How can nuclear reactions be used for in-space propulsion?
A number of ideas have been proposed using
There are a number of technologies which could be used for using nuclear power for space propulsion. Some have actually been used.
Nuclear pulse propulsion involves setting of a sequence of atomic bombs to propel the space craft. The spacecraft would need a substantial shield behind it to protect the craft and occupants from the radiation and the blast. It would involve a lot of sudden accelerations which would damaging. It couldn't really be used in the atmosphere because of fallout.
A nuclear thermal rocket would use a nuclear reactor to heat up liquid Hydrogen or other reaction mass which would be ejected to cause thrust. This involves having a nuclear reactor on the ship - but this is how nuclear submarines work. It has in fact been tested.
The Bussard ramjet idea uses a magnetic field to scoop hydrogen from space and compress it and heat it to start a fusion reaction. There is actually too little Hydrogen in space to make this effective.