...as long as nothing is slowing you down like drag or gravity in theory you should just keep speeding up as long as you are firing your rockets and you shouldn't slow down unless you applied force against that or you got hit by some gravity from a star or a planet or something.
The physics in this game are a bit backwards from reality. In real life you could accelerate to relativistic speed just pushing tiny electrons away from you at a constant rate. However, in this game, inertia or cumulative momentum doesn't apply to top speed. Even without flight assist, we still slow down for no reason. There is nothing causing this but a fake game engine that trying to create earth-like intuitive gameplay.
In short, don't try to use logic here. It will only give you a false impression of physics. I'm afraid that it will distort people's concept of gravity and orbital mechanics quite horribly. In this game gravity inhibits all speed. In reality, it increases speed as you head towards it, and decreases it as you head away.