Atomic Rockets

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 these are all NASA style exploration vehicles, they are not very suited for interplanetary combat.

Avatar ISV Venture Star

The good starship ISV Venture Star from the movie Avatar is one of the most scientifically accurate movie spaceships it has ever been my pleasure to see. When I read the description of the ship, I got a nagging feeling that something was familiar. A ship with the engines on the nose, towing the rest of the ship like a water-skier? Wait a minute, that sounds like Charles Pellegrino and Jim Powell's Valkyrie starship.

Well, as it turns out, there was a good reason for that. James Cameron likes scientific accuracy in his movies. So he looked for a scientist who had experience with designing starships. Cameron didn't have to look far. As it turns out he already knew Dr. Pellegrino. This is because Dr. Pellegrino had worked with Cameron on a prior movie, since Dr. Pellegrino is one of the worlds greatest living experts on the Titanic.

After James Cameron had designed all the technical parameters of the Venture Star, master artist Ben Procter worked within those parameters to bring it to life.

Departing from Earth

In the upper diagram is a green arrow at the ship's nose, indicating the direction of flight. The ship is 1.5 kilometers long. In the Sol departure phase, a battery of orbital lasers illuminates a 16 kilometer diameter photon sail attached to the ship's nose (sail not shown). A mirror shield on the ship's rear prevents the laser beams from damaging the ship. The lasers accelerate the ship at 1.5 g for 0.46 year. At the end of this the ship is moving at 70% the speed of light (210,000 kilometers per second).

Keep in mind that battery of orbital lasers is going to have to be absolutely huge if it is going to push a lightsail at 1.5 g. This is not going to be a tiny satellite in LEO.

I cannot calculate the exact power rating since figures on the mass of the ISV Venture Star are conspicuous by their absence. The equation is Vs = (2 * Ev) / (Ms * c) where Vs is the starship acceleration, Eb is the energy of the beam, Ms is the mass of the starship, and c is the speed of light in a vacuum. Dr. Geoffrey Landis says is boils down to 6.7 newtons per gigawatt.

In Dr. Robert Forward's The Flight of the Dragonfly (aka Rocheworld), his starship's light sail is illuminated by a composite laser beam with a strength of 1500 terawatts. This pushes the starship with an acceleration of 0.01g (about 150 times as weak as the acceleration on the Venture Star). The beam is produced by one thousand laser stations in orbit around Mercury (where solar power is readily available in titanic amounts). Each station can produce a 1.5 terawatt beam, 1500 terawatts total. By way of comparison, in the year 2008, the entire Earth consumed electricity at a rate of about 15 terawatts. Since the Venture Star appears to be more massive than Forward's starship, and is accelerating 150 times as fast, presumably its battery of laser cannons is orders of magnitude larger.

As a side note, it is good to remember Jon's Law for SF authors. and The Kzinti Lesson. While technically this laser array is a component of a propulsion system, not a weapon; in practice it will have little difficulty vaporizing an invading alien battlefleet. Or hostile human battlefleet, for that matter (with the definition of "hostile" depending upon who actually controls the laser array). As Commander Susan Ivanova said in the Babylon 5 episode Deathwalker: "Our gun arrays are locked on to your ship, and will fire the instant you come into range. You will find their firepower most impressive ... for a few seconds."

Anyway, after the laser boost period is over, the sail is then collapsed along molecular fold lines by service bots, and stowed in the cargo area. The ship then coasts for the next 5.83 years to Alpha Centauri.

Breaking at Alpha Centauri

There are no batteries of laser cannon at Alpha Centauri so the lightsail cannot be used to brake to a halt. Instead, the twin hybrid fusion/matter-antimatter engines are used. These engines are not used for the Sol departure phase because that would increase the propellant requirement by about four times with a corresponding decrease in cargo capacity. The engines burn for 0.46 year, producing 1.5 g of thrust, thus braking the ship from a velocity of 70% c to zero.

Matter and antimatter is annihilated, and the energy release is used both in the form of photons and to heat up hydrogen propellant for thrust. A series of thermal shields near the engines protect the ship's structure from the exhaust heat. The engines are angled outwards a few degrees so that the exhaust does not torch the rest of the ship (exhaust path indicated in diagram by red arrows). This does reduce the effective thrust by an amount proportional to the cosine of the angle but is acceptable.

Why is most of the ship behind the engine exhaust? Because this reduces the mass of the ship. And when you are delta-Ving a ship up to and down from 70% c, every single gram counts. Conventional spacecraft have the engines on the bottom and the rest of the ship build on top like a sky scraper. This design has the engines on the top and the rest of the ship is dragged behind on a long tether (the "tensile truss" on the diagram). The result is a massive reduction in structural mass.

The engines are topped by monumental heat radiators used to get rid of waste heat from the matter-antimatter reaction. According to the description, after the burn is finished, the radiators will glow dull red for a full two weeks.

Cargo Modules

Immediately stern ward of the engines is the cargo section. It is arranged in four ranks of four modules each. Each module contains 6 cargo pods. A mobile transporter with a long arm moves within the cargo section in order to load and unload the shuttles.

Interface Craft

Next comes Two Valkyrie trans-atmospheric vehicles, aka "surface to orbit shuttles." They are docked to pressurized tunnels connected to the habitation section. Each is capable of transporting either:

  1. the contents of two cargo pods and 100 passengers OR
  2. the contents of six cargo pods and no passengers

Habitation Modules

Next come the habitation module. This holds the passengers in suspended animation for the duration of the trip. This is constructed almost totally from non-metallic materials, to prevent secondary radiation from galactic cosmic radiation.

The habitation module's life support system can only support all the passengers being awake for a limited time. There is no problem for the short period when the passengers are woken up and shuttled to the planet's surface. However, if the suspended animation system malfunctioned half-way through the multi-year voyage, life support could not handle it. In theis case, the passengers would be "euthanized" instead of being awakened.

Crew Modules

Next is the two on-duty crew modules. These are spun on the ends of arms to provide artificial gravity. When the ship is under thrust, the spin is taken off, and the arms are folded down along their hinges so that the direction of gravity is in the proper direction.

Shield

Finally comes the shield. While the ship is being boosted by the laser batteries, the shield protect the ship (but not the sail) from the laser beams. After boost, while the ship is coasting at 70% c, the ship is rotated so that the shield is in the direction of travel. The shield is constructed as a Whipple shield, and protects the ship from being damage by grains of dust.

At 70% c relative, each dust grain would have 4,900,000,000 freaking Ricks of damage. This means a typical interstellar dust grain with a mass of 4 x 10-6 grams will hit with the force of 20 kilograms of TNT, or about the force of four anti-tank mines.

When the ship wants to depart Alpha Centauri and return to Sol, it re-fills its antimatter and propellant tanks from the local fueling stations, uses the matter-antimatter engines to boost up to 70% c again, coasts for five-odd years, and is decelerated to a halt by the laser batteries at Sol.

Basic Solid Core NTR

Overview

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)
Payload15,0004.579.14
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 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."

Reactor

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.

Discovery II

Discovery II
PropulsionHelium3-Deuterium Fusion
Specific Impulse35,435 s
Exhaust Velocity347,000 m/s
Wet Mass1,690,000 kg
Dry Mass883,000 kg
Mass Ratio1.9
ΔV223,000 m/s
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. 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.

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

Liberty Ship
PropulsionNTR-GAS/closed
Specific Impulse3060 s
Exhaust Velocity30,000 m/s
Wet Mass2,700,000 kg
Dry Mass1,600,000 kg
Mass Ratio1.6875
ΔV15,000 m/s
Mass Flow1246 kg/s
Thrust37,380,000 newtons
Initial Acceleration1.4 g
Payload172,000 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.

HOPE

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

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.

HOPE (FFRE)

HOPE (FFRE)
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
Acceleration3×10-4

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

HOPE (MPD)

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
PropulsionMPD thrusters
Specific Impulse8,000 s
Exhaust Velocity78,500 m/s
Wet Mass242,000 kg
Dry Mass182,000 kg
Mass Ratio1.3
ΔV20,600 m/s
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

OPE Tanker
PropulsionMPD thrusters
Specific Impulse8,000 s
Exhaust Velocity78,500 m/s
Wet Mass244,000 kg
Dry Mass184,000 kg
Mass Ratio1.3
ΔV20,600 m/s
Mass Flow1.4 x 10-4 kg/s
Thrust11 n
Initial Acceleration4.6 x 10-6 g
Payload103,000 kg
Length135 m
Diameter55

HOPE Crew vehicle

Piloted Callisto Transfer Vehicle
PropulsionMPD thrusters
Specific Impulse8,000 s
Exhaust Velocity78,500 m/s
Wet Mass262,000 kg
Dry Mass188,000 kg
Mass Ratio1.4
ΔV26,400 m/s
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

Note the similarity to this 1962 Ernst Stuhlinger design for a Mars ion-drive rocket.

HOPE (VASIMR)

Revolutionary Concepts for Human Outer Planet Exploration (HOPE) (PDF file).

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
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 (PDF). 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.

ICAN-II

ICAN-II
PropulsionAntimatter Catalyzed Micro-Fission
Specific Impulse13,500 s
Exhaust Velocity132,000 m/s
Wet Mass707,000 kg
Dry Mass345,000 kg
Mass Ratio2
ΔV100,000 m/s
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

Kuck Mosquito
PropulsionH2-O2 Chemical
Specific Impulse450 s
Exhaust Velocity4,400 m/s
Wet Mass350,000 kg
Dry Mass100,000 kg
Mass Ratio3.5
ΔV5,600 m/s
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 tonnes 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.

Lighter and Tanker

Lighter
PropulsionH2-O2
Chemical
Specific Impulse450 s
Exhaust Velocity4,410 m/s
Wet Mass73,710 kg
Dry Mass33,900 kg
Mass Ratio2.17
ΔV3,410 m/s
Mass Flow31.8 kg/s
Thrust140 kiloNewtons
Initial Acceleration1.9 g
Payload25,000 kg
Length18.3 m+
Diameter≈4.57 m
Tanker
PropulsionOpen-cycle
gas core NTR
Specific Impulse3,600 s
Exhaust Velocity35,000 m/s
Wet Mass567,000 kg
Dry Mass351,540 kg
Mass Ratio1.61
ΔV16,730 m/s
Mass Flow100 kg/s
Thrust3,500 kiloNewtons
Initial Acceleration0.6 g
Payload209,790 kg
Length37 m+
Diameter≈18 m

These two designs are from The Resources of the Solar System by Dr. R. C. Parkinson. 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.

For no particular reason the report decided to use modular cryogenic hydrogen tanks that would fit in the Space Shuttle's cargo bay (its not like we would be using the Shuttle to ship hydrogen to California). 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 25,000 kg. I calculate that 300 m3 of liquid hydrogen would have a mass of about 20,352 kg, but I digress. Examining the drawing of the tanker, the front cluster is composed of seven tanks while the rear has ten, for a total of seventeen. 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 engine length. It will need a large enough 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.


All the other figures 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 17 tanks each of 25,000 kg, then the total is 425,000 kg. If this is 75% of the wet mass, the actual wet mass is 567,000 kg. If the payload is 37% of the wet mass, it is 209,790 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 73,710 kg. If the electrolysis plant is 25,000 kg, the lighter (with no payload) must be 48,710 kg. The lighter payload is one payload tank at 25,000 kg. So the lighter wet mass is 73,710 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 33,900 kg. Subtract the 25,000 kg payload, and there is 8,900 kg for the structure and the engine.

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

Umbrella Ship
PropulsionIon
Specific Impulse8,200 s
Exhaust Velocity80,000 m/s
Wet Mass660,000 kg
(730,000 kg)
Dry Mass328,000 kg
Mass Ratio2.0
ΔV55,000 m/s
Thrust490 Newtons
Initial Acceleration7.6 × 10-5 g
(6.7 × 10-5 g)
Payload136,000 kg
(150,000 kg)
Crew size20
Length102 m
Diameter152 m
Hab ring Dia39 m

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.

NASA Space Tug

NASA Space Tug
PropulsionChemical
Specific Impulse450 s
Exhaust Velocity4,400 m/s
Wet Mass32,000 kg
Dry Mass14,000 kg
Mass Ratio2.3
ΔV3,800 m/s
Mass Flow5 kg/s
Thrust22,400 n
Initial Acceleration0.17 g (lunar g)
Payload5,900 kg
Length?
Diameter?

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 DC-X

Nuclear DC-X
Propulsionpebble-bed NTR
PropulsionLANTR
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.

PARTS Plasma Accelerated Resuable Transport System

P.A.R.T.S. (PDF file) is a 2002 study by the Embry-Riddle Aeronautical University for a resuable 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

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.

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.

LevelGravity
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.

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
6
height of
one deck
8'4"
rate of
rotation
~4 x/min
launch vehicle2-stage Saturn V
orbit260-n-mi, 29.5° incl
mission
duration
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 Battleship

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 the dreaded Casaba Howitzer, which is basically a ray gun that shoots nuclear flame (the technical term is "nuclear shaped charge").

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.

Reusable Nuclear Shuttle

Reusable Nuclear Shuttle
PropulsionNTR-solid
Specific Impulse816 s
Exhaust Velocity8,000 m/s
Wet Mass170,000 kg?
Dry Mass30,000 kg?
Mass Ratio5.3?
ΔV13,000 m/s?
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
PropulsionNTR-solid
Specific Impulse1000 s
Exhaust Velocity9,810 m/s
Wet MassM kg
Dry MassM/4.6 kg
Mass Ratio4.6
ΔV15,000 m/s
Mass Flow? kg/s
Thrust? n
Initial Acceleration? g
Payload< M/4.6 kg
Length?
Diameter?

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)

Super Nexus

Super Nexus
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 ΔV2,440 m/s?
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 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 ΔV19,500 m/s?
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)

Water Truck

Water Truck
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
Nozzles
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
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.

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