[chris_va]

Surprisingly, I actually did a fair bit of research into this field.

Both rail guns and light gas guns cap out at around 6km/s, and you really want 8km/s (more actually, for hypersonic drag loss) for this to be economical. Otherwise you need to launch a large enough rocket to make up the 2km/s difference, and you stuck solving the rocket equation again. Except you've subjected your rocket to 1000g, and the odds of it being reusable are really small.

For rail guns, the plasma discharge on the rails limits your speed. Around 6km/s.

For light gas guns, the working temperature of the gas cannot exceed the melting point of the barrel (or get close), otherwise tungsten/steel will get into the working gas and slow down the speed of sound. With hydrogen+tungsten, also around 6km/s.

There is a better solution, though :).

[rakoo]

> and you stuck solving the rocket equation again

Yes, but only for 2km/s, so there's that. Also this system can be used on other bodies such as Mars and the Moon.

But yeah, I'm more worried about the 1000g than anything.

[chris_va]

2km/s means you are limited to about 20% payload. 20% is 10x better than a normal rocket, but there are actually easier ways to get 20%.

The beamed power method gets about 20%, and works for humans (no g load). Escape Dynamics is trying it: http://escapedynamics.com/edispacelaunch/.

The difficulty here is more technical/scientific than engineering. Synchronizing microwave beams is hard, and if you don't you get destructive interference. I don't know if they have solved that problem yet (there have been a few proposed solutions, we'll see if any work).

Edit: Yes, rail would work great on the moon/mars. Actually there was a movie (Moon, with Sam Rockwell) where fuel is harvested on the moon and shot into orbit. Fun, crazy movie.

[iwwr]

Still, g-hardening may not be a showstopper. Artillery shells routinely have electronics in them nowadays. It doesn't have to be reusable, but needing only 2km/s from outside the atmosphere basically.

Edit: have you looked at these people's work? https://en.wikipedia.org/wiki/Quicklaunch

[chris_va]

Yep, very true. My notes, if you find them inspiring: https://docs.google.com/document/d/1tD8H8B4Umfmqf-8uOH2zoR72...

I think the oil derrick idea was a bit crazy (why make the problem harder than it has to be?), but the concept is sound provided that the rockets are fairly inexpensive.

I think the idea of launching to an arbitrary orbit appeals to engineers, but economically it would be more efficient to launch to a common orbit, and use cheap fuel to change orbits later if necessary. Of course, in-space refueling is only in the concept stage (but getting solved!).

To get above 6km/s, the only way I know of would be to use induction tracks (see Lawrence Livermore patent in doc). The original problem with the EM methods was getting high power switches to work repeatably. They used to cost a fortune, and last only for 10s of cycles. However this problem was solved by a completely different industry in the last couple of years (connecting wind power to the grid), and now you can get them cheap on Alibaba.

The major difficulty, actually, would be: 1) Sound pollution (not many places have high mountains, easy industrial access, and no local population). 2) Hypersonic drag (which is quadratic with velocity at that reynolds number). This is half as hard at 6km/s than 8km/s.

[aggieben]

+1 for that. I didn't know railguns were limited thusly.