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Comment by cousin_it | Hacker News Reader

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.

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

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.

Actually I am thinking of the EMDrive thing now. Would that be possible?

Isn't EMDrive thoroughly debunked by now?

No. Fails to account for the conservation of momentum nor does provide an explanation for why it doesn't apply.

The EMDrive thing was just measurement error. It's not providing any thrust.

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.

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.

> 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

(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.

I'm talking about flying away from the Earth. And yeah, it's making a more theoretical point. All it proves is that "recapturing propellant", so to speak, is possible in principle - it's not prohibited by conservation of energy or momentum or whatever. Like sailing against the wind faster than the wind. Given that, there are probably more efficient ways to exploit this.

You'd still need to get your spaceship pair into an orbit (or at least a sharp sub-orbital hop) in order to excecute your plan though. the spaceship pair wouldn't just hang there in space. Also, it's simply not possible to mechanically push the two ships away from each other with sufficient velocity, and I imagine the explosives needed to blast your two ships apart would be better used in a smaller slower controlled explosion, often called a rocket engine.

There isn't a 'hack' for defeating orbital mechanics sadly. It takes expenditure of energy, and significant amounts of it.

I don't imagine using explosives for it. More like one ship is a railgun and the other a projectile. Then when they meet again, they can convert their leftover relative velocity back to electricity, by having the railgun catch the projectile and charge up.

Or the two ships could be a spinning pair tied by a cable, and then the cable is severed. To make a fast spinning pair, start two spinning pairs in parallel planes and opposite directions, and make them accelerate against each other electrically.

There are probably objections to those too, like size of railguns and strength of cables, and then engineering can find answers to that, and so on.

It doesn’t work - when combining the velocities sum to zero. Redirecting them in up isn’t free, it needs additional energy.

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