I only have a few minutes before bedtime, but I got a lot of thesis stuff today, and since I was a good boy, I’m going to blog.
This was a quick idea mentioned by a commenter on one of my previous posts. It’s crazy, and has a good chance of not working, but was rather clever nonetheless. Basically the idea is to have an atmospheric collector mounted on a station in LEO. Basically, even though the atmosphere at say 400km is really thin, a 1 m^2 collector area travelling at a relative velocity of 29000km/hr will have a flow rate of roughly .03 km^3 / hr flowing into it. At that rate even a minuscule density can add up. The inherent drag at that altitude is also low enough to be possibly handleable by an electrodynamic tether.
The commenter claims that over the course of a year, such a system could collect nearly 7.8 tons of oxygen, as well as ~1 ton of water, 0.7 tons of nitrogen, and 0.5 tons of helium. It would need to overcome a constant drag of ~2N, which is well within what an ED tether could handle. For comparison, the frontal area of a Sundancer module is going to be about 28m^2 by my estimation. If this commenter’s claims are even remotely close, that would amount to nearly 220mT of LOX per year. That’s enough to refill the LOX tanks on a Centaur stage once per month. It would need a decent ED tether to counteract the drag, but the gravity gradient stabilization it would provide would also help with fluid settling. Interesting no?
An interesting point the commenter missed is that with how hard helium is to get, it might actually end up being almost as valuable as the LOX. Many stages use helium for lox pressurization and purges. Being able to top up on that in orbit would be helpful as well.
Anyhow, I have to wrap up in the next minute or so, so I want to mention the potential problems I see with this idea before I go. All of those numbers assume that you’re getting 100% efficiency of collection–ie that none of the molecules that hit your collector can escape. I’m not sure how real that is. And if turns out that you can only get a relatively low efficiency, it might not make the idea worthwhile. Also molecular pumps like that are rather power hungry. I’m not sure if a Sundancer module would actually be able to pump out enough power to run both the pumps as well as the ED tether. And who knows how well the thing will hold up in orbit.
But there’s something rather nifty about the idea of on-orbit ISRU propellant gathering isn’t there?
[Update: the volume flow error was mine, the original commenter had only specified the velocity. So I fixed and clarified it. That’s what I get for trying to write a post in 10 minutes before bed time. Also the group working on the idea is at Worcester Polytechnic Institute, under a Dr John Wilkes–supposedly they’ll be presenting more info in the near future at some conferences]