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Comment by radford-neal | Hacker News Reader

You'd need to reach the height of 36,000 km, but not the velocity needed for geosynchronous orbit - only the velocity needed to match the angular motion of the moon, which I think would be 28 times less.

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So it's more like "use the moon as part of a non-rotating skyhook."

https://en.wikipedia.org/wiki/Skyhook_(structure)

I’m not very good at physics but - intuitively- would not attaching a payload that have not reached geostationary velocity pull this space line thing to the side and down with the force for which it was not designed

Attaching the payload would indeed pull on the line, since the payload wouldn't have the velocity to maintain itself in that orbit, and so would need an upward force from the line to not fall down.

The line would of course need to be designed to handle that, as well as the increased load as the payload is hoisted up to the moon.

Why is it less energy expended to hoist a load on a line than to use rocket propulsion?

Well, whether it takes less energy would obviously depend on how the hoisting is done. But one advantage is that you can hoist slowly if that's advantageous, whereas you can't do things slowly with a rocket, at least until you've achieved orbit.

Also, if we start with a rocket+fuel on earth, and end with a rocket on the moon, there's an inevitable input of energy to move the rocket to a location of higher gravitational potential, that would have to be provided with hoisting as well, but there's also be a lot of rocket exhaust (formerly fuel) that's now zipping around the solar system, carrying away energy.

To attempt a generalized version of sibling comment from radford-neal, the energy that goes specifically into changing the position of the payload is the same, but how much of the total energy spent goes into that, depends on the method. For the rocket energy goes into so many things other than lifting the payload: kinetic energy of the exhaust, high temperature, sound, and - most importantly - lifting the fuel itself, quickly.

Or to think another way think of all the challenges with human powered helicopter where the record is reaching 3.3 meters in 64 second flight, whereas any moderately physically fit person can climb the same height on the rope much faster and hang out there indefinitely, because they did not have to lift the weight of the helicopter and send the big masses of swooshing air down from the blades just to combat the force of gravity.

You don't need to carry rocket fuel to do it. You can use solar energy.

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