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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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 rad iation
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 fue l 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.
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