA Mass Flow13 kg/s
J2 Mass Flow25 kg/s
NERVA Thrust110,000 newtons
J2 Thrust110,000 newtons
Initial Acceleration? g
Payload? kg
Length30 m + boom
Diameter46 m

The Pilgrim Observer was a plastic model kit issued by MPC back in 1970 (MPC model #9001) designed by G. Harry Stine. Many of us oldster have fond memories of the kit. It was startlingly scientifically accurate, especially compared its contemporaries (ST:TOS Starship Enterprise, ST:TOS Klingon Battlecruiser, Galactic Cruiser Leif Ericson).

The model kit included a supplemental booklet just full of all sorts of fascinating details. NERVA engine design, mission plan, all sorts of goodies with the conspicuous absence of the mass ratio and the total delta-V.

The kit has been recently re-issued, and those interested in realistic spacecraft design could learn a lot by building one. If you do, please look into the metal photoetched add-on kit, and alternate decals. Round 2 Models (the company who re-issued the kit) have some detailed kit building instructions here.

The Pilgrim makes a cameo appearance in Jerry Pournelle's short story "Tinker", in the role of the Boostship Agamemnon, and in Allen Steele's short story The Weight as the Medici Explorer.

The design is interesting, and has a lot of innovative elements. For one, it uses a species of gimbaled centrifuge to deal with the artificial gravity problem. It also uses distance to augment its radiation shielding, in order to save on mass and increase payload. This is done by mounting the NERVA solid core nuclear rocket on a telescoping boom.

One major flaw with the Pilgrim's design is the fact that one of the three spinning arms is the power reactor. This means that all the ship's power supply has to be conducted through a titanic slip-ring, since there can be no solid connection between the spin part and the stationary part. Another flaw is if you are going to all the trouble to put the NERVA reactor on a boom to get the radiation far away from the crew, why would you put the radioactive power reactors on an arm right next to the crew?

Anyway, the Pilgrim is an orbit-to-orbit spacecraft that is incapable of landing on a planet. It has a ten man crew (four crew and six scientists), and has enough life support endurance to keep them alive for five years. It could also be used as a space station, in LEO, GEO, or lunar orbit. In launch configuration the NERVA boom is retracted and the spinning arms are locked down. In this configuration it is 100 feet long and 33 feet wide, which fits on top of the second stage of a Saturn V booster. A disposable shroud is placed over the top of the spacecraft to make it more aerodynamic during launch.

Level 60.05g
Level 50.06g
Level 40.07g
Level 30.08g
Level 20.09g
Level 10.10g

After launch, the shroud is jettisoned, the spinning arms deploy, and the NERVA engine's boom telescopes out. The spinning arm array has a diameter of 150 feet. The arms will rotate at a rate of two revolutions per minute (safely below the 3 RPM nausea limit). This will produce about one-tenth Earth gravity at the tips of the arms (Level 1), which fades to zero gravity at the rotation axis. Not much but better than nothing. The spherical center section does not spin, a special transfer cabin is used to move between the spin and non-spin sections.

One arm is the crew quarters, one is a hydroponic garden for the closed ecological life support system, and the third is a stack of advanced Space Nuclear Auxiliary Power (SNAP) reactors using Brayton cycle nuclear power units.

The center section is divided into the Main Control Center at the top and the Service Section at the bottom. The very top of the Control Center has the large telescope, radar, and other sensors. By virtue of being mounted on the non-spin section, the astronomers and astrogators can make their observations without having to cope with all the stars spinning around. Also mounted here is the antenna farm for communications and telemetry.

The Pilgrim carries two auxiliary vehicles: a modified Apollo command and service module, and a one-man astrotug similar to the worker pods seen in the movie 2001 A Space Odyssey. They mate with Universal Docking Adaptors on the non-spin section.

The chemical propulsion system consists of three up-rated J-2 rocket engines with a thrust of 250,000 lbs, fueled by liquid hydrogen and liquid oxygen.

The nuclear thermal propulsion system consists of one solid-core NERVA 2B, using liquid hydrogen as propellant. The NERVA has a specific impulse of 850 seconds, a thrust of 250,000 pounds, and an engine mass of 35,000 pounds (the fact that both the J-2 and the NERVA have identical thrust makes me wonder if that is a misprint). It uses a de Laval type convergent-divergent rocket nozzle. The reactor core has a temperature of 4500°F. The core of the reactor is encased in a beryllium neutron reflector shell. Inside the reflector and surrounding the reactor core are twelve control rods. Each rod is composed of beryllium with a boron neutron absorber plate along one side. By rotating the control rods, the amount of neutrons reflected or absorbed can be controlled, and thus control the fission chain reaction in the reactor core.

There is a dome shaped shadow shield on top of the NERVA to protect the crew from radiation. In addition, the NERVA is on a long boom, adding the inverse square law to reduce the amount of radiation. And finally, the cosmic ray shielding around the crew quarters provides even more protection.

Various attitude control and ullage rockets are located at strategic spots, they are fueled by hypergolic propellants.

The mission will start in June of 1979. Mission is an Earth-Mars-Venus-Earth swing-by. It will have a mission duration of 710 days, as compared to the 971 days required for a simple Mars orbiting round trip. This is done with clever gravitational sling-shots, and use of the NERVA 2B.

Mission starts with an orbital plane change to a 200 nautical mile circular Earth orbit inclined 23°27' (i.e., co-planar with the ecliptic). Transarean insertion burn is made with the three J-2 chemical engines (D+0). At this point the Pilgrim 1 becomes the Pilgrim-Observer space vehicle. It will coast for 227 days. Then it will perform a retrograde burn with the NERVA to achieve a circumarean orbit (Mars orbit) with a periapsis of 500 nautical miles and a high point of 5,800 nautical miles (D+227).

The Pilgrim-Observer will spend 48 days in Martian orbit (including several close approaches to Phobos). Then the NERVA will thrust into a transvenerian trajectory (D+275). It will coast for 246 days, including a close approach and fly-by of the asteroid Eros occurring 145 days after transvenerian burn (D+320).

The NERVA will burn into a circumvenarian orbit of of 500 nautical miles (D+521). It will spend 55 days studying Venus.

The NERVA will thrust into a transearth injection (D+576). It will coast for 140 days. Upon Earth approach, it will burn into a 200 nautical mile Earth orbit (D+710). The crew will be out shipped by a shuttle craft following extensive debriefing.

I did some back of the envelope calculations, and the numbers look fishy to me. An Earth-Mars Hohmann and Mars capture orbit will take a delta V of about 5,200 m/s. This is done with the J-2 chemical engine, and will require a mass ratio of 3.3. That is not a problem.

The problem comes with the NERVA burns. The Mars-Venus burn and the Venus-Earth burns have a total of about 14,800 k/s. With a NERVA exhaust velocity of 8,300 m/s, this implies a mass ratio of 5.9. I'm sorry but without staging you are going to be lucky to get a mass ratio above 4.0.

The plastic model kit is allegedly 1:100 scale according to the kit instructions. However, expert model builders who did measurements figured out that various parts are clumsily in different scales. The "arms folded mode" diameter is supposed to be 33 feet, to fit on top of a Saturn V, that is 1:127 scale. The rotating arms and the Apollo M are more like 1:144 to 1:200 scale. At 1:100 the arms have a deck spacing of a cramped 5 feet, the passage connecting the arm to the ship proper is only 2.5 feet in diameter, and the command module on the Apollo M is 20% smaller than the real Apollo CM. So the scale of the plastic model kit is a mess.

BoostShip Agamemnon

The Agamemnon is basically the Pilgrim Observer with the NERVA solid core NTR swapped out for an ion drive powered by a deuterium fusion reactor.

A Step Farther Out
IBS Agamemnon
Total ΔV280,000 m/s
Specific Power39 kW/kg
(39,000 W/kg)
Thrust Power1.1 terawatts
Exhaust velocity220,000 m/s
Thrust10,000,000 n
Wet Mass100,000 mt
Dry Mass28,000 mt
Mass Ratio3.57
Ship Mass8,000 mt
Cargo Mass20,000 mt
Length400 m
Length spin arm100 m
T/W >1.0no

IBS Agamemnon (Interplanetary Boost Ship) masses 100,000 tons as she leaves Earth orbit. She carries up to 2000 passengers with their life support requirements. Not many of these will be going first-class, though; many will be colonists, or even convicts, headed out steerage under primitive conditions.

Her destination is Pallas, which at the moment is 4 AU from Earth, and she carries 20,000 tons of cargo, mostly finished goods, tools, and other high-value items they don't make out in the Belt yet. Her cargo and passengers were sent up to Earth orbit by laser-launchers; Agamemnon will never set down on anything larger than an asteroid.

She boosts out at 10 cm/sec2, 1/100 gravity, for about 15 days, at which time she's reached about 140 km/second. Now she'll coast for 40 days, then decelerate for another 15. When she arrives at Pallas she'll mass 28,000 tons. The rest has been burned off as fuel and reaction mass. It's a respectable payload, even so.

(ed note: in reality, the maximum amount of thrust a single ion drive could put out is about 10,000 newtons, not 10,000,000 like the Agamemnon is cranking out.)

The reaction mass must be metallic, and it ought to have a reasonably low boiling point. Cadmium, for example, would do nicely. Present-day ion systems want cesium, but that's a rare metal—liquid, like mercury—and unlikely to be found among the asteroids, or cheap enough to use as fuel from Earth.

In a pinch I suppose she could use iron for reaction mass. There's certainly plenty of that in the Belt. But iron boils at high temperatures, and running iron vapor through them would probably make an unholy mess out of the ionizing screens. The screens would have to be made of something that won't melt at iron vapor temperatures. Better, then, to use cadmium if you can get it.

The fuel would be hydrogen, or, more likely, deuterium, which they'll call "dee." Dee is "heavy hydrogen," in that it has an extra neutron, and seems to work better for fusion. We can assume that it's available in tens-of-ton quantities in the asteroids. After all, there should be water ice out there, and we've got plenty of power to melt it and take out hydrogen, then separate out the dee.

(ed note: 1,100 gigawatts requires burning about 0.014 kilograms of deuterium per second. For 30 days total burn time this will require about 36 metric tons of deuterium.)

If it turns out there's no dee in the asteroids it's not a disaster. Shipping dee will become one of the businesses for interplanetary supertankers.

From Life Among the Asteroids by Jerry Pournelle, collected in A Step Farther Out (1975)


The other screen lit, giving us what the Register knew about Agamemnon. It didn't look good. She was an enormous old cargo-passenger ship, over thirty years old—and out here that's old indeed. She'd been built for a useful life of half that, and sold off to Pegasus Lines when P&L decided she wasn't safe.

Her auxiliary power was furnished by a plutonium pile. If something went wrong with it, there was no way to repair it in space. Without auxiliary power, the life-support systems couldn't function.

I switched the comm system to Record. "Agamemnon, this is cargo tug Slingshot. I have your Mayday. Intercept is possible, but I cannot carry sufficient fuel and mass to decelerate your ship. I must vampire your dee and mass, I say again, we must transfer your fuel and reaction mass to my ship.

"We have no facilities for taking your passengers aboard. We will attempt to take your ship in tow and decelerate using your deuterium and reaction mass. Our engines are modified General Electric Model five-niner ion-fusion. Preparations for coming to your assistance are under way. Suggest your crew begin preparations for fuel transfer. Over."

The Register didn't give anywhere near enough data about Agamemnon. I could see from the recognition pix that she carried her reaction mass in strap-ons alongside the main hull, rather than in detachable pods right forward the way Slinger does. That meant we might have to transfer the whole lot before we could start deceleration.

She had been built as a general-purpose ship, so her hull structure forward was beefy enough to take the thrust of a cargo pod—but how much thrust? If we were going to get her down, we'd have to push like hell on her bows, and there was no way to tell if they were strong enough to take it.

The refinery crew had built up fuel pods for Slinger before, so they knew what I needed, but they'd never made one that had to stand up to a full fifth of a gee. A couple of centimeters is hefty acceleration when you boost big cargo, but we'd have to go out at a hundred times that.

They launched the big fuel pod with strap-on solids, just enough thrust to get it away from the rock so I could catch it and lock on. We had hours to spare, and I took my time matching velocities. Then Hal and I went outside to make sure everything was connected right.

Slingshot is basically a strongly built hollow tube with engines at one end and clamps at the other. The cabins are rings around the outside of the tube. We also carry some deuterium and reaction mass strapped on to the main hull, but for big jobs there's not nearly enough room there. Instead, we build a special fuel pod that straps onto the bow. The reaction mass can be lowered through the central tube when we're boosting.

Boost cargo goes on forward of the fuel pod. This time we didn't have any going out, but when we caught up to Agamemnon she'd ride there, no different from any other cargo capsule. That was the plan, anyway. Taking another ship in tow isn't precisely common out here.

Everything matched up. Deuterium lines, and the elevator system for handling the mass and getting it into the boiling pots aft; it all fit.

Ship's engines are complicated things. First you take deuterium pellets and zap them with a big laser. The dee fuses to helium. Now you've got far too much hot gas at far too high a temperature, so it goes into an MHD system that cools it and turns the energy into electricity.

Some of that powers the lasers to zap more dee. The rest powers the ion drive system. Take a metal, preferably something with a low boiling point like cesium, but since that's rare out here cadmium generally has to do. Boil it to a vapor. Put the vapor through ionizing screens that you keep charged with power from the fusion system.

Squirt the charged vapor through more charged plates to accelerate it, and you've got a drive. You've also got a charge on your ship, so you need an electron gun to get rid of that.

There are only about nine hundred things to go wrong with the system. Superconductors for the magnetic fields and charge plates: those take cryogenic systems, and those have auxiliary systems to keep them going. Nothing's simple, and nothing's small, so out of Slingshot's sixteen hundred metric tons, well over a thousand tons is engine.

Now you know why there aren't any space yachts flitting around out here. Slinger's one of the smallest ships in commission, and she's bloody big. If Jan and I hadn't happened to hit lucky by being the only possible buyers for a couple of wrecks, and hadn't had friends at Barclay's who thought we might make a go of it, we'd never have owned our own ship.

When I tell people about the engines, they don't ask what we do aboard Slinger when we're on long passages, but they're only partly right. You can't do anything to an engine while it's on. It either works or it doesn't, and all you have to do with it is see it gets fed.

It's when the damned things are shut down that the work starts, and that takes so much time that you make sure you've done everything else in the ship when you can't work on the engines. There's a lot of maintenance, as you might guess when you think that we've got to make everything we need, from air to zweiback. Living in a ship makes you appreciate planets.

Space operations go smooth, or generally they don't go at all.

When we were fifty kilometers behind, I cut the engines to minimum power. I didn't dare shut them down entirely. The fusion power system has no difficulty with restarts, but the ion screens are fouled if they're cooled. Unless they're cleaned or replaced we can lose as much as half our thrust—and we were going to need every dyne.

His face didn't change. "Experienced cadets, eh? Well, we'd best be down to it. Mr. Haply will show you what we've been able to accomplish." They'd done quite a lot. There was a lot of expensive alloy bar-stock in the cargo, and somehow they'd got a good bit of it forward and used it to brace up the bows of the ship so she could take the thrust. "Haven't been able to weld it properly, though," Haply said. He was a young third engineer, not too long from being a cadet himself. "We don't have enough power to do welding and run the life support too."

Agamemnon's image was a blur on the screen across from my desk. It looked like a gigantic hydra, or a bullwhip with three short lashes standing out from the handle. The three arms rotated slowly. I pointed to it. "Still got spin on her."

"Yes." Ewert-James was grim. "We've been running the ship with that power. Spin her up with attitude jets and take power off the flywheel motor as she slows down."

I was impressed. Spin is usually given by running a big flywheel with an electric motor. Since any motor is a generator, Ewert-James's people had found a novel way to get some auxiliary power for life-support systems.

Agamemnon didn't look much like Slingshot. We'd closed to a quarter of a klick, and steadily drew ahead of her; when we were past her, we'd turn over and decelerate, dropping behind so that we could do the whole cycle over again.

Some features were the same, of course. The engines were not much larger than Slingshot's and looked much the same, a big cylinder covered over with tankage and coils, acceleration outports at the aft end. A smaller tube ran from the engines forward, but you couldn't see all of it because big rounded reaction mass canisters covered part of it.

Up forward the arms grew out of another cylinder. They jutted out at equal angles around the hull, three big arms to contain passenger decks and auxiliary systems. The arms could be folded in between the reaction mass canisters, and would be when we started boosting. All told she was over four hundred meters long, and with the hundred-meter arms thrust out she looked like a monstrous hydra slowly spinning in space.

The fuel transfer was tough. We couldn't just come alongside and winch the stuff over. At first we caught it on the fly: Agamemnon's crew would fling out hundred-ton canisters, then use the attitude jets to boost away from them, not far, but just enough to stand clear.

Then I caught them with the bow pod. It wasn't easy. You don't need much closing velocity with a hundred tons before you've got a hell of a lot of energy to worry about. Weightless doesn't mean massless.

We could only transfer about four hundred tons an hour that way. After the first ten-hour stretch I decided it wouldn't work. There were just too many ways for things to go wrong.

"Get rigged for tow," I told Captain Ewert-James. "Once we're hooked up I can feed you power, so you don't have to do that crazy stunt with the spin. I'll start boost at about a tenth of a centimeter. It'll keep the screens hot, and we can winch the fuel pods down."

He was ready to agree. I think watching me try to catch those fuel canisters, knowing that if I made a mistake his ship was headed for Saturn and beyond, was giving him ulcers.

First he spun her hard to build up power, then slowed the spin to nothing. The long arms folded alongside, so that Agamemnon took on a trim shape. Meanwhile I worked around in front of her, turned over and boosted in the direction we were traveling, and turned again.

The dopplers worked fine for a change. We hardly felt the jolt as Agamemnon settled nose to nose with us. Her crewmen came out to work the clamps and string lines across to carry power. We were linked, and the rest of the trip was nothing but hard work.

We could still transfer no more than four hundred tons an hour, meaning bloody hard work to get the whole twenty-five thousand tons into Slinger's fuel pod, but at least it was all downhill. Each canister was lowered by winch, then swung into our own fuel-handling system, where Singer's winches took over. Cadmium's heavy: a cube about two meters on a side holds a hundred tons of the stuff. It wasn't big, and it didn't weigh much in a tenth of a centimeter, but you don't drop the stuff either.

Finally it was finished, and we could start maximum boost: a whole ten centimeters, about a hundredth of a gee. That may not sound like much, but think of the mass involved. Slinger's sixteen hundred tons were nothing, but there was Agamemnon too. I worried about the bracing Ewert-James had put in the bows, but nothing happened.

Three hundred hours later we were down at Pallasport.

From Tinker by Jerry Pournelle (1975)


The Wayfarer is basically a stock Pilgrim Observer, all the way down to the NERVA engine. Except that the arms do not extend and rotate for artificial gravity.

Exiles To Glory

Their first impression was of a bundle of huge cigars. Those were the big fuel tanks almost a hundred meters long. They were so large that they dwarfed the rest of the ship, and ran the entire length of midsection. Behind the "cigars" was a solid ring that held three rocket motors. Then at the end of a spine as long as the main body of the ship was the nuclear reactor and another rocket motor.

This was the real drive. The three chemical rockets were only for steering and close maneuvering. Wayfarer's power came from her atomic pile. The cigar-shaped tanks held hydrogen, which was pumped back to the reactor where it was heated up and spewed out through the rear nozzle. A ring of heavy shielding just forward of the reactor kept the pile's radiation from getting to the crew compartment. The rest of the pile wasn't shielded at all.

Despite the large size of the ship, the crew and cargo sections seemed quite small. There were some structures reaching back from the forward ring where the control room was. Two of those were passenger quarters. The other was another nuclear power unit to make electricity to run the environmental control equipment, furnish light for the plants, power to reprocess air, and all the other things the ship and passengers and crew would need. There was a big telescope and a number of radar antennae on the forward section.

The scooter pilot was careful not to get near the reactor in the ship's "stinger."

The ship had been designed for sixty passengers. She carried twice that number plus eight crew.

The internal space was constructed in a series of circular decks. Each deck had an eight-foot hole in its center, so that from the forward end, just aft of the separately enclosed control cabin, Kevin could look all the way aft to the stern bulkhead. Although there was a long and rather flimsy-appearing steel ladder stretching from aft to forward bulkhead, no one used it.

"F deck," the crewman said "A deck is the bridge. B is the wardroom. C, D, and E are the three aft of that. E happens to be the recreation and environmental control. Yours is the one beyond that. They're marked."

Finally he reached F deck, which he found to be sectioned into slice-of-pie compartments arranged in a ring around the central well, fifteen of them in all. He found the one marked "12" and went in.

His "stateroom" was partitioned off with a flexible, bright blue material that Kevin thought was probably nylon. The door was of the same stuff and tied off with strings. It didn't provide much privacy.

Inside the cramped quarters were facilities for two people. There were no bunks, but two blanket rolls strapped against the bulkhead indicated the sleeping arrangements. It made sense, Kevin thought. You didn't need soft mattresses in space. "Sleeping on a cloud" was literally true here. You needed straps to keep you from drifting away, but that was all.

One viewscreen with control console, a small worktable, and two lockers about the size of large briefcases completed the furnishings.

The incident reminded Kevin that he was in free fall, and his stomach didn't like it much. He gulped hard. "I'll be glad when we're under way," he said. "It won't last long, but it will be nice to have some weight again. Even for a day or so."

Norsedal frowned and rolled his eyes upward for a moment. "Not that long, I'm afraid," he said. "Let's see, total velocity change of about five kilometers a second, at a tenth of gravity acceleration—five thousand seconds." He took a pocket computer off his belt and punched numbers. "An hour and a half. Then we're back in zero-gravity."

Weight felt strange. The ship boosted at about ten percent of Earth's gravity, but Kevin found that quite enough. All over the ship loose objects fell to the decks.

Ninety minutes later the acceleration ended. Wayfarer was now in a long elliptical orbit that would cross the orbit of Ceres. Left to itself, the ship would go on past, more than halfway to Jupiter, before the Sun's gravity would finally turn it back to complete the ellipse and return it to its starting point. In order to land on Ceres, the ship would have to boost again when it got out to the orbit of the asteroid.

There would also be minor course-correction maneuvers during the trip, but except for those the ship's nuclear-pile engine wouldn't be started up until they arrived at Ceres's orbit. Then the ship would accelerate to catch up with the asteroid. That wouldn't happen for nine months.

The heart of the system was a series of large transparent tanks filled with green water and tropical fish. Once Wayfarer was under way the crew erected large mirrors outside the hull. The mirrors collected sunlight and focused it through Plexiglas viewports onto the algae tanks. A ventilation system brought the ship's air into the tanks as a stream of bubbles. Other pumping systems collected sewage and forced it into chemical processors; the output was treated sewage that went to the algae tanks as fertilizer.

Wayfarer had two airlocks. One was right in the bows, a large docking port that allowed smaller space capsules to link up with the ship, and could also be used to link with an airtight corridor connecting the ship with the Ceres spaceport, or even with another ship. The other was a smaller personnel lock on the side of the hull just aft of the bows. Kevin and Ellen went out that way. There was a small ladder leading forward. It wasn't needed as a ladder, but it provided handholds.

The telescope was large, over a foot in diameter, with flexible seals that let it pass through the ship's hull and into the control bridge.

The ship's engines started. There was no sound and no flame. Hydrogen was pumped from the tanks and into the nuclear pile on its sting at the end of the ship. The nuclear reactor heated the hydrogen and forced it back through nozzles. The ship drove forward at a tenth of a gravity.

From Exiles to Glory by Jerry Pournelle (1977)

Medici Explorer

The Weight

(ed note: the Medici Explorer is basically the Pilgrim Observer with the NERVA solid core NTR swapped out for a gas-core NTR)

The Medici Explorer was fifty-six meters in length, from the gunmetal-grey nozzle of its primary engine to the grove of antennae and telescopes mounted on its barrel-shaped hub module; at the tips of its three arms—which were not yet rotating—the spacecraft was about forty-six meters in diameter. Pale blue moonlight reflected dully from the tube-shaped hydrogen, oxygen, and water tanks clustered in tandem rows between the hub and the broad, round radiation shield at the stern. Extended on a slender boom aft of the shield, behind the three gimbal-mounted maneuvering engines, was the gas-core nuclear engine, held at safe distance from the crew compartments at the forward end of the vessel. Although the reactor stack in Arm Three was much closer to the hub, it was heavily shielded and could not harm the crew when it was in operation.

The Medici Explorer was already awake and thriving. Two days earlier, it had departed from Highgate, the lunar-orbit spacedock where it had been docked since the completion of its last voyage six months ago. During the interim, while its crew rested at Descartes City and the precious cargo of Jovian helium-3 was unloaded from the freighters and transported to Earth, the Medici Explorer had undergone the routine repairs necessary before it could make its next trip to Jupiter. Now, at long last, the giant spacecraft had been towed by tugs to a higher orbit where it was reunited with its convoy.

The shuttle made its final approach toward the vessel’s primary docking collar on the hub module. On the opposite side of the docking collar, anchored to a truss which ran through a narrow bay between the outboard tanks, was the Marius, a smaller spacecraft used for landings. The fact that the ship’s boat was docked with the larger vessel was evidence that the Medici Explorer’s crew had returned from shore leave; more proof could be seen from the lights which glowed from the square windows of Arm One and Arm Two.

Red and blue navigational beacons arrayed along the superstructure illuminated more details: an open service panel on the hub where a robot was making last-minute repairs; a hardsuited space worker checking for micrometeorite damage to the hull; the round emblem of Consolidated Space Industries, the consortium which owned the vessel, painted on the side of the hub. Then the shuttle slowly yawed starboard, exposing its airlock hatch to the docking collar, and the Medici Explorer drifted away from its windows.

So on and so forth, barely pausing for breath, as we dropped down the hub’s access shaft to the carousel which connects the hub to the ship’s three arms. Since the arms were not presently rotating, we didn’t need to make the tricky maneuver of reorienting ourselves until the appropriate hatchway swung past us. The carousel’s hatches were aligned with their appropriate arms, so all we had to do was squirm through the upward-bending corridor—passing the sealed tiger-striped hatch which led to the reactor stack in Arm Three—until we reached the open hatch marked Arm 1.

The arm’s central shaft resembled a deep well, fifteen meters straight down to the bottom. Although I consciously knew that I couldn’t fall in zero-gee, I instinctively rebelled at the thought of throwing myself into a neck-breaking plummet. While I paused at the edge of the hatch, still visually disoriented by the distance, Young Bill dove headfirst through the hatch, scarcely grabbing the rungs of the ladder which led down the blue-carpeted wall of the shaft. I shut my eyes for a moment, fighting a surge of nausea, then I eased myself feet-first into the shaft, carefully taking each rung a step at a time.

There were six levels in Arm One, each accessed by the long ladder. Still babbling happily about rain forests and South American Indian tribes, Young Bill led me past Level 1-A (the infirmary and life sciences lab) Level 1-B (the Smith-Tate residence), Level 1-C (Smith-Makepeace) and Level 1-D (Smith-Tanaka). The hatches to each deck were shut, but as we glided past Level 1-D, its hatch opened and a preadolescent boy recklessly rushed out into the shaft and almost collided with Young Bill.

Young Bill shut the hatch, then led me down one more level to Deck 1-E, the passenger quarters.

He opened the hatch to Deck 1-E and pulled himself inside, hauling my duffel bag behind him. The deck was divided into four passenger staterooms, along with a common bathroom; not surprisingly, it was marked Head, retaining the old nautical term. The small compartment Bill led me to had its own foldaway bed, desk, data terminal and screen, along with a wide square window through which I could see the Moon.

Don’t bother making yourself at home,” he said as he stowed my duffel bag in a closet. “After we launch, you won’t see this place again for nine months.”

I nodded. “The other passengers… they’re already in hibernation?”

Yep. I helped Uncle Yoshi dope ’em up a few hours ago. They’re zombified already. You’ll be joining them after we—”

The Medici Explorer’s command center was shaped like the inside of a Chinese wok. Located on the top deck of the hub, Deck H-1 was the largest single compartment in the vessel: about fifteen meters in diameter, the bridge had a sloping, dome-shaped ceiling above a shallow, tiered pit. Two observation blisters, each containing an optical telescope, were mounted in the ceiling at opposite ends of the pit; between them were myriad computer flat-screens and holographic displays, positioned above the duty stations arranged around the circumference of the pit. In the center of the bridge, at the bottom of the pit between and slightly below the duty stations, was the captain’s station, a wingback chair surrounded by wraparound consoles. On one side of the bridge was the hatch leading to the hub’s access tunnel; on the opposite side was a small alcove, a rest area furnished with three chairs and a small galley.

It may sound claustrophobic and technocratic, but the bridge was actually quite spacious and comfortable. The floors were carpeted, allowing one to comfortably walk on them provided that one was wearing stikshoes, and the holoscreens provided a variety of scenes from outboard cameras as well as the main telescope, giving the illusion of cathedral windows looking out upon the grand cosmos.

The major technological breakthrough which made Jupiter reachable was made in 2028 by a joint R&D project by Russian and American physicists at the Kurchatov Institute of Atomic Energy and the Lawrence Livermore National Laboratory: the development of a gas-core nuclear engine, resulting in an impulse-per-second engine thrust ratio twice as high as even the thermal-fission engines used by Mars cycleships.

He went out in the ship’s service bug, a tiny gumdrop-shaped vehicle with double-jointed RWS arms, used for in-flight repair operations. Bill had been thoroughly trained and checked out for the bug; indeed, this was the third time he had piloted it during a flight. While Betsy, his dad, and Saul monitored from the bridge, he took the bug out from its socket on the hub, jetted around the ship’s rotating arms, and gently maneuvered the little one-person craft until he reached the maneuvering engines behind the radiation shield.

From The Weight by Allen Steele (1995)

Pilgrim Observer Roots

When creating the Pilgrim Observer, G. Harry Stine started with a 1960's study on creating a self deploying space station. Mr. Stine added the propellant tanks and the NERVA NTR to make it into a spacecraft. You will note the box cover says "Space Station", not "Spacecraft". David Portree identified the space station study in question. Actually studies plural, the Pilgrim was based on an amalgam of several.

MSC Station
total weight manned
& supplied
250,000 lbs
diameter (deployed)150 ft
diameter of
pressurized cabin
15.25 ft
diameter of
access tubes
5 ft
diameter of hub33 ft
length of spoke50 ft
# of decks
in spokes
height of
one deck
rate of
~4 x/min
launch vehicle2-stage Saturn V
orbit260-n-mi, 29.5° incl
5 yrs
launch date1968-1970
crew size24-36
Spin Gravity
Level2 rpm3 rpm4 rpm
Level 60.05g0.10g0.18g
Level 50.06g0.13g0.23g
Level 40.07g0.15g0.27g
Level 30.08g0.18g0.32g
Level 20.09g0.21g0.36g
Level 10.10g0.23g0.41g

David Portree said the design below is from an NASA Manned Spacecraft Center team under Owen Maynard and dates from 1962. The pressurized cabins and the access tubes are covered with a meteor bumper for protection (0.99 probability of not more than one penetration per month).

GE came up with a modified 35-kw SNAP-8 power system for this design in 9/64. They looked at placing the reactor at the center of rotation, down below the hub, or at the end of one of the arms. Oddly enough (from a balance standpoint), they favored placing the reactor at the end of one of the arms. I think they did this because the nadir surface of the hub was supposed to carry Earth-observation instruments.

You will notice that locating the reactor in one of the arms was copied in the design for the Pilgrim. This is foolish, since unlike the space station the Pilgrim has no Earth-observation instruments on its nadir surface. As a matter of fact, the Pilgrim already has a reactor on its nadir, inside the NERVA.

If was to re-design the Pilgrim Observer, I would not waste an entire rotating arm on the reactor. Instead I'd make the NERVA into a Bimodal NTR, and use the third arm for extra labs or something. The NERVA is not going to be thrusting during the months the ship coasts, so it might as well do something useful. The Bimodal switch would require the addition of some heat radiators, a turbine, a generator, and a condensor, but that should not be hard to incorporate.

However, the fact that the Pilgrim also had the reactor in one of the arms is yet more proof it was copied from the design of this space station.

The 150 foot diameter of the rotating section is the same figure quoted in the Pilgrim plastic model booklet. The Pilgrim however only rotated at 2 rpm, instead of 4 rpm. The patent #3300162 specfied 3 rpm (citing the spin nausea limit). Take your pick.

In the pressurized cabin, each level had an internal floor to ceiling height of 84 inches, an external deck to deck spacing of 100 inches, and the floor had a diameter of 183 inches.

The patent notes that the advantage of the folding arms is that when the station is boosted into orbit the direction of acceleration is the same as when the arms are spinning. This means that the cargo does not shift. I'm sure G. Harry Stine noted that thrust can occur in a deep space exploration ship as well as a station being boosted into orbit.

Project Orion

RocketCat sez

Project Orion sets the mark by which all other propulsion systems are measured, in the category of OMG! That's bat-poop insane! You have GOT to be joking!.

It is like a naughty little boy lighting a firecracker under a tin can. Except the tin can is a spaceship and the firecracker is a nuclear warhead.

But before you hurt yourself sputtering with indignation, be told that not only would the accursed thing work, but it is the closest thing to a torchship that we could actually build.

You know my slogan Every Gram Counts? Not with this monster. Unlike almost every other rocket, Orion does not scale down worth a darn. It actually works better if you make it bigger! The Saturn V had enough delta V to send about 118 metric tons to Luna and back. A 1959 Orion design could send 1,300 freaking tons to Saturn and back! Instead of miserly trying to shave off grams from your rocket by things like omitting the Lunar Module's astronaut chairs, the Orion was talking about bringing along 100 kg Barber Chairs just in case any of the astronauts needed a shave.

The fly in the ointment is that Orion works best to boost outrageous payloads from the surface into orbit. Which is exactly what makes the anti-nuclear crowd go hysterical. Once in orbit, there are other propulsion systems with better specific impulse / exhaust velocity. But when it comes to huge thrust and huge specific impulse, Orion is near the top. A pity the limited nuclear test ban treaty killed the Orion dead.

No, Orion does not make tons of deadly radioactive fallout. If you launch from an armor plated pad covered in graphite there will be zero fallout. And No, it would not create the apocalyptic horror of EMP making the world's cell phones explode and wiping out the Internet. That's only a problem with one megaton nukes, the Orion's charges are only a few kilotons. Just launch from near the North Pole (at least 276 kilometers from anything electronic) and you'll be fine.

But if the truth must be known, anybody who is freaking out about the concept of Orion is only doing so because they never heard about Zubrin's nuclear salt water rocket, which is a million times worse. Orion just goes bang...bang...bang. Zubrin's NSWR is a blasted continuously detonating nuclear explosion!

For the inside details about the Orion propulsion system go here.

Please note that Orion drive is pretty close to being a torchship, and is not subject to the Every gram counts rule. It is probably the only torchship we have the technology to actually build today.

If you want the real inside details of the original Orion design, run, do not walk, and get a copies the following issues of of Aerospace Projects Review: Volume 1, Number 4, Volume 1, Number 5, and Volume 2, Number 2. They have blueprints, tables, and lots of never before seen details.

If you want your data raw, piled high and dry, here is a copy of report GA-5009 vol III "Nuclear Pulse Space Vehicle Study - Conceptual Vehicle Design" by General Atomics (1964). Lots of charts, lots of graphs, some very useful diagrams, almost worth skimming through it just to admire the diagrams.

The following table is from a 1959 report on Orion, and is probably a bit optimistic. But it makes for interesting reading. Note that 4,000 tons is pretty huge. The 10-meter Orion (the one in all the "Orion" illustrations) is only about 500 tons.

In other words, if you can believe their figures, the advanced Orion could carry a payload of 1,300 tons (NOT kilograms) to Enceladus and back!

Gross Mass4,000 tons10,000 tons
Propulsion System Mass1,700 tons3,250 tons
Specific Impulse4000 sec12,000 sec
Exhaust Velocity39,000 m/s120,000 m/s
Diameter41 m56 m
Height61 m85 m
Average accelerationup to 2gup to 4g
Thrust8×107 N4×108 N
Propellant Mass Flow2000 kg/s3000 kg/s
Atm. charge size0.15 kt0.35 kt
Vacuum charge size5 kt15 kt
Num charges for 38,000 m200200
Total yield for 38,000 m100 kt250 kt
Num charges for 480 km orbit800800
Total yield for 480 km orbit3 mt9 mt
Δv 10 km/s
Mass Ratio (Payload)
1.2 (1,600 tons)1.1 (6,100 tons)
Δv 15.5 km/s
Mass Ratio (Payload)
1.4 (1,200 tons)1.1 (5,700 tons)
Δv 21 km/s
Mass Ratio (Payload)
1.6 (800 tons)1.2 (5,300 tons)
Δv 30 km/s
Mass Ratio (Payload)
2.1 (200 tons)1.3 (4,500 tons)
Δv 100 km/s
Mass Ratio (Payload)
cannot2.2 (1,300 tons)
10 km/sTerra surface to 480 km Terra orbit
15.5 km/sTerra surface to soft Lunar landing
21 km/sTerra surface to soft Lunar landing to 480 km Terra orbit or
Terra surface to Mars orbit to 480 km Terra orbit
30 km/sTerra surface to Venus orbit to Mars orbit to 480 km Terra orbit
100 km/sTerra surface to inner moon of Saturn to 480 km Terra orbit

USAF 10 Meter Orion

Most of the information and images in this section are from Aerospace Project Review vol 1 no 5. I am only giving you a "Cliff Notes" executive summary of the information, and only a few of the images and those in low resolution. If you want the real deal, get a copy of APR v1n5.

Orion drive spacecraft scale up quite easily. However, unlike other propulsion systems, they do not scale down gracefully. Surprisingly it is much more of an engineering challenge to make a small Orion. It is difficult to make a nuclear explosive below a certain yield in kilotons, and small nuclear explosives waste most of their uranium or plutonium. But it is relatively easy to make them as huge as you want, just pile on the megatons.So in the 1960's when General Atomic made their first pass at a design, it was for a titanic 4,000 metric ton monster.

Alas for General Atomic, neither the United States Air Force (USAF) nor NASA wanted it. USAF had no need for a ship sized for being a space going battleship (they thought they did but President Kennedy smacked them down). NASA wanted nothing to do with a spacecraft that would make the Saturn V and its infrastructure obsolete and pitifully inadequate overnight. So General Atomic heaved a big sigh, and started designing a tiny Orion drive craft with only a 10 meter diameter pusher plate.

However, recently declassified documents reveal that the USAF's decision to cancel plans for the 4000 ton Orion was a near thing. If some of the high-ranking USAF officers had slightly different personalties, today there would be a US Space Force with Orion spacecraft sending expeditions to Enceladus.

Since General Atomic was trying to sell the design to a couple of organizations with vastly different missions in mind, GA made the design modular. There was a basic propulsion system that one could attach any number of different payloads, and customizing the amount of propellant was as easy as stacking poker chips.

In this section we will be focusing on the USAF design. After the USAF lost interest, General Atomic started working with NASA to customize the Orion to their needs. This made the NASA design quite different from the USAF design. NASA was losing intererest even before the partial test-ban treaty of 1963 killed the Orion dead.

ΔV31,800 m/s
32,900 m/s
107,900 kg
(Full Magazine
294,260 kg
Payload Stack
73,075 kg
Wet Mass475,235 kg
Dry Mass180,975
Mass Ratio2.63

The USAF 10M Orion had three main components: the Orion Drive propulsion module, the stacks of magazines containing the nuclear pulse units (the Propellant), and the Payload Stack.

The Propulsion Module containes the cannon firing the nuclear pulse charges. It also has the massive array of shock absorbers allowing the spacecraft to absorb the nuclear explosion without being crushed like a bug. It also contains 138 "starter" nuclear pulse units. These are half strength units used to initiate a period of acceleration.

The Magazine Stack holds the (full-strength) nuclear pulse units. Each magazine holds 60 units. There are six magazines in a layer, holding 360 units. This design can hold up to ten layers depending upon how much delta V it needs, but if it has an odd number of magazines they must be balanced around the thrust axis. A full load of ten layers contains 3,600 pulse units.

The Payload stack has three components: Powered Flight Station, Personnel Accommodations, and the Basic 12-Meter Spine.

The Spine rests on the propulsion module and has the magazine stack frame attached. The spine contains the spare parts, the repair shack, and mission specific payload. Some of the mission payload is attached outside the spine, such as the Mars Lander.

On top of the spine is the Personnel Accomodations. This holds the life support, the crew quarters, laboratories, and workshops.

On top of the Accomodations is the Powered Flight Station. This contains the anti-radiation storm cellar, which contains the flight controls used when the ship is accelerating (since exploding nuclear bombs make radiation). In an emergency, the entire section can turn into a large escape life-boat rocket and fly away from the rest of the spacecraft. The life-boat has about 600 m/s of delta V and enough life support to keep the 8 person crew alive for 90 days.

Note that in the table the mission specific payload is not included. The more of that which is added, the lower becomes the delta V.

Nuclear Pulse Unit

Nuclear Pulse Unit
Container and
electronic mass
6.9 kg
Nuclear device
72.1 kg
Total mass79 kg
Diameter0.36 m
Height0.6 m
Yield1 kt
per pulse
34.3 kg
per pulse
2.0×106 N
Specific Impulse3,350 seconds
Exhaust velocity32,900 m/s
0.8 to 1.5 sec
0.86 sec is std

The USAF nuclear pulse units are atom bombs. They were about 0.6 meters tall, had a mass of 79 kilograms, produced a 1 kiloton nuclear explosion, and produced 2.0×106 Newtons of force per pulse unit. They were basically nuclear shaped charges. 80% of the blast was focused on the pusher plate instead of being wastefully sprayed everywhere.

The latter NASA pulse units had more mass, more Newtons of force, but a lower specific impulse.

The propulsion system also carries 138 "starter" nuclear pulse units. These are half-strength (1.0×106), used to start a period of acceleration. A starter pulse is used on a stationary pusher plate, a full strength pulse is used on a pusher plate in motion. The first shot will be a starter pulse, and the remaining pulses will be full-strength for the rest of the acceleration period.

You see, a half-strength push is enough to push the plate from the neutral position up to the fully compressed position. A full-strength push is enough to stop a plate moving downward and start it moving upward. Using a full-strength push on a stationary plate will give it twice as much as it need, driving the pusher hard into the body of the spacecraft and gutting it like a trout.

Naturally if one of the full-strength units misfires, the pilot will wait for the pusher plate to settle down then start anew with a fresh half-strength unit.

The layer of tungsten propellant should be as thin as possible. However, there are limits to how wide it can be (or a pulse unit will have an inconveniently large diametr) and it should be thick enough to stop most of the neutron and gamma radiation (to reduce the radiation exposure on the ship in general and the neutron activation on the propulsion module in particular). The mass ratio of the tungsten propellant to the beryllium oxide channel filler should be about 4:1.

Each unit had two copper bands, that are bitten into by the rifling of the cannon that shoots these little darlings. The rifling spins the pulse units like rifle bullets, for gyro-stabilization.

Magazine Stack

Magazine Stack
Empty magazine181 kg
Single pulse unit79 kg
Pulse units
in magazine
Total pulse
4740 kg
Loaded magazine4,921 kg
1 stack layer
(6 magazines)
29,526 kg
Pulse units
in 1 stack layer
Full stack
(10 stack layers)
294,260 kg
Pulse units
in full stack

Pulse units were packaged in disk shaped magazines, 60 nukes per magazines. The magazines were stacked like poker chips on top of the propulsion module, held in a hexagonal truss. There are six stacks, with a maximum height of 10 magazines.

The bottom six magazines attach directly to the propulsion module's feed system. The open end of a magazine fits onto one of the propulsion module's six "pulse system conveyors". On the magazine, a slot in the side called a "sprocket opening" allowed one of the propulsion system's sprockets to be inserted into the magazine. As it spins, the star-shaped sprocket grabs the next pulse unit and feeds it into the pulse system conveyor. From there the pulse system travels deep inside the propulsion module to the launch position.

Pulse units are drawn simultaneously from the bottom six magazines. "Bottom" because that is the layer which attaches to the propulsion system's pulse system conveyor. "Simultaneously" because you do not want the spacecraft's center of gravity straying from the thrust axis. Those pulse units are heavy, and they do not automatically redistribute the mass like fluid propellant in a tank.

When the bottom six magazines are empty (360 pulse units expended), propulsion is momentarily halted, and the six "ejection actuators" (pistons) push on the ejection pad of their respective empty magazine and catapult the empty into space. Sort of like flicking a bottle-cap off the bar room table with your finger. The "stack drive pinions" then engage the racks on the magazine stack and lower the entire stack down until it engages the propulsion system's pulse system conveyor. Propulsion is restarted.

Propulsion Module

Propulsion Module
Pusher plate mass47,800 kg
Shock absorber
and launcher mass
42,900 kg
Engine mass
90,700 kg
Structural mass17,200 kg
Total module mass107,900 kg
Module diameter10 m
Module height
Mode IB
26 m
per pulse
34.3 kg
per pulse
2.0×106 N
Specific Impulse3,350 seconds
Exhaust velocity32,900 m/s
0.8 to 1.5 sec
0.86 sec is std

The propulsion module is build around a compressed gas cannon that fires nuclear pulse units downward through a hole in the pusher plate. Once the pulse unit reaches a point 25 meters below the pusher plate, the it detonates. The shaped charge channels the explosion into a 22.5° cone perfectly covering the pusher plate.

Since premature detonation of a pulse unit would probably utterly destroy the entire spacecraft, there are incredibly stringent controls on them. The units are locked into safe mode and as such are as impossible to detonate as the designers can possibly make them. Otherwise no astronaut is going to set foot inside a spacecraft carrying enough nuclear warheads to totally vaporize the entire thing. 3.6 megatons is nothing to sniff at.

If everything is nominal, the arming signal is transmitted to a launched unit when it approaches the 25 meter detonation point. If the engine control computer determines that the synchronization between the pusher, the shock-absorber system, and the pulse unit are within tolerances; the detonation signal is sent when the unit arrives at the detonation point.

If anything is wrong, the computer instead transmits the "safety" signal and the unit enter safe mode again. When the pulse unit is a safe distance away, the computer sends a destruct signal. You don't want unattended nuclear explosives just flying through space.

If the computer sends the standard detonation signal but the pulse unit fails to do so, it is automatically disarmed (we hope). Again the destruct signal is sent once the unit is safely away, hopefully the unit will oblige. But since the unit has already failed to detonation on command once already, something is obviously wrong with it. Whether it will actually disarm then destruct is anybody's guess.

It goes without saying that the various pulse unit radio signals will be heavily encryped to prevent sabotage. Especially if the spacecraft in question is a military vessel. Otherwise an enemy ship could send the detionation code to every single pulse unit on board, and cackle as your ship did its impression of a supernova.

Because the pusher plate has a hole in the center, part of the blast will sneak through and torch the business end of the cannon. The cannon has a plasma deflector cone (with 1.27 centimeters of armor) on the end to protect it. When a pulse unit emerges from the cannon, the deflector cone opens for a split second to let it out. The deflector cone has its own tiny shock absorbers, of course. The rest of the conical base of the propulsion module is also armored, since the deflector cone will be deflecting plasma all over the base.

When the blast hits the pusher plate it gives thrust to the spacecraft, like a nuclear powered boot kicking you in the butt at 32 kilometers per second. To prevent this thrust from flattening the ship like a used beer can, two stages of shock absorbers do their best to smooth out the slam.

The first stage is a stack of inflated flexible tubes on top of the pusher plate. It takes the 50,000 g of acceleration and reduces the peak acceleration to a level that can be handled by a rigid structure. Such as the second stage shock absorbers.

The second stage is a forest of linear shock absorbers. They reduce the peak acceleration further to only a few gs.

The structural frame is welded out of T-1 steel I-beams to take the jolt and transfer the thrust from the shock absorbers to the payload. A set of six torus tanks pressurizes the linear shock absorbers and lubricates their interiors with large amounts of grease. You need that grease, those linear shock absorbers are working real hard. If one seizes up the results will be ... unfortunate.

In between blasts the inflatable first stage shock absorbers oscillates through 4.5 cycles (no doubt making a silent sad cartoon accordion noise), while the second stage absorbers go through one half cycle. The first stage oscillates between being 0.6 normal height to 1.4 height.

As each pulse unit is fired, a fine spray of ablative silicone oil coats the pusher plate to help it survive the blast. With the oil coating, each nuclear charge raises the temperature of the plate by only 0.07° Celsius. Typically acceleration periods use only 1,000 pulse units at a time, which would raise the pusher plate temperature by only 70° C.

The effective thrust is the thrust-per-pulse divided by the detonation interval. So 2.0×106 / 0.8 = 2.5×106 N effective thrust. This means a series of nuclear bombs going off every 4/5ths of a second. Boom Boom Boom Boom Boom!

The compressed gas cannon uses ammonia (NH3), stored in the "gas collector and mixing tank". This is the top-most of the torus (donut) shaped tanks around the core. The tank holds about 8 metric tons of ammonia, and 2 kilograms are used for each shot. Which means the tank is good for about 4,000 shots. A full set of magazine stacks + the start and restart pulse system has 3,600 + 138 = 3,738 total pulse units so this should be ample. If more ammonia is needed, there is room to spare inside the propulsion system. Alternatively extra ammonia tanks could be stored inside the Basic Spine.

The cannon barrel is 12 meters long, aimed straight down. The cannon accelerates the pulse unit at 45 g giving them a velocity of 90 meters per second. The barrel is rifled to spin the pulse unit at 5 rps. When the nuclear charge reaches the 8 meter point inside the barrel, exhaust manifolds frantically try to suck out all the ammonia gas and spit it out the ejector gas exhaust tubes. The idea was to have no ammonia between the pusher plate and the detonating nuclear charge. The shaped charge blast could accelerate the ammonia and damage the pusher plate.

The main source of nuclear pulse units is from the magazines stacked on top of the propulsion module. However, the module carries 138 pulse units internally in its "start and restart pulse system." They are stored at the top of the module in six curving channels holding 23 pulse units apiece. These are special half-strength pulse units (0.5 kt, 1×106N). They are used as the first shot for engine start or in the event of a regular pulse unit misfire. A half-strength unit is used on a stationary pusher plate, a full strength unit is used on a pusher plate in motion.

Payload Stack

Mission Payload Stack
Structural Mass
Powered flight
station (8 crew)
1,730 kg
(8 crew)
7,600 kg
Basic 12-m
2,270 kg
11,600 kg
Payload Mass
(including structural)
Powered flight
station (8 crew)
(8 crew)
36,950 kg
Basic 12-m
6,170 kg
Total Operational
Payload Mass
73,075 kg
Mission Specific
Payload Mass
Terra ⇒ Mars750 kg to
150,000 kg

Since General Atomic was trying to market the 10 meter Orion to both the USAF and NASA, they made it modular instead of integrated. That way they could have a common propulsion system for both, with customized payload stacks for each. The example payload stack shown here is for a Mars mission. A chemical rocket using a Hohmann trajectory would take at least nine months to travel to Mars. But the Orion drive rocket could go to Mars and back in four months flat! However the mission that General Atomic finally settled on was a more pedestrian fifteen month mission requiring only 22.2 km/s of delta V. This would only need a mass ratio of 1.93. If my slide rule is not lying to me, this means it needs about 2149 pulse units (36 magazines or 6 layer magazine stack).

All the payload stacks started with a Basic 12-Meter Spine at the bottom, resting on the top of the propulsion module with the magazine supports tied to it. In the Mars mission, this contained the space parts and the repair shack.

On top of the Basic Spine was the Personnel Accommodations. This contains the life support system, crew living quarters, and laboratories.

At the very top is the Powered Flight Station. This contains the anti-radiation storm cellar. The crew shelters inside in case of space radiation storms. The crew also shelters inside while the Orion drive is operating, since a series of nuclear detonations is also very radioactive. This is why the flight deck is located inside. Finally the entire level can detach and turn into an emergency life boat if something catastrophic happens to the main ship.

Usually you have a 10-meter propulsion module topped with a Basic 12-m Spine, topped with an 8-crew Personnel Accomodation, topped with an 8-crew Powered Flight Station.

However it is possible to replace the last two items with a 20-crew Personnel Accomodation topped with a 20-crew Powered Flight Station. The 20-crew Accomodation needs its base modified to attach to the Basic Spine, it was originally designed to attach to the larger diameter spine of a 20-meter propulsion module.

Since the spacecraft is long and skinny, it uses the "tumbling pigeon" method of artificial gravity. This is where the spacecraft rotates end over end, at four revolutions per minute. For a 50 meter long spacecraft this would give about 0.45 g at the tip of the nose, gradually diminishing to zero at the point where the basic spine joins the propulsion module. The amount of gravity will change as pulse units are expended, thus shifting the center of gravity, rotation point, and rotation radius.

This does pose a problem in the internal arrangement. While under acceleration the direction of "down" is towards the pusher plate. But while tumbling, the direction of "down" is where the nose of the ship is pointing. So if you are standing on the "floor" during acceleration, when it switches over to tumbling you will find yourself falling "upwards" and end up standing on the ceiling.

As it turns out, if the ship is accelerating it also means that everybody is huddling inside the storm cellar (or dying of radiation poisoning). Therefore the storm cellar is built with "pusher-plate is down" orientation, and the rest of the ship is build with "nose is down" orientation.

This also means that the entire mission payload stack has to have a structure that can handle tension as well as compression.

Powered Flight Station
8 Crew
Powered Flight
Payload Mass
Structure1,730 kg
18,170 kg
Escape Rockets600 kg
Escape Propellant4,500 kg
RCS680 kg
Vehicle total25,680 kg
Content at
emergency separation
Life Support
880 kg
90 day life
680 kg
90 day
food supply
1,080 kg
navigation system
360 kg
230 kg
power supply
910 kg
Communication135 kg
Content total4,275 kg
Content total
+ Crew (8)
725 kg
5,000 kg
content + operational
29,955 kg
Diameter3 m
Inside diameter2.5 m
Height9 m
control center
2 m
Bunk room height1.6 m

This section contains the flight controls and the reaction control system. The unshielded point at the top is the navigation station. The unshielded room below is full of the emergency supplies.

Since this is the section farthest from the detonating nuclear pulse units, it makes sense to locate the anti-radiation storm cellar here. In the diagram it is the rooms inside the thick radiation shielding. You get extra protection via the inverse square law at no cost in shield mass. The radiation created by operating the Orion drive is also the reason why all the flight controls are located inside the storm cellar. Otherwise the pilots will be forced to be at flight controls during flight which are located inside the deadly radioactive flux from the drive, making it impossible to recruit Orion drive pilots. The storm cellar will also be used in case of solar proton storms.

It is unwise to put holes in the part of the radiation shield protecting the crew from the pulses, radiation will spray through. This is why access to the Flight Station is from the sides not the bottom, via two pressurized passageways attached laterally. The right hand passageway is attached to an airlock in the emergency supply room, and just has a pressure-tight hatch down at the Personnel Accommodation module, at the other end. The left hand passageway is attached to a pressure-tight door on the Propulsion Control center, and has a full airlock down at the Personnel Accommodation module.

The main radiation shield on the "floor" is composed of 55 grams per square centimeter of lead. Below that is 120 g/cm2 of hydrogenous material, probably water. The side walls and ceiling have 25 g/cm2 of water to protect against backscatter. There actually is some extra protection inside each pulse unit in the form of the channel filler and propellant. The estimate was that the shield would keep the crew exposure down to 0.5 Sievert from the Orion drive, and 0.5 Sieverts from solar flares, for a total of 1.0 Sievert per Mars mission. More recent analysis shows that only 0.5 Sieverts from solar flares is a bit optimistic.

The storm cellar will also have lots of fiberglas sound-proofing. The tungsten propellant striking the pusher plate will make a tremendously huge noise, transmitted by conduction to the entire spacecraft. From freqencies of 7,000 cps to 50 cps the noise will be about 100 to 140 decibels. Without sound-proofing it will damage the hearing of the crew members.

The Flight Station can also detach to become an emergency lifeboat if catastrophe strikes the main ship. It has about 600 m/s of delta V and about 90 days worth of life support for the 8 crew members. Part of the floor radiation shield is from the emergency rocket fuel tanks. A bank of solid rocket booster ejects the Flight Station, and liquid rockets are used for maneuvers. The RCS is already a part of this module.

There was some analysis about angling the nuclear pulse units slightly off-center instead of using a RCS, but thankfully cooler heads prevailed.

Personnel Accommodation
8 Crew
Personnel Accommodation
Operational Payload Mass
Structural Mass7,600 kg
Furnishings2,400 kg
Main power supply3,470 kg
Guidance &
3 kg
1 kg
Spin gravity system
tankage & nozzles
386 kg
Spin Propellant4,540 kg
Life Support2,977 kg
Life Support
3,515 kg
Reserve Life Support1,170 kg
Food Supply5,398 kg
x4 Space Taxi625 kg
x4 Space Taxi Propellant825 kg
Crew (8)725 kg
Contingency (~5%)3,315 kg
Total Operational
Payload Mass
36,950 kg
General Dynamics 2-Man Space Taxi
Specific Impulse450 s
Exhaust Velocity4,500 m/s
Wet Mass361 kg
Dry Mass155 kg
Propellant Mass206 kg
Mass Ratio2.3
ΔV3,750 m/s
Height3.5 m

This section contains crew living quarters, the main power supply, repeaters for the navigation instruments and communication gear in the Powered flight station, the tumbling pigeon jets, the life support system, and the food.

If there were several Orion vessels in the mission they would have space taxis, since trying to maneuver and dock with an Orion Drive is like trying to thread a needle with a bulldozer.

Since this is an Orion drive and not every gram counts, this section is built solid. The decks are pressure-tight bulkheads, not non-pressure tight walls. Compartments are accessed via airlocks, so a space suited crew member can enter an area that was vented by a meteor strike without killing everybody. The two passageways at the top lead to the Powered Flight Station above. The left hand passageway has an airlock in this module, the right hand passageway just has a pressure-tight hatch.

The module is 7.2 meters in diameter and had two compartments. Each had a center cylindrical section 3.2 meters in diameter (a continuation of the Basic 12 M Spine). Both compartments could be divided into eight wedges via non-structural partitions. The center sections are for labs and workshops. The wedge rooms listed in the table. Each stateroom is double occupancy, to accommodate the 8 crew persons.

Wedge Rooms
Deck 1Deck 2
Stateroom AlfaStateroom Charlie
Stateroom BravoStateroom Delta
GalleyCommand &
Rec RoomEmergency
Gear Storage

The Powered Flight Station plus the Personnel Accommodation has a total pressurized volume of 200 cubic meters, or 25 cubic meter per crew member (not counting the two passageways flanking the Flight Station). This is actually pretty luxurious. NASA figures a bare minium is 17 m3 per person, and a wet Navy enlisted man is lucky to have 8.3 m3. In addition the Basic Spine is available, but it is only pressurized when needed.

A 20-person Powered Flight Station and a 20-person Personnel Accommodation from a 20-meter Orion can be mounted on a 10-meter spine on a 10-meter Orion. In that case the sum of the Flight Station and Accommodation pressurized volume is 490 m3 or 24.5 m3 for each of the 20 crew.

Basic 12-Meter Spine
Basic 12-Meter Spine
Operational Payload Mass
Structural Mass7,600 kg
Spares1,130 kg
Repair Equipment2,270 kg
500 kg
Total Operational
Payload Mass
6,170 kg
Diameter3.2 m
Height12 m

The spine has an internal volume of about 97 cubic meters. It contains spare parts, a repair bay, and miscellaneous payload.

There is also an airlock on the bottom allowing repair crews to enter the propulsion module. It is constructed out of materials with a low neutron activation potential. In addition, each pulse unit is only about 1kt (not a lot of neutrons), they are detonated 25 meters away from the propulsion unit (inverse square law), and the pulse unit channel filler plus tungsten propellant will provide shielding. It will be radiologically safe for crews to enter the propulsion module a couple of hours after the the most recent nuclear detonation.

USAF 4000 Ton Orion

USAF 4000 Ton Orion
Pusher Plate
26 m
Height78 m
Wet Mass3,629,000 kg
(4,000 short tons)
Payload Shell
11,000 m3
Pulse unit mass
bare unit
1,150 kg
Pulse unit mass
w/support rollers, etc.
1,190 kg
Pulse unit dim.80 cm dia × 87 cm high
Pulse unit
detonation dist.
52.4 m ± 2 m
Pulse unit
specific impulse
4,300 sec
effec: 3,600 sec
Detonation delay1.1 sec
Pulse Unit Storage
storage levels
Number conveyors
per level
Number pulse units
per conveyor
Total pulse
unit capacity
15 km/s ΔV30 km/s ΔV
Average initial accel1.25 g1.25 g
Total engine
weight (dry)
1,233,000 kg1,252,000 kg
pulse units
Pulse system
(incl. coolant)
1,280,000 kg2,068,000 kg
Payload mass1,115,000 kg308,000 kg

Most of the information and images in this section are from Aerospace Project Review vol 2 no 2. I am only giving you a "Cliff Notes" executive summary of the information, and only a few of the images and those in low resolution. If you want the real deal, get a copy of APR v2n2.

Orion drive spacecraft scale up quite easily. However, unlike other propulsion systems, they do not scale down gracefully. Surprisingly it is much more of an engineering challenge to make a small Orion. It is difficult to make a nuclear explosive below a certain yield in kilotons, and small nuclear explosives waste most of their uranium or plutonium. But it is relatively easy to make them as huge as you want, just pile on the megatons.

So in the 1960's when General Atomic made their first pass at a design, it was for a titanic 4,000 ton monster. By this time they realized that they would never get permission to launch an Orion from the ground under nuclear-bomb power, so the baseline was Mode III: a gargantuan chemical booster boosts the fully loaded Orion into LEO. So it does not carry the pulse units required to achieve orbit. For that the engine section would have to be taller.

This became the basis for the USAF Orion Battleship. They took the 11,000 cubic meters of the payload shell and stuffed it full of weapons.

Orion Battleship

RocketCat sez

This thing rulz. Period.

It can stomp both the Michael and the Thuktun Fishithy into the dirt and still have enough firepower left over to blow the Soviet Union into the Stone Age.

Orion Battleship
Pusher Plate
26 m
Height78 m
Wet Mass3,629 tonnes
(4,000 short tons)
Payload Shell
11,000 m3
Pulse unit
specific impulse
4,300 sec
effec: 3,600 sec
Detonation delay1.1 sec
(20 crew)
Space Taxi
15 km/s
30 km/s
Average initial
1.25 g1.25 g
Total engine
weight (dry)
pulse units
Pulse system
(incl. coolant)
Payload mass1,115
Mk-42 5-inch
naval turret
Missiles w/20
Missiles Silos3 banks of 30 each
90 total
Casaba Howitzer
Several hundred
Casaba Howitzer
20mm CIWS

When the Orion nuclear pulse propulsion concept was being developed, the researchers at General Atomic were interested in an interplanetary research vessel. But the US Air Force was not. They thought the 4,000 ton version of the Orion would be rightsized for an interplanetary warship, armed to the teeth.

And when they said armed, they meant ARMED. It had enough nuclear bombs to devastate an entire continent (500 twenty-megaton city-killer warheads), 5-inch Naval cannon turrets, six hypersonic landing boats, and several hundred of the dreaded Casaba Howitzer weapons — which are basically ray guns that shoot nuclear flame (the technical term is "nuclear shaped charge").

This basically a 4,000 ton Orion with the entire payload shell jam-packed with as many weapons as they could possibly stuff inside.

Keep in mind that this is a realistic design. It could actually be built.

The developers made a scale model of this version, which in hindsight was a big mistake. It had so many weapons on it that it horrified President Kennedy, and helped lead to the cancellation of the entire Orion project. The model (which was the size of a Chevrolet Corvette) was apparently destroyed, and no drawings, specifications or photos have come to light.

Scott Lowther has painstakingly done the research to recreate this monster. If you want all the details, run, do not walk, and purchase a copy of Aerospace Projects Review vol2, number 2. He also made a model kit of the battleship for Fantastic Plastic, you can order one here.

Rhys Taylor's 3D Orions

Rhys Taylor is a scientist who is also a master of the 3D modeling package Blender. His animation of a launching Orion drive spacecraft is quite famous, and has been seen by most people who type "Orion" into Google. His more recent project is a battle between US and Russian Orion drive ships out around Jupiter, and a rendition of the proposed Orion Discovery from preproduction of 2001 A Space Odyssey.

William Black's 3D Orions

Here are some more CGI 3D rendering of Orion concepts created by Master Artist William Black. Click for larger images.

Orion-powered Discovery from 2001

Like everything else in 2001, the good ship Discovery passed through many transformations before it reached its final shape. Obviously, it could not be a conventional chemically propelled vehicle, and there was little doubt that it would have to be nuclear-powered for the mission we envisaged. But how should the power be applied? There were several alternatives — electric thrusters using charged particles (the ion drive); jets of extremely hot gas (plasma) controlled by magnetic fields, or streams of hydrogen expanding through nozzles after they had been heated in a nuclear reactor. All these ideas have been tested on the ground, or in actual spaceflight; all are known to work.

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.

There was also a digression — to the great alarm, as already mentioned, of the Art Department — into a totally different form of propulsion. During the late 1950’s, American scientists had been studying an extraordinary concept (“Project Orion”) which was theoretically capable of lifting payloads of thousands of tons directly into space at high efficiency. It is still the only known method of doing this, but for rather obvious reasons it has not made much progress.

Project Orion is a nuclear-pulse system — a kind of atomic analog of the wartime V-2 or buzz-bomb. Small (kiloton) fission bombs would be exploded, at the rate of one every few seconds, fairly close to a massive pusher plate which would absorb the impulse from the explosion; even in the vacuum of space, the debris from such a mini-bomb can produce quite a kick.

The plate would be attached to the spacecraft by a shock-absorbing system that would smooth out the pulses, so that the intrepid passengers would have a steady, one gravity ride — unless the engine started to knock.

Although Project Orion sounds slightly unbelievable, extensive theoretical studies, and some tests using conventional explosives, showed that it would certainly work — and it would be many times cheaper than any other method of space propulsion. It might even be cheaper, per passenger seat, than conventional air transport — if one was thinking in terms of million-ton vehicles. But the whole project was grounded by the Nuclear Test Ban Treaty, and in any case it will be quite a long time before NASA, or anybody else, is thinking on such a grandiose scale. Still, it is nice to know that the possibility exists, in case the need ever arises for a lunar equivalent of the Berlin Airlift...

When we started work on 2001, some of the Orion documents had just been declassified, and were passed on to us by scientists indignant about the demise of the project. It seemed an exciting idea to show a nuclear-pulse system in action, and a number of design studies were made of it; but after a week or so Stanley decided that putt-putting away from Earth at the rate of twenty atom bombs per minute was just a little too comic. Moreover — recalling the finale of Dr. Strangelove — it might seem to a good many people that he had started to live up to his own title and had really learned to Love the Bomb. So he dropped Orion, and the only trace of it that survives in both movie and novel is the name.

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

Project Orion Battleship

This section has been moved here.

Reusable Nuclear Shuttle

Reusable Nuclear Shuttle
ΔV13,000 m/s?
Specific Power45.9 kW/kg?
(45,870 W/kg?)
Thrust Power1.4 gigawatts?
Specific Impulse816 s
Exhaust Velocity8,000 m/s
Wet Mass170,000 kg?
Dry Mass30,000 kg?
Mass Ratio5.3?
Mass Flow41.7 kg/s
Thrust344,000 n?
Initial Acceleration0.16 g
Payload 8-burn45,000 kg?
Payload 4-burn58,000 kg?
Length49 m
Diameter33 m

This is a 1970's era NASA concept for a nuclear shuttle. Note that in many of the images the shuttle has a Space Tug crew module perched on top. Design is very similar to the Basic Solid Core NTR. David Portree wrote a nice history of the nuclear shuttle: The Last Days of the Nuclear Shuttle.

Phase I design was for an expendable vehicle with a 200,000-pound-thrust NERVA II engine. It was to be used for several rocket stages on their planned Mars mission vehicle.

The Phase II design is what is pictured below. Specifically the Phase II class 1 hybrid model. Phase II design was for a reusable vehicle with a 75,000-pound-thrust NERVA I engine and a payload capacity of 50 tons. This was dubbed the Reusable Nuclear Shuttle (RNS). NASA had an optimistic RNS traffic model calling for 157 Terra-Luna flights between 1980 and 1990 by a fleet of 15 RNS vehicles. The shadow shield casts a 10 degree half-angle shadow, shielding was intended to reduce the radiation exposure to 10 REM per passenger and 3 REM per crew member per round trip to Luna and back.

The little attachable crew module has a mass of 9,000 kg. The NERVA engine is 18 meters long and 4.6 meters wide, intended to fit inside a Space Shuttle's cargo bay (the propellant tank can be lofted into orbit on a big dumb booster, but a nuke requires the human supervision). The propellant tank is 31 meters long and 10 meters wide.

The NERVA has a 1360 kilogram shadow shield on top, but it also relied upon propellant, structure, and distance to provide radiation shielding for the crew. Obviously as the propellant was expended, the shielding diminished. North American Rockwell tried to solve the problem with a "stand-pipe", in which a cylindrical “central column” running the length of the main tank stood between the crew and the NERVA I engine. The central column would remain filled with hydrogen until the surrounding main tank was emptied.

McDonnell Douglas Astronautics Company dealth with the radiation problem by developing a “hybrid” RNS shielding design that included a small hydrogen tank between the bottom of the main tank and the top of the NERVA I engine. In the first diagram below, you can see the small tank included in the propulsion module length.

D. J. Osias, an analyst with Bellcomm, pointed out that the radiation dosage received by the astronauts riding the RNS was unacceptable. Osias stated that the maximum allowable radiation dose for an astronaut from sources other than cosmic rays of between 10 and 25 REM per year (0.1 and 0.25 Sievert). But the luckless astronaut on board the RNS would get 0.1 Sieverts every time the NERVA did a burn. Any external astronauts (not in the cone of safety cast by the shadow shield) at a range of 16 kilometers from a RNS operating at full power would suffer a radiation dose from 0.25 to 0.3 Sieverts per hour. Osias suggested that external astronauts not approach a burning RNS closer than 160 kilometers.

Nowadays the yearly limit of radiation exposure for astronauts is set at 3 Sieverts, with a career limit of 4 Sieverts. Which means an astronaut piloting a RNS through 40 total burns would be permanently grounded by reaching his career limit of radiation.

The RNS is assumed to have an operational life of 10 Terra-Luna round trips. After that the RNS is attached to a chemical booster and thrown into the Sun or somewhere remote.

You will find more information than you ever wanted to know about the RNS in these PDF files here, and here.

There are two mission types: the 8-burn mission and the 4-burn mission.

8-burn mission disadvantage: requires 4 extra burns for change-of-plane maneuvers. This increases the required ΔV to 8,495 m/s, and reduces the payload size to 45,000 kg. Advantage: you do not have to wait for a launch window, you can launch anytime you want.

4-burn mission disadvantage: mission launch windows occur only at 54.6 day intervals. Advantage: since you are not required to perform change-of-plane maneuvers the required ΔV is reduced to 8,256 m/s and the payload size is increased to 58,000 kg.

In both of these missions, it is assumed that the full payload is carried to Luna, where the payload is dropped off EXCEPT for the 9,000 kg that is the crew module. Presumably the crew wants something to live in for the trip back to Terra.

Interpretation by master artist William Black
Interpretation by master artist Tom Peters

Lunar Ferry

Lunar Ferry
ΔV15,000 m/s
Specific Power?
Thrust Power?
Specific Impulse1000 s
Exhaust Velocity9,810 m/s
Wet MassM kg
Dry MassM/4.6 kg
Mass Ratio4.6
Mass Flow? kg/s
Thrust? n
Initial Acceleration? g
Payload< M/4.6 kg

This is a 1965 design from NUCLEAR SPACE PROPULSION by Holmes F. Crouch. It seems to be the father of the NASA Nuclear Shuttle design. According to the book, it would have a single solid-core NTR engine with a specific impulse of 1000 seconds (i.e., an exhaust velocity of 9,810 m/s) and a ΔV capability of 15,000 m/s (which implies a mass ratio of about 4.6, which is a bit over the rule-of-thumb maximum of 4.0). The book estimates that an Terra to Luna Hohmann trajectory would take about 12,000 m/s ΔV, after you add in all the change-of-plane maneuvers and added an abort reserve. This would require about 60 hours to travel from the Terra to Luna, but that can be reduced to 20 hours by spending an extra 900 m/s.

In the second diagram, the ship is shown docked to something that looks suspiciously like the Space Tug. Note that they dock nose-to-nose so the lunar shuttle vehicle can stay inside the radiation shadow area.

One really exciting nuclear rocket potiential lies in Earth-Moon transport. The Moon is 208,000 n mi from the Earth. The mission concept simply is one of ferrying back and forth between Earth and Moon terminal orbits. We can think of the ferry terminals as 300 n mi Earth orbits and 100 n mi lunar orbits.

The essence of the lunar ferry concept is presented in Figure 11-8 (the one with the Earth-Moon orbits). the lunar vehicle would do all the propulsive legwork in the the terminal orbits and between the terminal orbits. Chemical systems would be employed as shuttle vehicles at the Earth terminius and at the lunar terminus. This would permit specialization in chemical systems where they are most capable: planetary launch and entry.

The nuclear ferry would have one rocket reactor with capability for multiple reuses, in-orbit replenishment, multiple restarts, and full nozzle maneuverability. We would expect the reactor to have a proven Isp on the order of 1000 seconds. It would have proven reliability, man-rating, pilot control, and long life. We would not expect the ultimate in solid-fueled reactor technology but we should be headed in that direction.

Note in Figure 11-8 that the ferry trajectory is in the form of a "figure-8." This is because it is necessary to transfer from one gravitational force center to another. Each section of the figure-8 can be thought of as an elliptical orbit: one focus at Earth and one focus at the Moon. The two ellipses "join" each other at a transfer region which is about 85% of the distance from Earth (the crossover occurs at about 180,000 n mi from Earth or about 28,000 n mi from the Moon). When going from Earth to Moon, the transfer point is called translunar injection. When going from the Moon to Earth, the transfer is called transearth injection. The injection maneuvers actually start well in advance of the trajectory crossover.

Caution is required when interpreting Figure 11-8. It gives the impression that the launching/entry trajectories, the rendezvous/docking orbits, and translunar/transearth ellipses are all in the same orbit plane with each other. This is not the case. We are dealing with noncoplanar orbit trajectories. Furthermore, they are variable noncoplanar trajectories which change from day to day and from month to month. As a consequence, the target plane — that plane connecting the Earth and Moon centers — "corkscrews" around the major axis of the figure-8 flight path. The corkscrewing of the ferry trajectory introduces fluctuations in the ΔV requirements.

Table 11-4 Nuclear Ferry ΔV Requirements
ManeuverFeet per second
Earth Orbit Docking1,750
Earth-Space Plane Changes3,500
Earth to Translunar Injection10,000
Translunar to Lunar Orbit3,500
Lunar-Space Plane Changes1,500
Lunar Orbit Docking750
Lunar to Transearth Injection3,500
Transearth to Earth Orbit10,000
Midcourse Corrections500
Abort Reserve5,000
Total ΔV40,000

A representative summary of the round trip ΔV requirements is given in Table 11-4. This listing includes all contingencies (a lunar mission can be performed with less ΔV than table 11-4 but the risk-potential increases). Note that total ΔV is 40,000 feet per second (fps). A single stage nuclear vehicle with an Isp of 1000 sec would have a ΔV capability of nearly 50,000 fps. Hence, there is some excess ΔV available.

The unused nuclear ΔV can be applied to reducing the trip time. The normal one-way trip time for a chemical propulsion system is about 60 hours (2 ½ days). Because chemical lunar missions border on marginal ΔV capabilities, the chemical trip time cannot be reduced much below 60 hours. In the case of nuclear systems, for an additional expenditure of 3,000 fps, the one-way trip time can be reduced to 20 hours. The effect of other ΔV expenditures on trip time is shown in Figure 11-9 (not shown), It can be seen that if an attempt is made to reduce the trip time below 20 hours, the extra ΔV requirements are disproportionate to the time gained. Therefore, a value of 20 hours will be selected as the nuclear ferry time base.

If the lunar terminal orbit is 100 n mi altitude, the orbit period is about 2 hours. If the lunar terminal activities necessitate as much as two orbit periods fur completion, the nuclear ferry turnaround could be made within 24 hours of Earth departure. If two nuclear ferry vehicles were used, we could have daily service to the moon and back! All-chemical lunar rocket systems could not possibly compete with this schedule.

The advantages of reduced lunar trip time are self-evident There is reduced time of confinement of astronaut, scientific, and technical personnel to the limited quarters of spacecraft. In-transit boredom and monotony are reduced. Less life support equipment is required: less oxygen, less food, less waste disposal. There is less exposure to weightlessness and less exposure to space radiation. The less the life protection equipment required, the more transport capacity for lunar basing supplies.

In the lunar terminal orbit, all exchange activities would take place at the pilot end of the nuclear ferry. This is because the propulsion reactor would be kept idling. The major features involved are presented in Figure 11-10 (middle image above). One feature not always self-evident is the need to off-load chemical propellants from the nuclear ferry to the lunar shuttle. To make the propellaut transfer, special cargo tanks on the nuclear ferry and special piping on the chemical shuttle would be required, It is assumed that chemical propellants for the shuttle vehicle probably could not be manufactured on the Moon and therefore would have to be transported from Earth.

From NUCLEAR SPACE PROPULSION by Holmes F. Crouch (1965)

RM-1 Lunar Reconnaissance Craft

2,800 m/s
314 s
Length23 m
Max Width7.4 m

This design was the result of a nice bit of collaboration between Walt Disney and Dr. Wernher von Braun (architect of the Saturn V).

Disney's TV show "The Wonderful World of Color" had decades of material for the segments Fantasyland, Frontierland, and Adventureland, but zero for Tomorrowland. Disney's concept executive Ward Kimball had been following Collier magazine's awe inspiring series Man Will Conquer Space Soon, detailing von Braun's plans for manned spaceflight. This series would be perfect for a set of Tomorrowland episodes.

Kimball quickly discovered that he was in over his head, but Disney allowed him to hire technical experts. Kimball proceeded to enlist the main tech experts from the Collier's series: Willey Ley, Heinz Haber, and of course Wernher von Braun. Kimball realized that when it got down to the fine details, you'd have to get help from The Man himself. When Kimball made a tentative inquiry to von Braun, the latter jumped in with both feet. von Braun desperately needed favorable publicity for his Moon mission. The Colliers article reached barely three million viewers. A Disney show could reach tens of millions!

The three Tomorrowland episodes were Man in Space, Man and the Moon, and Mars and Beyond. The middle episode is where the RM-1 makes its appearance.

The RM-1's mission was a simple loop around Luna, with no landing (the same as the Apollo 8 mission). The only things you needed was a few days of life-support for the crew, and about 2,700 m/s of delta V. And a bit under 100 m/s to brake back into Terra's orbit. So the spacecraft can be built out of bits and pieces of the existing cargo and passenger ferry rockets.

The front part of the RM-1 was the top stage of the passenger ferry minus the wings but including the passenger section, life support, and engine. Six standard propellant tanks were attached to increase the delta V to 2,800 m/s. When the extra tanks were empty, they were retained as protection from meteors (unnecessarily, meteors are not that common), but jettisoned just before the braking burn into Terra orbit to reduce the ship's mass.

On a nose spike was attached a nuclear reactor, for on-board power. A conical shadow shield protects the crew from reactor radiation. The reactor is ludicrously tiny, in reality it would be quite a bit bigger. And the spike would be a bit longer as well.

A dish antenna for radar and communication is on a set of tracks around the ship's waist. Unfortunately the propellant tanks block the view aft.

It also has a belly docking port for a bottle suit, the port is already standard on the passenger ferry.

Rocketpunk Large Fast Transport

The deep space ship above (click on the image for full sized view) was inspired by the Travel Planner spreadsheet in the previous post, and modeled in the wonderfully simple and handy DoGA 3D modeler. The shuttle alongside is a rough approximation of the NASA shuttle, and thus a thorough anacronism in this image, but provided as a scale reference.

Of course you want some specifications of the ship. Even if you don't, you get them anyway:

Length Overall300 meters
Departure Mass10,000 tons
Propellant Load H25000 tons
Drive Mass2000 tons
Keel and Tankage1000 tons
Gross Payload2000 tons
Flyway Cost$5 billion (equivalent)

The payload includes a hab with berthing space for 50-200 passengers and crew, depending on mission duration, and a pair of detachable pods for 500 tons of express cargo, plus service bays and the like.

What this ship can do depends on its drive engine performance. If the drive puts out 2 gigawatts of thrust power — my baseline for a Realistic [TM] nuke electric drive — the ship can reach Mars in three months, give or take. (The sim gave 92 days for a 0.8 AU trip in flat space.) With a later generation drive putting out 20 gigawatts it can reach Mars in a little over a month, or Saturn in eight months.

The general arrangement of this ship is driven by design consideration — a nuclear drive that needs to be a long way from the crew, with large radiators to shed its waste heat; tanks for bulky liquid hydrogen; and a spinning hab section. Most serious proposals for deep space craft in the last 50 years have had more or less this arrangement — the movie 2001 left off the radiator fins, because in those days the audience would have been puzzled that a deep space ship had 'wings.'

A large, long-mission military craft, such as a laser star, might not look much different overall — replace the cargo pods with a laser installation and side-mounted main mirror, and perhaps a couple of smaller mirrors on rotating 'turret' mounts. Discussions here have persuaded me that heavy armor is of little use against the most likely threats facing such a ship.

Within these broad constraints, however, spaceships offer a great deal of design freedom, more than most terrestrial vehicles. Ships, planes, and faster land vehicles are all governed by fluid dynamics, and even movable shipyard cranes must conform to a 1-g gravity field. A spaceship, unless built for aerobraking, will never encounter fluid flow, and the forces exerted by high specific impulse drives — even torch level drives — are relatively gentle.

This ship might have had two propellant tanks, or half a dozen, instead of four. And the entire industrial assemblage of tanks and girders might be concealed, partly or entirely, within a 'hull' of sheeting no thicker than foil, protecting tanks and equipment from shifting heat exposure due to sunlight and shadow. Much of the ISS keel girder has a covering of some sort — in close-ups it looks a lot like canvas — that in more distant views gives the impression of a solid structure.

In fact the visual image of the ISS is dominated by its solar wings and radiators. The hab structure is fairly inconspicuous by comparison, like the hull of a sailing ship under full sail. This would be true to an extreme of solar electric ships; a 1-gigawatt solar electric drive would need a few square kilometers of solar wings. Even nuclear drives, fission or fusion, require extensive radiators — probably more than I showed — with other ship systems needing their own radiators, at varied operating temperatures. Unless the ship has an onboard reactor it must also have solar collectors for use when the drive is shut down.

All of which may do more to catch the eye than heavier but smaller structures such as the hab or even propellant tankage. And then there is color: the gold foil of the main ISS solar wings, for example.

Hollywood knows nothing of this (though I'm surprised they haven't picked up on the gold foil). Hollywood is no more interested in what real spaceships look like than it is in how they maneuver. This is only natural, even though we hard SF geeks complain. Hollywood doesn't care because its audience has almost no clue of what spaceships look like, or act like, getting most of their impressions from Hollywood itself.

The one actual spacecraft to have iconic visual status, the Shuttle, essentially looks like an airplane. The ISS has not yet acquired iconic status, though it may, especially after the Shuttle is retired. And perhaps it looks so unlike terrestrial vehicles that our eye does not yet know quite what to make of it.

As a point of comparison, watch aviation scenes in old movies, especially from before World War II. You'll see airplanes whooshing past (sometimes in pretty unconvincing special effects shots), but you will rarely see what is now a standard shot — a plane filmed from another plane in formation, hanging 'motionless' on the screen, clouds and distant landscape rolling slowly past, until perhaps the plane banks and turns away.

It is a standard shot because it is so very effective. But older movies rarely used it, because audiences would have had no idea what they were seeing. Everyone knew that airplanes were fast, and had at least some idea that their speed is what kept them in the air. A plane apparently hanging in midair would make no sense.

What changed all this, I would guess, is World War II. A flood of newsreel footage included many formation shots, and audiences gradually absorbed a feeling for what midair footage really looks like. When a postwar Jimmy Stewart enlisted for Strategic Air Command (1955), Hollywood — and its audience — were ready to see the B-36 and B-47 showcased in all their glory, including airborne formation shots.

I know what you bloodthirsty people are thinking — one good space war, and everyone will grok the visual language of space travel. Shame on you. Given enough civil space development, and time, people will get the hang of it.

The beauty of spaceships is in the eye of the beholder. The familiar aesthetics of terrestrial vehicles are as irrelevant to them as to Gothic cathedrals (which in some broad philosophical sense are themselves spaceships of a sort). General principles of design will provide some guidance. Even in making the quick thrown-together model above I found that slight changes in proportion could make the difference between a jumble of parts and a unity.

But the real visual impact of spaceships is something we will only learn from experience, by the glint of a distant sun.

From The Aesthetics of Space Travel by Rick Robinson (2010)

Rocketpunk Orbital Patrol Ship

Orbital Patrol Ship
Exhaust Velocity4,400 m/s
Specific Impulse449 s
Thrust3.5×106 N
Thrust Power7.7 gigawatts
Total ΔV6,100 m/s
Mass Budget
Engine Mass7 mton
Heat Shield Mass15 mton
(15% re-entry mass)
Terra Recovery
parachute, retro,
landing gear
5 mton
(5% landing mass)
NonTerra Recov
landing legs
Luna, Mars
5 mton
(5% landing mass)
attitude jets,
electrical, etc.
20 mton
(20% dry mass)
farings, etc.
5 mton
(5% dry m)
Tankage body18 mton
(6% of
300 mton
hab module
cargo bays
25 mton
DRY MASS100 mton
300 mton
WET MASS400 mton
Mass Ratio4.0
booster rocket
? mton

This is a splendid spacecraft designed by Rick Robinson, appearing on his must-read blog Rocketpunk Manifesto. This was designed for his Orbital Patrol service, which he covered in three previous posts.

The important insight he noted was that if you can somehow get your spacecraft into orbit with a full load of fuel/propellant, it turns out that most cis-Lunar and Mars missions have delta V requirements well within the ability of weak chemical rockets. So you make a small chemical rocket and lob it into orbit with a huge booster rocket (heavy lift launch stack). This will be the standard Orbit Patrol ship.

It can also be boosted into orbit by a smaller booster rocket, then using the patrol ship's engines for the second stage. So as not to cut into the ship's mission delta V, it will need access to an orbital propellant depot to refuel. At a rough guess, you'll need 9,700 m/s delta V to boost the patrol ship into orbit (7,900 m/s orbital velocity plus gravity and aerodynamic drag losses). So the booster will need 9,700 m/s with a payload of 400 metric tons. Bonus points if the booster is reusable.

Actually, it reminds me a bit of the old Three Man Space Scout.

At a rough guess, Rick figures that if the ship is capsule shaped it will be about 12 meters high by 14 meters in diameter. If it is wedge shaped, it will be about 40 meters high by 25 meters wide by 8 meters deep.

In both cases, total interior volume of 1,200 m3 (of which 900 m3 is propellant), and a surface area of 800 m2

Present day expandable propellant tanks have a mass of about 6% of the mass of the liquid propellant. Rick is assuming that in the future the 6% figure will apply to reusable tanks as well.

If my slide rule is not lying to me, the 300 metric tons of H2-O2 fuel/propellant represents 33.3 metric tons of liquid hydrogen and 266.7 metric tons of liquid oxygen. About 470 m3 of liquid hydrogen volume (sphere with radius of 4.8 m) and 234 m3 of liquid oxygen volume (sphere with radius of 3.8 m). This is a total volume of 704 m3 which falls short of Rick's estimate of 900 m3 so I probably made a mistake somewhere.

Landing on Terra will use retro-rockets, the heat shield for aerocapture, maybe a parachute, and aircraft style landing gear for belly landing. Landing on Luna or Mars will be by tail-landing on rear mounted landing legs. That will also mean reserving some of the propellant for landing purposes.

Note that the heat shield is rated for the ship's unfueled mass (heat shield mass = 15% of ship's re-entry mass), there is not enough to brake the ship if it has propellant left. This assumes a "low-high'low" mission profile: start at LEO, go outward to perform mission while burning most of the propellant, then return to LEO or even land on Terra. So 15 metric tons for heat shield is for a ship with a mass of 100 metric tons at re-entry (ship's total dry mass).

If the ship is going to aerobrake then return to higher orbit, it will need more heat shield mass to handle the extra mass of get-home propellant. This will savagely cut into the payload mass, which is only 25 metric tons at best. For example, if the mission had the ship heading for translunar space from LEO after aerobraking, the extra propellant mass at aerobrake time will increase the heat shield mass from 15 metric tons to 31. This will reduce the payload from 25 metric tons to 8. But by the same token a ship that will not perform any aerobraking can omit the heat shield entirely, using the extra 15 metric tons for more propellant or payload.

Payload includes habitat module (if any) as well as cargo, since hab modules are optional for short missions. The gross payload is 25 metric tons, of which 20 is cargo and the other 5 mtons are payload bay structure and fittings. If you assume two tons of life support consumables per crew per two week mission; then the ship could carry a crew of five plus 12 mtons of removable payload, or a crew of 10 and 4 mtons of payload (the more that payload is consumables, the less mass needed for payload bay structure).

Patrol Missions
MissionDelta V
Low earth orbit (LEO) to geosynch and return5700 m/s powered
(plus 2500 m/s aerobraking)
LEO to lunar surface (one way)5500 m/s
(all powered)
LEO to lunar L4/L5 and return
4800 m/s powered
(plus 3200 m/s aerobraking)
LEO to low lunar orbit and return4600 m/s powered
(plus 3200 m/s aerobraking)
Geosynch to low lunar orbit and return
4200 m/s
(all powered)
Lunar orbit to lunar surface and return3200 m/s
(all powered)
LEO inclination change by 40 deg
5400 m/s
(all powered)
LEO to circle the Moon and return retrograde
3200 m/s powered
(plus 3200 m/s aerobraking)
Mars surface to Deimos (one way)6000 m/s
(all powered)
LEO to low Mars orbit (LMO) and return6100 m/s powered
(plus 5500 m/s aerobraking)

Representative samples of small space craft atop booster rockets:

Rocketpunk Patrol Ship

Hab Module100 tons
Consumables25 tons
Other Payload75 tons
Total Payload200 tons
Propulsion Bus
Engine+Radiator200 tons
Tankages+Keel100 tons
Dry Mass475 tons
Loaded Mass500 tons
Propellant Mass500 tons
Wet Mass1000 tons

The discussion thread about 'Industrial Scale of Space' veered, among other things, into a discussion of patrol missions in space. My first reaction was that (so long as you aren't dealing with an interstellar setting) there is no place in space for wartime patrol missions. But the matter might be more complicated, and for story purposes probably should be.

According to The Free Dictionary, patrol is The act of moving about an area especially by an authorized and trained person or group, for purposes of observation, inspection, or security. This fits my own sense of the word, and is in fact a bit broader, 'security' including SSBN patrols, which are not observing or inspecting anything, just waiting for a launch order if it comes.

In a reductionist way you could say that all military spacecraft are on patrol, since they are all on orbit, and if they are orbiting a planet they have a very regular 'patrol area.' But this is not what most of us have in mind. We picture a patrol making a sweep through an area, looking for anything unusual, ready to engage any enemy they encounter, or report it and shadow it if they cannot engage it.

Back in the rocketpunk era it was plausible that, say, Earth might send a patrol past Ceres to see if the Martians had established a secret base there. But (alas!) telescopes 'patrolling' from Earth orbit can easily observe the large scale logistics traffic involved in establishing a base; watch it depart Mars and track it to Ceres. If you want a closer look you can send a robotic spy probe. If you engage in 'reconnaissance in force' by attacking Ceres, that is a task force, not a patrol.

In an all out interplanetary war there may be plenty of uncertainty on both sides, but very little of it can be resolved by sending out patrols.

But of course all-out war is not the context in which the Space Patrol became familiar. I associate it with Heinlein's Patrol; apparently the 1950s TV series had an independent origin (unlike Tom Corbett, who was Heinlein's unacknowledged literary child).

The rocketpunk-era Patrol, which in turn gave us Starfleet, was placed in the distinctly midcentury future setting of a Federation. This is as zeerust as monorails. But plausible patrolling is not confined to Federation settings. It can justified in practically any situation but all out war.

Orbital patrol in Earth orbital space will surely be the first space patrol, and could be imagined in this century. It might initially be a general emergency response force, because travel times in Earth orbital space are short enough for classical rescue missions. On the interplanetary scale, with travel times of weeks or more likely months, rescue is rarely possible. But eventually power players will want some kind of police presence or flag showing in deep space.

As so often in these discussions, I picture a complex and ambiguous environment in which policing, diplomacy, and sometimes low level conflict blur together. To take again our Earth-Mars-Ceres example, there are kinds of reconnaissance that cannot be carried out by robots (short of high level AIs). If Ceres closes its airlocks to liberty parties from a visiting Earth patrol ship, that conveys some important intelligence information.

The ships that perform these missions will be fairly large (and expensive). They must carry a hab pod providing prolonged life support for a significant crew: at least a commander and staff, SWAT team of espatiers, and some support for both.

Let us say a crew of 25—which is cutting the human presence very fine. Now we can venture a mass estimate. Allow 100 tons for the hab compartment plus 25 tons for crew and stores plus 75 tons other payload, for a total payload of 200 tons. Let the drive bus be 200 tons for the drive, including radiators, and 100 tons for tankage, keel, and sundry equipment.

Our patrol ship with a crew of 25 thus has a dry mass of 475 tons, mass fully equipped 500 tons, plus 500 tons propellant for a full load departure mass of 1000 tons. Cost by my usual rule of thumb is equivalent to $500 million, perhaps $1 billion after milspecking, expensive compared to military planes, cheaper than major naval combatants.

This is no small ship. If the propellant is liquid hydrogen the tanks have a volume of about 7000 cubic meters, equivalent to a 7000 ton submarine. The payload section is about two thirds the mass of the ISS and of roughly comparable size, though the hab is probably spun giving the prolonged missions.

Armament is necessarily modest. The 75 tons of additional payload allowance probably must include a ferry craft for the espatiers and an escort gunship or two, plus their service pod, leaving perhaps 15-20 tons each for kinetics and a laser installation. The laser might be good for 20 megawatts beam power, with plug power from the 200 megawatt drive engine.

This ship is no laser star, but the laser is respectable. Assuming a modest 5 meter main mirror and a near IR wavelength of 1000 nanometers, at a range of 1000 km it can burn through Super Nano Carbon Stuff at rather more than 1 centimeter of per second. Its armament is also rather 'balanced.' My model shows that this laser can just defeat a wave of about 1000 target seekers, each with a mass of 20 kg, closing at 10 km/s—thus a total mass of 20 tons, comparable to its kinetics payload allowance.

Deploying troops, or personnel in general, is impressively expensive: About three fourths of the payload and cost of a billion dollar ship goes to support and equip a crew of 25, with perhaps a dozen espatiers. For comparison the USS Makin Island (LHD-8) displaces 41,000 tons full load, carries a crew of 1200 plus 1700 Marines, and costs about $1.8. So by my model it costs about as much to deploy one espatier as 80 marines.

And this ship is about the minimum patrol package, so standing interplanetary patrol is a costly and somewhat granular business, something not everyone can afford.

From Space Patrols by Rick Robinson (2010)
Ray McVay version of Rocketpunk Patrol Ship
Ray McVay
Rocketpunk Patrol Ship
Dry Mass76.2 metric tons
Wet Mass384.6 metric tons
Mass Ratio5
Length Z73 meters
Length Y20.1 meters
Length X15.2 meters
Enginex2 F-26-A LH/LOX
Thrust7.7×106 N
Acceleration0.5 g
ΔV8,200 m/s

This is the same one from the other day, only dressed up with a nice logo and some stats. These are realistic capabilities made courtesy of the charts and other information available from Atomic Rocket and inspiration from Rick Robinson's Rocketpunk Manifesto.

My PL differs from the one in Rick Robinson's article in a few key areas. The main difference is that it is not made for long hauls. It only has a delta v of about 8200 m/s. This will not get one far in the solar system but it allows a forward deployed Patrol Craft a sufficient "range" to perform many of the missions we discussed in the last post on Building a Space Navy. Our little A-Class has enough Delta V to shape a light-second orbit around a convoy in deep space, conduct SAR missions anywhere in cis-lunar space, or to reach any moon of Saturn from any other moon. Obviously, this rocket is mostly propellant (mass ratio 5). If you drew lines through the side view of the rocket that bracket the docking rings, you would encompass the entire pressurized volume. I've got to say, it's nice to work on a warship for a change — I don't have to make it economical to run!

One of the interesting things about this design is actually the freedom the little carried craft gives me. It was a throw-away touch, originally — a design borrowed from another project. But as I got to looking at the little thing, I realized that it's about the size of the Saturn V stage/Apollo/LM stack. That means it should be able to go from Earth Departure to Lunar orbit. That means that it has the Delta V to ferry crew to and from a Patrol Craft on station away from the convoy. That means, like submarines, our Patrol Craft can have two crews and stay out for a lot longer than otherwise. This is one of those realistic touches that I hope add to the charm of the rocket's design.

ed note: a 1500 nanometer near infrared laser with a 10 meter fixed mirror can have a 4 centimeter spot size out to 220 kilometers or so. A 4 meter mirror can have a 4 centimeter spot size out to 87 kilometers or so.

Rocketpunk Solar-Electric Ship

Solar-electric deep space drive engines, according to Isaac Kuo at sfconsim-l, may soon achieve a power output density of about 400 watts per kilogram, when operating near Earth distance from the Sun. If you do not see what this sort of technical information could possibly have to do with so lovely an image as gossamer wings, you probably reached this blog by accident, have no poetry in you, or both.

What makes it potentially relevant as well as beautiful is that 400 watts/kg is in hailing distance of the 1 kW/kg that Isaac and I independently chose as a benchmark for nuclear-electric drive, and generally as needed for relatively fast interplanetary travel. A spacecraft using solar electric drive can thus reach the same interplanetary speeds as its cousin, though it will take somewhat longer to reach cruising speed, and somewhat longer to slow down. It is a fair prospect that with a few decades' further progress, by the time we're actually building interplanetary ships the performance of the two drives will be comparable.

This is a big deal, because solar-electric space drive is technically and operationally elegant, while nuclear-anything drive, and especially nuclear-electric drive, is not. A solar electric drive has almost no moving parts. A nuclear-electric drive has lots of complex internal plumbing to draw energy from the reactor and incidentally keep it from melting. This plumbing operates under very nasty conditions, radioactivity being nothing to sheer high temperatures.

Plumbing is a big part of what makes spaceships so expensive, because it is complicated, full of parts that can jam, and as there is never a plumber around when you need one, it has to work perfectly for months at a time. (Even if you have a plumber in the crew, taking a nuclear reactor apart en route is a pain.) Robinson's Second Law: For each gram of physics handwavium in futuristic space tech, expect about a ton of plumbing handwavium.

Nuclear drives are also full of nasty fissionable stuff, tricky and dangerous to work with, requiring heavy shielding to get anywhere near (and radiation goes a long ways in space), requiring extreme security measures in handling and storage, and socially uncomfortable no matter how careful your procedures are.

In short, anything that gets rid of nuclear reactors in space is a huge plus on every level of operation, from spacecraft construction and maintenance to obtaining funding. Solar electric drive with comparable performance banishes nuclear reactors from the inner Solar System. You don't need them for travel, and you certainly don't need them for anything else, because one thing the inner Solar System has an ample and endless supply of is sunshine. Those skies are never cloudy all day.

Solar electric power does gasp for air, or for sunshine, as you move outward from the Sun. At Mars, thrust is about half as much as near Earth. In the asteroid belt it is about a fifth to a tenth, at Jupiter one twenty-fifth, at Saturn one percent. To give this some context, a one-milligee drive, baseline performance near Earth, nudges a ship along at about 1 km/s per day, reaching orbital transfer speeds in a week or two. At Jupiter, the drive delivers some 40 microgees, and a ship puts on about 1 km/s per month, thus the better part of a year for orbital transfer burns.

The time lost due to sluggish acceleration is only half as much, some six months, and a Jupiter mission would likely be upwards of a year each way even for a nuke-electric ship. So until we have regular bus service to Jupiter, the time cost is not dreadful. The inner Solar System, through the asteroid belt, can be efficiently traveled by solar-electric drive, which ought to hold us through this century and into the next.

Of course nuclear-electric ships can be built, but Isaac also pointed out a subtle effect that could sideline them. Over the decades to come we will build solar-electric probes, and later ships, steadily developing the technology, while nuke-electric remains a paper tech, falling further and further behind. A serious advance into the outer system will require a faster drive in any case—by that time perhaps a fusion drive, which can still be two orders of magnitude below the magical performance level of a 'torch.'

Let's mentally sketch-design a solar electric ship. Departure mass with full propellant load is 400 tons, broken down as follows:

  • Payload, 100 tons
  • Structures and fitting, 50 tons
  • Drive engine, 100 tons
  • Propellant, 150 tons

The drive engine we make an advanced one, meeting the baseline standard of 1 kW/kg. Thus rated drive power is 100 megawatts. If the exhaust velocity is 50 km/s (specific impulse ~5000 seconds), 80 grams of propellant is shot out the back each second. Thrust is 4000 Newtons, about 1000 lbs, giving our ship the intended 1 milligee acceleration at full load. Mass ratio is 1.6, so total ship delta v available on departure is 23.5 km/s, enough for a pretty fast orbit to Mars.

We could 'overload' this ship with a much bigger payload, another 400 tons (thus 500 tons total payload). Max acceleration falls to half a milligee, and mission delta v to 10 km/s—still ample for the Hohmann trip to Mars, for slow freight service. Since we want to go there ourselves, we will stick with the faster version and configure it as a passenger ship. Each passenger/crewmember requires cabin space, fittings, life support equipment, provisions and supplies for the trip, plus the mass of the passenger and baggage—in all, say, about 3 tons per person, so our ship carries some 30-35 passengers and crew.

The cabin structure of this ship might be about the size of a 747 fuselage, divided into berthing compartments or roomettes, diner/lounge area, galley, storage spaces, and life support plant. If the propellant is hydrogen, the tankage will be about the same size; if other stuff is used, the tankage will be smaller. All in all, the hull portion of our ship is comparable in size and mass to a jumbo jet. As space liners go this is a modest-sized one, as its modest passenger/crew capacity shows.

Now, finally, the gossamer wings part. We accounted for the mass of the drive engine, including solar collectors, but have not yet looked at the physical size of the solar panals. They are big. Big. If we assume that about 35 percent of the sunlight that hits them is converted into thrust power, they capture some 500 watts per square meter at 1 AU—meaning that for a 100 megawatt drive you need 200,000 square meters of solar panels, a fifth of a square kilometer.

This trim little interplanetary liner is physically enormous, or at least its solar wings are. The 'wingspan' might well be one kilometer, 'wing chord' then being 200 meters. In sheer size our ship is much bigger than any vehicle ever built (though freight trains can be up to about 2 km long).

Angular, squared-off, an instrument of technology—but how can this ship be anything but a thing of beauty, an immense gleaming-black butterfly? If that is too fluttery, say a dragonfly, or to be prosaic an equally immense gleaming-black kite. Indeed the prototype configuration is much like a box kite, likely for later versions as well.

Something is magical about such ships and travel aboard them. The drive thrust and power performance is the same as for a nuke-thermal ship, but now the milligee acceleration feels appropriately gentle, not merely weak, as our ship glides from world to world on its great sun-wings. (This is not, however, solar sailing, but a sun-powered 'steamship.')

The modest capacity of this immense little ship adds to the charm. With only about 35 passengers and crew this is no tawdry impersonal cruise ship. It all has somewhat the flavor of airship travel as we imagine it—perhaps encouraged by the zeppelin-like proportions of the vehicle, the gondola dwarfed by the feather-light structure that carries it. In early decades the ship will be much more utilitarian, a transport rather than a liner—don't ring for the steward; it's your turn in the galley. But if we go to the planets we will eventually go in liners.

The scenery out the viewports* won't change much after the first week or so spiraling out from Earth. (In fact you probably ride a connecting bus up through the Van Allen belts.) By then it is time for reading, cards, conversation, and flirting, till Mars looms close and the ship begins its long graceful swoop down to parking orbit.

Bon voyage!

* I disagree with Winch. All but the most utilitarian spaceships will have a few viewports, because while there is often nothing to see, when there is it is breathtaking. And fundamentally, why else are we going into space?

From On Gossamer Wings by Rick Robinson (2008)


Cargo Tug Slingshot
Jefferson contract
Total ΔV6,000 m/s
Specific Power1.5 kW/kg
(1,524 W/kg)
Thrust Power764.4 gigawatts
Exhaust velocity280,000 m/s
Thrust5,460,000 n
Wet Mass512,600 mt
Ship Mass1,600 mt
Payload Mass500,000 mt
Dry Mass501,600 mt
Mass Ratio1.02
Deuterium Fuel16 mt
Initial acceleration0.01 m/s2

The Cargo Tug Slingshot is from Jerry Pournelle's short story Tinker. In the story, it rescues the BoostShip Agamemnon.

The spacecraft's spine is a strong hollow tube built to transmit thrust from the aft engines to the fore array. The array is composed of detachable fuel pods of deuterium fuel and cadmium reaction mass. Fuel and remass are fed to the engines through the center of the ship's spine. The cargo goes fore of fuel pod. There are a couple of pods of fuel/remass attached to the hull.

Crew cabins are torus-shaped, arranged around the outside of the spine. Foremost torus is control deck. Next aftwards is living quarters for crew. Next comes deck with office and passenger quarters. Furthest aft is deck with shops, labs, and main entryway to the ship. Entryway doubles as a small store catering asteroid miners, to supplement the ship's income. Decks are connected by airlocks for safety.

There wasn't much doubt on the last few trips, but when we first put Slingshot together out of the wreckage of two salvaged ships, every time we boosted out there'd been a good chance we'd never set down again. There's a lot that can go wrong in the Belt, and not many ships to rescue you.

I shrugged and began securing the ship. There wasn't much to do. The big work is shutting down the main engines, and we'd done that a long way out from Jefferson (asteroid colony). You don't run an ion engine toward an inhabited rock if you care about your customers.

The entryway is a big compartment. It's filled with nearly everything you can think of: dresses, art objects, gadgets and gizmos, spare parts for air bottles, sewing machines, and anything else Janet or I think we can sell in the way-stops we make with Slingshot. Janet calls it the "boutique," and she's been pretty clever about what she buys. It makes a profit, but like everything we do, just barely.

(Nine tons of beef) I donated half a ton for the Jefferson city hall people to throw a feed with. The rest went for about thirty francs a kilo. That would just about pay for the deuterium I burned up coming to Jefferson.

(ed note: approimately 16,000 francs per metric ton of deuterium)

"I don't think you understand. You have half a million tons to boost up to what, five, six kilometers a second?" I took out my pocket calculator. "Sixteen tons of deuterium and eleven thousand reaction mass. That's a bloody big load. The fuel feed system's got to be built. It's not something I can just strap on and push off—"

I switched the comm system to Record. "Agamemnon, this is cargo tug Slingshot. I have your Mayday. Intercept is possible, but I cannot carry sufficient fuel and mass to decelerate your ship. I must vampire your dee and mass, I say again, we must transfer your fuel and reaction mass to my ship.

"We have no facilities for taking your passengers aboard. We will attempt to take your ship in tow and decelerate using your deuterium and reaction mass. Our engines are modified General Electric Model five-niner ion-fusion. Preparations for coming to your assistance are under way. Suggest your crew begin preparations for fuel transfer. Over."

The Register didn't give anywhere near enough data about Agamemnon. I could see from the recognition pix that she carried her reaction mass in strap-ons alongside the main hull, rather than in detachable pods right forward the way Slinger does. That meant we might have to transfer the whole lot before we could start deceleration.

The refinery crew had built up fuel pods for Slinger before, so they knew what I needed, but they'd never made one that had to stand up to a full fifth of a gee. A couple of centimeters is hefty acceleration when you boost big cargo, but we'd have to go out at a hundred times that.

They launched the big fuel pod with strap-on solids, just enough thrust to get it away from the rock so I could catch it and lock on. We had hours to spare, and I took my time matching velocities. Then Hal and I went outside to make sure everything was connected right.

Slingshot is basically a strongly built hollow tube with engines at one end and clamps at the other. The cabins are rings around the outside of the tube. We also carry some deuterium and reaction mass strapped on to the main hull, but for big jobs there's not nearly enough room there. Instead, we build a special fuel pod that straps onto the bow. The reaction mass can be lowered through the central tube when we're boosting.

Boost cargo goes on forward of the fuel pod. This time we didn't have any going out, but when we caught up to Agamemnon she'd ride there, no different from any other cargo capsule. That was the plan, anyway. Taking another ship in tow isn't precisely common out here.

Everything matched up. Deuterium lines, and the elevator system for handling the mass and getting it into the boiling pots aft; it all fit.

Ship's engines are complicated things. First you take deuterium pellets and zap them with a big laser. The dee fuses to helium. Now you've got far too much hot gas at far too high a temperature, so it goes into an MHD system that cools it and turns the energy into electricity.

Some of that powers the lasers to zap more dee. The rest powers the ion drive system. Take a metal, preferably something with a low boiling point like cesium, but since that's rare out here cadmium generally has to do. Boil it to a vapor. Put the vapor through ionizing screens that you keep charged with power from the fusion system.

Squirt the charged vapor through more charged plates to accelerate it, and you've got a drive. You've also got a charge on your ship, so you need an electron gun to get rid of that.

There are only about nine hundred things to go wrong with the system. Superconductors for the magnetic fields and charge plates: those take cryogenic systems, and those have auxiliary systems to keep them going. Nothing's simple, and nothing's small, so out of Slingshot's sixteen hundred metric tons, well over a thousand tons is engine.

Now you know why there aren't any space yachts flitting around out here. Slinger's one of the smallest ships in commission, and she's bloody big. If Jan and I hadn't happened to hit lucky by being the only possible buyers for a couple of wrecks, and hadn't had friends at Barclay's who thought we might make a go of it, we'd never have owned our own ship.

When I tell people about the engines, they don't ask what we do aboard Slinger when we're on long passages, but they're only partly right. You can't do anything to an engine while it's on. It either works or it doesn't, and all you have to do with it is see it gets fed.

It's when the damned things are shut down that the work starts, and that takes so much time that you make sure you've done everything else in the ship when you can't work on the engines. There's a lot of maintenance, as you might guess when you think that we've got to make everything we need, from air to zweiback. Living in a ship makes you appreciate planets.

Space operations go smooth, or generally they don't go at all.

When we were fifty kilometers behind, I cut the engines to minimum power. I didn't dare shut them down entirely. The fusion power system has no difficulty with restarts, but the ion screens are fouled if they're cooled. Unless they're cleaned or replaced we can lose as much as half our thrust—and we were going to need every dyne.

Agamemnon didn't look much like Slingshot. We'd closed to a quarter of a klick, and steadily drew ahead of her; when we were past her, we'd turn over and decelerate, dropping behind so that we could do the whole cycle over again.

Some features were the same, of course. The engines were not much larger than Slingshot's and looked much the same, a big cylinder covered over with tankage and coils, acceleration outports at the aft end. A smaller tube ran from the engines forward, but you couldn't see all of it because big rounded reaction mass canisters covered part of it.

Finally it was finished, and we could start maximum boost: a whole ten centimeters, about a hundredth of a gee. That may not sound like much, but think of the mass involved. Slinger's sixteen hundred tons were nothing, but there was Agamemnon too.

From Tinker by Jerry Pournelle (1975)

Stuhlinger Hybrid NERVA-Ion

Hybrid NERVA-Ion
Wet Mass388 metric tons
Mars Lander mass57 metric tons
Mars Lander
ascent stage mass
27 metric tons
Ion propulsion mass
(with propellant)
123 metric tons
Cesium ion
153 metric tons
Propulsion reactor
20 megawatts
Rotation rate1 rpm
Artificial gravity0.2 g
Spin radius179 meters
NERVA-II stage
Length54 meters
Diameter10 meter
Wet mass309 metric tons
Hydrogen propellant
226 metric tons

This is from Ernst Stuhlinger 1966 hybrid NERVA-Ion Mars mission proposal.

The idea is to avoid the drawback of the ion drive, the fact that the pathetic thrust of around 100 Newtons means it had an equally pathetic acceleration of about 0.0001 meters per second. Ordinarily this would not be a problem, except it means the spacecraft takes over twenty days to crawl through that glowing blue field of radioactive death they call the Van Allen Belts. A NERVA style nuclear thermal rocket can zip through the belt in a couple of hours, but its abysmal exhaust velocity makes it a propellant hog.

Stuhlinger's plan was a two-stage spacecraft. The NERVA-II stage gets the spacecraft through the radiation belt before the astronauts are fried, then that stage is ditched. The ion drive with its vastly superior exhaust velocity then takes over and gets the expedition to Mars using only a tea-cup's worth of propellant.

In Phase 1, for each of the four spacecraft in the expedition, 3 Saturn V will boost the ion-drive stage components into orbit, where the components will be assembled (12 Saturn V launches total).

In Phase 2, for each of the four spacecraft, 2 Saturn V will boost the NERVA components into orbit (one for the NERVA, one for the propellant tank), where the components will be assembled (8 Saturn V launches total). The NERVA stages will be attached to the ion stages.

There are four spacecraft in the expedition, in case one or more have to be abandoned for whatever reason. In a pinch a single spacecraft can carry all 16 expedition members home, abet in cramped conditions.

The mission starts with the crew inside the landers. If anything goes wrong during the initial burn, the landers will be the crew's abort-to-Terra vehicles. The NERVA-II stage burns for 30 minutes, passing through the Van Allen belts in 2 hours. About 17 minutes into the burn, exhaust is vented to spin up the spacecraft to 1 revolution per minute, for artificial gravity. The burn terminates when the spacecraft is at an altitude of 3450 kilometers.

The crew leaves the lander, and climbs down the 179 meter arms to the habitat modules. The NERVA stage is jettisoned, and the ion engines are started. They will burn for a while, then the ship will coast.

145 days into the mission, the ion engines are restarted to decelerate into high Mars orbit. The crew enters the Mars lander and land on Mars.

The unmanned spacecraft will continue the ion burn 24 days to move the ship to a low 1000 kilometer orbit. It would take even longer if the spacecraft had to deal with the mass of the lander.

After a month on Mars frantically doing sciene, the crew enters the lander's ascent stage and blast of to rendezvous with the orbiting ion spacecraft. The ascent stage is discarded to save on mass. This allows the spacecraft to spiral out to Terra transfer orbit in only 18 days.

The trip home will take 255 days, with deceleration starting halfway through.

Super Nexus

Super Nexus
1st stage ΔV2,440 m/s?
1st stage Specific Power4.3 kW/kg
1st stage PropulsionChemical, plug nozzle
1st stage FuelLO2/LH2
1st stage Specific Impulse382 to 439 s
1st stage Exhaust Velocity3,750 to 4,310 m/s?
1st+2nd stage Wet Mass10,900,000 kg
1st+2nd stage Dry Mass5,940,000 kg?
1st stage Mass Ratio1.83?
1st stage Mass Flow3,160 kg/s?
1st stage Thrust13,600,000 n
1st stage Initial Acceleration1.25 g?
Staging velocity2,440 m/s
2nd stage ΔV19,500 m/s?
2nd stage Specific Power28 kW/kg
2nd stage PropulsionOC Gas Core NTR
2nd Engine size3500K
2nd Number of engines4
2nd stage Specific Impulse2,000 s
2nd stage Exhaust Velocity19,600 m/s?
2nd stage Wet Mass5,940,000 kg
2nd stage Dry Mass2,190,000 kg?
2nd stage Mass Ratio2.7?
2nd stage Mass Flow324 kg/s?
2nd stage Thrust6,350,000 n
2nd stage Initial Acceleration1 g?
Total Wet Mass10,900,000 kg
total ΔV21,800 m/s
Payload453,000 kg
Total height134 m
1st stage Diameter45-52 m
2nd stage Diameter36 m

This is a heavy-lift vehicle designed to boost absurd amounts of payload from the surface of Terra, using deadly open-cycle gas-core nuclear thermal rockets in the second stage. If you want all the hard details, run and purchase a downloadable copy of Aerospace Projects Review vol. 3 no. 1. You get a lot of info for your downloading dollar.

This monster is the Uprated GCNR Nexus grown to three times the size. The document says that it can deliver 453 metric tons (one million pounds) not to LEO, but to Lunar surface. Doing some calculations on the back of an envelope with my slide rule, I estimate that it can loft 4,600 metric tons into LEO. But also with a proportional increase in radioactive exhaust. The data in the table is for the Terra lift-off to Lunar landing mission.

Translunar Space Patrol

This is from NUCLEAR SPACE PROPULSION by Holmes F. Crouch (1965). It is a solid-core nuclear thermal rocket used by the outer space version of the Coast Guard to rescue spacecraft in distress. In the diagram below, note how the rear fuel tanks are cut at an angle. This is to prevent any part of the tank from protruding outside of the shadow cast by the nuclear shadow shield. Also note that while the central tank must be load-bearing, the strap on tanks do not. This means the side tanks can be of lighter construction.

With the advent of lunar exploration and round trip lunar transport, both chemical and nuclear, there inevitably will arise malfunctions and emergencies. There will arise communication difficulties, navigational errors, propulsion breakdowns, and structural failures. There are possibilities of collisions between spacecraft and of fatal damage from matter in space. More likely, however, are onboard concerns of life-support malfunctions, auxiliary power irregularities, compartment over pressurization (in some cases, explosions), cargo shifting, and unforeseen disorders. These are the realities of increased space travel.

In anticipation of spaceflight realities, there would be need for a nuclear rescue ship operating in translunar space. The primary role of such a ship would be to save human life and those extraterrestrial specimens aboard any ill-fated lunar vehicle. A secondary role would be to salvage the spacecraft if at all possible.

This means that the rescue ship would require propulsive capability to drastically change orbit planes and altitudes. It would require excess ΔV to proceed with dispatch to rendezvous with a disabled spacecraft. In addition, capability would be required for transferring personnel and equipment, making repairs to a disabled vehicle, and even taking it in tow if conditions warranted. The latest advances in crew facilitation, passenger accommodations, repair shops, navigational devices, and communication equipment would be required. As an introductory concept, one arrangement of a nuclear rescue ship is presented in Figure 11-11 (see above).

A particular feature to note in Figure 11-11 is the use of two nuclear engines. Each engine would be of the lunar ferry vintage and, therefore, would be sufficiently well developed and man-rated for rescue ship design. These engines would be indexed by a nominal Isp of 1000 seconds; they would have a short time overrating of, perhaps 1100 seconds. This overrating implies conditional melting of nuclear fuel in the reactor for emergency maneuvers and dispatch.

A rescue ship would be characterized by a large inert weight compared to a regular transport vehicle. This means that large magnitudes of engine thrust would be required. However, during periods of non-emergencies, low thrusts could be used. The vehicle F/Wo characteristics (Thrust-to-weight ratio) would vary over a wide range: possibly from 0.1 during non-emergencies to 1 during emergencies. Two engines would provide the high thrust capacity for emergencies. During non-emergencies, one engine could be left idling; the other engine could provide low thrust for economic cruise. Furthermore, two engines would provide engine-out capability for take-home in the event of malfunction in one of the engines. For reactor control reasons, the two reactors would have to be neutronically isolated from each other. For this purpose, note the neutron isolation shield in Figure 11-11.

(ed note: Nuclear reactors are throttled by carefully controlling the amount of available neutrons within the reactor. A second reactor randomly spraying extra neutrons into the first reactor is therefore a Bad Thing. "Neutronically isolated" is a fancy way of saying "preventing uninvited neutrons from crashing the party.")

A suggested patrol region for the rescue ship is indicated in Figure 11-12 (see above). Note that a rendezvous orbit has been designated so that the rescue ship could replenish its propellant from the nuclear lunar transport system. By having rendezvous missions with nuclear ferry routes, rescued personnel, lunar specimens, and damaged spacecraft parts could be returned to Earth without the need for the rescue ship returning. Also, rescue ship crew members could be duty-rotated this way. This would increase the on-station time of a nuclear rescue ship.

From NUCLEAR SPACE PROPULSION by Holmes F. Crouch (1965)


Wet Mass6,000,000 kg
Dry Mass1,835,000 kg
Mass Ratio3.27
ΔV200,000 m/s
Thrust2.4 × 105N
Exhaust Velocity170,000 m/s
Thrust Power20.4 gigawatts
Specific Power11.1 kW/kg
D-T Fusion
Width170 m
Height100 m

VISTA is the Vehicle of Interplanetary Space Transport Application, from a study by the Lawrence Livermore National Laboratory. It looks like a tiny flying saucer in the diagrams but it is actually freaking huge. Blasted spacecraft is taller than Godzilla.

Tiny pellets with a deuterium-tritium compount core surrounded by about 50 grams of propellant drop out of the bottom of the cone. At the pellet target position a battery of laser modules zap the pellet with enough energy to initiate a fusion explosion. The propellant blast bounces off the 12-Tesla superconducting magnetic coil to provide thrust. Thrust is throttled by varying the pellet detonation rate from 0 to 30 detonations per second.

With 100 metric tons of payload, VISTA can travel to Mars and back to Terra in six months flat.

Unfortunately about 75% of the fusion energy is wasted, creating no thrust (escaping as neutrons and x-rays). But the remaining 25% is more than powerful enough to give the ship 200 kilometers per second of delta V. The spacecraft is shaped like a cone in an attempt to minimize how much of the wasted energy hits the ship as deadly radiation (only about 4% of the wasted energy irradates the spacecraft).

Water Truck

Water Truck
Specific Power9.6 kW/kg
PropulsionSolid core NTR
Specific Impulse198 s
Exhaust Velocity1,942 m/s
Wet Mass123,000 kg
Dry Mass30,400 kg
Mass Ratio4.1
Total ΔV2,740 m/s
Total Propellant92,600 kg
Boost Propellant75,700 kg
Landing Propellant16,900 kg
Boost ΔV1,859 m/s
Landing ΔV881 m/s
Mass Flow155 kg/s
Thrust301,000 newtons
Initial Acceleration0.25 g
Payload20,000 kg
Tank Length8.5 m
Total Length11.9 m
Diameter3.38 m
Structural Mass
Guidance Package0.45 tons
Tank1.6 tons
Thrust Structure
and Feed Lines
0.91 tons
Primary and
Secondary Structure
1.82 tons
Landing System0.68 tons
25% Growth Factor2.09 tons
Reactor1.82 tons
Turbopumps and
Rocket Nozzles
0.23 tons
Reaction Control
0.68 tons
Total10.3 tons

The Lunar ice water truck is a robot propellant tanker design by Anthony Zuppero. Its mission is to boost 20 metric tons of valuable water from lunar polar ice mines into a 100 km Low Lunar Orbit (LLO) cheaply and repeatably. It is estimated to be capable of delivering 3,840 metric tons of water into LLO per year.

This design uses a nuclear thermal rocket with currently available materials, and using water as propellant (a nuclear-heated steam rocket or NSR) instead of liquid hydrogen). This limits it to a specific impulse below 200 seconds which is pretty weak. However, numerous authors have shown that a NSR could deliver 10 and 100 times more payload per launched hardware than a H2-O2 chemical rocket or a NTR using liquid hydrogen. This is despite the fact that the chemical and NTR have much higher specific impulses. NSR work best when [1] the reactor can only be low energy, [2] there are abundant and cheap supplies of water propellant, and [3] mission delta-Vs are below 6,500 m/s.

The original article describes the water extraction subsystem at the lunar pole. It is a small reactor capable of melting 112.6 metric tons of ice into water (92.6 metric tons propellant + 20 metric tons payload) in about 45 hours. This will allow the water truck to make 192 launches per year, delivering a total of 3,840 metric tons of water per year.

Since the water truck is lifting off under the 0.17 g lunar gravity, its acceleration must be higher than that or it will just vibrate on the launch pad while steam-cleaning it. The design has a starting acceleration of 0.25 g (about 1.5 times lunar gravity).

The landing gear can fold so the water truck will fit in the Space Shuttle landing bay, but under ordinary use it is fixed. The guidance package mass includes radiation shielding. In addition, the guidance package is on the water truck's nose, to get as far as possible away from the reactor. The thrust structure and feed lines support the tank and anchor the reactor. The 25% growth factor is to accommodate future design changes without having to re-design the rest of the spacecraft. The reaction control nozzles perform thrust vector control. They take up more mass than a gimbaled engine, but by the same token they are not a maintenance nightmare and additional point of failure.

The reactor supplies about 120 kilowatts to the tank in order to prevent the water from freezing. The reactor mass is 50% more than minimum. The lift-off burn is about 20 minutes durationa and consumes 0.7 kg of Uranium 235.

Water Ship

Water Ship
Specific Power31 W/kg
PropulsionSolid core NTR
Specific Impulse190 s
Exhaust Velocity1,860 m/s
Wet Mass299,030,000 kg
Water tank mass25,000 kg
Nuclear Engine+
structural mass
123,000 kg
Sans Payload Mass148,000 kg
Payload mass50,000,000 kg
Dry Mass50,148,000 kg
Mass Ratio5.96
ΔV[1] 802 m/s
[2] 1280 m/s
[3] 752 m/s
Mass Flow[1,2] 903 kg/s
[3] 2,684 kg/s
Thrust[1,2] 1,680 kiloNewtons
[1,2] 4,990 kiloNewtons
Nozzle Power[1,2] 4.9 gigawatts
[3] 1.6 gigawatts
Engine Power[1,2] 12.1 gigawatts
[3] 4.1 gigawatts
Initial Acceleration[1] 0.0006 g
[2] 0.0009 g
[3] 0.005 g
Payload50,000,000 kg
Length85 m
Diameter85 m

The Water Ship is a robot propellant tanker design by Anthony Zuppero. Its mission is to deliver 50,000 metric tons of valuable water from the Martian moon Deimos to orbital propellant depots in Low Earth Orbit (LEO) cheaply and repeatably. It is not much more than a huge water bladder perched on a NERVA rocket engine. It might have integral water mining equipment as does the Kuck Mosquito, or it might depend upon a seperate Deimos ice mine.

Mass of water bladder is 25 metric tons (rated for no more than 0.005 g). Mass of nuclear thermal rocket plus strutural mass is 123 metric tons (struture includes computers, navigation equipment, and everything else). Mass without payload is 25 + 123 = 148 metric tons. Payload is 50,000 metric tons of water. Dry mass is 148 + 50,000 = 50,148 metric tons. Propellant mass is 248,882 metric tons. Wet mass is 50,148 + 248,882 = 299,030 metric tons.

At Deimos, only about 4.55 megawatts will be needed to melt 299,000 metric tons of ice into water (50,000 tons for payload + 249,000 tons for propellant). The engine nuclear reactor can supply that with no problem. The water must be distilled, because mud or dissolved salts will do serious damage to the engine nuclear reactor. By "serious damage" I mean things like clogging the heat-exchanger channels to cause a reactor meltdown, or impure steam eroding the reactor element cladding resulting in live radioactive Uranium 235 spraying in the exhaust plume.

Nuclear thermal rocket was designed to be a very conservative 100 megawatts per ton of engine. Engine will have a peak power of 12,142 Megawatts (for stage [1] and [2]). This works out to a modest engine temperature of 800° Celsius, and a pathetic but reliable specific impulse of 190 seconds. A NERVA could probably handle 300 megawatts per ton of engine, but the designer wanted to err on the side of caution. This will require much more water propellant, but there is no lack of water at Deimos.

This design uses a nuclear thermal rocket using water as propellant (a nuclear-heated steam rocket or NSR) instead of liquid hydrogen). This limits it to a specific impulse below 200 seconds which is pretty weak. However, numerous authors have shown that a NSR could deliver 10 and 100 times more payload per launched hardware than a H2-O2 chemical rocket or a NTR using liquid hydrogen. This is despite the fact that the chemical and NTR have much higher specific impulses. NSR work best when [1] the reactor can only be low energy, [2] there are abundant and cheap supplies of water propellant, and [3] mission delta-Vs are below 6,500 m/s.

It is true that electrolyzing the water into hydrogen and oxygen then burning it in a chemical rocket will get you a much better specific impulse of 450 seconds. But then you need the energy to electrolyze the water, and equipment to handle cryogenic liquids. These are just more things to go wrong.

In the table, [1], [2], and [3] refer to different segments of the journey from Deimos to LEO.

  • [1] Start at Deimos. 497 m/s burn into Highly Eccentric Mars Orbit (HEMO). At apoapsis, 305 m/s burn into Low Mars Orbit (LMO)
  • [2] At LMO periapsis, 1,280 m/s burn using the Oberth Effect to inject the water ship into Mars-Earth Hohman transfer orbit
  • [3] 270 days later at LEO periapsis, 752 m/s burn using the Oberth Effect to capture the water ship into Highly HEEO
  • [x] Water ship does several aerobrakes until it reaches an orbital propellant depot in LEO

Total thrust time is about 10 hours.

Water ship's propellant has 15,137 metric tons extra as a safety margin. When it arrives, hopefully some of this will be available. It will take 322 metric tons of propellant for the empty water ship to travel from HEEO to Deimos, or 1,992 metric tons to travel from LEO to Deimos. Plus 0.139 gigawatts of engine power and 10 hours of thrust time.

Traveling from Deimos to LEO will consume about 12.7 kg of Uranium 235. Given the fact that Hohmann launch windows from Mars to Earth only occur every two years, the fuel in the engine nuclear reactor will probably last the better part of a century before it has to be replaced. The engine will be obsolete long before then.

For more details, refer to the original article.

Widmer Nuclear Mars Mission

RocketCat sez

Another ship that will give old rocket fans a sense of haunting familiarity. Whether you saw it in the old Life Science Library volume Man in Space or as the Project SWORD toy, it is another bit of your childhood that would actually work.

This is from a 1963 study called Application Of Nuclear Rocket Propulsion To Manned Mars Spacecraft by Thomas Widmer. Unfortunately I cannot seem to find a copy, so most of the data comes from abstracts. It is an expansion of an earlier 1960 Lewis Research Center study.

Lewis Study

Lewis Nuclear Mars Mission
PropulsionSolid core NTR
Delta V19,800 m/s
Mars lander mass40,000 kg
Terra lander mass13,600 kg
Terra lander wingspan6.7 m
Crew size7
Wet mass614,000 kg
Mass per crew102,000 kg

The Lewis vehicle would have a habitat module with two levels, and 35 square meters of floor per level (3.3 meter radius). The storm cellar is a cyliiner at the centerline, and doubles as a sleeping quarters. The mass of the storm cellar depended upon the maximum allowable radiation exposure for the 420 day mission:

Lewis storm cellar mass
Max 1 Sievert , no solar flares21,400 kg
Max 1 Sievert , one solar flare74,500 kg
Max 0.5 Sievert , no solar flares127,000 kg

I believe the Lewis design went with the 21,400 kg storm cellar.

The Lewis mission would use an opposition-class trajectory. Terra-Mars trajectory takes 150 days, Mars surface mission takes 40 days, Mars-Terra trajectory takes 240 days. Total mission time is 420 days. Spacecraft requires seven Saturn V launches to boost all components into orbit, each launch boosting 100,000 kg.

Widmer Study

Widmer Nuclear Mars Mission
Specific Power15 kW/kg
PropulsionSolid core NTR
Specific Impulse820 s
Exhaust Velocity8,000 m/s
Thrust580,000 newtons
Wet Mass400,000 kg
Dry Mass150,000 kg
Mass Ratio2.7
Delta V>7,900 m/s
Num of propellant tanks12
Hydrogen per tank20,000 kg
Length80 meters
Crew size4

The Widmer vehicle was sized to have four crewmen for a 15 month mission to Mars. Just like the Bono Mars Glider, it was optimistically scheduled to depart in 1971, to take advantage of the next Hohmann launch window.

The solid core nuclear thermal rocket used a fast spectrum refractory metal core, with an inherent re-start capability and resistance to fuel cladding erosion allowing long burnning times. Long engine life and multiple restarts are extremely important factors in reducing gross vehicle weight, since they permit a low initial thrust to weight ratio (small engine), and eliminate the need for staging engines after each firing interval.

Furthermore, the smaller size of a fast metallic core provides an engine weight advantage of at least two to one over a thermal-graphite core engine of the same thrust rating. Smaller core frontal area also permits a similar reduction in shield weight. That is, the smaller the top of the nuclear reactor core, the smaller the anti-radiation shadow shield has to be, and thus the lower the shield mass.

The spacecraft components are boosted into orbit by four Saturn V boosters, one launch for the propulsion/payload module and three launches containing 4 loaded propellant tanks each. There will be a total of twelve propellant tanks. Each tank contains 20,000 kilograms of liquid hydrogen. A SNAP-9 or SNAP-50 nuclear power unit provides electricity to the cryogentic re-condensation system. The SNAP radiator is the cone shaped area just forward of the rocket engine.

The minimum delta V I hand calculated as 7,900 m/s. It will actually be larger, since the spacecraft jettisons spent propellant tanks and the first stage of the Mars excursion module.

Step 1

The propulsion and payload module is shown in its launch configuration. The hydrogen tank and crew compartment secions are 6.7 meters in diameter. Attached to the forward end of the tank, a chemically propelled Mars excursion module will permit the landing of a two man exploration party, after the spacecraft has attained Mars orbit.

Click for larger image

Step 2

One of the three tanker vehicles is shown in the launch configuration. A structural shell supports four nearly spherical tanks, each of which contains over 20,000 kilograms of liquid hydrogen. By employing auxiliary structure to reinforce the tanks during booster ascent, the weight of the tankage can be minimized. After installation on the nuclear rocket spacecraft, the light weight tanks will be exposed to only moderate acceleration (less than 1g), rather than the 7 or 8g experienced in attaining initial orbit.

Click for larger image

Step 3

The separate hydrogen tanks are being attached to the propulsion module in low Earth orbit. Each tank is insulated with multi-layer radiation foils to minimized hydrogen boil-off. In addition, a cryogentic re-condensation system may be employed for those tanks which are not emptied until the later phases of the mission. This system would be powered by a SNAP-9 or SNAP-50 type nuclear electric generating system located between the main propulsion reactor and the aft end of the central tank. The radiator for the SNAP powerplant can be seen just aft of the tank. In practice, it may be necessary to move this radiator into a position well to the rear of rocket engine during coast periods, so that head load on the hydrogen tank will be minimized. An attractive possibility exists for eliminating the auxiliary power reactor by integrating a liquid metal heat exchange loop with the rocket reactor core. This approach not only reduces system weight, but also tends to minimize the problem of after-heat removal from the engine.

Click for larger image

Step 4

In this view, the general arrangement of the crew quarters can bee seen. A two deck command module will contain the life support system, living accommodations, communications gear, experimental equipment, and a control center. Solar flare protection is provided by a vacuum jacketed capsule projecting downward into the main hydrogen tank. This "storm cellar" is lined with carbon shielding to augment the 2.4 meter thick annulus of liquid hydrogen which surrounds the capsule. Shielding is designed to restrict the integrated crew dose to less than 1 Sievert for the complete mission.

Note that the proposed configuration does not provide an artificial "g" capacity. If zero "g" cannot be tolerated for the long duration of an interplanetary mission, a rotating cabin section could be factored into the design. However, this approach would result in a substantial increase in spacecraft gross weight due to structural integration problems with an artificial "g" design.

Click for larger image

Step 5

The orbital launch maneuver is shown here. A total of six tanks will be emptied to depart from Earth orbit and achieve the Mars transfer ellipse. In the event of an abort during the escape maneuver, the chemically propelled Mars landing craft could be used for return to Earth orbit.

Click for larger image

Step 6

Staging of tanks during Earth escape propulsion is shown. Total propellant consumed up to injection for the Mars transfer is about 127 metric tons. In coast configuration two of the six tanks emptied during Earth escape will remain attached. This provides a degree of redundancy against the possibility of a meteoroid puncture in any of the loaded tanks, since propellant could be transferred into the remaining empty tanks. If no puncture occurs, the empty tanks are released immediately prior to the firing interval for Mars capture.

Transit time to Mars is about 180 days.

Click for larger image

Step 7

The Mars capture maneuver produces an eccentric orbit of about 560 kilometer perigee and about 5,000 kilometers apogee; thereby minimizing propellant requirements, while still providing a close view of the planet for final evaluation of landing sites. Four of the last six external propellant tanks are emptied during capture, but only two are jettisoned. Two are retained for meteoroid puncture redundancy until just before the Mars escape firing interval.

Click for larger image

Step 8

After transferring to the Mars excursion module, two of the four crew members fire braking rockets to bring the entry vehicle orbit perigee into the planetary atmosphere. The major portion of the deceleration is then accomplished by aerodynamic drag. After maneuvering to an altitude of about one kilometer, the landing craft is maneuvered into a vertical attitude for final approach. One minute of hovering capacity allows for some possible changes in landing site, and three shock absorbing struts are extended for the final touchdown. The winged entry vehicle represents one of several possible shapes, and lenticular or conical configurations might also be employed, depending upon the degree of aerodynamic maneuvering desired during entry.

Click for larger image

Step 9

The Mars excursion module is shown in its landing position. In addition to the two man crew capsule, approximately 2,300 kilograms of scientific equipment and portable life support gear can be transported to the Martian surface. Equipment will include a portable meteorological station, a powerful radio for communication with Terra, and a tracked car for exploration.

Gross weight of the excursion module prior to departure from orbit will be about 15,900 kilograms if hydrogen/oxygen propulsion is used. Stay time on the planet is restricted to about 5 days, due to limited payload and the rapid deterioration in launch window for the Earth return phase of the mission.

Note that the upper part of the Mars excursion module is a modified Gemini.

Click for larger image

Step 10

All equipment, except for the minimum life support capsule and 150 kilograms of soil samples, will be abandoned on the surface. The chemically propelled second stage of the landing vehicle uses the first stage structure as a launching platform for the return to Mars orbit.

Click for larger image

Step 11

After rendezvous with the nuclear rocket spacecraft, the excursion module second stage is abandoned in the eccentric parking orbit.

Click for larger image

Step 12

This illustration shows the Mars escape configuration of the spacecraft. During this maneuver, the last two external tanks are emptied, as is the aft compartment of the main tank. The forward end of the main tank, which surrounds the solar flare shelter, still contains hydrogen throughout the Mars-Earth transfer.

Transit time to Terra is about 200 days

Click for larger image

Step 13

Upon approaching Earth, the two empty tanks are released, and the nuclear rocket engine is used to brake the vehicle into a high altitude parking orbit. The crew will then transfer to a ferry vehicle for Earth re-entry. Alternatively, it would be possible to reduce the velocity increment required of the interplanetary spacecraft by employing direct re-entry from the Mars transfer ellipse. However, this would require that an Earth re-entry vehicle be transported through the entire mission, thereby increasing the weight carried on the spacecraft. Since direct re-entry alleviates the need for a large propulsion maneuver at the terminal end of the mission, little or no propellant would be available for solar flare shielding during the return flight coast period. The flare shield weight would then have to be increased to insure crew protection in the "empty" vehicle.

Click for larger image

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