> Hybrid nuclear fusion–fission power plants have been already proposed and studied in theory.
I have a hand-wavy hard sci-fi universe I've been rolling around my head for years and I eventually came to the conclusion that fission-fusion drives would be really handy for spacecraft, since it would be much easier to start a fission reaction in a cold/dark ship than fusion because of the power requirements. Otherwise you need some other way to generate 10s or 100s of MW to start the fusion reaction.
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