When I was much younger in college (back in the dark ages to hear some of my cow-orkers talk :p) one of the things I recall being talked about were skyhooks. As I was having a discussion today with some old friends, the topic of launching into orbit came up, and I mentioned this…and drew a blank. There is a very brief Wiki on it, but it doesn’t really answer the question…why don’t we use them and what happened to the concept?
The concept is pretty simple if you don’t know what it is. It’s like a poor mans space elevator. Basically, just a non-rotating or rotating tether that is in orbit (preferably, IIRC, a geosynchronous orbit). You fly a hypersonic jet with payload or maybe a rocket up to the hook end of the tether then use it’s inertial energy to move you up to a higher orbit. You use some sort of station keeping system to recover the lost energy (some sort of thrust or propellant, possibly brought up as part of the payload each time). It’s fairly low tech, and as I recall the cost per kilogram put in orbit was relatively close to what was possible with something like a space elevator…but without all the grubby reality stuff that we can’t make the space elevator and it’s got a pretty bit capital cost even if we could. From memory even the materials available when I was in college (i.e. in the early 80’s) could do the trick…and today we have a lot better materials. So…why has no one tried this out? Is there some issue or show stopper?
Work out the total mass of the skyhook, then tell me how we get that much mass into orbit and impart it with the correct orbital velocity etc. Even if on paper it all works out and it would reduce cost to orbit 1000x theres still the enormous cost of building a completely unproven technology. Who’s going to pay for it?
If the end of the cable is in low Earth orbit, then the cable is moving at low Earth orbit speeds. If the cable is moving at low Earth orbit speeds, then that’s the speed you have to get your airplane up to. And if you can get an airplane up to that kind of speed, then you can just put the entire plane into orbit.
And if the end of the cable is in geosynchronous orbit, then you don’t need the plane, and it’s exactly a space elevator.
It’s conceivable that there’s some happy medium between the two, but I suspect not, because the in-between case would have most of the complexities of the two systems combined, plus some extra complexities like the rendezvous.
IIRC The idea of the rotating skyhook is that the center of mass is in low Earth orbit, but the rotation of the skyhook around the center of mass allows the end of the hook to (briefly) be moving at very low speed relative to the ground. Fun to think about, but probably less fun to ride aboard…
I believe they are waiting on developing a carbon fiber cable light enough to support it’s own weight dangling from a satellite. More cables would be attached until they can support an elevator lift.
Modern research shows the materials needed don’t yet exist. Diamond nanotubes may do the trick.
However there is a massive problem: sabotage. You’ve got a massive stationary target that, if sabotaged, could cause massive devastation to the planet as detailed in Red Mars by Kim Stanley Robinson.
A rotating skyhook is a good alternative to a space elevator on a slow-rotating, airless world, like Luna or Mercury. On a world with an atmosphere, though, like the Earth, the air resistance will kill your effectiveness.
This is the key. To make a rotating skyhook viable, you need to put the rendezvous point for the skyhook/rotovator as far above the dense atmosphere as possible. Because a skyhook/rotovator can be theoretically stationary at the bottom of its cycle, the speed of the rendevous can be negligible.
You should be able to make up the loss of momentum due to air-resistance + the mass of the payload by pumping solar power into the system. This might even be achievable using electrodynamic tethers, although how that could be made to work in a rotating skyhook would be a challenging engineering problem.
These concepts are described in the wiki page on Momentum Exchange Tethers
…the most significant sentence on that page is probably
"Unfortunately an Earth-to-orbit rotovator cannot be built from currently available materials since the thickness and tether mass to handle the loads on the rotovator would be uneconomically large. "
but other, less demanding options are also discussed.
The Tether-Launch-Assist concept seemed promising at one point; http://www.tethers.com/LaunchAssist.html
but perhaps some of the momentum has gone out of this project since the death of Robert Forward (so to speak).
