3 ms·
As well as the wrecking ball analogy there’s a second effect: if you fall in with one mass but emerge with a lighter mass, and energy is conserved, you’ll come
by gorgoiler 7y ago
As well as the wrecking ball analogy there’s a second effect: if you fall in with one mass but emerge with a lighter mass, and energy is conserved, you’ll come out faster. So how do you lose mass at the bottom of the gravity well? Burn some fuel!
Another good reason to burn fuel at the bottom of the gravity well is that it’s more efficient to eject rocket gas when you are moving faster. Burning and ejecting gas gives it energy (the exhaust velocity from your chemical reaction) but 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.
Put another way: if you propel yourself through deep space by throwing bricks behind you at 30mph, then this method of propulsion becomes most efficient at 30mph. Each brick you throw backwards ends up standing still, as you sail off at what is now 30.1mph.
(Presumably it’s only inefficient to burn fuel when you are moving slower than exhaust velocity, and not if you are moving at or faster than exhaust velocity?)
- Sharlin 7y agoThis is called a powered flyby or Oberth manouver: https://en.wikipedia.org/wiki/Oberth_effect https://en.wikipedia.org/wiki/Oberth_effect
- effie 7y ago> 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).