Momentum Exchange Tethers — An Introduction

Anyone who’s interested in going to the Moon ought to take a serious look at the technology of momentum-exchange tethers. My own interest began back in 1998, when as a summer intern on the X-33 program at Lockheed Martin Skunk Works, I spent time after work trying to come up with a new lunar exploration architecture. My ideas were based on the heavy expectation of ISRU, a heavy-lift rocket, and helium-3 mining at the Moon and later the gas giant Uranus.

After working on the effort for a few weeks, I summoned up the courage to show my ideas to my coworkers. One of them read through my work, and said to me “that’s it? can’t you come up with something better than that?”

I was really crushed and I asked him what he meant. He said, “Don’t you think that after 30 years that we can come up with a better way to go to the Moon than big throwaway rockets?”

Like someone who had just been told that their baby was ugly, I went back to the drawing-board, so to speak, humbled by my colleague’s response to my idea.

I started poking around the internet and found a company called Tethers Unlimited, owned by Robert Hoyt and Robert Forward, the late science fiction writer. This company (TUI) was talking about a kind of space tether I had never heard of before. It spun round and round and caught and threw a payload from one orbit to another. It sounds fantastic (as in fantasy) and I sat there and thought about, fresh from my orbital mechanics course a year earlier. After a few hours I had convinced myself that it didn’t violate the laws of physics or orbital mechanics, and that, in theory, it should be possible to do what they claimed. But I thought it was hopelessly complicated from an engineering perspective. The “catch maneuver” in particular seemed nigh-unto impossible.

Little did I know what I would be getting myself into–some of the most professionally rewarding and frustrating years were to follow that little discovery…

…as a preview, enjoy this animation, which a team of animators and I spent the better part of a year (2004) working on, so that we might make momentum-exchange tether principles easier to understand.

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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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10 Responses to Momentum Exchange Tethers — An Introduction

  1. john hare says:

    The neat thing about a momentum excjange tether in the orbit and use that you have here is that it has tip velocities of 1,500-1,700 m/s, which is well within spectra or kevlar capabilities. Nanotubes and fairy dust are not necessary.

    Do the lifetime trips through the high radiation zones affect the tether reliability or expected service life?

  2. John, the trips through the radiation belts were a real concern to us, especially with regards to the solar arrays. We had a lot of “space environment” materials issues to work through, including atomic oxygen degradation to the tether, radiation degradation, and orbital debris/micro-meteoroid damage to the tether. It was going to be a significant challenge.

  3. Eric Collins says:

    Nice teaser post. I’ve been hoping Jon, or John, or somebody would post about tethers sooner or later. I’ve been a fan of momentum exchange tethers since I read Dr. Forward’s book, “Indistinguishable from Magic”. There are a lot of far out technologies put forth in that book (most of them completely plausible if not terribly practical), but I knew as soon as I came to the part which talked about tethers that it was the only sensible way to do routine orbital transfers. There’s something elegant about being able to completely conserve (and reuse) angular momentum without expending (i.e. wasting) precious propellant.

    For the near term future, propellant depots may be among the most important technologies that needs to be developed and deployed to enable meaningful exploration beyond LEO. However, in the longer term, momentum depots are going to be what really enables routine travel within the solar system. I have a picture in my mind of momentum depots at strategic locations/orbits throughout the solar system acting like on/off ramps to interplanetary destinations.

  4. Mike Puckett says:

    “Crack that Whip!….Give the past the slip…….”

  5. john hare says:

    From the post title, I take it that you will be doing a series including Lunar surface pick up and deliver? Also tether applications to interplanetary travel? Using unprocessed Lunar downmass to lift and send Earth payloads on etc?

  6. tom.cuddihy says:

    awesome! I did my thesis on military applications of tether systems (NPS fall 2006). The orbital mechanics of tracking and predicting a tether system is fascinating stuff. Given what i’ve learned, I think a lot of work still needs to be done on tether construction and survivability, especially when you add in electric tether propulsion. It’s amazing what TUI has accomplished with no significant support though.

  7. Oh TUI had significant support–much of it came from my department!

    There’s a lot of other groups who advanced the technology tremendously who haven’t gotten much recognition, like Dr. Stephen Canfield and his team at Tennessee Tech, and Dr. Eugene Levin.

  8. Eli Doane says:

    What does the jerk (dA/dT) profile look like for a system like this? Seems like it might be a killer?

  9. I’ve read many MX papers. A few of which have been really good. The majority, however, spend so much time discussing the plethora of options that they fail to go into significant detail of any one option.

    As such, can you recommend a paper that describes an LEO-to-GTO tether boost system and includes actual mass numbers? :)

    Best I’ve found is:

  10. SimonDM says:

    I did my PhD research on dynamic yarn simulation (for weaving machines). First thing I can tell you is that you’ll NEVER be able to fully model tethers with the inclusion of bending and torsion effects. Simplifying with only tension, the biggest problem I see is that these tether concepts seems thought up by people ignoring the finite speed of sound in materials and damping/hysteresis inherent in any cable, no matter how low. Even not considering the catching of payload (which will send tension waves at finite speeds of over 1 km/s over tens to hundreds of kilometers of distance), the simple rotation of a few tons of tether alone will dissipate energy every single rotation in a gravity field with a gradient. I’m too lazy to do a back of the envelope calculation to find roughly how much and whether this impacts the viability of the design (I’m surprised I haven’t seen such basic calculations yet at, but suffice to say there will be continuously waves running in such a system.

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