Nice game - you got the physics spot on. Even the mastiff engine has a rough time once the mass gets high :) I have a similar control mechanic in my game - hope you have time to try it out.
By the way, I got to max mass (in about 30 mins). Was there an end-game? I wondered about the little ship that was pootling around, though he seemed to disappear somehow. Didn't want to miss anything.