Space Forum / Space Flight / August 2006

I propose a subcritical fission fragment rocket where the fuel assembly
is never of sufficient density to undergo a self sustaining reaction.
It would be necessary to use a pulsed neutron beam to start fission
which would then "pulse" and die out.  Thus the fission fragments would
be emitted as a much longer pulse than the starting neutron pulse.
This would give the fuel assembly time to cool between neutron pulses.
This would not be nearly as efficient as a critical fuel assembly but
has the advantage of being easier to engineer.  You might use a nuclear
reactor as the power source for the neutron generator or even a large
solar collector near the sun.