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Comment by skj | Hacker News Reader

> and a space elevator from the moon does nothing to reduce this cost.

How do you figure? My extensive Kerbal Space Program experience tells me that freeing oneself from the moon's gravity well is far cheaper than doing the same from the earth, and then you're approximately MGH of the way to freeing yourself from Earth's well.

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Perhaps you jest but clearly we can’t launch anything from the moon until we bring it there, from Earth.

Unless it's something that was mined there, like Helium-3 or some other mineral resource that we may find it's cheaper to extract from the moon.

You need more dv to get to the moon than to leo. An elevator helps with this.

Or (I hope people have a sense of humor here), space Nazis [1]

[1] https://en.m.wikipedia.org/wiki/Iron_Sky

But once we have this space elevator it's much more feasible to manufacture on the moon.

The moon has negligible atmosphere, so you can use a relatively cheap electromagnetic gun to reach the moon’s escape velocity. If all your doing is sending raw material back that’s a cheaper option.

True. But if you're doing raw material extraction on the moon, you probably also want a cheap way to bring mining equipment down to the lunar surface.

Just run the rail gun in reverse! (Just no fragile humans please unless it's a very long barrel.)

If the equipment is G-force sensitive, an elevator could launch science equipment that previously couldn't be put into space at all.

> science equipment that previously couldn't be put into space at all.

The large amount of science equipment manufactured on the moon?

The large amount of science equipment that could be manufacturer or assembled on the moon.

The paper actually points out and hints some very interesting implications. It could also serve to stabilize a station at the L1 lagrange point. Such a station could be permanent as it would not need reaction mass to maintain orbit. If you couple this with cheap material from the lunar surface, you could create truly massive habitats from thin plastic sheeting from earth and regolith from the moon.

Launching from the moon would be easier and cheaper than launching from the Earth, unless you include the cost of reaching the moon. But according to the table in [1], going from the Earth's surface to geostationary orbit (to rendezvous with a lunar elevator) requires about 14km/s delta-v, and from there to the moon's surface only about 4km/s delta-v. Of the 18km/s total delta-v required, a lunar space elevator only eliminates the last 4km/s.

1: https://en.wikipedia.org/wiki/Delta-v_budget#Earth%E2%80%93M...

I haven't done the math, but are you sure that last 4km/s isn't way more significant than you're estimating due to the rocket equation?

A small mass savings near the end of the journey might translate to significant mass savings near the start.

Also I think (a series of) rotating elevators to get from LEO to GSO are also possible?

The end of the lunar space elevator wouldn't be traveling at geostationary orbital speed, it would be going 28x slower. A rocket could sync with the end of the elevator at this lower speed, which would significantly reduce the delta-v budget. [1]

Not sure how practical all of this, but that is at least the theory.

[1] Credit to mnw21cam: https://news.ycombinator.com/item?id=20996246

The really interesting part is that the elevator would not be in a true geostationary orbit traveling at ~3000m/s. instead it would be traveling at 100m/s at the same height.

The delta-V to reach GEO without the radial velocity to maintain it is closer to 10 km/s if you use the partial escape velocity equation. This is pretty huge as mass required to achieve delta-V is nonlinear.

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