[VariousRen]

I don't see any problem at all, we do the same thing on earth. Oil refineries require some amount of energy (say 1 unit) to produce fuel. That fuel provides enough energy to power the refinery and have some extra left over (for example, the fuel provides 3 units of energy). This means infinite power, up until the point where you run out of raw materials.

For your spaceship you need access to raw fuel components to generate fuel, so really you are turning cargo space and a factory into an inefficent fuel tank.