D.5 Bulk Matter Engines

45 Rotary Flinger

Alternate Names:


Description: A one or two stage rotary mechanism mechanically

accelerates a small amount of reaction mass, then releases it. In the two

stage version, top speeds of 6 km/s are possible.




46 Coilgun Engine

Alternate Names: Mass Driver Reaction Engine


Description: A carrier, or bucket, is accelerated by interaction of

magnetic fields from 'driver' coils. The carrier holds a reaction mass,

which is released. The bucket is slowed down and reused.




47 Railgun Engine

Alternate Names:


Description:The interaction of the fields in current carrying rails and a

plasma short circuit of the rails accelerates the plasma, and anything in

front of it.




D.6 Ion and Plasma Engines

48 Arc Jet

Alternate Names:



Sunlight is converted to electricity by a photovoltaic array. The electricity

is arced through a propellant stream, heating it. The propellant is then

expanded through a nozzle.




[D34] Hardy, Terry L.; Curran, Francis M. "Low Power DC Arcjet

Operation with Hydrogen/Nitrogen/Ammoinia Mixtures", NASA

Technical Memorandum 89876, 1987.

[D35] Stone, James R.; Huston, Edward S. "NASA/USAF Arcjet

Research and Technology Program", NASA Technical Memorandum

100112, 1987.

[D36] Kagaya, Y. et al "Quasi-steady MPD Arc-jet for Space

Propulsion", Symposium for Space Technology and Science, Tokyo,

Japan, 19 May 1986, pp 145-154, 1986.

[D37] Manago, Masata et al "Fast Acting Valve for MPD Arcjet", IHI

Engineering Review, v 19 no 2 pp 99-100, April 1986.Ê Ê

[D38] Pivirotto, T. J.; King, D. Q. "Thermal Arcjet Technology for

Space Propulsion", Chemical Propulsion Information Agency, Laurel,

Maryland, 1985.

49 Electrostatic Ion

Alternate Names:






[D39] Rawlin, Vincent K; Patterson, Michael J. "High Power Ion Thruster

Performance", NASA Technical Memorandum 100127, 1987.

49a Solar-Electric Ion

Sunlight is converted to electricity by a photovoltaic array. The electricity

is used to ionize and electrostatically accelerate the propellant.

[D40] Mitterauer, J. "Liquid Metal Ion Sources as Thrusters for Electric

Space Propulsion", J. Phys. Colloq. (France) vol 48, no C-6, pp 171-6,

Nov. 1987.

[D41] Mitterauer, J. "Field Emission Electric Propulsion - Emission Site

Distribution of Slit Emitters", IEEE Trans. on Plasma Sci. vol PS-15, pp

593-8, Oct. 1987.

[D42] Stuhlinger, E. et al "Solar-Electric Propulsion for a Comet Nucleus

Sample Return Mission" presented at 38th Congress of the

International Astronautical Federation, Brighton, England, 10 Ocotober


[D43] Nakamura, Y.; Kuricki, K. "Electric Propulsion Test Onboard the

Space Station", Space Solar Power Review vol 5 no 2 pp 213-9, 1985.

[D44] Voulelikas, G. D. "Electric Propulsion: A Review of Future Space

Propulsion Technology" Communications Research Centre, Ottawa,

Ontario, report number CRC-396, October 1985.

[Dnn] Bartoli, C. et al

"A Liquid Caesium Field Ion Source for Space Propulsion", J. Phys. D vol

17 no 12 pp 2473-83, 14 Dec. 1984.

[D45] Imai, R.; Kitamura, S. "Space Operation of Engineering Test

Satellite -III Ion Engine", Proceedings of JSASS/AIAA/DGLR 17th Intl.

Electric Propulsion Conf. pp 103-8, 1984.

[D46] Jones, R. M.; Poeschel, R. L. "Primary Space Propulsion for 1995-

2000 - Electrostatic Technology Applications" AIAA/SAE/ASME 20th Joint

Propulsion Conference, AIAA paper number 84-1450, 1984.

[D47] Bartoli, C. et al "Recent Developments in High Current Liquid

Metal Ion Sources for Space Propulsion", Vacuum vol 34 no 1-2 pp 43-6,

Jan. -Feb. 1984.

[D48] Brophy, J. R.; Wilbur, P. J. "Recent Developments in Ion Sources

for Space Propulsion", Proceedings of the Intl. Ion Engineering Congress

vol 1 pp 411-22, 1983.

[Dnn] Anon. "Ion Propulsion Engine Tests

Scheduled", Aviation Week and Space Technology, v 116 no 26 pp 144-5,


[D49] James, E.; Ramsey, W., Sr.; Steiner, G. "Developing a Scaleable

Inert Gas Ion Thruster", AIAA paper number 82-1275 presented at

AIAA/SAE/ASME 18th Joint Propulsion Conference, Cleveland, OH, 21-

23 June 1982.

[D50] Zafran, S. et al "Aerospace Highlights 1982: Electric

Propulsion", Astronautics and Aeronautics, v 20 no 12 pp 71-72, 1982.

[D51] Clark, K. E.; Kaufman, H. B. "Aerospace Highlights 1981: Electric

Propulsion", Astronautics and Aeronautics, v 19 no 12 pp 58-59, 1981.

[D52] Kaufman, H. R. "Performance of Large Inert-Gas Thrusters",

AIAA paper number 81-0720 presented at 15th International Electric

Propulsion Conference, Las Vegas, Nevada, 21-23 April 1981.

[D53] Byers, D. C.; Rawlin, V. K. "Critical Elements of Electron-

Bombardment Propulsion for Large Space Systems", J. Spacecraft and

RocketsÊ vol 14 no 11 pp 648-54, Nov. 1977.

[D55] Mutin, J.; Tatry, B. "Electric Propulsion in the Field of Space",

Acta Electron. (France) vol 17 no 4 pp 357-70, Oct. 1974 (in French).

49b Thermoelectric Ion

Radioactive isotope decay produces heat. Heat is converted to electricity by

semiconductors. Electricity ionizes and accelerates atoms in engine.Ê

49c Laser-Electric Ion

Laser tuned to optimum absorption wavelength of photovoltaic cells. Cells

convert laser light to electricity, which is used to power ion engine. Ion

engine accelerates ionized propellants electrostatically.

[D56] Maeno, K. "Advanced Scheme of CO2 Laser for Space

Propulsion", Space Solar Power Review vol 5 no 2 pp 207-11, 1985.

49d Microwave-Electric Ion

A microwave receiving antenna (rectenna) on spacecraft converts

microwaves to electricity. Electricity is used to ionize and accelerate


[D57] Nordley, G. D.; Brown, W. C. "Space Based Nuclear-Microwave

Electric Propulsion", 3rd Symposium on Space Nuclear Power Systems,

Albuquerque, New Mexico, 13 January 1986, pp 383-95, 1987.

49e Nuclear-Electric Ion

Nuclear reactor generates heat, which is converted to electricity in

thermoelectric or turbine/generator cycles. Electricity is used to ionize

propellant and accelerate it by electrostatic voltage.

[D58] Cutler, A. H. "Power Demands for Space Resource Utilization",

Space Nuclear Power Systems 1986 pp 25-42.

[D59] Buden, D.; Garrison, P. W. "Space Nuclear Power Systems and

the Design of the Nuclear Electric Propulsion OTV", presented at

AIAA/SAE/ASME 20th Joint Propulsion Conference, AIAA paper number

84-1447, 1984.

[D60] Powell, J. R.; Boots, T. E. "Integrated Nuclear Propulsion/Prime

Power Systems", AIAA paper number 82-1215 presented at

AIAA/SAE/ASME 18th Joint Propulsion Conference, Cleveland, Ohio,

21-23 June 1982.

[D61] Powell, J. R.; Botts, T. E.; Myrabo, L. N. "Annular Bed Nuclear

Power Source for Electric Thrusters", AIAA paper number 82-1278

presented at AIAA/SAE/ ASME 18th Joint Propulsion Conference,

Cleveland, Ohio, 21-23 June 1982.

[D62] Ray, P. K. "Solar Electric versus Nuclear Electric Propulsion in

Geocentric Space", Trans. Am. Nucl. Soc. vol 39 pp 358-9, Nov.-Dec.


[D63] Hsieh, T. M.; Phillips, W. M. "An Improved Thermionic Power

Conversion System for Space Propulsion", Proceedings of the 13th

Intersociety Energy Conversion Engineering Conference pp 1917-1923,


[D64] Reichel, R. H. "The Air-Scooping Nuclear-Electric Propulsion

Concept for Advanced Orbital Space Transportation Missions", J. British

Interplanetary Soc. vol 31 no 2 pp 62-6, Feb. 1978.

50 Electron Beam Heated Plasma

Alternate Names:


Description: A high voltage (hundreds of keV) electron beam is injected

axially into a propellant flow. The electron beam heats the flow to plasma

temperatures, which produces high specific impulse. Cool gas is injected

along the chamber walls to provide film cooling and protect the chamber

from the very high temperature plasma.




51 Microwave Heated Plasma

Alternate Names: Electron-Cyclotron Absorption Rocket


Description:Ê Partially ionized gas directly absorbs microwaves,

becomingÊhot, then expands through rocket nozzle.




52 Fusion Heated Plasma

Alternate Names:


Description: Exhaust of pure fusion rocket is a thin, extremely hot

plasma. If higher thrust is needed, hydrogen can be mixed with plasma.

This increases thrust at the expense of performance.




52a Reactor leakage mixed

52b Plasma Kernal Mixed

53 Antimatter-Heated Plasma

Alternate Names:


Description: Exhaust of pure antimatter rocket is a charged particles. If

higher thrust is needed, hydrogen can be mixed with plasma. This

increases thrust at the expense of performance.




D.7 High Energy Particles

D.7a Particle Rockets

54 Pulsed Fission Nuclear

Alternate Names: Orion


Description: A series of small atomic bombs yield debris/particles

which pushes against plate/shock absorber arrangement. The shock

absorber evens out the explosion pulses to an even acceleration for the





55 Microfusion

Alternate Names:


Description: A conventional atomic bomb requires a certain

minimum size to operate with reasonable efficiency (a few kilotons). In the

microfusion approach, a fuel pellet consists of a fusion core material

(deuterium/tritium) surrounded by a fission shell (uranium 235). This is

similar to the arrangement of a fusion atomic bomb. Instead of chemical

explosives, which are what trigger a fusion bomb, a set of lasers or a heavy

ion beam are used to compress and set off the fission shell, which in turn

sets off the fusion core. A laser or ion compression can get higher

compressions than a chemical explosion, thus can set off smaller pellets. It

is easier to set off a fission shell than directly causing the fusion core to

ignite (as in the inertial fusion program). If explosions in the

ton range rather than kiloton range can be achieved, it will produce a more

useful vehicle than the pulsed fission concept in the previous item.




56 Alpha Particle

Alternate Names:



Radioactive element coats one side of thin sheet which is capable of

absorbing alpha particles. Particles emitted into sheet are absorbed,

particles emitted in opposite direction escape, providing net thrust.




57 Fission Fragment

Alternate Names:


Description: Thin wires containing fissionable material are at the heart of

this concept. Thin wires are used to allow the nuclear fragments from the

fission to escape. They are aimed by electrostatic or electromagnetic fields

to mostly go out the back end of the thruster. The performance is very high

because of the high speed of the fragments.




58 Fusion Particle

Alternate Names:


Description: Various thermonuclear fusion reactors have been proposed.

The results of a fusion reaction are high energy particles which can, in

priniple, be harnessed for propulsion.



58a Magnetic Confinement

Plasma in chamber similar to fusion power reactor is

intentionally leaked to magnetic nozzle.


[D65] Freeman, M. "Two Days to Mars with Fusion Propulsion", 21st

Century Science and Technology, vol 1, pp 26-31, Mar.-Apr. 1988.

[D66] Kammash, T.; Galbraith, D. L. "A Fusion-Driven Rocket

Propulsion Scheme for Space Exploration", Trans. Am. Nucl. Soc. vol 54

pp 118-9, 1987.

[D67] Mitchell, H. M.; Cooper, R. F.; Verga, R. L. "Controlled Fusion

for Space Propulsion. Report for April 1961-June 1962", US Air Force

report number AD-408118/8/XAB, April, 1963.

58b Inertial Confinement

Fuel pellet is heated and compressed by lasers, electron beam, or ion

beam. After fusing, the resulting plasma is directed by a magnetic



[D68] Kammash, T.; Galbraith, D. L. "A Fusion Reactor for Space

Applications", Fusion Technology, v. 12 no. 1 pp 11-21, July 1987.

[D69] Orth, C. D. et al "Interplanetary Propulsion using Inertial Fusion",

report number UCRL--95275-Rev. 1: 4th Symposium on Space Nuclear

Power Systems, Albequerque, New Mexico, 12 January 1987.

[D70] Hyde, Roderick, "A Laser Fusion Rocket for Interplanetary

Propulsion" , LLNL report UCRL-88857. (Fusion Pellet design: Fuel

selection. Energy loss mechanisms. Pellet compression metrics. Thrust

Chamber: Magnetic nozzle. Shielding. Tritium breeding. Thermal modeling.

Fusion Driver (lasers, particle beams, etc): Heat rejection. Vehicle

Summary: Mass estimates. Vehicle Performance: Interstellar travel required

exhaust velocities at the limit of fusion's capability. Interplanetary

missions are limited by power/weight ratio. Trajectory modeling. Typical

mission profiles. References, including the 1978 report in JBIS, "Project

Daedalus", and several on ICF and driver technology.)

[D71] Bussard, Robert W., "Fusion as Electric Propulsion", Journal of

Propulsion and Power, Vol. 6, No. 5, Sept.-Oct. 1990. (Fusion rocket

engines are analyzed as electric propulsion systems, with propulsion thrust-

power-input-power ratio (the thrust-power "gain" G(t)) much greater than

unity. Gain values of conventional (solar, fission) electric propulsion

systems are always quite small (e.g., G(t)<0.8). With these, "high-thrust"

interplanetary flight is not possible, because system acceleration (a(t))

capabilities are always less than the local gravitational acceleration. In

contrast, gain values 50-100 times higher are found for some fusion

concepts, which offer "high-thrust" flight capability. One performance

example shows a 53.3 day (34.4 powered; 18.9 coast), one-way transit

time with 19% payload for a single-stage Earth/Mars vehicle. Another

shows the potential for high acceleration (a(t)=0.55g(o)) flight in

Earth/moon space.)

58c Electrostatic Confinement

The fusion fuel is confined by a spherical potential well of order 100 kV.

When the fuel reacts, the particles are ejected with energy of order 2 MeV,

so escape the potential well. The potential well is at the focus of a

paraboloidal shell, which reflects the fusion particles to the rear in a narrow

beam (20-30 degree width).


[D72] Bussard, Robert W., "The QED Engine System: Direct Electric

Fusion-Powered Systems for Aerospace Flight Propulsion" by Robert W.

Bussard, EMC2-1190-03, available from Energy/Matter Conversion Corp.,

9100 A. Center Street, Manassas, VA 22110. (This is an introduction to

the application of Bussard's version of the Farnsworth/Hirsch electrostatic

confinement fusion technology to propulsion. 1500<Isp<5000 sec.

Farnsworth/Hirsch demonstrated a 10**10 neutron flux with their device

back in 1969 but it was dropped when panic ensued over the surprising

stability of the Soviet Tokamak. Hirsch, responsible for the panic, has

recently recanted and is back working on QED. -- Jim Bowery)

58d Plasma Mantle Confinement

The fusion fuel is contained in a toroidal/poloidal current pattern, similar to

a Tokamak except all the currents are in the plasma. The current pattern is

surrounded by a plasma sheath which isolates the fuel from a surrounding

working fluid. The fluid provides mechanical compression, which heats the

fuel to fusion ignition. After the fuel burn is completed, the energy

generated heats the working fluid to high temperature, which then goes out

a nozzle producing thrust.


[D73] Koloc, Paul M., "PLASMAKtm Star Power for Energy Intensive

Space Applications", Eighth ANS Topical Meeting on Technology of

Fusion Energy, Fusion Technology , March 1989. (Aneutronic energy

(fusion with little or negligible neutron flux) requires plasma pressures and

stable confinement times larger than can be delivered by current approaches.

If plasma pressures appropriate to burn times on the order of milliseconds

could be achieved in aneutronic fuels, then high power densities and very

compact, realtively clean burning engines for space and other special

applications would be at hand. The PLASMAKª innovation will make this

possible; its unique pressure efficient structure, exceptional stability, fluid-

mechanically compressible Mantle and direct inductive MHD electric power

conversion advantages are described. Peak burn densities of tens of

megawats per cc give it compactness even in the multi-gigawatt electric

output size. Engineering advantages indicate a rapid development schedule

at very modest cost. [I strongly recommend that people take this guy

seriously. Bob Hirsch, the primary proponent of the Tokamak, has recently

declared Koloc's PLASMAKª precursor, the spheromak, to be one of 3

promising fusion technologies that should be pursued rather than Tokamak.

Aside from the preceeding appeal to authority, the PLASMAKª looks like

it finally models ball-lightning with solid MHD physics. -- Jim Bowery])

59 Neutral Particle Beam Thruster

Alternate Names:


Description: A high energy (order 50 MeV) particle accelerator generates

a proton beam. This beam is neutralized (turned into atoms), then ejected.

The exhaust is moving at a substantial fraction of the speed of light, so

performance is very high. This type of machine was explored under the

SDI program as a way of destroying missiles (with the beam).




60 Antimatter Annihilation

Alternate Names:


Description: Protons and antiprotons annihilate, producing pions, then

muons, then gamma rays. The charged particles can be acted upon by a

magnetic nozzle. Antimatter provides the highest theoretical energy fuel

(100% matter to energy conversion), although the overhead involved with

storing antimatter may reduce the practical efficiency to a level comparable

to other propulsion methods.




[D74] Forward, Dr. Robert L. "Antiproton Annihilation Propulsion",

AFRPL TR-85-034 from the Air Force Rocket Propulsion Laboratory

(AFRPL/XRX, Stop 24, Edwards Air Force Base, CA 93523-5000). NTIS

AD-A160 734/0 [Quote: Technical study on making, holding, and using

antimatter for near-term (30-50 years) propulsion systems. Excellent

bibliography. Forward is the best-known proponent of antimatter. This

also may be available as UDR-TR-85-55 from the contractor, the University

of Dayton Research Institute, and DTIC AD-A160 from the Defense

Technical Information Center, Defense Logistics Agency, Cameron Station,

Alexandria, VA 22304-6145. And it's also available from the NTIS, with

yet another number.]

[D75] G. D. Nordley, "Application of Antimatter - Electric Power to

Interstellar Propulsion", Journal of the British Interplanetary Society, June


D.7b External Particle Interaction

61 Magsail

Alternate Names:


Description: The magsail operates by placing a large superconducting

loop in the solar wind stream. The current loop produces a magnetic field

that deflects the solar wind, producing a reaction force.




62 External Particle Beam

Alternate Names:


Description: A fixed particle beam source aims it at a target vehicle. The

particles are absorbed or reflected generating thrust at the vehicle.




63 Interstellar Ramjet

Alternate Names: Bussard Ramjet


Description: Compressing and fusing interstellar hydrogen for

propulsion. Because of the low density of the interstellar medium, an

extraordinarily large scoop is required to get any useful thrust. Performance

is limited by the exhaust velocity of the fusion reaction to a few percent of

the speed of light.




[D76] R. W. Bussard, "Galactic Matter and Interstellar Flight",

Astronautica Acta 6 (1960): 179 - 194.

[D77] A. R. Martin, "The Effects of Drag on Relativistic Spacefight",

JBIS 25 (1972):643-652

[D78] N. H. Langston, "The Erosion of Interstellar Drag Screens", JBIS

26 (1973): 481-484.

[D79] D.P. Whitmire, "Relativistic Spaceflight and the Catalytic Nuclear

Ramjet", Acta Astronautica 2 (1975): 497 - 509.

[D80] C. Powell, "Flight Dynamics of the Ram-Augmented Interstellar

Rocket", JBIS 28 (1975):553-562

[D81] D.P. Whitmire and A.A. Jackson, "Laser Powered Interstellar

Ramjet", JBIS 30 (1977):223 - 226.

[D82] G. L. Matloff and A. J. Fennelly, "Interstellar Applications and

Limitations of Several Electrostatic/Electromagnetic Ion Collection

Techniques", JBIS 30 (1977):213-222

64 Interstellar Scramjet

Alternate Names:


Description: Similar to the interstellar ramjet, the interstellar medium is

compressed to fusion density and temperature. In this concept it is only

compressed laterally, then re-expanded against a nozzle. Incredible vehicle

sizes and lengths are required to reach fusion conditions, but speed may

reach a substantial fraction of the speed of light.




D.8 Photon Engines

D.8a Photon Sails

65 Solar Sail

Alternate Names: Lightsails


Description: Sunlight reflecting off a large area sail produces force

because momentum of photons is reversed by refelection. Force is

(1+r)(E/c) for normal reflection, where r is the reflectivity of the sail, E is

the incident power, and c is the speed of light. At the distance of the Earth

from the Sun, the incident power is 1370 MW per square kilometer. This

produces about 8 Newtons/square kilometer for high-reflectivity sails.




[D83] Marchal, C. "Solar Sails and the ARSAT Satellite - Scientific

Applications and Techniques", L'Aeronautique et L'Astronautique, no 127,

pp 53-7, 1987.

[D84] Louis Friedman, Starsailing. Solar Sails and Interstellar Travel. ,

Wiley, New York, 1988, 146 pp., paper $9.95.

66 Laser Lightsail

Alternate Names:


Description: Laser photons are reflected off sail material. Reflection of

photons reverses their momentum vectors' component which is normal to

the sail. By conservation law, the sail gains momentum. Laser sails can

have higher performance than solar sails because the laser beam intensity is

not limited like the brightness of the sun.




[D85] Forward, Robert L., "Roundtrip Interstellar Travel Using Laser-

Pushed Lightsails" Journal of Spacecraft and Rockets , vol. 21, pp. 187-

95, Jan.-Feb. 1984

67 Microwave Sail

Alternate Names: Starwisp



Microwaves are reflected off very thin, open mesh. Momentum change of

photons bouncing off of mesh provides thrust. Because an open mesh of

thin wires can have a very low weight, in theory this propulsion method can

give high accelerations.




D.8b Photon Rockets

68 Thermal Photon Reflector

Alternate Names:


Description: A heat generating device, such as a nuclear reactor, is at the

focus of a paraboloidal reflector. The thermal photons are focussed into a

near parallel beam, which propells the vehicle. Another high-energy source

is a matter-antimatter reaction, which is absorbed by a blanket of heavy

metals and converted to heat.




69 Quantum Black Hole Generator

Alternate Names:


Description: In theory, a quantum black hole will emit particles as if it

were a black body of a certain temperature. If new matter is added to the

black hole at a rate sufficient to offset the emission losses, effectively 100%

conversion of matter to energy can be achieved. Black holes, quantum or

otherwise, are very massive, so the utility of such for propulsion is

questionable for anything smaller than an asteroid sized spaceship.




70 Gamma Ray Thruster

Alternate Names:



Gamma rays produced by antimatter annihilation behind vehicle can be

absorbed by a thick layer of heavy metals. Momentum of gamma ray

photons produces thrust.




D.9 External Interactions

71 Ionospheric Current Loop

Alternate Names: Electrodynamic Engine


Description: A current-carrying wire in a planetary magnetic field feels an

IxB force. The current loop is closed through an ionosphere. The wire

accelerates in one direction (pulling a vehicle along), and the ionosphere

accelerates in the other direction. Per unit of power input a current loop

thruster produces more thrust than an ion engine. No propellant is

consumed directly, although some material is consumed to produce a

plasma that enables good electrical contact with the ionosphere. Effectively

this gives a specific impulse in the 25,000 range.




[D86] Belcher, J. W. "The Jupiter-Io Connection: an Alfven Engine in

Space", Science vol 238 no 4824 pp 170-6, 9 Oct 1987.

72 Gravity Assist

Alternate Names: Planetary Flyby, Celestial Billiards


Description: Momentum exchange between planetary body and

vehicle allow changing direction, and velocity in other reference frames.




73 Dumb-Waiter

Alternate Names:


Description: Matter falling down a gravity well can be an energy source to

power payloads going up the gravity well.




74 Aerobrake

Alternate Names:


Description: Using drag against a planetary atmosphere to slow down.



74a Single pass aerobrake

74b Multi-pass aerobrake


75 Rheobrake

Alternate Names: Lithobrake, Crashportation


Description: Using drag against a planetary surface to slow down. For

example, imagine a rail made of cast basalt on the lunar surface. It is laid

level to the ground, and is shaped like a conventional steel railroad rail. A

landing vehicle is in a low grazing orbit. It aligns with the rail, just above

it, then exends some clamps over the rail. By applying clamping pressure,

the vehicle can brake from lunar orbit to a stop. Obviously the brake will be

dissipating a lot of heat, and will therefore have to be made of high

temperature material such as graphite.

Another approach is to have a 'runway' which is a smoothed area on the

lunar surface. The arriving vehicle slows down to below orbital speed, then

gravity puts it down on the runway, and friction on the bottom of the

vehicle slows it down.


Variations: Creating an aritifical 'atmosphere' of particles to

slow down against. A cloud of lunar dust could be raised by electrostatic

forces and an arriving vehicle slows by impact of the dust particles.


D.10 Comparisons Among Methods

Propulsion concepts can be sorted in various ways. One is by performance.

Measures of performance include specific impulse and thrust to weight

ratio. Another sorting is by technology maturity. It is hoped in a later

verion of this survey that these types of sortings or rankings can be