AAWSAP DIRD, Aneutronic Fusion Propulsion I, November 2010



        


             
   
     

     UNCLASSIFIED/,'P81t 8ffl01ilrt YSE 8HLlf Aneutronic Fusion Propulsion The Defense Intelligence Reference Document provides nonsubstantive but authoritative reference information related to intelli ence to ics or methodolo ies. Prepared by: Technology Warning Division (DW0-4) Defense Warning Office Directorate for Analysis Defense Intelligence Agency Author: AAP Person 83 Administrative Notes: (U) COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized. This product is one in a series of advanced technology reports produced in FY 2010 under the Defense Intelligence Agency, Defense Warning Office's Advanced Aerospace Weapon System Applications....--....... (AAWSA) Program. Comments or questions pertaining to this document should be addressed to ( · ---14A-.E..!'erson IAAP Person 1 j AAWSA Program Manager, Defense Intelligence Agency, ATTN: JUIAF - DI/DWb-3,i'- • "'Bfclg 6000, Washington, DC 20340-5100. UNCLASSIFIED//FQA QFFIGltliL W&& 9HL¥ UNCLASSIFIED//POlt Offl@IAL WS& &NkY Contents Introduction ............................................................................................................v Chapter 1: Theory ................................................................................................. 1 Rocket Propulsion .............................................................................................. 1 Comparison of Specific Impulse for Various Rocket Designs .............................. 5 Radiation Shielding ............................................................................................ 6 Subatomic Particle Mass, Velocity, and Energy .................................................. 9 Nuclear Fission Rockets .......................................................................................................................... 10 Chapter 2: Nuclear Fusion Rocket Design ............................................................ 13 Classic Nuclear Fusion Schemes ....................................................................... 13 Fusion Initiation Methods................................................................................. 14 Gravitational Confinement ............................................................................... 16 Magnetic Confinement Fusion (MCF) ................................................................ 16 Inertial Confinement Fusion (ICF) ................................................................... 16 Muon-Catalyzed Fusion .................................................................................... 17 Cavitation (Bubble) Fusion ............................................................................... 17 Rocket Design Using Fusion Energy ................................................................. 17 Chapter 3: Aneutronic Nuclear Fusion Schemes .................................................. 18 Chapter 4: Antimatter Propulsion ........................................................................ 20 Chapter 5: Aneutronic Fusion Propulsion Projects............................................... 21 Nuclear Pulse Propulsion ................................................................................. 21 Other Aneutronic Rocket Designs ..................................................................... 22 Commercial Development....................................................................................22 Chapter 6: Speculation on Research Needs Over the Next 30 Years ..................... 25 Surface to Low-Earth Orbit (100 miles) .................................................................................................. 26 LEO to Mars (34 to 249 million miles) .............................................................. 27 UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY ii UNCLASSIFIED//POR. Offl@IAL WSli Qptk\f LEO to the Moons of Jupiter and Saturn (460 to 940 million miles) .................. 28 LEO to Alpha Centauri (4.22 light-years or 24.8 trillion miles) ......................... 28 Chapter 7: Conclusions........................................................................................ 29 Appendix A: Relativistic Rockets ......................................................................... 30 Appendix B: Aneutronic Fusion Rocket ................................................................ 32 Appendix C: Antimatter Annihilation Rocket........................................................ 36 Appendix D: Relativistic Rocket Worksheet ......................................................... 38 Appendix E: Endnotes.......................................................................................... 41 Figures Figure 1. Liquid-Fueled Chemical Rocket. ....................................................... vi Figure 2. Mass Ratio Increases as Objects Approach Speed of Light ................ 2 Figure 3. Spherical Radiation Shield Surrounding Point Source ....................... 8 Figure 4. Nuclear Fission Rocket Design ........................................................ 11 Figure 5. Schematic Design of a Magnetohydrodynamic (MHD) Generator ..... 13 Figure 6. Nuclear Fusion of Deuterium and Tritium ....................................... 14 Figure 7. Low-Temperature Fusion Reactions................................................ 15 Figure 8. Fusion Ignition Energies for D-T, D-D, and D-He3 ........................... 15 Figure 9. Aneutronic Fusion Schemes ............................................................ 18 Figure 10. Fusion of Hydrogen-1 and Boron-11 Produces 3 Alpha Particles .. 19 Figure 11. QED Rocket Design ....................................................................... 23 Figure 12. Time Dilation at Relativistic Velocities .......................................... 31 Figure 13. Maximum Achievable Velocity Versus Fuel Usage ......................... 33 Figure 14. Acceleration and Rocket "Clock" Ratio Vs Mission Time on Earth .. 33 Figure 15. Spreadsheet for Sample Mission to Alpha Centauri........................34 Figure 16. Antihydrogen Atom........................................................................36 UNCLASSIFIED/fF9A 9FFI&I:.l.k W&li Qptk\f iii UNCLASSIFIED//POlt Offl@IAL WS& &NkY Tables Table 1: Specific Impulse for Various Rocket Engine Types ............................. 3 Table 2: Rest Mass of Various Subatomic Particles ........................................ 10 Table 3: Specific Impulse for Selected Drives ................................................ 35 UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY iv UNCLASSIFIED//POlt Offl@IAL WS& 9Nk¥ Aneutronic Fusion Propulsion Introduction Space exploration is limited by existing propulsion technology. Up to now, chemical rockets have been used to reach low-Earth orbit, the Moon, and the outer regions of the solar system. Chemical rockets can use either solid or liquid fuel. Regardless of the type of fuel, their design is similar to that shown in Figure 1. Oxygen is combined with hydrogen or a hydrocarbon fuel in a combustion chamber where high temperatures and pressures cause the exhaust to be ejected through a supersonic nozzle to provide thrust to the rocket. The momentum of the fuel ejected through the nozzle provides the force or thrust that accelerates the rocket forward. There are many variations of chemical rockets, but they all suffer from the need to carry copious amounts of fuel. Other methods have been proposed to decrease the need to carry such a significant mass of fuel into space. Ion drives, for example, are used to provide the very low thrust required to maintain satellites in Earth orbit. The "fuel" that they carry is xenon gas accelerated by electric fields. Nuclear fission propulsion has been proposed for space missions, and thermal nuclear fission reactor rockets were constructed and tested at the Nevada Test Site through Project Rover between 1956 and 1971. 1 In these rockets, a nuclear reactor provides heat to liquid hydrogen through nuclear fission and ejects the hydrogen gas through a Laval nozzle to generate thrust. While these rockets must still carry hydrogen fuel as a propellant, these rockets can provide more than twice the performance of chemical rockets by using heat through fission rather than reactive chemicals. The results of the 72 reactor tests conducted under Project Rover were very promising and culminated in the successful 12-minute test of the Phoebus-2A NERVA (Nuclear Engine for Rocket Vehicle Application) reactor that generated over 4 gigawatts of thermal power. One problem associated with nuclear fission rockets is radioactive contaminants. These contaminants in the exhaust make this technology impossible to use in launching payloads from Earth. Additionally, for applications in space, radiation protection must be provided for the crew by adding heavy shielding materials or by locating the crew as far as possible from the reactor propulsion system. Nuclear fusion, as opposed to fission, provides another potential propulsion technology. In a fusion propulsion system, isotopes of light elements are fused together under extreme conditions to form heavier elements, releasing large amounts of thermal energy. This thermal energy can be used to heat liquid hydrogen to high temperatures and expand it through a Laval nozzle to provide thrust in a manner similar to that shown in Figure 1. Typically, isotopes of hydrogen and helium would be used in fusion propulsion systems. Deuterium is an isotope of hydrogen and can be separated from the hydrogen in water. Fusion reactions are difficult to initiate due to the high temperatures and pressures required. Thermonuclear bombs, for example, combine a fusion device with a nuclear fission bomb to provide the high temperatures required to initiate the fusion reaction. Regardless of the conditions required to induce nuclear fusion, the energy release is large. From propulsion standpoint, an advantage of fusion over fission is that for a given amount of thrust, the fusion reaction requires less fuel than either fission or chemical propulsion systems. Fusion reactors using deuterium or tritium fuels are easiest to initiate; however, they generate significant amounts of neutron radiation. This is a hazard for the crew on a UNCLASSIFIED/fF9A 9FFI&I:.l.k W&li 9Nk¥ V UNCLASSIFIED//POR. Offl@IAI:: ~S& Qptk\f space ship, and there is a high probability that neutrons produced by fusion reactors would escape into space without providing much of their energy to a hydrogen propellant. Other fusion reactions using isotopes of helium and lithium will generate only charged particles, such as protons, that travel very short distances before giving up all of their energy as heat. These "aneutronic fusion" reactions take place without neutron production and decrease the need to carry large amounts of radiation shielding material for the crew. The energy from charged particles generated by aneutronic fusion can also be captured in conductive coils and converted directly into electricity. Aneutronic fusion promises to be an important mechanism for future space propulsion, although novel accelerator or laser systems must be researched and developed in order to initiate, sustain, and control the fusion reaction. Another futuristic method of spacecraft propulsion involves the use of antimatter. Antimatter includes antiprotons, antineutrons, and positrons (anti-electrons). Although this propulsion process may be the most efficient, antimatter has some drawbacks. For example, antimatter is generated in only minute quantities at accelerator facilities around the world. Although it has been captured and stored, containment remains a problem. When antimatter combines with matter, it completely annihilates and converts to energy, which then can be converted into heat for a propulsion system. Antimatter reactions provide the greatest amount of energy per unit mass of any potential fuel, but the ability to generate significant quantities of antihydrogen or similar antimatter fuels at any accelerator facility is very limited. The focus of this study is on aneutronic fusion propulsion. Integral to this study are the topics of fuel, rocket design, and the organizations that research aneutronic fusion development. Uquld H, LOX I.Jwal- Figure 1. Liquid-Fueled Chemical Rocket. UNCLASSIFIED/fF&A &FFI€il.t.k W&& Qptk\f vi UNCLASSIFIED//POlt Offl@IAL WS& &NkY Chapter 1: Theory ROCKET PRO PU LSION It is difficult to compare propulsion technology without talking about how objects are accelerated in space. Within Earth's atmosphere, aircraft use the air to generate lift and thrust. Propellers or turbofans move a mass of air rearward and Newton's second and third laws require that the momentum in this exhausted air is equal to a thrust in the opposite direction. In equation form, the thrust, F, is equal and opposite to the change in momentum over time. m d(mv' F =- _ ' / e ..x.lrausr ( 1.1) dt The momentum of the exhausted air is equal to the mass of air times its velocity and is provided by the propulsion system. The thrust can be used to accelerate a payload according to the following equation: w m F = m payload a (1.2) Here, thrust is equal to the payload mass times its acceleration. This method of momentum transfer works well for aircraft operating within the Earth's atmosphere; however, operating in space presents special problems. Space is nearly a complete vacuum, and there is no air mass to accelerate, i.e., no "reaction mass" that can be accelerated and exhausted at high speeds. In space, the reaction mass is carried by the rocket in the form of propellant mass, which is expended as the rocket accelerates. m m dm m F = ma +-V (1.3) dt In this equation, the thrust is provided by the momentum ejected from the rear of the rocket, but the total mass of the rocket is decreasing as the fuel is burned up and as propellant is lost. Examining equation 1.3, we see that there are two ways to increase rocket thrust. The first is to increase the mass flowrate, dm/dt, typically measured in kg/s (if mass flowrate is given in kg/s, then thrust, F, is given in newtons to maintain consistency). Unfortunately, this requires carrying increasing quantities of fuel. For flights to Mars, the outer planets, or to other star systems, it would not be possible to carry such large quantities of propellant. A second choice would be to increase V, i.e., the velocity of the ejected propellant reaction mass. There is an upper limit, however, to how fast we can eject the propellant. In his Special Theory of Relativity, Albert Einstein demonstrated that no object that has any mass when at rest can be accelerated beyond the speed of light or 2.998 x 108 m/s in a vacuum. Einstein's relationship between an object's mass (m), its velocity (v), and the speed of light, (c) is given in equation 1.4: UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 1 UNCLASSIFIED//POlt Offl@IAL WS& &NkY (1.4) m=g ?­ 9 8 7 ' 3 2 I / - ..--- 0.2 0.4 0.6 0.8 v/c Figure 2. Mass Ratio (m/mo) Increases as Objects Approach the Speed of Light. Equation 1.4 is plotted in Figure 2; we see that as an object's velocity approaches light speed its mass approaches infinity. Coupled with this, we see that as mass approaches infinity, the energy required to move the mass also approaches infinity. Thus, it would take an infinite amount of energy to accelerate an object to the speed of light. Equations 1.2 and 1.3 indicate that the best propulsion system would use the least amount of propellant but exhaust it at the highest possible velocity. Therefore, the rocket engine that has the highest exhaust velocity requires that the rocket be the least massive or carry the least amount of propellant. Unfortunately, this combination corresponds to the minimum efficiency in terms of rocket power. The point here is that for spaceflight to anywhere other than near destinations, our present rocket technology is insufficient. UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 2 UNCLASSIFIED//POlt Offl@IAL WS& &NkY Specific Impulse To compare various propulsion systems and their fuel, rocket efficiency can be represented by the amount of momentum that can be obtained per unit weight of the propellant that is used. This is defined as the "specific impulse" (lsp) and is measured in units of seconds: I = Li(m v • . ,1w 11s,) = v ,x/11111S/ (1.5) ,.,, ( ) • Li ni gearth g ea1·1/t Here, earth's gravitational acceleration, gearth, is 9.81 m/s2. As described earlier, the best propulsion system is typically one that has the highest possible exhaust or propellant velocity. High specific impulse, or correspondingly high exhaust velocity, also produces low energy efficiency. Table 1: Specific Impulse for Various Rocket Engine Types En<1ine Tvpe Ve (m/s) Isa (s) Saturn V Rocket 2nd and 3rd Staqes 4 130 421 LH2/LOX Liquid Fuel Solid Rocket 2,500 255 Best Chemical Rocket Tested 5 320 542 F2/Li/H2 Chemical Rocket Nuclear Thermal Rocket 8 340 850 Ion Thruster 29 000 3 000 VASIMR 290,000 30,000 The efficiency of a rocket can be defined as the ratio of rocket thrust to the power required to generate the thrust. The resulting equations follow: Power = dE 2 (1.6) = ½(dm) V dt 2 dt e Thrust = (1.7) (dm)V dt ' UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 3 UNCLASSIFIED//POlt Offl@IAL WS& &NkY r, = ( Thrust) = ]_ (1.8) Power Ve In choosing a rocket engine, as the specific impulse (Isp ) or the exhaust velocity (Ve) increase, the corresponding efficiency (TJ) decreases. The specific impulse for various fuels varies widely as shown in Table 1.2, 3 Tsiolkovsky Rocket Equation For a spaceflight to Mars or other body, the exhaust velocity affects the time of flight and the amount of fuel that is needed to change the velocity of the spacecraft. The change in velocity (t-.V) can be expressed in terms of the initial mass of the rocket (m ;n;t1a1, rocket+ propellant) and the final total rocket mass (mt;na1 rocket only): ~ V = V In (minilia/) (1.9) exhaust mfinal The mass of propellant used to generate thrust is given by: mpropellanr = minirial - mfinal (1.10) (1.11) m - m e~V / Vt'.r:lr ,rns r (1.12) ~ V = gO Jsp [In [ m i11i1ial]- ..!_ (1 _mfina l J] (1.13) fina l - 'initial mfi11a/ R mi11irial (1.14) In these equations, R is the thrust-to-weight ratio and t is the time required for an engine burn to produce a desired t-.V. These are the standard equations used for rocket engine performance. UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 4 UNCLASSIFIED//POlt Offl@IAL WS& &NkY COMPARISON OF SPECIFIC IMPULSE FOR VARIOUS ROCKET DESIGNS Chemical Rockets Chemical rockets burn solid or liquid propellant. The exhaust gas exits the rocket through a Laval nozzle and generates thrust. Figure 1 provided the outline of a liquid­ fueled rocket using liquid hydrogen as the fuel and liquid oxygen (LOX) to combust the fuel. Unlike turbofan and RAM engines, rockets are not airbreathing and must carry their own oxidizer. The Laval nozzle is a principle component of chemical rockets; its design is based on compressible fluid flow theory. 4 In general, the nozzle is made up of contoured convergent-divergent cross sections. Conical cross sections are also sometimes used. Its purpose is to transform pressure energy into kinetic energy. Nuclear fusion rockets may also make use of Laval nozzles by heating up a liquid propellant and ejecting it as a supersonic gas. In subsonic flow, fluid can only be accelerated by decreasing the cross-sectional area of the duct section t hat it is traveling through, as in a Venturi tube. Once the velocity in a fluid reaches the speed of sound (Mach 1), the fluid can continue to accelerate only if it is expanded. The Laval nozzle combines a converging section where the flow is subsonic, a throat where the flow is accelerated to sonic speed (Mach 1), and a diverging cone where the flow is accelerated to supersonic speed. The performance of a rocket is based on its thrust, where T = (dm/dt) x Vexhaust, and by maximizing the exit velocity, the thrust reaches a maximum. The pressure of the combusting gases in the combustion chamber directly affect the amount of thrust that the rocket can achieve. Whether a rocket is propelled by gases from combusting propellant or by gases heated through a nuclear fission or fusion reaction, two equations determine the thrust of the Laval nozzle. The maximum mass flow rate through the nozzle can be computed in terms of the nozzle area (Athroat) the combustion or heated gas pressure (po), and the heated gas temperature (To). ( d mJ = Po ~hroat (1.15) d t _ 'T mu1m11m --..Jl. o V ­ (1.16) exhaust ­ In these equations, R is the gas constant of the propellant gas and y is a thermodynamic property of the gas called the ratio of specific heats. These equations make it possible to compute the maximum thrust generated by a propellant gas through a Laval nozzle based on the pressure and temperature of the gas in the combustion chamber or heating tank. 5 UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY UNCLASSIFIED//POlt Offl@IAL HS& QptkY Ion Drives Ion thruster electrical propulsion provides a convenient and efficient method of generating thrust. A gas that is easily ionized, such as xenon, is carried onboard as a propellant. The voltage difference between an electrode and a metal screen accelerates xenon ions toward the screen and out the back of the spacecraft, generating thrust. The specific impulse of this kind of drive is about 3,000 seconds. It is relatively common for satellites to use ion drives, generating minute forces measured in millinewtons, to maintain orbit. Solar energy and radioactive decay are possible sources of electric power for satellites in Earth orbit. In deep space, fusion or fission reactors could provide electrical power. However, once the xenon propellant has been expended, the ion drive is no longer useful. Ion drives include the VASIMR (Variable Specific Impulse Magnetoplasma Rocket) designed by the Ad Astra Rocket Company in Webster, Texas. This system uses two RF radiowave antennae to couple energy into an ionized gas that is used for propulsion in space. While ion drives are often used to help maintain orbit for satellites circling the Earth, the VASIMR technology has been proposed for use in moving payloads throughout the solar system. The specific impulse for this technology is cited as 5,000 seconds compared to ~3,300 seconds for typical ion drives. 5 Photonic Propulsion Photons of visible light, infrared radiation, or x-rays can produce thrust through momentum transfer, where the momentum of each photon is given by p = h/A (h = Planck's constant, A = radiation wavelength). Photonic propulsion has been explored by Y. K. Bae Corporation (http://www.ykbcorp.com) who holds a patent on a photonic laser thruster. These drives generate no contaminants and require a source of electricity to produce photons. Their photonic laser thruster (PLT) uses an active resonant optical cavity formed between two mirrors on a pair of spacecraft to generate thrust. Photonic drives would be viable on fusion or fission -powered spacecraft if their power output were used to generate electrical power that could provide light. RADIATION SHIELDING Radiation shielding will be important for astronauts traveling to the Moon, to the other planets, and to other star systems. Radioactive particles and cosmic rays left over from the big bang, radiation from supernovae, x-ray emissions from black holes, and a constant flux of energetic protons from our own sun all contribute to the radiation dose received by humans in space. Radiation levels are typically measured in sieverts (Sv), expressing the amount of energy deposited in human tissue from radiation. One sievert is equivalent to 1 joule of energy absorbed for every kilogram of tissue. 6 • 7 An older unit, the rem (Roentgen equivalent, man) is still in common use: 1 rem = 0.01 Sv. Sieverts are now the international standard unit. UNCLASSIFIED/fF&A 9FFI€il.t.k Y&li QptkY 6 UNCLASSIFIED//POlt Offl@IAI:: HS& 8Ptk¥ The energy absorbed is strongly related to the amount of radiation damage done to the tissue. On Earth, the magnetic field of the planet helps to shield people from most of the effects of radiation from the sun, but cosmic radiation and terrestrial sources of radiation (granite, potassium, radon gas) all contribute to an annual background dose that everyone receives. The average annual dose of radiation in the United States is about 2.5 mSv from background and another 1.0 mSv from other source, such as dental x-rays, commercial jet flights, and radiopharmaceuticals. The total annual dose in the United States is approximately 3.5 mSv (millisieverts) per person. In space, away from the protection of the Earth's magnetic field, the radiation dose increases substantially to about 250 mSv per year. The radiation dose in space is continuous, and the effects of being in space for extended periods of time may be cumulative. As a comparison, 2,000 mSv of radiation in an acute dose can cause significant medical problems and 5,000 mSv is usually fatal. Leukemia and other forms of cancer are possible for people exposed to chronic doses of radiation at the levels encountered in space. The logical conclusion would be to carry radiation shielding into space to protect the astronauts. The problem is that shielding is typically heavy and expensive. Four types of radiation must be shielded: 1. Gamma Rays (y) These are energetic forms of electromagnetic radiation (photons) and tend to penetrate most materials. High-density metals, such as iron, lead, and uranium are usually used to shield gamma rays. 2. Beta Particles (~+, W) These are electrons or positrons, the antimatter counterpart to electrons. They are emitted by the radioactive decay of certain isotopes and through nuclear fission. Because these are charged particles, they are fairly easy to stop with minimal shielding. 3. Neutrons (n) These uncharged particles are generated by nuclear fission and fusion. They may penetrate metals, yet they can be slowed down until they decay in light materials that contain hydrogen or carbon. Typical shielding material includes water, paraffin wax, and polyethylene blocks. 4. Heavy Charged Particles (p, a) Ions are atoms that have one or more of their electrons stripped from their outer orbital. Due to their positive electric charge, ions are generally easy to stop within any kind of material, unless the ions are very energetic. Typical ions include protons, which are ionized hydrogen atoms, and alpha particles, which are ionized helium nuclei. Cosmic radiation includes heavy ions emitted by exploding supernovae and may include ions as heavy as iron nuclei at extremely high energy. UNCLASSIFIED/fFOA OFFI&I:.l.k W&& ONk¥ 7 UNCLASSIFIED//POR. Offl@IAI:: ~S& Qptk\f Long-duration spaceflights will require copious amounts of water for the crew, and water can be used to provide some shielding from neutrons for the astronauts. Shielding material for gamma rays presents a weight problem. Lead is one of the best shielding materials for gamma, but at a cost of about $10,000/lb to launch material into space, lead shielding is expensive to use. The International Space Station and other spacecra~ designed for long-term human habitation usually have a small area that is heavily shielded to prevent excessive radiation exposure to the crew during solar events. In addition to the dangers of natural sources of radiation in space that can endanger human health and safety, the propulsion techniques of nuclear fusion and fission generate large fluxes of radiation. Neutron production is of special concern because neutrons can penetrate metals and the structural material of space habitats. The general equations that govern radiation shielding can help develop spacecraft designs that will minimize radiation exposure. The intensity of gamma rays will attenuate according to the following equation: A..()-A.. - µI' ( 1.17) 'f-' r - 'f-'i11irial e In equation 1.17, the flux of gamma rays or neutrons, given in particles per unit area per unit time, is represented by <P(r); ¢ 1n1t1a1 represents the initial flux without the shielding; and µ is the linear attenuation coefficient, a function of the gamma ray or neutron energy and the type of shielding material. The thickness of the material is represented by r. The radiation flux decreases with distance since photons or radioactive particles typically expand outward through a spherical area of 4rcr2 as shown in Figure 3. Equation 1.18 shows the relationship between total attenuation, particle flux, and radiation exposure. Figure 3. Spherical Radiation Shield Surrounding a Point Source. UNCLASSIFIED/fF&A &FFI€il.t.k W&& Qptk\f 8 UNCLASSIFIED//POR. Offl@IAI:: ~S& Qptk\f e - p r ¢ (r) = </J;,,;,;,,1- 2 B(µ r) (1.18) 4 Tr r The radiation flux is inversely proportional to the square of the distance from a point source of radiation, such as a nuclear rocket engine. It also decreases through any intervening radiation shielding material. The last term in equation 1.18, B(µr), is called the "buildup factor"; it represents the process of reradiation following atomic collision with shielding material, thus contributing to the total radiation dose. This secondary radiation is a problem for all spacecraft since cosmic radiation impacting the spacecraft structural material can produce a cascade of secondary particles that can irradiate the crew. A standard technique to decrease the radiation exposure to the crew on a spacecraft using nuclear fusion as an energy source will be to locate the crew as far away from the engine as possible and place as much liquid hydrogen or other light shielding material between the crew and the engine as designs allow. A simpler solution would be to use nuclear fusion schemes that do not generate neutrons. These are the so-called "aneutronic fusion" propulsion techniques. SUBATOMIC PARTICLE MASS, VELOCITY, AND ENERGY Atoms are composed of a small nucleus containing neutrons and protons, along with electrons orbiting the nucleus in shells. The atomic number (Z) is equivalent to the number of protons or electrons in a stable atom.8 The atomic mass number (A) is the total number of neutrons and protons in the nucleus. The number of neutrons (N) can be found by subtracting Z from A. Atoms or nuclei are represented by a standard nomenclature based on A and Z. ; Atom Since chemical properties are governed by how many electrons circle the nucleus, Z defines the element and atoms with the same value of Z, but differing numbers of neutrons are referred to as "isotopes" of the same element. Some common isotopes of hydrogen are shown below: hydrogen I IH deuterium ~H , or :D tritium iH , or ~T Subatomic particles, such as a, /3, and neutrons, have a mass described in atomic mass units (amu). One amu is defined as the mass of one atom of carbon-12, and it roughly represents the mass of one neutron or proton. In terms of amu, the mass of various particles are included in Table 2. Due to relativistic effects, particle mass increases as the velocity of the particle approaches the speed of light, and because of special relativity, the mass of particles listed in the table is the "rest" mass corresponding to a particle that is not moving. UNCLASSIFIED/fF&A &FFI&I:.l.k W&& Qptk\f 9 UNCLASSIFIED//POlt Offl@IAL WS& &NkY Table 2: Rest Mass of Various Subatomic Particles Subatomic Particle Particle Name Mass (amu) a alpha 4.001506 13 beta 0.000549 ri neutron 1.008665 p proton 1.007276 D deuterium 2.014102 T tritium 3.016049 U-235 uranium 235.0439 Gamma rays, or photons, have no rest mass and only move at the speed of light (c = 3 x 108 m/s). Photons do have an effective mass since their momentum is given by p = me = h/A where h is Planck's constant (h = 6.626 x 10-34 J·s) and ,.\ is the wavelength of the photon. Particles also have energy, given by E = mc2 . Mass (m) was defined in equation 1.4. Particle or photon energy is usually expressed in terms of electronvolts (eV). Typical powers of eV are also used, including keV (1,000 eV) and MeV (1 million eV). For reference, 1 eV = 1.602 x 10-19 joules. NUCLEAR FISSION ROCKETS During the Cold War, the United States and the USSR developed designs for intercontinental ballistic missiles to carry nuclear weapons. Conventional rockets used highly reactive chemicals and, as an alternative power source, the use of nuclear fission reactors was explored. Project Rover, supervised by the U.S. Atomic Energy Commission and the U.S. Air Force, developed specialized fission reactors with hydrogen propellant. A number of nuclear rockets were built and tested at the DOE Nevada Test Site (NTS) Area 25 in the 1950s. Nuclear fission reactors require a "moderator" to operate. The moderator slows down neutrons generated by fission and absorbs their energy. Hydrogen is nearly the perfect moderator and was chosen as the moderator and the propellant in the construction of the NERVA (1) rockets under Project Rover. Figure 4 shows the main components of a nuclear fission rocket. UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 10 UNCLASSIFIED//POlt Offl@IAL WS& &NkY ---+- Payload liquid H, Tank Turbopump Nuclea r Fuel Assembfy Figure 4. Nuclear Fission Rocket Design. Nuclear fission rockets had numerous problems. The fission of uranium-235 emits about 200 MeV for every nucleus that undergoes fission. Approximately 11% of this energy is in the form of neutrinos and is unrecoverable. Approximately 4.8 MeV shows up as kinetic energy in the two or more neutrons that are created for every fission. At least one neutron must be absorbed by another U-355 nucleus and cause fission in order for a chain reaction to be sustained. Most of the energy goes into the kinetic energy of large fission fragments that are created by the breakup of the U-235 nucleus. Fission products are highly radioactive and may be ejected out with the rocket exhaust. Neutrons pose a radiation hazard to any human close to the rocket when it operates. In the tests of the NERVA series of rockets, on at least one occasion, pieces of radioactive material were ejected over a small region of the Nevada Test Site and had to be manually retrieved. On the positive side, the NERVA rockets created large amounts of thrust, and the energy within the reactor was more than sufficient to send its payload to the desired location in the USSR. This prompted scientists to consider the use of thermal fission reactors for use in space exploration, although the persistent problem of radiation exposure to the crew remained unresolved. Two classical designs were proposed. In one design, a reactor would be constructed as shown in Figure 4, and liquid hydrogen propellant would be passed through the reactor to create a supersonic exhaust and to provide thrust. The hydrogen fuel would be located between the reactor and the crew to serve as a radiation shield for neutrons produced during fission. The spacecraft would be elongated to move the crew as far away as possible from the reactor, taking advantage of the 1/r2 attenuation of radiation with distance from a source. UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 11 UNCLASSIFIED//POlt Offl@IAI:: HS& 8Ptk¥ In a second design, a nuclear reactor would be used to generate electricity onboard a spacecraft. The electrical power would provide energy for life support and much of the reactor output could be used to power an ion drive where high voltages would accelerate an ionized gas (usually xenon) to generate thrust. The thrust generated by ion drives is typically small (millinewtons), but continuous acceleration could provide enough velocity to reach Mars or the outer planets. Nuclear reactors have been used on spacecraft in the past, although their use is controversial. On 24 January 1978, for example, the Cosmos 954 Soviet spy satellite, complete with onboard plutonium-fueled nuclear reactor, crashed into the Arctic region of Canada.9 The cleanup cost the Canadian government over U.S. $6 million, half of which was reimbursed by the USSR. The launch of a reactor into space always poses the danger of problems related to an accidental reentry that could cause significant hazards to populated areas. UNCLASSIFIED/fFOA OFFI&I:.l.k W&& ONk¥ 12 UNCLASSIFIED//POlt Offl@IAL HS& QptkY Chapter 2: Nuclear Fusion Rocket Design CLASSIC NUCLEAR FUSION SCHEMES Nuclear fusion, which powers the Sun and the stars, begins with the collision of two lightweight atomic nuclei to create two new particles with the release of energy. As an example, if two specific isotopes of hydrogen (tritium and deuterium) were to collide, the reaction would produce a neutron plus an alpha particle (ionized helium nucleus). (2.1) The 17.6 MeV of energy is split between the kinetic energy of the neutron (14.1 MeV) and the helium nucleus (3.5 MeV) based on conservation of energy and conservation of momentum. The kinetic energy is eventually converted into heat in a fusion reactor. The 14.1-MeV neutron will penetrate far into lead or steel shielding and can cause considerable material damage. The ionized helium nucleus, however, will not go very far through any material without being absorbed and dissipating its energy as heat. There is a novel way to capture the energy from the ionized nucleus. As shown in Error! Reference source not found., a magnetohydrodynamic (MHD) generator can b e used to harness the energy from the helium ions and convert it directly into electricity. The electricity could be used to power an ion drive on a spacecraft or provide power for life support. Equation 2.2, known as the Lorentz force equation, illustrates which parameters are involved and how they are related: F =e(Vx B) (2.2) Electodes on the Top and Bottom of the Channel Carry Electrical Current Away Di rection of the Magnetic Field, B Path of Ions Between the Magnets Generate the Magnetic Field Figure 5. Schematic Design of a Magnetohydrodyanamic (MHD) Generator. The velocity (V) of the ions interacts with the magnetic field (B) and forces positively charged ions to move downward in the channel to an electrode where they impart an UNCLASSIFIED/fF&A 9FFI€il.t.k Y&li QptkY 13 UNCLASSIFIED//POR. Offl@IAI:: ~S& Qptk\f electric current. The ion charge is given in the equation as e. MHD generators have been proposed for highly efficient generation of electricity from the combustion of coal, for example. On a spacecraft, MHD generators can generate power from both ions and electrons, which deflect in opposite directions due to the magnetic field. Even if the ion energy cannot be converted directly into electricity, neutrons or ions can be used to heat up a propellant gas to provide thrust. Hydrogen gas would be the most efficient propellant for fusion reactions producing neutrons because the neutron energy is easily absorbed through collisions with the hydrogen nuclei. Propulsion fusion reactors, however, still generate radiation, including neutrons, which pose a health hazard for the crew of any spacecraft. By carrying hydrogen propellant and locating the crew as far away as possible from the fusion reactor, some degree of shielding is possible. While fusion reactors have the potential to produce incredible amounts of energy from relatively inexpensive fuel (deuterium, tritium, helium-3), the problems of initiating, controlling, and sustaining the fusion reaction remain unsolved. FUSION INITIATION METHODS There are many possible fusion reactions that extend all the way up from hydrogen to the actinides (uranium). In each case, the two ions that "fuse" must collide to form a new nucleus that rapidly decays with the release of fusion energy, as shown in Figure 6. Both ions, however, are positively charged and tend to repel each other due to Coulombic repulsion: / He+ 3.5 MeV n + 14.1 MeV Figure 6. Nuclear Fusion of Deuterium and Tritium. The Sun emits vast amounts of thermal energy through the fusion of hydrogen isotopes; it overcomes Coulombic repulsion through the high pressures and temperatures that exist in its interior. High temperatures create high ion velocities and high-velocity collisions are more likely to cause two ions to fuse together. Controlled fusion reactions are difficult to achieve due to the temperatures required to initiate the process. The reactions that occur at the lowest temperatures are listed in Figure 7, including the D-T reaction which was discussed earlier. UNCLASSIFIED/fF&A &FFI&I:.l.k W&& Qptk\f 14 ---- --- UNCLASSIFIED/ /PO lt Offl@IAL WS& &NkY i2D + i3T ➔ 01n [14.1 MeV] + 24He [3.5 MeV] i2D + /D ➔ 0 1n [2.45 MeV] + 23He [0.82 MeV] i3T + i3T ➔ 2 x o1n + 24He + [11.3 MeV split between the neutrons and helium] Figure 7. Low Temperature Fusion Reactions. l E+-OJ ----------------------­ 1.E+OO +-----1-------'---------,....-----+----+-­ , 1 - - D-T ... ,,,, - D·D ,,,, ,,,, - • D-He3 1.E--01 +-----+-----+----------+------+-­ I C ! ~ C 0 -·­ ~ 1.E-02 e ~ V 1.E-04 -~ --+-----1_,_______________ 1 l .E-0.5 ,.___.__ I I · ' . ,,,, ,,,, " I,.,, . I - 20 40 60 80 100 Deuteron Energy(keVI Figure 8. Fusion Ignition Energies for D-T, D-D, and D-He3. Figure 8 shows the probabilities of these reactions occurring, expressed in units of barns (1 b = 10-28 m2). The D-T reaction becomes increasingly probable as the energy of the deuterium nucleus (deuteron) reaches about 5 keV. In terms of temperature, this equals about 10 million degrees kelvin. At 10 keV (100 million degrees kelvin), the two D-D reactions listed in Figure 7 become more probable and D-He3 fusion becomes viable at 30 keV (300 million degrees kelvin). There are several ways to achieve these temperatures in a controlled manner. For example, Edward Teller and Stanislaw Ulam developed a design for a fusion, or thermonuclear bomb, that used a plutonium fission atomic bomb to reach the ignition temperature for fusion. Deuterium is the most useful fuel for low-temperature fusion reactors, and there is a limitless supply available by centrifuging ordinary tap water to separate out heavy water (D2O). Because of deuterium's availability, a strong incentive exists to develop manageable ways to initiate nuclear fusion, concentrating on the fusion schemes outlined in Figure 6; UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 15 UNCLASSIFIED//POlt Offl@IAL WS& &NkY these methods are discussed below. Fusion reactors for the production of electricity have been a goal for over 50 years, yet no method has yet achieved "break even," where the amount of energy generated by fusion exceeds the energy required to initiate the fusion process. Fusion methods are often compared based on their ability to "break even." GRAVITATIONAL CONFINEMENT To initiate fusion, ions of hydrogen or its isotopes must be heated to high enough temperatures to increase the likelihood of a fusion reaction occurring during a collision. Another method is to increase the pressure of an ionized gas to a point where the number of collisions increase with an enhanced possibility of a fusion collision . This is the mechanism that stars employ to initiate fusion; for example, if the object were completely composed of deuterium, the minimum mass needed to generate gravitational pressures sufficient to initiate fusion would be equivalent to the mass of the planet Jupiter. MAGNETIC CONFINEMENT FUSION (MCF) Deuterium and other ions follow lines of magnetic flux, and tokamaks have been used to form a magnetic field in the shape of a torus to contain a plasma containing ions for fusion. Tokamaks contain powerful electromagnets that generate the magnetic field. Secondary electromagnets induce an electric current into the plasma to heat it to ignition temperature (ohmic heating). Other methods have been employed to heat the plasma, including the introduction of radiofrequency energy, magnetic compression, and neutral beam injection. INERTIAL CONFINEMENT FUSION (ICF) The Teller-Ulam thermonuclear bomb was an example of inertial confinement where x­ ray radiation pressure from a fission explosion is used to compress a mixture of deuterium and tritium to initiate fusion. The National Ignition Facility in Livermore, California, is an example of a laser-based ICF system. In this facility, a 287,000-lb, 10-meter-diameter target vacuum chamber is equipped with a small metal cylinder, or holraum, that contains a 2-mm pellet of D-T gas or "ice." An assembly of powerful lasers simultaneously fire 4 megajoules of infrared energy into a device that converts this energy into ultraviolet (UV) energy. The UV energy impacts the holraum, generating x-rays and rapidly heating the holraum. This induces an implosion that creates extremely high pressures and temperatures in the D-T pellet, initiating nuclear fusion. Less than 10% of the initial energy is imparted to the holraum. This is a pulsed system where multiple holraums and pellets would be required to sustain energy output. Other methods can be used to momentarily confine a plasma containing deuterium and tritium to initiate fusion. For example, instead of lasers, ion beams, electron beams, and conventional explosives could be employed. Several systems based on electron accelerators have also been used .. The Farnsworth-Hirsch fusor and the Polywell are examples of two tabletop devices used to demonstrate fusion. Another accelerator design is called the Dense Plasma Focus (DPF), where a pulsed accelerator drives a magnetic field within a diffuse mixture of deuterium and tritium gas to the top of an anode. When the moving magnetic field reaches the top of the anode, UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 16 UNCLASSIFIED//POlt Offl@IAL WS& &NkY it collapses and generates a magnetic pinch for a fraction of a second that has high enough temperatures to generate fusion in the diffuse gas. The production of neutrons in D-T fusion can be on the order of 10 13 neutrons. While this production may seem high, if this pulsed system were fired 10 times in 1 second, and all of the energy of the electrons and ions could be captured, the energy production would amount to only 280 watts. MUON-CATALYZED FUSION This method, sensationalized by Steven Jones at the University of Utah in the 1980s, makes use of the fact that certain material crystal shapes (hexagonal close pack, or HCP) tend to "hide" atoms of hydrogen in the interstitial space between atomic planes in the crystal. By diffusing deuterium into the crystal through electrolysis, fusion could be achieved. The famous scientist Sakarov observed this phenomenon as a way to account for the presence of He-3 in platinum. Platinum, palladium, and titanium are the materials that were used most often to demonstrate this technique. Initiating fusion by this method has been very poor. CAVITATION (BUBBLE) FUSION In water, vapor bubbles are produced when pressures drop below 2,300 pascals ( ~2% of atmospheric pressure) through a process called "cavitation." When these cavitation bubbles collapse, they produce high temperatures and pressures for a short period of time. Cavitation has been shown to release enough energy to pit ship propeller blades. In addition, heavy water (D2O) and deuterated acetone have been used to demonstrate that cavitation can cause particles from fusion. Unfortunately, to date, the performance of cavitation fusion systems has been low. ROCKET DESIGN USING FUSION ENERGY While nuclear fusion releases large amounts of energy for a minimal quantity of fuel, initiation of the fusion reaction means that fusion ignition dictates the design of the system . Possible spacecraft designs include pulsed nuclear explosions with impact plates or sails; plasma confinement methods that generate charged particles used directly for propulsion; and confinement methods that heat a propellant, like liquid hydrogen, to be ejected through a Laval nozzle. UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 17 UNCLASSIFIED//POlt Offl@IAL WS& &NkY Chapter 3: Aneutronic Nuclear Fusion Schemes Up to now, the fusion schemes described have the lowest fusion initiation temperatures ranging from 10 million kelvin to about 300 million kelvin. Most of the energy released from the fusion schemes in Figure 7 also release more than 80% of their energy in neutrons. Neutrons are difficult to shield and present a safety concern for the crew of a fusion-powered spacecraft. Unlike charged particles, their energy cannot easily be converted into electricity using magnetohydrodynamic generators and they cannot be focused into a propulsion beam to generate thrust. As a result, aneutronic fusion schemes have been explored for possible use in space propulsion . The schemes with the lowest temperature threshold are highlighted in Figure 9. Figure 9. Aneutronic Fusion Schemes Deuterium is readily available by centrifuging water, and protons are ionized hydrogen atoms. Helium-3, however, is very rare on earth, although quantities of it exist in lunar regolith due to ion impact on the Moon from the Sun. Over one million tons of helium-3 is estimated to exist on the lunar surface. Removing the helium-3 schemes does shorten the table, and one of the most attractive schemes uses boron-11. Boron is readily available on earth and 80.1 % of naturally-occurring boron is boron-11. A consistent method of comparing each fusion scheme is based on how difficult it is to initiate fusion. In 1955, John D. Lawson established a standardized measurement of the performance of each fusion scheme based on the conditions required to initiate or ignite fusion. Three terms occur in his performance number, referred to as the "Lawson criteria." The triple product includes the plasma density (ne), the energy confinement time (TE), and the plasma temperature. Lower values of the Lawson criteria indicate better fusion ignition performance. The Lawson criteria for D-T fusion is 34; this figure of merit is only 0.43 for the first aneutronic fusion scheme, D-He3. Two of the best performing schemes are p-Li6 at 0.005 and p-B11 at 0.014. The ion temperatures required for both of these schemes UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 18 UNCLASSIFIED/ /PO lt Of f l@IAL WS& &NkY are 800 keV and 300 keV, respectively, much higher than the 50 keV required for D-T neutronic fusion. /H 0 m /He ~ 0 / ffi /He /1B @ / ~ m /He 3x /He+8.7MeV Figure 10. The Fusion of Hydrogen-1 and Boron-11 Produces Three Alpha Particles Fortunately, both lithium-6 and boron-11 are readily available, while tritium must be manufactured due to its 12.6-year halflife. Lithium-6 represents 7.5% of all lithium on Earth. Lithium is mined at Silverpeak, Nevada. Another concern in choosing an effective aneutronic fusion scheme is bremmstrahlung, or "braking radiation." This term refers to the x-rays emitted as electrons pass through metals and interact with electrons in the outer shells of the metal atoms. In order to ignite a fusion reaction, the gases must be heated to the point where they ionize and form a plasma. Electrons stripped from the ions in the plasma will interact with the walls of the chamber that house the plasma and bremmstrahlung x-rays will result. Bremmstrahlung is a parasitic process that decreases the temperature of the plasma and reduces the chance of fusion. While the two favored fusion schemes, p-Li6 and p­ B11, can generate significant bremmstrahlung losses, there are techniques proposed using electromagnets to direct both electrons and ions to minimize x-ray production. UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 19 UNCLASSIFIED//POlt Offl@IAL WS& &NkY Chapter 4: Antimatter Propulsion Although the focus of this report is aneutronic fusion propulsion, another possible propulsion technology involves the use of antimatter. Antimatter is created in certain nuclear reactions and minute quantities have even been collected from particle accelerators and stored for a short time in magnetic bottles. Antimatter has the highest energy density of any material known. When particles of antimatter and matter collide, they completely annihilate and convert their mass into energy according to Einstein's famous equation, E = mc2. 10 There are various terrestrial sources of antimatter. Positrons are created spontaneously from high-energy gamma rays as they decay. Any gamma ray with an energy of more than 1.022 MeV can decay by pair production where an electron and a positron are generated. A positron is the antimatter counterpart to an electron and carries a positive charge. Due to the difference in charge between electrons (13-) and positrons (13+ ), they can be separated by magnetic fields and the positrons stored. Certain radionuclides decay through the emission of an antiproton, a negatively charged antimatter counterpart to a proton. If antiprotons are collected and combined with positrons, stable atoms of antihydrogen are produced. Antihydrogen has been created and stored in magnetic bottles. Up to 1012 antiprotons have been successfully stored for days at a time. Antihydrogen can also be potentially chilled to form liquid antihydrogen or "ice" to use as a rocket fuel. The specific impulse for antimatter rockets (Isp/c) are 1 for electron-positron annihilation and 0.60 for proton-antiproton annihilation. For nuclear fusion, lsp/c is only 0.119, followed by 0.04 for nuclear fission. Antimatter could be used to heat up a propellant gas and expel it through a Laval nozzle to generate thrust. If liquid hydrogen is carried onboard the spacecraft, the electrons and protons in the hydrogen can be annihilated by the antimatter to generate power. Annihilation products can also be reflected or directed by magnetic fields to be ejected as exhaust from the rocket to generate thrust. Antimatter is seen as the only fuel that can potentially accelerate rockets to near the speed of light, providing the potential for human flight to neighboring star systems. UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 20 UNCLASSIFIED//POlt Offl@IAL WS& &NkY Chapter 5: Aneutronic Fusion Propulsion Projects As discussed above, several techniques are currently being explored by research groups and private companies to employ nuclear fusion for space propulsion. Their efforts over the past 60 years have resulted in three classes of fusion drives, which are representative of magnetic, inertial, and antimatter schemes. These include magnetic confinement fusion (MCF), inertial confinement fusion (ICF), magnetized target fusion (MTF), inertial electrostatic confinement (IEC), and antimatter-catalyzed fusion applications. Magnetic confinement fusion employs an electromagnet system that forces ions in a plasma to follow a toroidal-shaped magnetic field. Tokomaks and spheromaks employ this method and, between 1987 and 2004, the NASA Glenn Research Center developed the concept for Discovery II vehicle designed to deliver payloads to Jupiter and Saturn in a 4- to 6-month journey. The simplest methods for fusion propulsion tend to use pulses from the detonation of nuclear devices. Other methods are based on the ejection of a propellant gas or ions to generate thrust. NUCLEAR PULSE PROPULSION In this method, nuclear explosions are used to provide rocket thrust. The explosions act upon a steel pusher plate attached to the rear of the rocket and shock absorbers cushion the impact to the crew and payload. General Atomics first proposed this technique in the late 1950s under Project Orion. 11 • 12 With a maximum specific impulse of 100,000 seconds, this is one of the few fusion technologies that can be built with existing technology. Radiation exposure to the crew and the high period of acceleration induced by this propulsion system poses significant problems, yet a mission to Mars could only take 4 weeks using this technology instead of the 12 months required for conventional chemical rockets. Project Orion led to Project Daedalus in the 1970s, pioneered by the British Interplanetary Society for missions to nearby stars. 1 In this design, a D-Li6 or D-He3 pellet would be imploded and the exhaust materials directed by an electromagnetic field to provide thrust for the rocket. The pellet would be ignited by multiple lasers that would strike the pellet and ablate the outer surface to generate a large implosive force. A concept known as "Medusa" was developed in the 1990s that employed a large "sail" ahead of the payload. Fusion explosions between the payload and the sail would carry the payload forward. Specific impulses of as high as 100,000 seconds were possible. Project Longshot, a conceptual spacecraft explored by the U.S. Navy and NASA in the 1990s, would have employed an electromagnetic funnel and ICF to power a rocket 1 Project Deadalus, led by Alan Bond, was a 5-year design study undertaken by the British Interplanetary Association between 1973 and 1978. The study focused on designing an unmanned, interstellar probe. Specifications were that the probe must use current (or near-term ) technology and be able to reach its destination within a human lifetime. The probe's chosen destination was Barnard's Star (5.9 light years away), estimated to take 50 years at speeds up to 12% of the speed of light. The major stimulus for the project was Friedwardt Winterberg's ICF concept. UNCLASSIFIED/fF&A &FFI&I:.l.k W&li &NkY 21 UNCLASSIFIED//POlt Offl@IAL WS& &NkY using D-Li6 fuel pellets. The estimated travel time of this system to Alpha Centauri was 100 years at an average velocity of about 0.5% of the speed of light. OTHER ANEUTRONIC ROCKET DESIGNS Antimatter-Catalyzed Fusion In the 1990s, Pennsylvania State University worked on a fusion rocket design that employed antimatter to catalyze fission reactions in uranium. As a comparison, in order to make a nuclear fission bomb for space propulsion, approximately 12 kg of uranium-235 is required to generate the three critical masses required. Using antimatter, this can be achieved with gram-quantities of uranium. Magnetized Target Fusion Plasma guns are used instead of lasers to generate heat in a low-density fusion fuel mixture confined by magnetic fields. The fuel is rapidly compressed to ignite fusion. The NASA/MSFC HOPE (Human Outer Planets Exploration) Group estimates that this propulsion system could transport payloads to Jupiter within about 300 days. Ion Drives The VASMIR engine is a highly efficient ion thruster that uses an RF resonant cavity to accelerate ionized argon or xenon gas as a propellant. One concept is to generate electricity from aneutronic fusion by capturing the energy of the emitted ions in a magnetohydrodynamic generator. The electricity would then be used to power the VISMIR ion drive. This direct conversion drive could capture useful energy from aneutronic fusion or from D-T fusion which is easy to ignite, but loses about 80% of its energy to neutrons. The neutrons can be used to generate secondary ions through impact on a target and the ion energy can be collected in the MHD generator. COMMERCIAL DEVELOPMENT In addition to teams from universities and national laboratories, several companies have been formed to develop aneutronic fusion propulsion systems. Several are discussed below . 1. EMC2 Fusion Development Corporation A prolific designer and author, Dr. Robert Bussard has explored inertial electrostatic confinement fusion as used in the Farnsworth-Hirsh Fusor .13, 14, 15, 16, 17 He and his colleagues formed EMC2, a private company based in Santa Fe, New Mexico, to test components of a practical fusion drive. Their work has been funded by DARPA, NASA, and the U.S. Navy. Outlined in Figure 11, his QED (charged particle electric discharge engine) is based on the Farnsworth-Hirsch Fusor, an ion accelerator patented in 1968. This accelerator works through the use of spherical electrodes that force ions toward the center of a spherical chamber by Loren