To sum up, I understand fully that fuel is an issue, I’m talking theoretical.
In space there is little to no resistance because of how few particles floating about. But if you were always running and engine, pushing matter out behind you, youd have a constant acceleration. So it seems like we should be able to travel near speed of light (combining this constant acceleration with abusing gravity for slingshot maneuvers). Yet people always say nothing can travel the speed of light.