Engine List

AM: Solid
Thrust Power	2.4 GW
Exhaust velocity	10,791 m/s
Thrust	440,000 n
T/W >1.0	yes

Basically a NERVA design where a tungsten target replaces the reactor. 13 micrograms per second of antiprotons are annihilated. The gamma rays and pions are captured in the tungsten target, heating it. The tungsten target in turn heats the hydrogen. Produces high thrust but the specific impulse is limited due to material constraints (translation: above a certain power level the blasted tungsten melts)

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  Image courtesy of Positronics Research, LLC

AM: Gas
Exhaust velocity	24,500 m/s

Microscopic amounts of antimatter are injected into large amounts of water or hydrogen propellant. The intense reaction flashes the propellant into plasma, which exits through the exhaust nozzle. Magnetic fields constrain the charged pions from the reaction so they heat the propellant, but uncharged pions escape and do not contribute any heating. Less efficient than AM-Solid core, but can achieve a higher specific impulse. For complicated reasons, a spacecraft optimized to use an antimatter propulsion system need never to have a mass ratio greater than 4.9, and may be as low as 2. No matter what the required delta V, the spacecraft requires a maximum of 3.9 tons of reaction mass per ton of dry mass, and a variable amount of antimatter measured in micrograms to grams.

Well, actually this is not true if the delta V required approaches the speed of light, but it works for normal interplanetary delta Vs.

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AM: Plasma
water
Thrust Power	30 GW
Exhaust velocity	980,000 m/s
Thrust	61,000 n
hydrogen
Thrust Power	192 GW
Exhaust velocity	7,840,000 m/s
Thrust	49,000 n
both
Engine mass	500 tonne
T/W >1.0	no
Power req	MW
Eff	%

Similar to antimatter gas core, but more antimatter is used, raising the propellant temperature to levels that convert it into plasma. A magnetic bottle is required to contain the plasma.

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AM: Beam
Thrust Power	500,000 GW
Exhaust velocity	100,000,000 m/s
Thrust	10,000,000 n
Engine mass	10 tonne

Microscopic amounts of antimatter are reacted with equal amounts of matter. The charged pions from the reaction are used directly as thrust, instead of being used to heat a propellant. A magnetic nozzle channels them. Without a technological break-through, this is a very low thrust propulsion system. All antimatter rockets produce dangerous amounts of gamma rays. The gamma rays and the pions can transmute engine components into radioactive isotopes. The higher the mass of the element transmuted, the longer lived it is as a radioisotope.

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  Image courtesy of NASA
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Positron Ablative
Exhaust velocity	49,000 m/s

This engine produces thrust when thin layers of material in the nozzle are vaporized by positrons in tiny capsules surrounded by lead. The capsules are shot into the nozzle compartment many times per second. Once in the nozzle compartment, the positrons are allowed to interact with the capsule, releasing gamma rays. The lead absorbs the gamma rays and radiates lower-energy X-rays, which vaporize the nozzle material. This complication is necessary because X-rays are more efficiently absorbed by the nozzle material than gamma rays would be.

Drawbacks include the fact that you need 1836 positrons to equal the energy of a single anti-proton, and only half the positrons will hit the pusher plate limiting the efficiency to 50%.

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  Image courtesy of Positronics Research, LLC

Laser Thermal
Thrust Power	0.065 GW
Exhaust velocity	40,000 m/s
Thrust	13,000 n
Engine mass	20 tonne
T/W >1.0	no
Power req	920 MW laser
Eff	30%

Similar to Solar Moth, but uses a stationary ground or space-station based laser instead of the sun. Basically the propulsion system leaves the power plant at home and relies upon a laser beam instead of an incredibly long extension cord.

With the mass of the power plant not actually on the spacecraft, more mass is available for payload. Or the reduced mass makes for a higher mass ratio to increase the spacecraft's delta V. The reduced mass also increases the acceleration. In some science fiction novels, combat "motherships" will have batteries of lasers, used to power hordes of ultra-high acceleration missiles and/or fighter spacecraft.

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  Mirror Steamer Robonaut patent card from the game High Frontier.

Solar Moth
Thrust Power	0.018 GW
Exhaust velocity	9,000 m/s
Thrust	4,000 n
Engine mass	0.1 tonne
T/W >1.0	no
Power req	Sunlight
Eff	63 %

Solar thermal rocket. 175 meter diameter aluminum coated reflector concentrates solar radiation onto a window chamber hoop boiler, heating and expanding the propellant through a regeneratively-cooled hoop nozzle. The concentrating mirror is one half of a giant inflatable balloon, the other half is transparent. Propellant is hydrogen seeded with alkali metal. The advantage is that you have power as long as the sun shines. The disadvantage is it doesn't work well past the orbit of Mars. The figures in the table are for Earth orbit.

The solar moth might be carried on a spacecraft as an emergency propulsion system, since the engine mass is so miniscule.

The energy density of sunlight at a given distance from the sun is:

S_ed = 1 / S_dist²

where:

• S_ed = sunlight energy density (Earth orbit density = 1.0)
• S_dist = distance from Sun (Astronomical Units, Earth = 1.0)

1.0 astronomical units is defined as 149,597,870,700 meters.

So in Earth's orbit, the density is 1.0, at Mars orbit it is 0.44 (44%), at Jupiter orbit it is 0.037, at Neptune orbit it is 0.001, at Mercury orbit it is 6.68

Multiply this by the solar constant to get the exact power density. The solar constant is about 1.361 kilowatts per square meter (kW/m²) at solar minimum and about 1.362 kW/m² at solar maximum. For example, during solar minimum the energy density at Jupiter orbit is 0.05 kW/m² and at Mercury orbit is 9.1 kW/m².

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  Art by Frank Tinsley
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  Painting by Professor Sol Dember
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  Mirror Steamer Robonaut patent card from the game High Frontier.

Chemical: Aluminum-Oxygen
Exhaust velocity	2,800 m/s
T/W >1.0	yes

Aluminum and oxygen are burned resulting in an unremarkable specific impulse of about 285 seconds. However, this is of great interest to any future lunar colonies. Both aluminum and oxygen are readily available in the lunar regolith, and such a rocket could easily perform lunar liftoff, lunar landing, or departure from a hypothetical L5 colony for Terra (using a lunar swingby trajectory). The low specific impulse is more than made up for by the fact that the fuel does not have to be imported from Terra. It can be used in a hybrid rocket (with solid aluminum burning in liquid oxygen), or using ALICE (which is a slurry of nanoaluminium powder mixed in water then frozen).

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  Image courtesy of NASA

Chemical: Methane-Oxygen
Exhaust velocity	3,700 m/s
T/W >1.0	yes

Methane and oxygen are burned resulting in an unremarkable specific impulse of about 377 seconds. However, this is the highest performance of any chemical rocket using fuels that can be stored indefinitely in space. Chemical rockets with superior specific impulse generally use liquid hydrogen, which will eventually leak away by escaping between the the molecules composing your fuel tanks. Liquid methane and liquid oxygen will stay put.

Chemical: Hydrogen-Oxygen
Exhaust velocity	4,600 m/s
T/W >1.0	yes

Hydrogen and oxygen are burned resulting in close to the theoretical maximum specific impulse of about 450 seconds. However, liquid hydrogen cannot be stored permanently in any tank composed of matter. The blasted stuff will escape atom by atom between the molecules composing the fuel tanks.

100% Atomic Hydrogen
Exhaust velocity	20,600 m/s
15% Atomic Hydrogen in solid H2
Exhaust velocity	7,300 m/s

Atomic hydrogen is also called free-radical hydrogen or "single-H". The problem is that it instantly wants to recombine. The least unreasonable way of preventing this is to make a solid mass of frozen hydrogen (H₂) at liquid helium temperatures which contains 15% single-H by weight.

Metallic Hydrogen
Exhaust velocity	17,000 m/s

Hydrogen subjected to enough pressure to turn it into metal, then contained under such pressure. Release the pressure and out comes all the stored energy that was required to compress it in the first place. It will require storage that can handle millions of atmospheres worth of pressure. The mass of the storage unit might be enough to negate the advantage of the high exhaust velocity.

Metastable He*
Thrust Power	1.4 GW
Exhaust velocity	43,000 m/s
Thrust	64,000 n
Engine mass	10 tonne
T/W >1.0	no

Spin-polarized triplet helium. Two electrons in a helium atom are aligned in a metastable state (one electron each in the 1s and 2s atomic orbitals with both electrons having parallel spins, the so-called "triplet spin state", if you want the details). When it reverts to normal state it releases 0.48 gigjoules per kilogram. Making the stuff is easy. The trouble is that it tends to decay spontaneously, with a lifetime of a mere 2.3 hours. And it will decay even quicker if something bangs on the fuel tank. Or if the ship is jostled by hostile weapons fire. To say the fuel is touchy is putting it mildly. The fuel is stored in a resonant waveguide to magnetically lock the atoms in their metastable state but that doesn't help much. There were some experiments to stablize it with circularly polarized light, but I have not found any results about that.

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  Metastable Helium Thruster patent card from the game High Frontier.

Metastable He IV-A
Exhaust velocity	21,600 m/s
Engine mass	10 tonne

Meta-helium would be such a worthwhile propulsion system that scientists have been trying real hard to get the stuff to stop decaying after a miserable 2.3 hours. One approach is to see if metastable helium can be formed into a room-temperature solid if bonded with diatomic helium molecules, made from one ground state atom and one excited state atom. This is called diatomic metastable helium. The solid should be stable, and it can be ignited by heating it. The exhaust velocity is about half that of pure He^* which is disappointing, but not as disappointing as a dust-mote sized meteorite blowing your ship into atoms.

Theoretically He IV-A would be stable for 8 years, have a density of 0.3 g/cm³, and be a solid with a melting point of 600 K (27° C). The density is a plus, liquid hydrogen's annoying low density causes all sorts of problems.

Electrodeless Plasma Thruster

Helicon Double Layer Thruster

Magnetoplasmadynamic
Thrust Power	3.1 GW
Exhaust velocity	314,000 m/s
Thrust	20,000 n
Engine mass	1540 tonne
T/W >1.0	no
Power req	4000 MW
Eff	79%
HOPE
Thrust Power	0.033 GW
Exhaust velocity	79,000 m/s
Thrust	98 n
Num above engines	x6

Magnetoplasmadynamic thruster, a travelling wave plasma accelerator. Propellant is potassium seeded helium.

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Pulsed inductive thruster

Pulsed plasma thruster

VASIMR
High Gear
Exhaust velocity	294,000 m/s
Thrust	40 n
Medium Gear
Exhaust velocity	147,000 m/s
Thrust	80 n
Low Gear
Exhaust velocity	29,000 m/s
Thrust	400 n
All
Thrust Power	0.006 GW
Engine mass	10 tonne + pp
T/W >1.0	no
Power req	10 MW
Eff	60 %

Some classify this as an electromagnetic plasma, some as an electrodeless electrothermal

The variable specific impulse magnetoplasma rocket is a plasma drive with the amusing ability to "shift gears." This means it can trade exhaust velocity for thrust and vice versa. Three "gears" are shown on the table. There are more details here and here.

VASIMR has been suggested for use in a space tug aka Orbital Transfer Vehicle. A VASIMR powered tug could move 34 metric tons from Low Earth Orbit (LEO) to Low Lunar Orbit (LLO) by expending only 8 metric tons of argon propellant. A chemical rocket tug would require 60 metric tons of liquid oxygen - liquid hydrogen propellant. Granted the VASIMR tug would take six month transit time as opposed to the three days for the chemical, but there are always trade offs.

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  VASIMR Thruster patent card from the game High Frontier.
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  Artwork by Nick Kaloterakis

ESTAT: Colloid
Thrust Power	0.17 GW
Exhaust velocity	43,000 m/s
Thrust	8000 n
Engine mass	20 tonne
T/W >1.0	no
Power req	200 MW
Eff	85 %

Similar to Ion, but utilizing tiny droplets instead of ions.

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Field-emission electric propulsion, a type of Colloid thruster.

Hall Effect Thruster

Ion
Thrust Power	1.05 GW
Exhaust velocity	210,000 m/s
Thrust	10,000 n
Engine mass	400 tonne
T/W >1.0	no
Power req	800 MW
Eff	96%

Gridded Electrostatic Ion Thruster. Potassium seeded argon is ionized and the ions are accelerated electrostatically by electrodes. Other propellants can be used, such as cesium and buckyballs. Though it has admirably high exhaust velocity, there are theoretical limits that ensure all Ion drives are low thrust.

It also shares the same problem as the other electrically powered low-thrust drives. In the words of a NASA engineer the problem is "we can't make an extension cord long enough." That is, electrical power plants are weighty enough to make the low thrust an even larger liability. A high powered ion drive will generally be powered by a nuclear reactor, Nuclear Electric Propulsion (NEP). Low powered ion drives can get by with solar power arrays, all ion drive space probes that exist in the real world use that system. Researchers are looking into beamed power systems, where the ion drive on the spaceship is energized by a laser beam from a remote space station.

If you are interested in the technical details about why ion drives are low thrust, read on.

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  NASA NSTAR for Deep Space 1

IBS Agamemnon Ion
Thrust Power	1,095 GW
Exhaust velocity	219,000 m/s
Thrust	10,000,000 n
T/W >1.0	no

Fictional Interplanetary Boost Ship Agamemnon from Jerry Pournelle's short story "Tinker". This fictional ship is a species of Ion drive utilizing cadmium and powered by deuterium fusion. Looking at its performance I suspect that in reality no Ion drive could have such a high thrust. The back of my envelope says that you'd need one thousand ultimate Ion drives to get this much thrust.

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ETHERM: ArcJet
Thrust Power	0.00002 GW
Exhaust velocity	20,000 m/s
Thrust	2 n
T/W >1.0	no
Power req	0.1 MW
Eff	35%

Hydrogen propellant is heated by an electrical arc.

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  Arcjet Robonaut patent card from the game High Frontier.

Microwave Electrothermal Thruster

Resistojet
Thrust Power	0.0000007 GW
Exhaust velocity	2900 m/s
Thrust	0.5 n
T/W >1.0	no
Power req	0.002 MW
Eff	80%

In a resistojet, ropellant flows over a resistance-wire heating element (much like a space heater or toaster) then the heated propellant escapes out the exhaust nozzle. They are mostly used as attitude jets on satellites, and in situations where energy is more plentiful than mass.

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MC-Fusion
Thrust Power	200 GW
Exhaust velocity	8,000,000 m/s
Thrust	50,000 n
Engine mass	0.6 tonne
T/W >1.0	yes

A magnetic bottle contains the fusion reaction. Very difficult to do. Researchers in this field say that containing fusion plasma in a magnetic bottle is like trying to support a large slab of gelatin with a web of rubber bands. Making a magnetic bottle which has a magnetic rocket exhaust nozzle is roughly 100 times more difficult.

Since the engine is using a powerful but tightly controlled magnetic field, it might be almost impossible to have a cluster of several magnetic confinement fusion engines. The magnetic fields will interfere with each other.

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  Tandem mirror engine.
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  "Field Reversed Configuration"

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  Discovery II from NASA report.
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  Discovery II
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  Discovery II
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  Discovery II magnetic nozzle.
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  Discovery II magnetic nozzle details.

D-T Fusion
Thrust Power	1.2 GW
Exhaust velocity	22,000 m/s
Thrust	108,000 n
Engine mass	10 tonne
T/W >1.0	yes

Fuel: deuterium and tritium. Propellant: lithium. 1 atom of Deuterium fuses with 1 atom of Tritium to produce 17.6 MeV of energy. One gigawatt of power requires burning a mere 0.00297 grams of D-T fuel per second.

Note that Tritium has an exceedingly short half-life of 12.32 years. Use it or lose it. Most designs using Tritium included a blanket of Lithium to breed more fresh Tritium fuel.

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H-B Fusion
Thrust Power	30 GW
Exhaust velocity	980,000 m/s
Thrust	61,000 n
Engine mass	300 tonne
T/W >1.0	no

Fuel is Hydrogen and Boron-11. Propellant is hydrogen. Bombard Boron-11 atoms with Protons (i.e., ionized Hydrogen) and you get a whopping 16 Mev of energy, three Alpha particles, and no deadly neutron radiation.

Well, sort of. Current research indicatates that there may be some neutrons. Paul Dietz says there are two nasty side reactions. One makes a Carbon-12 atom and a gamma ray, the other makes a Nitrogen-14 atom and a neutron. The first side reaction is quite a bit less likely than the desired reaction, but gamma rays are harmful and quite penetrating. The second side reaction occurs with secondary alpha particles before they are thermalized.

The Hydrogen - Boron reaction is sometimes termed "thermonuclear fission" as opposed to the more common "thermonuclear fusion".

A pity about the low thrust. The fusion drives in Larry Niven's "Known Space" novels probably have performance similar to H-B Fusion, but with millions of newtons of thrust.

It sounded too good to be true, so I asked "What's the catch?"

Professor N. Rostoker, et. al think they have the solution, utilizing colliding beams. Graduate Student Alex H.Y. Cheung is looking into turning this concept into a propulsion system.

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  Diagram from Alex H.Y. Cheung

He3-D Fusion
Thrust Power	192 GW
Exhaust velocity	7,840,000 m/s
Thrust	49,000 n
Engine mass	1,200 tonne
T/W >1.0	no

Fuel is helium³ and deuterium. Propellant is hydrogen. 1 atom of Deuterium fuses with 1 atom of Helium-3 to produce 18.35 MeV of energy. One gigawatt of power requires burning a mere 0.00285 grams of 3He-D fuel per second.

AV:T Fusion
cruise mode
Thrust Power	102 GW
Exhaust velocity	832,928 m/s
Thrust	245,250 n
T/W >1.0	no
combat mode
Thrust Power	2,540 GW
Exhaust velocity	104,116 m/s
Thrust	48,828,125 n
T/W >1.0	yes

Fictional magnetic bottle fusion drive from the Attack Vector: Tactical wargame. It uses an as yet undiscovered principle to direct the heat from the fusion reaction out the exhaust instead of vaporizing the reaction chamber. Like the VASIMR it has "gears", a combat mode and a cruise mode. The latter increases specific impulse (exhaust velocity) at the expense of thrust.

In the illustration, the spikes are solid-state graphite heat radiators, the cage the spikes emerge from is the magnetic bottle, the sphere is the crew quarters and the yellow rectangles are the retractable power reactor heat radiators. The ship in the lower left corner is signaling its surrender by deploying its radiators.

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  Radiators extended to signal surrender.
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Fusion Pulse low gear
Thrust Power	6 GW
Exhaust velocity	300,000 m/s
Thrust	40,000 n
Fusion Pulse high gear
Thrust Power	6 GW
Exhaust velocity	150,000 m/s
Thrust	80,000 n
Both
Engine mass	4 tonne
T/W >1.0	yes

Fictional inertial-confinement fusion drive from the game GURPS: Transhuman Space. Like the VASIMR it has "gears", one increases specific impulse (exhaust velocity) at the expense of thrust.

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  Artwork by Win Barker
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  Artwork by Win Barker

Solid Core NTR
3200° K
Exhaust velocity (H2)	8,093 m/s
Exhaust velocity (CH4)	6,318 m/s
Exhaust velocity (NH3)	5,101 m/s
Exhaust velocity (H2O)	4,042 m/s
Exhaust velocity (CO2)	3,306 m/s
Exhaust velocity (CO or N2)	2,649 m/s

Nuclear thermal rocket / solid core fission. It's a real simple concept. Put a nuclear reactor on top of an exhaust nozzle. Instead of running water through the reactor and into a generator, run hydrogen through it and into the nozzle. By diverting the hydrogen to a turbine generator 60 megawatts can be generated. The reactor elements have to be durable, since erosion will contaminate the exhaust with fissionable materials. The exhaust velocity limit is fixed by the melting point of the reactor.

Hydrogen gives the best exhaust velocity, but the other propellants are given in the table since a spacecraft may be forced to re-fuel on whatever working fluids are available locally (what Jerry Pournelle calls "Wilderness re-fuelling", Robert Zubrin calls "In-situ Resource Utilization", and I call "the enlisted men get to go out and shovel whatever they can find into the propellant tanks"). For thermal drives in general, and NTR-SOLID in particular, the exhaust velocity imparted to a particular propellant by a given temperature is proportional to 1 / sqrt( molar mass of propellant chemical ).

The value for "hydrogen" in the table is for molecular hydrogen, i.e., H2. Atomic hydrogen would be even better, but unfortunately it tends to explode at the clank of a falling dust speck (Heinlein calls atomic hydrogen "Single-H"). Another reason to avoid hydrogen is the difficulty of storing the blasted stuff, and its annoyingly low density (Ammonia is about eight times as dense!). Exhaust velocities are listed for a realistically attainable core temperature of 3200 degrees K for the propellants Hydrogen (H₂), Methane (CH₄), Ammonia (NH₃), Water (H₂O), Carbon Dioxide (CO₂), Carbon Monoxide (CO), and Nitrogen (N₂).

The exhaust velocities are larger than what one would expect given the molecular weight of the propellants because in the intense heat they break down into their components. Ammonia is nice because it breaks down into gases (Hydrogen and Nitrogen). Methane is nasty because it breaks down into Hydrogen and Carbon, the latter tends to clog the reactor with soot deposits. Water is most unhelpful since it doesn't break down much at all.

Dr. John Schilling figures that as an order of magnitude guess, about one day of full power operation would result in enough fuel burnup to require reprocessing of the fissionable fuel elements. (meaning that while there is still plenty of fissionables in the fuel rod, enough by-products have accumulated that the clogged rod produces less and less energy) A reprocessing plant could recover 55-95% of the fuel. With reprocessing, in the long term each totally consumed kilogram of plutonium or highly enriched uranium (HEU) will yield ~1E10 newton-seconds of impulse at a specific impulse of ~1000 seconds.

Dr. Schilling also warns that there is a minimum amount of fissionable material for a viable reactor. Figure a minimum of 50 kilograms of HEU.

One problem with solid-core NTRs is that if the propellant is corrosive, that is, if it is oxidizing or reducing, heating it up to three thousand degrees is just going to make it more reactive. Without a protective coating, the propellant will start corroding away the interior of the reactor, which will make for some real excitement when it starts dissolving the radioactive fuel rods. What's worse, a protective coating against an oxidizing chemical is worthless against a reducing chemical, which will put a crimp in your wilderness refueling. And trying to protect against both is an engineering nightmare. Oxidizing propellants include oxygen, water, and carbon dioxide, while reducing propellants include hydrogen, ammonia, and methane. Carbon Monoxide is neither, as the carbon atom has a death-grip on the oxygen atom.

Keep in mind that the oxidizing/reducing effect is only a problem with solid-core NTRs, not the other kinds. This is because only the solid-core NTRs have solid reactor elements exposed to the propellant (for heating).

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  "The enlisted men get to go out and shovel whatever they can find into the propellant tanks"
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  NERVA testing. From nuclearspace.com.

A useful refinement is the Bimodal NTR.

Say your spacecraft has an honest-to-Johnny NERVA nuclear-thermal propulsion system. Typically it operates for a few minutes at a time, then sits idle for the rest of the entire mission. Before each use, one has to warm up the reactor, and after use the reactor has to be cooled down. Each of these thermal cycles puts stress on the engine. And the cooling process consists of wasting propellant, flushing it through the reactor just to cool it off instead of producing thrust.

Meanwhile, during the rest of the mission, your spacecraft needs electricity to run life support, radio, radar, computers, and other incidental things.

So make that reactor do double duty (that's where the "Bimodal" comes in) and kill two birds with one stone. Refer to above diagram. Basically you take a NERVA and attach a power generation unit to the side. The NERVA section is the "cryogenic H2 propellant tank", the turbopump, and the thermal propulsion unit. The power generation section is the generator, the radiator, the heat exchanger, and the compressor.

Warm up your reactor once, do a thrust burn, stop the propellant flow and use the heat exchanger and radiator to partially cool the reactor to power generation levels, and keep the reactor warm for the rest of the mission while generating electricity for the ship.

This allows you to get away with only one full warm/cool thermal cycle in the entire mission instead of one per burn. No propellant is wasted as coolant since the radiator cools down the reactor. The reactor supplies needed electricity. And as an added bonus, the reactor is in a constant pre-heated state. This means that in case of emergency one can power up and do a burn in a fraction of the time required by a cold reactor.

Pretty ingenious, eh?

And the Pratt & Whitney company went one step better. They took the Bimodal NTR concept and merged it with the LANTR concept to make a Trimodal NTR. Called the Triton, it can be used in LANTR mode when thrust is more important than specific impulse, NTR mode when specific impulse is more important than thrust, and in power generation mode while coasting.

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  TRITON Trimodal
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  TRITON Trimodal schematic
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  TRITON Trimodal and Brayton radiator

Liquid Core
Thrust Power	56 GW
Exhaust velocity	16,000 m/s
Thrust	7,000,000 n
Engine mass	70 tonne
T/W >1.0	yes

Nuclear thermal rocket / liquid core fission. Similar to an NTR-GAS, but the fissionable core is merely molten, not gaseous.

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Fission Orion
Thrust Power	5.7 GW
Exhaust velocity	43,000 m/s
Thrust	263,000 n
Engine mass	200 tonne
T/W >1.0	no
Fusion Orion
Thrust Power	10.7 GW
Exhaust velocity	73,000 m/s
Thrust	292,000 n
Engine mass	200 tonne
T/W >1.0	no
1959 Orion 1st Gen
Thrust Power	1,600 GW
Exhaust velocity	39,000 m/s
Thrust	80,000,000 n
Engine mass	1,700 tonne
T/W >1.0	yes
1959 Orion 2nd Gen
Thrust Power	24,000 GW
Exhaust velocity	120,000 m/s
Thrust	400,000,000 n
Engine mass	3,250 tonne
T/W >1.0	yes
Orion MAX
Thrust Power	39,000 GW
Exhaust velocity	9,800,000 m/s
Thrust	8,000,000 n
Engine mass	8 tonne
T/W >1.0	yes

Orion AKA "old Boom-boom" is the ultimate consumable nuclear thermal rocket, based on the "firecracker under a tin can" principle. This concept has the spacecraft mounted with shock absorbers on an armored "pusher plate". A stream of small (5 to 15 kiloton) fission or fusion explosives are detonated under the plate to provide thrust. While you might find it difficult to believe that the spacecraft can survive this, you will admit that this will give lots of thrust to the spacecraft (or its fragments). On the plus side, a pusher plate that can protect the spacecraft from the near detonation of nuclear explosives will also provide dandy protection from any incoming weapons fire. On the minus side I can hear the environmentalists howling already. It will quite thoroughly devastate the lift-off site, and give all the crew bad backs and fallen arches. And they had better have extra-strength brassieres and athletic supporters. However, there is a recent report that suggests ways of minimizing the fallout from an ORION doing a ground lift-off (or a, wait for it, "blast-off" {rimshot}). Apparently if the launch pad is a large piece of armor plate with a coating of graphite there is very little fallout.

Mathematician Richard Courant viewed an Orion test and said "Zis is not nuts, zis is super-nuts."

If you want the real inside details of the original Orion design, run, do not walk, and get a copy of Aerospace Projects Review issue Volume 2, Number 2. It has blueprints, tables, and lots of never before seen details. If you want your data raw, piled high and dry, here (PDF file) is a copy of report GA-5009 vol III "Nuclear Pulse Space Vehicle Study - Conceptual Vehicle Design" by General Atomics (1964). Lots of charts, lots of graphs, some diagrams.

The sad little secret about Orion is that the mission it is best suited for is boosting heavy payloads into orbit. Which is exactly the mission that the enviromentalist and the nuclear test ban treaty will prevent. Orion has excellent thrust, which is what you need for lift-off and landing. Unfortunately its exhaust velocity is pretty average, which is what you need for efficient orbit-to-orbit maneuvers.

Having said that, there is another situation where high thrust is desirable: a warship jinking to make itself harder to be hit by enemy weapons fire. It is also interesting to note that the Orion propulsion system works very well with the bomb-pumped laser weapons system.

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Each pulse unit is a tiny nuclear bomb, encased in a "radiation case" that has a hole in the top. A nuclear blasts is initially mostly x-rays. The radiation case is composed of a material that his opaque to x-rays (depleted uranium). The top hole thus "channels" the flood of x-rays in an upwards direction (at least in the few milliseconds before the bomb vaporizes the radiation case). The channeled x-rays then strike the "channel filler" (beryllium oxide). This transforms the atomic fury of x-rays into an atomic fury of heat. Lying on top of the channel filler is the disc of propellant (tungsten). The heat flashes the tungsten into a jet of ionized tungsten plasma, traveling at high velocity (in excess of 1.5 × 10⁵ meters per second). This crashes into the pusher plate, accelerating the spacecraft. You will note that there are two stages of shock absorbers between the pusher plate and the spacecraft, preventing instant crew death. The jet is confined to a cone about 22.5 degrees. It is estimated that 85% of the energy of the nuclear explosion can be directed in the desired direction. The pulse units are popped off at a rate of about one per second.

The device is basically a nuclear shaped charge. A pulse unit that was not a shaped charge would of course waste most of the energy of the explosion. Figure that 10% at best of the energy of the explosion would actually hit the pusher plate.

Each charge accelerates the spacecraft by roughly 12 m/s. A 4,000 ton spacecraft would use 5 kiloton charges, and a 10,000 ton spacecraft would use 15 kiloton charges. For blast-off, smaller charges of 0.15 kt and 0.35 kt respectively would be used while within the Terra's atmosphere. The air between the charge and the pusher plate amplifies the impulse delivered. A 5 kiloton charge is about 1,152 kg.

According to Scott Lowther, the smallest pulse units were meant to propel a small ten-meter diameter Orion craft for the USAF and NASA. The units had a yield ranging from one-half to one kiloton. The USAF device was one kiloton, diameter 36 centimeters, mass of 86 kilograms, tungsten propellant mass of 34.3 kilograms, jet of tungsten plasma travels at 150,000 meters per second. One unit would deliver to the pusher plate a total impulse of 2,100,000 newton-seconds. Given the mass of the ten-meter Orion, detonating one pulse unit per second would give an acceleration well over one gee. According to my slide rule, this implies that the mass of the ten-meter Orion is a bit under 210 metric tons.

Pulse Unit	Yield	Mass	Diameter	Height	Propellant	Jet Velocity	Thrust
NASA Orion 10m plate - vacuum charge	1 kt	141 kg	0.36 m	0.6 m	90? kg	? m/s	3.5 × 106 n
USAF Orion 10m plate - vacuum charge	1 kt	79 kg	0.36 m	0.6 m	34.3 kg	150,000 m/s	2.0 × 106 n
4,000 ton Orion 41m plate - atm charge	0.15 kt	? kg	0.81 m	0.86 m	? kg	? m/s	? n
4,000 ton Orion 41m plate - vacuum charge	5 kt	1,152 kg	0.81 m	0.86 m	? kg	39,000 m/s	8.0 × 107 n
10,000 ton Orion 56m plate - atm charge	0.35 kt	? kg	? m	? m	? kg	? m/s	? n
10,000 ton Orion 56m plate - vacuum charge	15 kt	? kg	? m	? m	? kg	120,000 m/s	4.0 × 108 n
20,000? ton Orion ?m plate - vacuum charge	29 kt	1,150 kg	0.8 m	? m	? kg	? m/s	? n

Tungsten has an atomic number (Z) of 74. When the tungsten plate is vaporized, the resulting plasma jet has a relatively low velocity and diverges at a wide angle (22.5 degrees). Now, if you replace the tungsten with a material with a low Z, the plasma jet will instead have a high velocity at a narrow angle. The jet angle also grows narrower as the thickness of the plate is reduced. This makes it a poor propulsion system, but an effective weapon. Instead of a wall of gas hitting the pusher plate, it is more like a directed energy weapon. The process was examined in a Strategic Defense Initiative project called "Casaba-Howitzer", which apparently is still classified.

The following table is from a 1959 report on Orion, and is probably a bit optimistic. But it makes for interesting reading. For more in depth calculations of an Orion rocket's specific impulse, read page 1 and page 2. But be prepared for some heavy math.

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

	Interplanetary Ship	Advanced Interplanetary Ship
Gross Mass	4,000 tons	10,000 tons
Propulsion System Mass	1,700 tons	3,250 tons
Specific Impulse	4000 sec	12,000 sec
Exhaust Velocity	39,000 m/s	120,000 m/s
Diameter	41 m	56 m
Height	61 m	85 m
Average acceleration	up to 2g	up to 4g
Thrust	8e7 N	4e8 N
Propellant Mass Flow	2000 kg/s	3000 kg/s
Atm. charge size	0.15 kt	0.35 kt
Vacuum charge size	5 kt	15 kt
Num charges for 38,000 m	200	200
Total yield for 38,000 m	100 kt	250 kt
Num charges for 480 km orbit	800	800
Total yield for 480 km orbit	3 mt	9 mt
Δv 10 km/s Mass Ratio (Payload)	1.2 (1,600 tons)	1.1 (6,100 tons)
Δv 15.5 km/s Mass Ratio (Payload)	1.4 (1,200 tons)	1.1 (5,700 tons)
Δv 21 km/s Mass Ratio (Payload)	1.6 (800 tons)	1.2 (5,300 tons)
Δv 30 km/s Mass Ratio (Payload)	2.1 (200 tons)	1.3 (4,500 tons)
Δv 100 km/s Mass Ratio (Payload)	cannot	2.2 (1,300 tons)

Delta-V	Mission
10 km/s	Terra surface to 480 km Terra orbit
15.5 km/s	Terra surface to soft Lunar landing
21 km/s	Terra surface to soft Lunar landing to 480 km Terra orbit or Terra surface to Mars orbit to 480 km Terra orbit
30 km/s	Terra surface to Venus orbit to Mars orbit to 480 km Terra orbit
100 km/s	Terra surface to inner moon of Saturn to 480 km Terra orbit

NASA has been quietly re-examining ORION, under the new name of "External Pulsed Plasma Propulsion". As George Dyson observed, the new name removes most references to "Nuclear", and all references to "Bombs."

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Mini-Mag Orion
Thrust Power	66 GW
Exhaust velocity	210,000 m/s
Thrust	625,000 n
T/W >1.0	yes?

The Mini-MagOrion is a sort of micro-fission Orion propulsion system. The fuel and propellant are subcritical pellets of Curium-245. These are compressed electrodynamically by a Z-pinch magnetic field until they reach criticality and explode. The momentum from the explosion is transferred to the spacecraft by the magnetic field. The field coils are attached to a shock absorber Orion style. The detonations occur at a rate of 1 Hz.

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  Mini-Mag Orion Z-Pinch Fission Gigawatt Thruster patent card from the game High Frontier (Colonization Expansion).
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Medusa
Exhaust velocity	490,000 m/s to 980,000 m/s

Medusa is driven by the detonation of nuclear charges like Orion, except the charges are set off in front of the spacecraft instead of behind. The spacecraft trails behind a monstrously huge parachute shaped sail (about 500 meters). The sail intercepts the energy from the explosion. Medusa performs better than the classical Orion design because its pusher plate intercepts more of the bomb's blast, its shock-absorber stroke is much longer, and all its major structures are in tension and hence can be quite lightweight. It also scales down better. The nuclear charges will be from 0.025 kilotons to 2.5 kilotons.

The complicated stroke cycle is to smooth out the impulses from each blast, transforming it from a neck-braking jerk into a prolonged smooth acceleration.

Jondale Solem calculates that the specific impulse is a function of the mass and yield of the nuclear charges, while the thrust is a function of the yield and explosion repetition rate. In this case, the mass of the nuclear charge is the mass of "propellant".

Remarkably the mass of the spinnaker (sail) is independent of the size of its canopy or the number or length of its tethers. This means the canopy can be made very large (so the bomb blast radiation does not harm the canopy) and the tethers can be made very long (so the bomb blast radiation does not harm the crew). The mass of the spinnaker is directly proportional to the bomb yield and inversely proportional to the number of tethers.

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  (A) the payload capsule, (B) the winch mechanism, (C) the main tether cable, (D) riser tethers, and (E) the parachute mechanism
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  (1) Starting at moment of bomb / pulse unit firing, (2) As the bomb's explosion pulse reaches the parachute canopy, (3) Pushes the canopy, accelerating it away from the bomb explosion as the spacecraft plays out the main tether with the winch, braking as it extends, starting to accelerate the spacecraft, (4) And finally winches the tether back in.
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  Solem Medusa Tugged Orion Terawatt Thruster patent card from the game High Frontier (Colonization Expansion).

IC-Fusion
Thrust Power	500,000 GW
Exhaust velocity	10,000,000 m/s
Thrust	100,000,000 n
Engine mass	1000 tonne
T/W >1.0	yes

A pellet of fusion fuel is bombarded on all sides by strong pulses from laser or particle accelerators. The inertia of the fuel holds it together long enough for most of it to undergo fusion.

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  D-T VISTA Inertial Fusion Gigawatt Thruster patent card from the game High Frontier (Colonization expansion).
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Nuclear fission pulse drives like Orion scale up well, since it is relatively easy to design a bigger bomb than the last one. However, physics seem to prevent the creation of a nuclear device with a yield smaller than about 1/100 kiloton (10 tons, 42 GJ) and a fissionable material mass under 25 kilograms. This is due to critical mass restraints.

However, if a tiny sub-critical bit of fissionable material is bombarded by a few antiprotons, it will indeed create a tiny nuclear explosion. The antiprotons annihilate protons in uranium atoms, the energy release splits the atoms, creating a shower of neutrons, and a normal chain reaction ensues. Using antiprotons, yields smaller than 1/100 kiloton can be achieved. This can be used to create Antimatter catalyzed nuclear pulse propulsion

An E-Sail is a sail powered by solar wind.

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A Laser Sail is a photon sail beam-powered by a remote laser installation.

Magnetic Sail
Thrust per sail area	0.001 N/km2
Thrust by Sol dist	1/R2

A MagSail is a sail powered by the solar magnetic field.

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M2P2
Thrust per sail area	0.001 N/km2
Thrust by Sol dist	Constant Disk Inflates as 1/R2
Plasma use	0.25 kg/Day per N Thrust Isp = 35,000

A Mini-magnetospheric plasma sail (M2P2) is a MagSail inflated by an injection of plasma, powered by the solar magnetic field.

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A MagBeam is Mini-magnetospheric plasma sail beam-powered by a remote helicon plasma beam installation. PDF report here. Alternatively the spacecraft can use a plasma magnet instead of a M2P2 to intercept the beam. With the current design, the spacecraft mass cannot be larger than about 10,000 kg (10 metric tons).

The installation is called a High Power Platform (HPP). The HPP does not have much range, so the spacecraft will require a second HPP at the destination in order to slow down. For a Mars mission the HPP fires for about four hours before the spacecraft is out of range. By that time the spacecraft is travelling at about 20,000 m/s, which is fast enough to get to Mars in 50 days flat. The range is about 1×10⁷ meters (ten thousand kilometers).

After boosting a spacecraft, the HPP rotates the MagBeam in the opposite direction and uses it as an ion drive to move back into position. Newton's laws still hold, the recoil from the MagBeam is going to push the HPP way off base.

And I'm quite sure that at short ranges the MagBeam can be used as a weapon. It would also be a nifity thing for a warship to mount, so it can use it to boost missiles to ferocious velocities.

The main advantages seem to be increased acceleration levels on the spacecraft, and that one HPP propulsion unit can service multiple spacecraft. There are certain maneuvers that are impossible for low acceleration spacecraft, such as sub-orbital to orbital transfers, LEO to GEO transfers, LEO to escape velocity, and fast planetary missions.

Plasma beams as a general rule have short ranges. However, the system can take advantage of the fact that both the HPP and the spacecraft have magnetic fields. The MagBeam uses magnetic fields to focus the beam and the spacecraft has a MagSail to catch the beam. If they start off close enough to each other, the two magnetic field merge ("magnetic reconnection"), and gradually stretch as the spacecraft moves. This creates a long magnetic tunnel to confine the plasma stream, making the stream self-focusing.

This will be a problem when the HPP is faced with the task of slowing down an incoming spacecraft, since initially there will be no magnetic link. The spacecraft will have to temporarily inflate its MagSail, which can be done because it is an M2P2. Once the magnetic connection is made the M2P2 can be deflated to normal size.

Plasma will probably be argon or nitrogen. The beam range will a few thousand kilometers if the HPP or the beam passes through the ionosphere, tens of thousands of kilometers if in the magnetosphere. This is because of the ambient plasma and magnetic fields in the ionosphere.

Since the spacecraft does not carry its propellant, the standard rocket equation does not apply. Instead:

HPP_e = (0.25 * M * deltaV * V_e ) / HPP_eff

where:

• HPP_e = electrical energy expended by HPP (joules)
• M = mass of spacecraft (kg)
• deltaV = delta V applied to spacecraft (m/s)
• HPP_eff = efficiency of HPP at converting electricity into plasma energy (100% = 1.0, currently 0.6)

M_pb = HPP_e / (0.5 * V_e²)

where:

• M_pb = mass of propellant expended in HPP beam (kg)
• HPP_e = electrical energy expended by HPP (joules)
• V_e = velocity of HPP beam (m/s)

HPP_power = HPP_e / T_accel

where:

• HPP_power = miminum power level of HPP power plant (watts)
• HPP_e = electrical energy expended by HPP (joules)
• T_accel = duration of HPP beam usage (sec)

So if a HPP had to boost a 10,000 kg (10 metric ton) spacecraft to a deltaV of 3,000 m/s (3 km/s) using a plasma beam with a velocity of 19,600 m/s (2000 s) had only 300 seconds (5 minutes) to do so and had an efficiency of 0.6 (60%), then the electrical power used would be 2.5×10¹⁰ joules, the power plant would need a level of 82,000,000 watts (82 megawatts), and 127 kilograms of propellant would be expended.

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  MagBeam. Image credit: U. of Washington/Robert Winglee.
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  MagBeam. Image credit: U. of Washington/Robert Winglee.
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  MagBeam mothership launches a few Jupiter Probes. Image credit: U. of Washington/Robert Winglee

Photon Sail
Thrust per sail area	9 N/km2
Thrust by Sol dist	1/R2

A Photon Sail is a sail powered by solar photons. Commonly called a "solar sail", but that term does not make it clear if the sail is powered by solar photons, solar magnetic field, or solar wind.

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  Artwork by Frank Tinsley. Note spining habitat in center of sail. Click for larger image.
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  Artwork by Philippe “Manchu” Bouchet for a French edition of The Instrumentality of Mankind by Cordwainer Smith. Click for larger image.
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  Click for larger image.
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  Click for larger image.
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  Illustration for Arthur C. Clarke's "Sunjammer", aka "The Wind From The Sun"
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  Illustration for Arthur C. Clarke's "Sunjammer", aka "The Wind From The Sun"
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  Real world photon sail. Ikaros solar sail built by the Japan Aerospace Exploration Agency.

An plasma Magnet is a type of E-sail powered by solar wind.

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  Artwork by Ed Emshwiller
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  Artwork by Ed Emshwiller

Beer
Thrust Power	8 × 10-8 GW
Exhaust velocity	83 m/s
Thrust	84 n
T/W >1.0	no

In The Makeshift Rocket (also known as A Bicycle Built for Brew), the old geezer cobbles together a crude rocket out of hogs-heads of pressurized beer in order to escape to an adjacent asteroid.

Mass Driver
Thrust Power	0.3 GW
Exhaust velocity	30,000 m/s
Thrust	20,000 n
Engine mass	150 tonne
T/W >1.0	no
Power req	350 MW
Eff	90%

Mass drivers: magnetic buckets filled with packed rock dust are accelerated electmagnetically. Buckets are recovered for re-use. Propellant is rock dust or anything else you can stuff into the bucket. Popular with asteroid miners who want to nudge their claims into different orbits. However, their existence may prompt the creation of an Orbital Guard.

In Gerard O'Neill's plan for L5 colonies, mass drivers were used to deliver raw materials mined on Luna into orbit for colony construction. But instead of the mass driver being mounted on a cargo rocket, it was instead a ground installation near the lunar mine. The buckets were filled not with rock propellant, but instead with cargo cannisters of raw materials. The mass driver shot the cannisters into orbit. The cannisters were intercepted by a "catcher" at the colony site. So instead of needing a fleet of cargo rockets, you just needed a mass driver launcher and a catcher.

A mass driver is an electromagnetic mass accelerator that is optimized for propulsion. If you optimize it as a weapon instead, you have a coil-gun or rail gun. The weapons still have recoil and can be used as a crude propulsion system.

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  Mass Driver Thruster patent card from the game High Frontier.
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  O'Neill's lunar mass driver

Photon
Exhaust velocity	299,792,458 m/s

The exhaust is not a stream of matter. Instead it is a beam of Electromagnetic radiation, basically a large laser. The advantage is that it has the maximum possible exhaust velocity and thus the highest specific impulse. The disadvantage is the ludicrously high power requirements.

The momentum of a photon is p = E/c, where E is the energy of the photon. So the thrust delivered by a stream of photons is ∂p/∂t = ∂E/∂t/c. This boils down to:

F = P/c

P = F * c

where:

• F = thrust in Newtons
• P = power in joules
• c = speed of light in a vacuum (3e8 m/s)

In other words, one lousy Newton of thrust takes three hundred freaking megawatts!!

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  From Boeing's "Program for Astronomical Research and Scientific Experiments Concerning Space" (1960)
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  Art by Frank Tinsley