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Comment by t_fatus | Hacker News Reader

Without considering the fact that the orbit of the Moon is not circular, this only helps reduce the delta-v needed to go from the surface of the earth to the surface of the moon by ~3.2km/s, but we still need ~12-14km/s to go to GTO/GEO.

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However since the moon period is not the same as a GEO period, maybe the fact that you could juste 'catch' the end of the cable before falling back to earth without having the speed to maintain at GEO could help you save some delta-v.

That's essentially what GTO is - an orbit going close and fast at one side and going high and almost stopped at high side.

Going directly up means over 1h of fighting earth gravity. So need Isp > 3600. Much cheaper to go for orbit.

An elliptical orbit that intersects the surface of the massive body at the periapsis side is fine, if you can change the orbit when you reach apoapsis.

But if you miss the cable, you're boned.

i think the bigger upside here is that getting humans into orbit and then to the moon via elevator is likely safer than repeated landings back and forth.

Indeed this would avoid the tricky 'land to a place where nobody can help you' part.

Also would introduce the tricky no way to abort if something goes wrong. Unless you are willing to carry a fast abort rocket with you.

You don't need to hold the orbit. Just jump to the elevator and grab on.

Would be nice to get the savings in delta-v, I've no idea how to compute this.

A ballistic approach is enough so the kinematics are simple, too.

The gravity is only 3% up there, so more tricky. And reducing with square of the distance.

Also every second you are accelerating straight up costs you gravity worth of Delta-v because gravity losses. So need to be quite short and intense burn to be worth it.

Eh?

You don't need to get sideways velocity at all. It's enough to only reach the distance. This is a major and huge saving in fuel.

My estimate is ~9.5km/s from the Equator to the hook. Add some more for atmospheric losses.

you can use the partial escape velocity equation. ~10 km/s to get to GEO without the radial velocity. delta-V to the moon is about 18 km/s (one way). Keep in mind that kinetic energy is proportional to velocity squared.

Useful for refueling GTO sattelites from the Moon however.

Just need to build a trans Lunar railway from the poles to supply the water.

Or attach the damn thing to one pole because no atmosphere.

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