Compound Tethers

guest blogger john hare

This is not part of Kirk’s series on tethers.  Unlike his professional tether work that just needs  funding and hardware development, this is a concept that may have serious flaws that make it yet another hare brained scheme. If Kirk sees a flaw in this post, that  is probably good enough to take for granted that he’s right.

One issue that I have had with tethers is that they are difficult to use as an LEO delivery system. When a rotovator picks up a cargo from orbit or suborbit, it preferentially sends it to an orbit as much higher than the tether orbit as the pick up point was below. So a rotovator must always orbit independently of the nodes that it services. I would like a system that can pick up a cargo from a suborbital vehicle and deliver it to an LEO station. I’m sure there are other methods for doing this, but I think this one may have some benefits.

With a heavy rotovator in LEO with a tip speed of 1,200 m/s, hubs are attached to both tips with much lighter tethers from those hubs with tip speeds of 1,200 m/s relative to the respective hubs. At full extension, the additive velocity of the compound tether is 2,400 m/s, while at the cross point near the center of the main hub, the light tether tip velocity relative to the orbit and the main hub is momentarily zero. This allows the light tether to pick up a suborbital payload at 2,400 m/s below orbital velocity, and have it at rest relative to the station half a revolution later.

This is much like one of the carnival rides that have arms from the ride center holding smaller arms with several seats on the smaller arms with both sets of arms rotating. At one point you can almost shake hands with someone standing in the middle, and half a turn later the gees have you pressed against the side of your seat near the fence. Another type of ride has the outer arms contrarotating which gives a momentary stop near the fence with a really fast path near the main hub. If you have seen either of this type of ride in operation, then the compound tether becomes much easier to visualize.

The compound tether would allow the station to pick up a propellant tank from a suborbital vehicle and bring it to rest quickly for emptying. Then the system would be able to pick up the empty tank, and throw it back so that the only propellant used to maintain orbit would be for lifting the cargo. Co-orbiting with the ISS, the compound tether could fling the garbage astern to maintain orbit and supply the orbital energy to lift more supplies.

A second feature of this technique is that the tether vectors can be added in various ways to match other inclination orbits.  If the above cartoon were laying flat Earth relative in LEO, then it could pitch and catch cubesats into a whole range of LEO orbits. This would allow a commercial operator to launch a large  group of small payloads to an LEO distribution center to be dispensed into the orbits required by the customer(s).

An early application during development could be for orbital debri removal. A Falcon I could launch a system that could grab hundreds of small debri over a period of years and deorbit them. Each piece grabbed could be used for propulsion mass to reach the next one like a water bug on a pond. The B hub in the cartoon show a couple of the directions and velocities available to such a system. The system would  have to time an orbit crossing to grab the garbage and not actually have to match orbits with it. By using debri mitigation for development, missed catches would be an embarrassment rather than a lost payload.

One future application could be on the Lunar surface, A compound tether on a tower could catch payloads from Earth and bring them to rest on the surface. It could also send payloads to Lunar orbit by picking them up from a platform.

Compound tethers have some serious flaws. One is the constant load cycling. The gee stress on the outer tethers will be constantly changing as the combined force vectors of the two tethers add and subtract  on every revolution. Another is that when the gee forces add during full extension, it is probably worse than a straight tether. I don’t have the math to model all the stresses involved, but I believe that the system I drew above would have higher stress, and therefore more mass than a standard tether with the same tip speed.

A contra-rotating system though, cancels stresses to some extent. With contra-rotating, as in the carnival ride, the temporary motionless point would be on the extreme perimeter. As the small tether tip approaches the main hub, much of the centrifugal stress cancels, which might allow the compound tether to exceed the capabilities of the simple tether. I have read that current materials allow a tapered tether to have tip speeds of 3,000 m/s with a reasonable mass ratio. It might be possible for a compound tether system to have 2,500 m/s tip speeds compounded. If this is true, and it is sheer conjecture at this time, then it would allow a tether built with current materials to capture payloads from suborbital vehicles at 5,000 m/s under orbital velocity and bring the to rest at a co-orbiting LEO station. By using the down mass from the station, very little propellant would be needed to maintain the orbit after assembly complete.

A 5,000 m/s catcher would put orbit withing the reach of an upper stage from the current group of suborbital vehicles . And though buckytubes and nanotech would make it all so much easier, this just may be something that could be done while waiting for them to be available.

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johnhare

I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.

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17 Responses to Compound Tethers

1. A_M_Swallow says:

It should be possible to test this idea with a 100 metre tether. Although a length that is a simple multiple of pi may be easier to model.

2. john hare says:

I’m thinking that Kirk will mention some prior work on something like this, including problems or a solid conclusion on why it won’t work. Modeling it on the computer might be a little tricky. A fairly slow 10 meter compound could be instrumented and tested on the ground. A hundred meter in orbit as you say could be a secondary payload for some real confirmation. On the extreme off chance that I am completely correct, something real could be in use in a couple of years.

3. I’m still reading the original post, but John, you’re absolutely right that there’s no simple way to deliver a payload to an orbit that you’re in with a single tether system.

4. john hare says:

Kirk,
Thank you for taking the time to look it over. This is really such a dirty kludge compared to the clean elegance of the true rotovators. If it is worth modeling, it’s going to be a pain, and I don’t have the tools.

5. Eric Collins says:

There is some interesting dynamics to such a system. Like for instance, when you catch or release a payload, how is the momentum transferred. What does it take from the rotation of the primary tether vs. the secondary tether(s).

The other question is how will this thing behave in a gravity field. Depending on the rotational frequency of the primary and secondary tethers and the mass distribution of the tether-payload system, their could be some real non-linear motion. I’m thinking specifically of compound pendulums. The addition of the one additional pivot point is sufficient to produce fairly chaotic motion.

6. john hare says:

Eric, good points reenforcing that this thing would have to be seriously modeled before spending much on hardware. It is only the possible advantages of having a tether served transportation hub that makes it worth considering. I think I have a couple of workarounds for more obvious problems once we see what people can find wrong with the basic thought.

7. Rob says:

The main challenge with this dual-rotovator concept is that you need to keep the spin planes of the two tether stages perfectly aligned, or the second stage tether will smack into the first stage. And just the Earth’s oblateness will be enough to induce torques on the system that will cause misalignment between the spin vectors. Any electrodynamic thrusting performed on the system will also indue torques on the system.

8. gravityloss says:

Aren’t there huge changing loads even when the system just spins, never mind taking any payloads?

9. john hare says:

Rob,
Separation issues are crucial, 2,400 m/s smack can’t be good. This system couldn’t use electrodynamic thrusting as that requires proper alignment with the Earth, and this one is changing by the second.

Gravityloss,
Absolutely, which makes load cycling about the first issue to look into. I would put that at #1 on the list of things that could bust this concept.

10. gravityloss says:

Though if you have counterweights in the tip tethers as well then the loads could perhaps be evened. This gets a bit beyond intuition already…

11. john hare says:

Gravityloss,
It is the stresses in the tip tethers that concern me most. For mass, they are like the payload on a multistage rocket, every added pound is reflected and multiplied as you go down.

It is beyond reasonable intuition. I’m hoping that Kirk and company have already looked into something like this and can either point me to prior art, or tell me the definitive reasons why it won’t work. If there are just technical problems, I have a shot at out-thinking them. Frex, a tiny tip tether length relative to the main boom would experience far less of the cycling loads.

12. John, I can tell you that the NASA momentum exchange tether program never looked at compound tethers because none of us could believe that such a system would spin without hitting itself. Perhaps it’s possible, but I would need to see greater proof before going on an advocacy program for such a system.

The fact that the tether is in an intermediate orbit between the initial and destination orbit isn’t such a bad thing. For a LEO-GTO tether, you don’t really care anyway, and for a suborbital-GTO tether or suborbital-TLI tether, well, we don’t have the material strength to build those tethers yet anyway.

13. john hare says:

Kirk,
Thank you for looking it over. From your comment, it looks to me like this idea was thought of and stopped at the napkin sketch stage. I thought I replied last night, but apparently missed a button. I agree this idea is busted unless a bulletproof way of ensuring separation can be found. I didn’t think I was talking against conventional tethers so much as attempting to address a couple of limitations, material strength through staging, and servicing LEO.

I will give some thought to separation issues and will return to this if I think I see a way.

14. Bob Steinke says:

If you want to deliver a payload to the orbit that the tether is in the payload could climb up to the tether hub before releasing. This would require some kind of climber system, which might not be so simple, but there’s at least an initial proof of concept in the beamed power challenge.

15. John, I think it’s certainly an idea worth consideration. Sometimes even bad ideas have good outcomes. I’ll give you an example. I saw a very crude sketch in a paper by Bryan Tillotson of Boeing for a compound tether very similar in principle to what you described. But the sketch wasn’t for the tether–it was for a “catch mechanism” for a tether. It was very different that anything I had previously conceived for that purpose before, but it looked like it might work. Well, to make a long story…shorter…that sketch (which wouldn’t work) led me to a new catch mechanism design (which also wouldn’t work) which led me to another design (which would work, but not well) which led to to another design (which would work okay) which led me to another design (that would work well) which led to design a model that was actually tested successfully.

If I hadn’t read Tillotson’s paper (circa 1989) on that compound tether I wouldn’t have began walking down the road that led to a successful and simple tether catch mechanism design.

16. john hare says:

Kirk,

Thanks for the kind words. I try not to push ideas that have clear flaws pointed out as it irritates the people that give such good feedback. I found one possible workaround today that ‘should’ work for the separation issue. I’m not sure I want to keep pushing it here though. I’ve thrown out the original idea for people to look at and the details I’m playing with now would get boring fast.

17. john hare says:

I said that wrong. Pushing an idea with known flaws is just wrong unless you have a solution in mind.