Nuclear fusion of light elements like hydrogen or helium would permit approaching the speed of light. It seems very attractive to refuel your space ships where the fuel is.

I have to be alone very often. I'd be quite happy if I spent from Saturday night until Monday morning alone in my apartment. That's how I refuel.

I have to be alone very often. I'd be quite happy if I spent from Saturday night until Monday morning alone in my apartment. That's how I refuel.