Even a Microscopic Piece of 95% U-235 can produce Big Heat if it is Detonated by a Burst of Neutrons. Such a Burst Can Be Arranged By Polonium and Beryllium Metals Being Forced Together Hard and Fast.

By placing various metals on Graphene Tape, a Nozzle can operate by expelling matter such as water, or heavier material like Tungsten and/or Copper.

The heavy reaction mass metals can be placed in orbit using ordinary rockets (H2 + O2).

The fission nozzle must not be used inside Earth atmosphere as it expels radio-nucleides (Barium, Krypton, etc).

The fission nozzle is ideal for exploring the outer planets and their moons.

By 2036 there will be corporate projects to do that, and to visit asteroid Psyche in the belt.

What comes out of the nozzle is plasma anyway. So including some more massive, and not too costly atoms, gives more thrust. Copper sulfate or oxide, or many metal salts are all possible thrust adders.