You figure out how to put a fusion reactor on a spaceship without melting the whole thing into a blob of metal and use a particle accelerator as a super high isp engine.
Yeah you need to work very hard to lose heat without losing mass in space. Basically you can only radiate it away, which is slow; there are ways of concentrating heat in one spot so it is radiated more efficiently, but that’s where you get to the melting part.
Sci-fi but mostly in the ’not melting down’ part.