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.