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I was paging through a book called Islands in the Sky, then I came upon this article by Robert Zubrin:
In this article I present a third type of tether-skyhook system, which I call a hypersonic skyhook (figure 2.1). As I show, the hypersonic skyhook has the potential of relieving the severe strebgth-of-materials demands, which have thus far prevented the implementation of a practical skyhook of either the geostationary or rotating varieties. The hypersonic skyhook accomplishes this by keeping the tip of the extended tether outside of the tangible atmosphere and allowing it to move at hypersonic velocities with respect to the ground. This allows the skyhook's center of mass to be lowered from geostationary altitude, reducing the length of the tether, and cutting its mass and taper ratio by many orders of magnitude. Unlike the rotating skyhook, however, the hypersonic skyhook does not spin about its satellite center of mass, but keeps both tethers aligned along a constant radial directiopn toward and away from the center of Earth.
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I seem to recall in his "Mars Direct" book that, regarding the exact same topic, he pointed out how even a jet trying to rendevous with this thing would be suicide - how the HELL do you latch onto something moving a couple hundred miles an hour faster than you without sheer luck and survive...in less than a dozen pieces? :shock:
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I seem to recall in his "Mars Direct" book that, regarding the exact same topic, he pointed out how even a jet trying to rendevous with this thing would be suicide - how the HELL do you latch onto something moving a couple hundred miles an hour faster than you without sheer luck and survive...in less than a dozen pieces? :shock:
Read the description above again. The extremities of the tether are above the atmosphere. You need something like a scramjet which can approach MACH 15, and there working on that, you need some strong nanotube fibers, and there working on that too. There has been some significant progress in both carbon nanotubs and in scramjet testing. So far we haven't produced nanotube ribbons that are strong enough for a statinary space elevator, and we haven't produced a scramjet that can reach orbital velocity, but they say MACH 15 should be within reach. What we need to do is combine scramjets with space elevators so we can get into orbit. The scramjet would build momentum within the atmosphere, burning its fuel with ambient oxygen in a hypersonic stream, and it would leap out of the atmosphere on a ballistic trajectory matching the velocity of the lower extremity of the tether, and using some downward pointing rockets it would hold its altitude long enough to rendervous with the tether, open its cargo bay doors and attact its cargo to a vehicle attached to the tether so that it may be ferried up and into orbit. The scramjet would then drop back into the atmosphere, land at a runway and prep for the next launch.
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RedStreak Dec 16, 2007
... how the HELL do you latch onto something moving a couple hundred miles an hour faster than you without sheer luck and survive...in less than a dozen pieces
I suspect something similar to the lower end of the rotavator could be used: Reel the lower end out as it approaches rendezvous time, and reel it back in to extend the time available.
Above all, don't even try, if you can't cut your approach speed down to only maybe a few tens of meters/sec, instead of hundreds of km/hr.
Lots easier than trapping on a pitching carrier deck, at night, in bad weather, during a battle.
I suspect that the lower end of the hypersonic skyhook could look a lot like the helo landing deck on a warship. As it sits while waiting for an incoming craft, two sets of cables lie along the edges of the pad (fore & aft, port & starboard. The incoming craft lets out a cable with a hook. When the hook touches down, the cables along the 4 edges of the pad contract into the center, capturing the hook. The helo pilot applies positive collective pitch, while the cable underneath is reeled down, and the landing ear are trapped.
Navy pilots rely on this for their lives, navies to win a battle or the war, and their nation's survival.
The hypersonic skyhook may be looked at as a "cheat" to orbit, like "zero stages" such as a horizontal track along the ground: NASA has seen that if you accelerate a spaceplane horizontally up to mach .8 before firing the rockets, you decrease the fuel to reach LEO by ~30 %. Toss in airbreathing (not even necessarily a scramjet), and an upper stage like the hypersonic skyhook, and you've got a spaceplane that's effectively SSTO HTOHL, without even needing full orbital delta-V or thermal protection.
Each addition to the basic spaceplane is incapable of getting it up alone, but each applies its own strengths, and helps out where it can.
Another nice thing about the hypersonic skyhook, is that you don't need unobtanium like nanotubes. Kevlar is not too far away from what's needed.
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