Fission fragment rockets use the charged fission fragments that have
very high kinetic energy as the fuel of a rocket to achieve very high
isp. However, in order to allow the fission fragments to escape the
critical reaction area, the fuel must be in the form of nano-particles
held in suspension as a dusty plasma. This also allows for cooling of
the very hot nano-particles.
Fission fragment rockets have also been proposed using radioactive
materials without criticality although the energy density is much
lower.
I propose a fission fragment rocket that operates at low temperature
but also achieves high isp using a non-critical fuel nearly non-
radoiactive fuel, Thorium.
A thorium energy amplifier uses an accelerator to produce protons that
produce neutrons by impacting a suitable target. Each fissioned
thorium atom produces more neutrons than a conventional uranium
reactor so can be very efficient. However, as the thorium is not
naturally fissile, the reaction will stop without the input of
neutrons. The reactor produces far more power than is input.
Fission fragments exiting the reactor are used the same way as in a
normal fission fragment rocket, as fuel and to produce power except in
this case more of the power is used to produce the proton beam.
Unlike a conventional fission fragment rocket, heating of the fuel is
not a huge problem because the thorium cools faster than the fissile
uranium of a conventional fission fragment reactor. Furthermore, a
thorium fission fragment rocket can be more easily throttled by
varying the proton beam current.
Placing such a fission fraqment rocket into space from earth would not
be the same problem as a uranium fueled reactor because the thorium is
not naturally fissile as is uranium so that the destruction of the
rocket in an accident is not a major environmental issue.