Rotating momentum-exchange tethers are a very exciting technology, but one of my first thoughts after being exposed to the technology was the tricky rendezvous. The space industry has spent all kinds of money and time on satellite rendezvous, and these are typically slow, long, drawn-out affairs with two satellites in almost precisely identical orbits, slowly closing the distance between each other and finally making a solid connection.
The rendezvous required for a rotating tether and its payload is far more dramatic. The whole point of the operation is to have the tether and the payload in different orbits, so that the rendezvous can lead to an exchange in angular momentum and orbital energy between the two, resulting in a payload boosted to a higher energy orbit (or dropped to a lower energy one).
Thus, you can’t match orbits like you do in conventional rendezvous. The best that you can do is to instantaneously match position and velocity (but not acceleration). So you need an approach to rendezvous that is pretty tolerant of error.
So we threw out the book when it came to trying to think of how to do rendezvous, and came up with something totally different and designed to meet the specific needs of the mission. And I was pretty proud of the result, and still am. Because, you see, this is a bit of an anniversary for tether rendezvous technology. It was five years ago (February 2005) that we successfully demonstrated that the rendezvous technology we had postulated could work, at least at the lab scale.
We took advantage of the fact that the tether was under rotation and experiencing centrifugal acceleration, and that the payload was in free-fall. We simulated this (quite accurately) by hanging the tether’s “catch mechanism” from the ceiling of a racquetball court at Tennessee Tech, and then we “shot” our simulated payload up to the catch mechanism, with its boom positioned to be captured by the catch mechanism when it penetrated the aperture of the catch mechanism. Then the catch mechanism would release and close around the boom, quite quickly, allowing the simulated payload to be caught.
It all worked out a lot better than I thought it would–take a look at our results:
And here was the press release that came out months later announcing the accomplishment. Our video footage of successful testing got on NASA TV…once.
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