Actually, in order for a spacecraft to go, quote "serving the asteroid belt and nearby colonies", a form of propulsion, based off of nuclear energy, would be a must. Nuclear would provide much more energy than the fuel that we use now, and I'd sure feel a lot more confident if the fuel I used was something whose measurement wasn't so vital to the crew's safe arrival.
I think that the Valiant would most likely use a form of plasma or NERVA propulsion. She might even use them as a primitive alternative to impulse drive, even with warp drive installed.
After all, you don't want to run out of gas in space.