AAWSAP DIRD, Aneutronic Fusion Propulsion I, November 2010

This Defense Intelligence Reference Document, prepared in fiscal year 2010 by the Defense Intelligence Agency's Defense Warning Office under the Advanced Aerospace Weapon System Applications (AAWSA) Program, reviews aneutronic fusion as a way to propel spacecraft. It compares chemical, ion, fission, fusion and antimatter propulsion, and it also covers radiation shielding and relativistic rocket calculations. It looks at research needs over the next 30 years for missions from low Earth orbit to Mars, Jupiter and Alpha Centauri.

From the source:Release of 2026-09-18 Incident: 11/1/10, Las Vegas, Nevada. Released with redactions. 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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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, wh ich 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.
Propu lsion 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 sustain ing 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 t hermal 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.
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