7 ms·

Here's a random idea. The rocket equation is a problem because the propellant you throw backwards becomes just useless mass, right? What if instead you had two

Here's a random idea. The rocket equation is a problem because the propellant you throw backwards becomes just useless mass, right? What if instead you had two spacecraft, and one accelerated by throwing the other one backward, then the second one changed direction via a gravity assist or something, and caught up with the first one again? Maybe they could even subdivide recursively. Or the ones thrown backward could link up with the next rocket passing the same way, and become reaction mass for it in turn, and so on.

blueflow 4y ago
Are you aware of https://en.wikipedia.org/wiki/Conservation_of_energy https://en.wikipedia.org/wiki/Conservation_of_energy ?
- michaellee8 4y ago
  Actually I am thinking of the EMDrive thing now. Would that be possible?
  - blueflow 4y ago
    No. Fails to account for the conservation of momentum nor does provide an explanation for why it doesn't apply.
  - Beltiras 4y ago
    Isn't EMDrive thoroughly debunked by now?
  - simiones 4y ago
    The EMDrive thing was just measurement error. It's not providing any thrust.
- Rayhem 4y ago
  Conservation of energy doesn't torpedo this idea a priori. The "exhaust" mass would steal energy from the massive body/planet in the gravity assist. If you do this a million million times you might slow the planet down to where it has no more energy to usefully donate, but a one-off maneuver is negligible.
rocqua 4y ago
If the second one could change direction via a gravity assist, wouldn't it make more sense to use that gravity assist for the main rocket? I do think the idea of 'throw a solid mass rather than hot gas', but for a totally different purpose: stealthy change of orbits in space-warfare.
- cousin_it 4y ago
  Here's one way to imagine this. A pair of spacecraft above the south pole push off from each other horizontally, in opposite directions. Their trajectories bend around the Earth, and they meet again over the north pole, going partly sideways and partly up. Now they can join and continue traveling up. Or repeat the maneuver and meet again over the south pole, traveling up even faster. Rocket equation sidestepped.
  - JumpCrisscross 4y ago
    > pair of spacecraft above the south pole push off from each other horizontally, in opposite directions. Their trajectories bend around the Earth, and they meet again over the north pole Okay, they’re in polar orbits [1]. > going partly sideways and partly up Wat? (“Up” is ambiguous. Do you mean northward on the first half of their polar orbits? If so, it’s the same as sideways relative to a ground-based observer.) > they can join and continue traveling up As in parallel to the pole? “Up” away from the North Pole? How did they accelerate from an orbit to this trajectory? [1] https://en.m.wikipedia.org/wiki/Polar_orbit https://en.m.wikipedia.org/wiki/Polar_orbit
  - jetbooster 4y ago
    (Edit: these two paragraphs assume you want to put your rocket into earth orbit, which I realised you might not be attempting.) No matter how you spin it, if you want these two rockets to go in exactly opposite directions and meet up again, The point at which they meet would _not_ be partially sideways and partially "up" (away from the earth). at the point where they meet, they would be going 100% parallel to the ground, and 180degrees from each other (ie, direct head-on collision). This is ignoring the fact that minimal stable orbital velocity (the minimum speed for your rockets to go 'around' the planet, is about 7.8km/s, or 17 thousand miles per hour. and because they're going in opposite directions, thats 34 thousand miles per hour of effective collision speed. Now, if you're suggesting instead, a full 'escape' velocity from the earth that allows them to both enter parabolic trajectories, and meet at some arbitrary point in the distance, I hope you've thought of a way to allow your two spacecraft to survive an explosion that launches them from 0 m/s relative to each other to roughly 22km/s relative to each other (49 thousand mph), in some semi-instantaneous event. If we assume this 'push' from each other happens over 10seconds (this is generous), that's gaining 2.2km/s/s, which is 224G for 10 seconds.

blueflow 4y ago

Are you aware of https://en.wikipedia.org/wiki/Conservation_of_energy https://en.wikipedia.org/wiki/Conservation_of_energy ?

michaellee8 4y ago
Actually I am thinking of the EMDrive thing now. Would that be possible?
- blueflow 4y ago
  No. Fails to account for the conservation of momentum nor does provide an explanation for why it doesn't apply.
- Beltiras 4y ago
  Isn't EMDrive thoroughly debunked by now?
- simiones 4y ago
  The EMDrive thing was just measurement error. It's not providing any thrust.
Rayhem 4y ago
Conservation of energy doesn't torpedo this idea a priori. The "exhaust" mass would steal energy from the massive body/planet in the gravity assist. If you do this a million million times you might slow the planet down to where it has no more energy to usefully donate, but a one-off maneuver is negligible.

rocqua 4y ago

If the second one could change direction via a gravity assist, wouldn't it make more sense to use that gravity assist for the main rocket? I do think the idea of 'throw a solid mass rather than hot gas', but for a totally different purpose: stealthy change of orbits in space-warfare.

cousin_it 4y ago
Here's one way to imagine this. A pair of spacecraft above the south pole push off from each other horizontally, in opposite directions. Their trajectories bend around the Earth, and they meet again over the north pole, going partly sideways and partly up. Now they can join and continue traveling up. Or repeat the maneuver and meet again over the south pole, traveling up even faster. Rocket equation sidestepped.
- JumpCrisscross 4y ago
  > pair of spacecraft above the south pole push off from each other horizontally, in opposite directions. Their trajectories bend around the Earth, and they meet again over the north pole Okay, they’re in polar orbits [1]. > going partly sideways and partly up Wat? (“Up” is ambiguous. Do you mean northward on the first half of their polar orbits? If so, it’s the same as sideways relative to a ground-based observer.) > they can join and continue traveling up As in parallel to the pole? “Up” away from the North Pole? How did they accelerate from an orbit to this trajectory? [1] https://en.m.wikipedia.org/wiki/Polar_orbit https://en.m.wikipedia.org/wiki/Polar_orbit
- jetbooster 4y ago
  (Edit: these two paragraphs assume you want to put your rocket into earth orbit, which I realised you might not be attempting.) No matter how you spin it, if you want these two rockets to go in exactly opposite directions and meet up again, The point at which they meet would _not_ be partially sideways and partially "up" (away from the earth). at the point where they meet, they would be going 100% parallel to the ground, and 180degrees from each other (ie, direct head-on collision). This is ignoring the fact that minimal stable orbital velocity (the minimum speed for your rockets to go 'around' the planet, is about 7.8km/s, or 17 thousand miles per hour. and because they're going in opposite directions, thats 34 thousand miles per hour of effective collision speed. Now, if you're suggesting instead, a full 'escape' velocity from the earth that allows them to both enter parabolic trajectories, and meet at some arbitrary point in the distance, I hope you've thought of a way to allow your two spacecraft to survive an explosion that launches them from 0 m/s relative to each other to roughly 22km/s relative to each other (49 thousand mph), in some semi-instantaneous event. If we assume this 'push' from each other happens over 10seconds (this is generous), that's gaining 2.2km/s/s, which is 224G for 10 seconds.