Just a note that a rotating skyhook was used in Neil Stephenson’s recent SevenEves.
You do get some advantage, in that a non-rotating skyhook lower end will be going less than orbital speed, but it’s not by all that much. If you put the center of mass about 1500km up, then you are only decreasing the speed of the lower hook by a bit less than a km per second. Though, when going to space, every bit helps, and not having to carry the fuel for that last kps could result in pretty significant fuel savings or increase to payload.
Trying to bring the end of the skyhook to a stationary point would involves some pretty insane stresses and speeds on it. The ends would need to be rotating at the same speed as the center of mass is orbiting. A more practical approach is to simply lower the required speed to some happy middle where the skyhook is not at such a speed that it impractical or impossible to build with current materials, but gives enough of a boost into orbit to make it worthwhile.
Some videos on the concept. I don’t always agree with this guy, and he is somewhat optimistic in terms of materials science, but they go over the concept fairly thoroughly.
But… we did something similar in the 30’s:
How fast was that dirigible moving when the biplane attached to it? How fast would the skyhook be moving?
I could be wrong, but doesn’t what matters is the relative closing speed of the two objects during rendezvous?
Disclaimer: My linking of the dirigible stuff was mostly in jest, anyway. I think the USN dirigible skyhook stuff is pretty cool, and almost steam-punk, in it’s own way.
Both USS Macon and USS Akron crashed in bad weather.
I know. Are you saying that this fact invalidates the “skyhook” as a concept?
No, that dirigibles were invalidated as aircraft carriers. 
Relative speed is mostly all that matters. Militaries all around the world perform aerial refueling every day at jet cruise speeds.
Conversely, the dirigible stuff, as well as McDonnell XF-85 Goblin - Wikipedia from the immediately post WW-II era, didn’t fare too well. Youtube has some horrifying footage of this not working.
It takes a lot of stability to fly precisely. The faster you’re going, the more any given angular change in orientation translates into distance away from the desired spot per unit time. So you need very precise angular control to begin with and the tolerances get tighter & tighter as speed increases.
The flight control tech of the 1930s couldn’t really make it work at dirigible speeds. The flight control tech of the late 1940s couldn’t really make it work at propeller-driven bomber speeds. The flight control tech of the 1960s & subsequent make it work at slow jet speeds. And it works much better now with modern aircraft than it did with 1960s aircraft. We’re probably not able to do it at Mach 1.5 yet. At least not with humans driving.
Hypersonics are a whole 'nother kettle of worms.
I am no engineer. Could a “skyhook space elevator” be sub-sonic, ground-speed-wise? (Or does that defeat the whole purpose? :smack:)
Turbostratic carbon fiber, made from (polyacrylonitrile precursor) isn’t anywhere strong enough for this purpose. Carbon graphene nanotubes–essentially extemely long single crystals of hexagonal lattice carbon–may have sufficient tensile strength but would not survive the atmospheric heating in this hypersonic rotating/whipping motion, and the longest carbon nanotubes we’ve produced to date are about 50 cm in length and were not entirely free of crystallographic defects, reducing their tensile strength to a fraction of the theoretical maximum. It is not known how, when, or if we’ll be able to produce carbon nanotubes hundreds of kilometers in length suitable for ground-to-orbit structures or lifting, and it certainly won’t be by conventional fabrication means.
To summarize the correct answers as to why this skyhook concept is not feasible:
[ul]
[li]Insufficient tensile strength in conventional materials[/li][li]Atmoshperic heating due to relative mothion of the structure versus atmosphere[/li][li]Atmospheric drag losses [/li][li]Casualty hazard to ground[/li][li]Potential hazard by and to other orbiting objects[/li][li]Lack of ability to lift sufficient mass into orbit or use in-situ utilization of space materials to construct this system[/li][li]Cost versus perceived need[/li][/ul]
Orbital momentum exchange tethers and electrodynamic tethers are separate concepts with various applications including orbital maneuvering for stationkeeping and retirement, gradual ‘propellantless’ orbit raising, interplanetary impulse, et cetera. There are several companirs working on thse concepts such as Tethers, Unlimited but they are primarily focused on small satellites (smallsats, nanosats, picosat) applications because those forms have little room or mass to allocate for conventional propulsion systems.
A space elevator with a terminus at geostationary orbit (GSO) is stationary with respect to the ground and thus doesn’t experience the drag losses and attendant heating effects, and will also occupy a predictable region in orbit space that satellites in lower orbits can be projected to avoid. The ground hazard from a space elevator can be minimized by making the lowest section the weakest point and a counterweight above GSO such that breaking or releasing the tether results in the orbital terminus raising the remaining structure out of harms way, so the only falling hazard is in the lowest section which can be cleared out, or even with the anchor placed in mid-ocean, minimizing ground hazards. There is literally no way the main structure above the atmosphere can fall or be pulled down. This rotating ‘skyhook’, however, could break apart at any point (the most highly loaded regions are near the hub), flinging debris up to near orbital speeds on trajectories which may wrap around the globe with no control over where they land. But we lack materials with sufficient tensile strength in the necessary lengths to construct it.
I cannot imagine a scerario for which investing money and engineering effort into this rotating skyhook concept would be better than advancing the state of the art in conventional heavy lift rocket launch systems (including the use of sustainable renewable propellants, developing high efficiency pulse or and continuous wave detonation engines, developing more robust and logisitcally less complex vehicles and support systems, et cetera) for transport and autonomous space resource extraction/utilization systems and orbital manufacturing to reduce the need to ship bulk materials and large manufactured items. This skyhook would be a massive megastructure with the attendant costs, risks, and consequences of failure upon industries that would depend upon it as the single route for orbital access.
Stranger
Keep in mind that reaching orbital altitude is far easier than reaching orbital speed. The Germans got a V-2 to almost 110 miles during WWII. It took 13 more years to send a Sputnik into orbit.
So you can keep the skyhook completely above the atmosphere and still be theoretically useful.
One item not on Stranger On A Train’s list that’s a big issue for me: Stability. This thing is going to wiggle around a lot. That’s going to be a problem for many reasons. Magic carbon fibers aren’t going to do anything about this.
You still need to rendezvous with it somehow. If you’re planning on just launching a sounding rocket to the right height and having the cable zip by and hook onto your payload, that’s going to put an absolutely insane stress on your cable, orders of magnitude greater than what you’d have in a space elevator.
Plus, of course, the guidance difficulty of catching something on a hook moving at orbital speeds, but that’s a problem that could at least plausibly be solved with reasonably-anticipatable technology.
I think the skyhook dreamers envision the previously mentioned scenario of the rotational rate of the hook and the orbital speed of the center of mass matching in such a way that the hook is moving at 0 speed relative to the Earth at the bottom of the spin. (I.e., this is not the perpetually straight up and down scenario like a truncated space elevator. Which has even more issues, IMHO.)
So rocketing straight up and reaching max altitude at the hook’s lowest altitude and both will be essentially not moving with respect to each other. Still some serious trajectory tweaking to get it right.
The main stress involving hooking up will be the skyhook now having to absorb the extra mass on the end while maintaining orbital and spin stability. Good luck with that, of course.