Section 2.7 - High Energy Particle Engines
Particle engines generally have very good performance because of the high velocity of the particles. The high velocity also requires a lot of energy, since particle kinetic energy goes as the square of the velocity.
A. Particle Rockets
Particle rockets are distinguished by emitting particles from an internal source, while the next section, External Particle Interactions, covers methods that employ particles in the environment, either natural or man-made.
54 Pulsed Fission Nuclear Thruster
Alternate Names: Orion
Type: Atomic Particles by Nuclear Detonation
Description: A series of small atomic bombs yield debris/particles which push against a plate/shock absorber arrangement. The shock absorber evens out the explosion pulses to a steady acceleration for the vehicle. Issues with this method are not so much technical feasibility as risks of using it near Earth and having a shipload of nuclear weapons, not to mention the radiation levels for crew and cargo. Benefits are the enormous potential payload and high acceleration. A possible application is moving dangerous asteroids. That would not take very many bombs, and the pusher plate/shock absorber could be made from asteroidal metals, even from the asteroid you are moving.
Other applications might involve moving large amounts of material in the outer Solar System, or starting interstellar trips. Far away from Earth the issue of radiation added to the environment is not as significant, since solar and cosmic radiation are already present in large amounts.
Status: Atomic bombs are well tested (unfortunately). A demonstration of the pusher plate technology was done in small scale with conventional explosives.
- 54a Antimatter Catalyzed Pulsed Nuclear - This speculative method uses antimatter triggered implosion of plutonium targets. In theory this would lower the critical mass to ~ 2 grams significantly lowering the size threshold for pulsed nuclear.
- 54b Implosion Driver Pulsed Nuclear - Standard fission bombs are triggered by a chemical implosion. This method substitutes a laser, ion beam, or other external driver to collapse the fuel load. It potentially reduces the critical mass with a stronger collapse than chemicals can achieve, or by using a different isotope.
55 Inertial Confinement Nuclear Engine
Alternate Names: Microfusion Engine
Type: Atomic particles by Fission or Fusion Reactor
Description: There are two major methods being pursued for fusion power. Magnetic confinement holds a hot plasma in a more or less steady state long enough for fusion reactions to produce net power. Inertial methods use lasers or particle beams to compress a fuel pellet to high pressure and temperature such that the fusion reaction happens very fast, after which it rapidly expands. Both methods can be applied to space propulsion. Magnetic confinement was addressed at Fusion Heated Plasma Engine, and this method addresses Inertial confinement.
A fuel pellet consists of a fusion core material, such as a Deuterium/Tritium mix, surrounded by a liner to optimally absorb the laser or particle beam energy. Optionally a fission shell surrounds the fusion core. This is similar to the arrangement of a fusion atomic bomb. Alternately particle beams can consist of fissionable heavy ions or fusible light element ions, or even highly accelerated solid pellets of inert or reactive fuels. The end result in all cases is a rapidly expanding cloud of highly energetic particles. These are directed by a shaped magnetic field or pusher plate to produce thrust. Thrust can be varied by how often you generate the explosions.
Status: Inertial confinement fusion is being researched for power generation. A chemical implosion is also the method by which most nuclear bombs are set off.
Variations: Multiple variations are possible by combining:
- Compression sources: laser, particle/ion beams, or solid pellets,
- Type of central target: none in the case of colliding pellets, fission and/or fusion fuel ingredients,
- Type of pellet liner: none or what material, and
- Thrust method: physical or magnetic nozzle
56 Alpha Particle Emitter
Type: Atomic Particles by Radioactive Decay
Description: A radioactive element coats one side of a thin sheet which is capable of absorbing alpha particles. The particles emitted into the sheet are absorbed, while the particles emitted in the opposite direction escape, providing net thrust. Advantages are this is a simple device, and heating of the sheet can produce power as a side effect via thermoelectric or heat engine. Disadvantages are low thrust, and you cannot turn off radioactivity. You would have to close two plates so they face each other to neutralize thrust, and open them like a book to turn the thrust on.
Status: Alpha emitters are well known. Using them for propulsion is theoretical so far.
Variations: Choice of radioactive material allows a wide range thrust levels.
57 Fission Fragment Engine
Type: Atomic Particles by Fission Reactor
Description: Thin wires containing fissionable material are arranged to allow the nuclear fragments from the fission to escape. They are allowed to decay naturally for low thrust devices, or arranged in a nuclear critical arrangement for high thrust. The fragments 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. The fission decay tends to damage whatever the wires are made of, so provision for replacing or reforming the wires would be needed for long term use.
Status: Currently theoretical only.
- 57a Antimatter Sail - Antiprotons are directed at a sail coated with fissionable material such as Uranium. Their negative charge allows them to be captured in the electron shell of the atoms, and then causes fission by annihilation with one of the nuclear protons. One fission fragment is absorbed by the sail, while the other moves away at several percent of the speed of light, generating thrust.
- Dusty Plasma Based Fission Fragment Nuclear Reactor Clark and Sheldon, Joint Propulsion Conference, 2005.
58 Pure Fusion Engine
Type: Atomic Particles by Fusion Reactor
Description: Various thermonuclear fusion reactors have been proposed. The results of a fusion reaction are high energy particles which can, in principle, be harnessed for propulsion. This differs from 52 Fusion Heated Plasma Engine in that no additional fuel is added beyond the fusion reactants. Instead the high temperature plasma or escaping charged particles are used directly as rocket exhaust. Since a break even fusion reaction has not been achieved as of yet, this remains a theoretical concept.
The actual method of containing the fusion plasma will influence the design of the rocket and its feasibility, in particular the mass of the device. A tokamak or stellarator plasma containment device would present the largest problems in siphoning off plasma for propulsion and their large weight would lead to an uninspiring thrust to mass ratio. Inertial confinement fusion using high power lasers or x-rays on small targets would probably be easier to build into a propulsion device, however the enormous size of the lasers and x-ray generators of current internal fusion projects would lead to the same problem with the thrust to mass ratio.
Status: Significant research is in progress for fusion in general. Applications to space propulsion are theoretical. Besides the large funding for magnetic and inertial confinement fusion, there are several alternate approaches that have lower funding levels.
- 58a Magnetic Confinement - Plasma in a chamber similar to a tokamak fusion power reactor is intentionally leaked to a magnetic nozzle.
- 58b Inertial Confinement - The fuel pellet is heated and compressed by lasers, electron beam, or ion beam. After fusing, the resulting plasma is directed by a magnetic nozzle.
- 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).
- 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.
- 58e Dense Plasma Focus - A high current discharge in a radial arrangement causes the plasma to collapse to high temperature and pressure. This is being developed by Lawrenceville Plasma Physics in Middlesex, New Jersey. This device has a reported fusion energy of 0.044 Joules vs capacitor energy of 50 kJoules. The output vs input energy ratio is a measure of how close the device is to practical operation.
- 58f Magnetized Target Fusion - Colliding plasma balls are further compressed by acoustic shock waves generated by mechanical drivers. General Fusion near Vancouver, Canada is researching this method.
- Freeman, M. "Two Days to Mars with Fusion Propulsion", 21st Century Science and Technology, vol 1, pp 26-31, Mar.-Apr. 1988.
- Kammash, T.; Galbraith, D. L. "A Fusion-Driven Rocket Propulsion Scheme for Space Exploration", Trans. Am. Nucl. Soc. vol 54 pp 118-9, 1987.
- 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.
- Inertial Confinement
- Kammash, T.; Galbraith, D. L. "A Fusion Reactor for Space Applications", Fusion Technology, v. 12 no. 1 pp 11-21, July 1987.
- 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.
- Hyde, Roderick, "A Laser Fusion Rocket for Interplanetary Propulsion" , LLNL report UCRL-88857. Topics include:
- - 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.
- Bussard, Robert W., "Fusion as Electric Propulsion", Journal of Propulsion and Power, Vol. 6, No. 5, Sept.-Oct. 1990. Abstract: 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.)
- Electrostatic Confinement
- 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. Summary: 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.
- Plasma Mantle Confinement
- Koloc, Paul M., "PLASMAKtm Star Power for Energy Intensive Space Applications", Eighth ANS Topical Meeting on Technology of Fusion Energy, Fusion Technology , March 1989.
This note is 20 years old and needs updating: 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)
- Dense Plasma Focus
- Lerner, Eric et al, Fusion Reactions from >150 keV Ions, Physics of Plasmas, v 19 issue 3, March 2012.
59 Neutral Particle Beam Thruster
Description: A high energy (order 50 MeV) particle accelerator generates a proton beam. This beam is neutralized (combined with electrons to make neutral 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 US missile defense program as a way of destroying missiles (with the beam). The energy required for space propulsion with this method exceeds the energy available from nuclear fusion, so it only makes sense with antimatter or external power sources such as a laser.
Status: Particle accelerators of this energy have existed since the mid-20th century. What limits this method is lack of a power supply, so it remains untested.
- 59a Near Lightspeed Probe - If the particle accelerator produces very high energy protons, such that relativistic mass increase is significant, and the power source is a very powerful laser located at the origin star, and focused using the star as a gravitational lens, the rocket equation no longer constrains the final velocity. Velocities close to the speed of light would be possible, and stopping would be possible by pointing the accelerator in the opposite direction. Acceleration in this case is limited by the power to mass ratio of the power conversion and particle accelerator hardware.
60 Antimatter Annihilation
Description: Atoms and anti-atoms (or their constituent particles) annihilate, producing pions, then muons, then gamma rays. The charged particles can be acted upon by a magnetic nozzle. The gamma rays can be absorbed by a container, and the resulting heat used to supply power. 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.
Status: Small amounts of antimatter have been created and temporarily stored at particle accelerator labs. At present, antimatter is not thought to exist naturally in large quantities.
- Wikipedia article: Redshift Rocket
- 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 Note: This is a 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.
- G. D. Nordley, Application of Antimatter - Electric Power to Interstellar Propulsion, Journal of the British Interplanetary Society, June 1990.
B. External Particle Interactions
Description: The magsail operates by placing a large superconducting loop in the solar wind or planetary ion stream. The current loop produces a magnetic field that deflects the ions, producing a reaction force. Because of the large area covered by the loop, which is mostly empty, it can develop a relatively large force for a given mass. It is limited in direction and strength by the local solar wind or ion flow.
Status: Magnetic field interaction with an ion stream is well understood. Practicality of this method depends on an efficient enough design. Has not been tested as of 2012.
62 External Particle Beam
Description: A remote particle beam source aims it at a target vehicle. The particles are either absorbed or reflected to generate thrust directly, or their kinetic energy is used as a power source for some other type of propulsion. The major issue is keeping the beam narrow enough to be a useful energy transfer method.
Status: Untested as of 2012
63 Interstellar Ramjet
Alternate Names: Bussard Ramjet
Description: A large funnel or inlet is used to compress interstellar gas, which is then fed to a fusion reactor 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. In other words, collecting the gas causes drag, and at the exhaust velocity of the reactor there will be no net thrust. This makes the system an efficient decelerator by pointing the exhaust forward. In that case the drag and reverse thrust both act to slow the vehicle. Variable density of interstellar gas affects the viability of this method. By running the reactor partly off of internal fuel, this type of vehicle can be brought to high enough velocity for the collector to start to function, and then to go to higher velocities. By having the reactor use less fuel than collected, it can be self-refueling.
Status: Although the concept dates back to 1960, it is untested due to a lack of practical fusion reactors.
[D77] R. W. Bussard, "Galactic Matter and Interstellar Flight", Astronautica Acta 6 (1960): 179 - 194.
[D78] A. R. Martin, "The Effects of Drag on Relativistic Spacefight", JBIS 25 (1972):643-652
[D79] N. H. Langston, "The Erosion of Interstellar Drag Screens", JBIS 26 (1973): 481-484.
[D80] 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 since lateral compression does not cause as much drag as full compression, it is not limited by the exhaust velocity. Thus speed may reach a substantial fraction of the speed of light.
65 External Fuel Supply
Description: Any propulsion system which stores all it's fuel at the start has an exponential fuel requirement as a function of velocity. If added fuel is delivered by a particle or pellet stream, either from behind or positioned ahead, that exponential requirement is turned into a linear one. This is because at each point the vehicle is only accelerating itself (plus a small amount of fuel), instead of the vehicle plus all the fuel for the entire mission. In the latter case you are using fuel early to accelerate fuel for later, which causes the exponential overhead.
Variations: Lots of variations are possible in terms of what fuel is delivered, and if it is formed into a particle stream, discrete pellets, or container tanks of fuel. Incidentally this can also be used for other types of supplies than fuel, but that belongs in the Engineering Methods section.
- "So You Wanna Build A Rocket?" at Project Rho gives quick estimates for a variety of atomic rockets -- including the Bussard Ramjet, the Nuclear Lightbulb, the Nuclear Salt Water Rocket, the magsail, etc. -- and related engineering challenges -- including radiation sheilding, space suits, etc.
- "The Relativistic Rocket" in the Physics FAQ describes some of the relativistic effects of traveling close to the speed of light, and one way to calculate "How much fuel is needed?".