[tqc]

Most of it I can see working - going faster/further does basically come down to adding more fuel or reducing payload.

The navigation and communication would be where you run into problems. I note that you have specified that as rock solid, unlike the other aspects. Over long distances, communication needs to be directional - you have to know exactly where the spacecraft is (perfect navigation) or it sends no data back and therefore isn't doing anything useful.

Interestingly, this issue doesn't apply to a manned spacecraft that can make unexpected changes without being in contact with Earth, but that doesn't fit too well with the redundant/disposable concept.