A Tether Technology Anniversary

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:

First Catch Mechanism Test

Second Catch Mechanism Test

More Testing with Animation

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.

NASA Engineers, Tennessee College Students Successfully Demonstrate Catch Mechanism for Future Space Tether

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MS, nuclear engineering, University of Tennessee, 2014, Flibe Energy, president, 2011-present, Teledyne Brown Engineering, chief nuclear technologist, 2010-2011, NASA Marshall Space Flight Center, aerospace engineer, 2000-2010, MS, aerospace engineering, Georgia Tech, 1999

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About Kirk Sorensen

MS, nuclear engineering, University of Tennessee, 2014, Flibe Energy, president, 2011-present, Teledyne Brown Engineering, chief nuclear technologist, 2010-2011, NASA Marshall Space Flight Center, aerospace engineer, 2000-2010, MS, aerospace engineering, Georgia Tech, 1999
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8 Responses to A Tether Technology Anniversary

  1. Those movies are pretty sweet. Good work. Hopefully some day you’ll get a chance to try out the real thing.

    ~Jon

  2. What happened to the hardware? Sitting in someone’s garage?

    I wonder if such a mechanism could be used for mid-air recovery of payloads by a suborbital vehicle.

  3. Trent, I think that the catch mechanism hardware is still at Tennessee Tech, actually still in that same converted racquetball court.

  4. Chris (Robotbeat) says:

    Wow, those are some really impressive videos! This is still going to require some rather long* tethers to catch a suborbital crew module and haul it to orbit without crushing the crew.

    *~500 km radius for a 7 km/s delta-v and just over 10 gees of acceleration
    radius of tether=velocity^2/acceleration
    (velocity is the difference of velocity of the tether center of mass and the tether tip)
    This relation shows us that with a lower delta-v, design becomes much easier (the tether length and/or acceleration decrease).

  5. Unfortunately Chris, we don’t have the materials to build a tether with a 7 km/s tip velocity. More about that in an upcoming post.

  6. Kirk, do you and John Hare just pretend to be stupid on Transterrestrial Musings for kicks or what? Cause if so I’ll stop replying to you over there 🙂

  7. No Trent, do you offer up cliched arguments that have no grounding in reality just to defend human spaceflight? Cause if you do I’ll stop trying to point out that your argument holds no water.

  8. john hare says:

    Who said I was pretending?

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