This is a rocket that
expels heated gas at high velocity. The difference between this
and a chemical rocket is that instead of being heated by a chemical
reaction, the gas in this case has been heated by thermalizing
interaction with neutrons in a controlled fission reactor.
The other form of fission rocketry involves the use
of an
uncontrolled fission
explosive behind the spacecraft. It is called
Fission Explosive
Rocketry.
This is a difficult task, and early models will have
a relatively high probability of disaster.