Defense Intelligence Reference Document Aneutronic Fusion Propulsion(1)

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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
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