> if you fall in with one mass but emerge with a lighter mass, and energy is conserved, you’ll come out faster. It doesn't work that way. If a body loses mass,

> if you fall in with one mass but emerge with a lighter mass, and energy is conserved, you’ll come out faster. It doesn't work that way. If a body loses mass, it loses also kinetic energy that mass was carrying. To make losing mass helpful for acceleration, the mass must be given high enough momentum, i.e. fuel must be burned, not just ejected. > if you do so while moving forward at the same velocity, you effectively leave the exhaust gas behind you standing still with respect to the universe, which is maximally efficient. That standing still happens only at one instant of an idealized scenario. It is not the maximum fuel efficiency either. When the rocket accelerates past that point, it gets faster and the fuel efficiency becomes even better. The higher the speed, the better the fuel efficiency (because the same rocket motor force acts on a faster body, generating more kinetic energy per second).