DOW-UAP-D145, AAWSAP DIRD, Aneutronic Fusion Propulsion I, November 2010

This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys aneutronic fusion as a possible advanced space-propulsion method, focusing on fusion reactions that release most of their energy in charged particles rather than neutrons and therefore offer potential advantages over more neutron-intensive fusion concepts, especially in radiation shielding, direct energy conversion, and thrust generation. The report reviews the underlying rocket physics, compares candidate fusion fuels and ignition conditions, and gives particular attention to proton-boron and related schemes, while also discussing Bussard’s concepts and other fusion projects as representative development paths. It also makes clear that the central obstacle remains ignition and sustained net-energy fusion under practical conditions, and it notes additional problems such as x-ray energy losses from the hot plasma, extreme temperature requirements, and the gap between theoretical specific impulse and what proposed systems had demonstrated experimentally. Overall, the document presents aneutronic fusion propulsion as an attractive long-range concept for deep-space travel, but one whose practical realization still depended on major unresolved advances in fusion engineering.

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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 ( · ---14-A.E..!'erson
IAAP
Person 1 j AAWSA Program Manager, Defense Intelligence Agency, ATTN: JUIAF - DI/DWb-3,i'- •
"'Bfclg 6000, Washington, DC 20340-5100.
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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
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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
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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
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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
V
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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.
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Figure 1. Liquid-Fueled Chemical Rocket.
vi
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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.
d(mv'
m
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:
m
w
F = a (1.2)
m payload
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