[leonidasrup]

Using the energy of fission for spacecraft propulsion has been studied and some prototypes have been already constructed. Most of these nuclear engines should be used only outside of Earths atmosphere.

https://en.wikipedia.org/wiki/Nuclear_propulsion

https://en.wikipedia.org/wiki/NERVA

Most interesting and promising are direct nuclear propulsions, like fission-fragment rockets.

https://en.wikipedia.org/wiki/Fission_fragment_rocket