D.5 Bulk Matter Engines
45 Rotary Flinger
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
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
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
[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,
49 Electrostatic Ion
[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,
[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
[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
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
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
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
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
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
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
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
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
59 Neutral Particle Beam Thruster
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
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
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
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
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
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
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
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
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.
Description: Matter falling down a gravity well can be an energy source to
power payloads going up the gravity well.
Description: Using drag against a planetary atmosphere to slow down.
74a Single pass aerobrake
74b Multi-pass aerobrake
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