4 ms·

I do not understand why the Oberth effect works: https://en.wikipedia.org/wiki/Oberth_effect https://en.wikipedia.org/wiki/Oberth_effect I understand the math

by Imnimo 5y ago

I do not understand why the Oberth effect works: https://en.wikipedia.org/wiki/Oberth_effect https://en.wikipedia.org/wiki/Oberth_effect I understand the math, but I cannot form an intuitive understanding of why it works. My brain just insists that it shouldn't matter how fast you're going, you have a certain amount of energy in your fuel, and that's all there is to it.

magneticDuck 5y ago
Your fuel is measured in delta-V—the velocity you can accelerate by in an inercial reference frame—and kinetic energy scales with the square of your velocity.
- kubanczyk 5y ago
  Yes, Wikipedia offers a grokable example of increasing speed by 1 m/s: > at 1 m/s, adding 1 m/s increases the kinetic energy from 1 J to 4 J, for a gain of 3 J; > at 10 m/s, starting with a kinetic energy of 100 J, the rocket ends with 121 J, for a net gain of 21 J
verve_rat 5y ago
But the energy of the spacecraft is not static. As the craft falls towards a body it is trading gravitational potential energy for kinetic energy. Thus the fuel/reaction mass gains kinetic energy along with the rest of the craft. So at the lowest point to the body that reaction mass has the most kinetic energy. From here it could go two ways: a) The craft does nothing. The full mass continues to get higher and the kinetic energy the reaction mass gained is turned back into gravitational potential energy. b) The craft ejects (maybe by burning) the reaction mass. The kinetic energy of the reaction mass is transferred to the craft. Ejecting the reaction mass slows it down and that speed is transferred to the craft. Now the craft continues with all the energy it had going in, but because it is much lighter less of that kinetic energy is transformed back into gravitational potential energy. Or think of it this way. You have a big rock at the top of a hill. You roll that rock down the hill and convert 100 units of energy in to kinetic energy. At the bottom of the hill is a see-saw. The big rock hits it and launches a small pebble. That pebble still only has 100 units of energy, but it's moving much faster than the big rock did. It is not about getting more energy, it is about shedding mass at the right time to transfer (what starts as gravitational) energy from the fuel to the craft.
- magneticDuck 5y ago
  That's not an accurate explanation, since the Oberth effect also holds for a fictitious rocket that doesn't lose mass when it thrusts.
  - verve_rat 5y ago
    Well shit, I guess I'm in the same boat as Imnimo then.

by Imnimo 5y ago

magneticDuck 5y ago

Your fuel is measured in delta-V—the velocity you can accelerate by in an inercial reference frame—and kinetic energy scales with the square of your velocity.

kubanczyk 5y ago
Yes, Wikipedia offers a grokable example of increasing speed by 1 m/s: > at 1 m/s, adding 1 m/s increases the kinetic energy from 1 J to 4 J, for a gain of 3 J; > at 10 m/s, starting with a kinetic energy of 100 J, the rocket ends with 121 J, for a net gain of 21 J

verve_rat 5y ago

But the energy of the spacecraft is not static. As the craft falls towards a body it is trading gravitational potential energy for kinetic energy. Thus the fuel/reaction mass gains kinetic energy along with the rest of the craft. So at the lowest point to the body that reaction mass has the most kinetic energy. From here it could go two ways: a) The craft does nothing. The full mass continues to get higher and the kinetic energy the reaction mass gained is turned back into gravitational potential energy. b) The craft ejects (maybe by burning) the reaction mass. The kinetic energy of the reaction mass is transferred to the craft. Ejecting the reaction mass slows it down and that speed is transferred to the craft. Now the craft continues with all the energy it had going in, but because it is much lighter less of that kinetic energy is transformed back into gravitational potential energy. Or think of it this way. You have a big rock at the top of a hill. You roll that rock down the hill and convert 100 units of energy in to kinetic energy. At the bottom of the hill is a see-saw. The big rock hits it and launches a small pebble. That pebble still only has 100 units of energy, but it's moving much faster than the big rock did. It is not about getting more energy, it is about shedding mass at the right time to transfer (what starts as gravitational) energy from the fuel to the craft.

magneticDuck 5y ago
That's not an accurate explanation, since the Oberth effect also holds for a fictitious rocket that doesn't lose mass when it thrusts.
- verve_rat 5y ago
  Well shit, I guess I'm in the same boat as Imnimo then.