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Comment by Cthulhu_ | Hacker News Reader

I saw a Scott Manley clip earlier about a newish rocket company that was building a launcher using rotational energy (flinging rockets towards space, their idea being that they can do away with the first stage of a rocket entirely). I wonder if a device like that would be able to launch a payload from the moon back to earth without needing fuel (besides maybe small maneuvers / course corrections).

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This is a bit more flexible than a rail (which can only throw payloads on a single direction), but the Earth is always more or less on the same place in the sky from the Moon, so a rail /linear induction motor is probably the simplest way to do it. No need to have moving parts and, if the rail is long enough, the acceleration can be quite mild.

For safety, I'd not build it aimed directly at Earth, but require some trajectory corrections not to miss the planet. Otherwise it's just a weapon.

"Otherwise it's just a weapon. "

Everything that is in space and big enough and can be roughly guided, can be a devastating weapon.

Which is why I love the idea of peaceful space exploration and am not so happy about the latest developements to arm the space again.

> Everything that is in space and big enough and can be roughly guided

When it's guided, you may be able to see it coming, see the jets for changing trajectory, and it can evade an impactor designed to turn it into a lot of smaller pieces that will burn up in the atmosphere. If it's just a rock covered in F22 paint it'll be a lot harder to see before it hits.

True up until the plasma sheath has burned away the radar absorbant coating and then generates a magnitude or more EMF than the object does at ambient space temperature.

By that time there isn't much you can do to prevent impact.

Certainly you can. At that point it is on final trajectory and we simply have to put a missile under the object heading straight up. It depends on the type of rock we're talking about. If it's sufficiently large to be highly destructive upon splintering then we are in the realm of science fiction.

If it comes in at 10 km/s you'll have about 10 seconds to do that.

From the moon, the 'traditional' idea is a mass driver:

https://en.wikipedia.org/wiki/Mass_driver

First time I heard about it was in:

https://en.wikipedia.org/wiki/The_High_Frontier:_Human_Colon...

Anybody interested in this should play Policenauts. It is an amazing game by Hideo Kojima of Metal Gear fame. It features lots of details like that about life in space, very fleshed out, and is just altogether one of the best games ever made.

There is also an amazing English translation, and a blog on how it was done: https://lparchive.org/Policenauts/Update%2042/

The big advantage of the rotating launcher is that you can aim it towards other targets, not only the Earth. You can take your shuttle from LEO to the Moon and then board the big spaceliner in her maiden voyage (because that's the last time it'll be on a planetary surface) towards Mars.

Where you aim the ground launcher (given you're aiming "over the horizon" at least) doesn't have a lot to do with where you wind up in orbit.

Would it not take more dV to cirularize if we fired it straight up?

The better aligned to the desired trajectory you launch, the less delta-V you need to get where you want to be. If we launch a payload and, in order to get to where you want it'll need to do a u-turn, our launcher isn't helping.

Precisely. Therefore a 45 or 30 degree launch angle gets more of the velocity in the orbital vector.

Unless you are talking about launching it at lunar escape velocity directly at earth orbit.

We were discussing the installation of these on the Moon as a way to launch spacecraft. A straight rail would be aimed at a fixed point, but a centrifugal launcher could release its payload at any point and send it in any direction (as most interesting destinations in the solar system are on a single plane).

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