perfidy

Rand over there at Transterrestrial Musings has had some good stuff recently. One that caught my eye was the post I just linked, regarding the use of rotating tethers to fling payloads and passengers from Earth orbit to the moon. There's a lot of good commentary in the, uh, comments - so read it all.

This is, I think, a very good idea. It's practically all upside. A 100km tether would be able to fling a payload to the moon, and then use the Earth's own magnetic field to recharge so that it can launch the next payload. Essentially, we can get one fourth of the momentum needed for a lunar round trip for absolutely nothing. Okay, for the amortized cost of building and launching the tether.

But there is more to it than that. Before learning how to make large, 100km tethers, we would need to practice on baby tethers. However, they would not be merely experimental artifacts with no practical use. Smaller tethers would serve an immediate need in allowing payloads to be shifted to higher orbits. And the skills learned in building and operating these tethers are obviously directly applicable to constructing larger ones.

Larger tethers have multiple uses. A rotovator could dip into the upper atmosphere and grab a vehicle like Rutan's SpaceShipOne, reducing the cost to get off the earth. A tether in lunar orbit could catch incoming payloads from the Earth, or coming up from the Lunar surface. While Lunar tethers could not use magnetic fields to recharge (the Moon has a very weak magnetic field) one of the niftiest properties of a rotating tether is that it can serve as a momentum bank, like a flywheel. A ship inbound from Earth will have momentum, which is transferred to the tether when it is caught and lowered to the center. That linear momentum is stored in the spinning tether as angular momentum. Sometime later, that ship or another could use that energy to launch itself back toward the earth. Naturally, there'd be some losses. But it would mean an end to spending energy to launch each ship individually. Just do it once, and use that energy over and over. A high traffic rate between the Earth and moon would only improve matters, making it easier to balance the load.

The real advantage would lie in having a system of rotating tethers, located around the solar system. Tethers to catch relatively low-powered suborbital launch vehicles coming up from Earth and pass them up to orbit. Inter-orbit tethers to fling payloads back and forth. Larger tethers to fling ships to and from the earth, and even further afield. And practice building rotating tethers is in the end practice building space elevators.

There are some problems, most notably the issue of guidance and "catching" incoming payloads. Launching, by comparison, is easy. But like most technical problems, there will certainly be a technical solution. The only way to get to the point where that matters, though, is to go ahead and start building tethers - and given NASA's current state, that isn't terribly likely.