A few moments ago, I did the experiment. I've gone as far as about 36,000,000,000,000 kilometers or 22,000,000,000,000 miles or about 244,000 AU away from the sun and I got the speed up to 1,500,000 kilometers per second and still no problems whatsoever!!! I used time acceleration for this experiment. After I did the experiment, I scrap it, open the saved scenario which is my saved space travels and now I'm doing the space travel where I left off using the real time which I'm currently now 42,553 AU or 6,355,915,000,000 kilometers away from the sun and moving at the speed of 627,894 kilometers per second.
You got the point there! I'm waiting for the next version of orbiter comes out before I try the starship enterprise! I hope by then the starship for orbitersim will have fully functioning systems like on Star Trek.