If you could project a gravitational distortion from a vehicle in the direction you wanted to go. Then being pulled by the distortion,you would theoretically accelerate continuosly until you shut the distortion off. Either reaching the speed of light or actually breaking it,that is if its possible to break the speed of light. Of course since we really don't know how gravity really works other than it just does, but if it does work by gravitons then it would seem feasible that these particles could be synthesised. Also as a side note I did come up with this if somebody else has already said this, I'm sorry and I did search for it on this forum.