[inglor_cz]

Well, depends on what you mean by "space".

The orbit around the Earth is the closest and travel there is the cheapest, but manufacturing fuel out of nothing (vacuum) is pretty much impossible. Technically, what you have on lower Earth orbits isn't vacuum, but a very, very thin gas. It is still probably too thin to be used as a source of stuff for industrial processes.

The closest solid body we have as of now is the Moon. A nice source of minerals, where fuel will be almost certainly produced in situ in the future, but then you still need to fly from Earth to Moon orbit and get the produced fuel to the Moon orbit to tank the ship. (Possibly using a space elevator.) Still quite a lot of delta v.

The best solution could be to intercept a comet full of volatile materials and drag it somehow to the Earth orbit, say, 1000 km above the planet. Of course, if you mess up and hit the Earth instead, you will cause a nice extinction event down there...

[noduerme]

I love HN just for the fact that this conversation started with shipping a laptop and ended up with the dangers of space refueling.

[dylan604]

By "space", I meant not from Earth. Extra-terrestrial might have been a better choice. Clearly, I wasn't propsing hoovering up parts of the atmo or from the vacuum of space itself (do the Brits call it the hoover of space???).

>Of course, if you mess up and hit the Earth instead, you will cause a nice extinction event down there...

Why does it need to be in Earth's orbit. If we have the ability to capture a comet and place it in a more friendly orbit, we'd surely have the ability to park it in the moon's orbit negating a SimCity-esque ELE.

[jhgb]

> but then you still need to fly from Earth to Moon orbit and get the produced fuel to the Moon orbit to tank the ship. (Possibly using a space elevator.

You mean a space elevator from the Moon (since on Earth it seems to be infeasible)? It's probably doable but I suspect that an electromagnetic accelerator would be far more affordable and technologically less demanding.

[inglor_cz]

From the Moon, yes. The cost and technological demand would certainly be significant, but unlike elmg accelerator, a space elevator would be suitable for transport of people as well.

[jhgb]

There's no reason why you couldn't use an accelerator for transporting people. The velocities are fairly low so acceleration can be moderate as well. Also there's no air to complicate things with aerodynamics, so all you have to do is to design a high-speed maglev with auxiliary thrusters and RCS that could lift from the tracks at orbital speed and then land back on them again (which is much easier in the complete absence of atmosphere which turns it into an orbital mechanics problem).

[inglor_cz]

So I put the numbers into a calculator. Let us say that 3G for a minute would be tolerable for most humans, at least those that venture onto the Moon.

You need approximately 60 seconds of 3G acceleration to reach orbit around the Moon. That would mean some 54 km of an accelerator. That is more feasible than I instinctively expected. The main question is how much metal would be needed for 1 m of such accelerator track.

[jhgb]

The real question is how much material you'd need for a ~80000-km-sized vertical structure that the lunar elevator is expected to be. The ~1:1000 factor in the largest dimension required can't be good for a practical elevator.

As for the track, I suspect it might be (order-of-magnitude-wise) similar to existing maglevs on Earth. On one hand, you need to reach higher velocities, higher precisions, etc. On the other hand, any supporting structure can be substantially more lightweight (much smaller gravity, no corrosion resistance required, etc.).

[rocqua]

Would a properly counter-weighted space elevator not have much lower power requirements than an electromagnetic accelerator?

[jhgb]

If you can't realistically build an elevator, the question is meaningless. As for if you could build one, I don't know. Maybe, but chances are that an electromagnetic accelerator working in reverse could recover a lot of the energy. Also, it wouldn't be constrained by having to keep the upmass and downmass ratio bounded. And third, energy is really easy to come by on the surface of the Moon, so it's not like it would be a severe limitation.