A skyhook is a proposed momentum exchange tether that aims to reduce the cost of placing payloads into space.
An engineer speculated in 1994 that the skyhook could be cost competitive with what is realistically thought to be achievable using a space elevator, but the skyhook is not competitive with other rotating tether concepts. In addition, it has been suggested that the rotating skyhook is "not engineeringly feasible using presently available materials". While no skyhook has yet been built, there have been a number of flight experiments exploring various aspects of the space tether concept in general.
A skyhook differs from a geostationary orbit space elevator in that a skyhook would be much shorter and would not come in contact with the surface of the Earth. A skyhook would require a suborbital launch vehicle to reach its lower end, while a space elevator would not.
Different synchronous non-rotating orbiting skyhook concepts and versions have been proposed, starting with Isaacs in 1966, Artsutanov in 1967, Pearson and Colombo in 1975, Kalaghan in 1978, and Braginski in 1985. The version with best potential involve a much shorter tether in low Earth orbit which rotates in its orbital plane and whose ends brush the upper Earth atmosphere, with the rotational motion cancelling the orbital motion at ground level. These "rotating" skyhook versions were proposed by Moravec in 1976, and Sarmont in 1994.
When the Italian scientist Giuseppe Colombo proposed in the early 1970s the idea of using a tidally stabilized tether for downward-looking Earth observation satellites, NASA officially begun to assess in 1979 the possible scientific applications for long tethers in space and whether the development of a tethered system was justified. This resulted in a Shuttle-based tether system: the TSS-1R mission, launched 22 February 1996 on STS-75 that focused in characterizing basic space tether behavior and space plasma physics. The Italian satellite was deployed to a distance of 19.7 km (12.2 mi) from the Space Shuttle.
Types of skyhooks
A non-rotating skyhook is a vertically oriented, gravity-gradient-stabilized tether whose lower endpoint would not reach the surface of the planet it is orbiting. As a result, it would appear to be hanging from the sky, hence the name skyhook.
In 1990, E. Sarmont proposed using a non-rotating skyhook as part of a space transportation system. Sub-orbital launch vehicles would fly to the bottom end of the tether, and spacecraft bound for higher orbit or returning from higher orbit would use the upper end of the tether. He expanded on the idea in a second paper published in 1994. Other scientists and engineers have investigated and added to the concept.
The non-rotating skyhook is not the same as a surface to geostationary orbit space elevator. The non-rotating skyhook is a much shorter version that would not touch the surface of the Earth and would be much lighter in mass. It would work by hanging a cable from a relatively low altitude orbit to just above the Earth's atmosphere. Since the lower end of the cable would be moving at less than orbital velocity for its altitude, a launch vehicle flying to the bottom of the skyhook would be able to carry a larger payload while being assisted into orbit by the device. This type of skyhook would start out as short as 200 km and grow to over 4,000 km in length using a bootstrap method that would take advantage of the reduction in launch costs that come with increases in tether length. With a long enough cable, single-stage to skyhook flight with a reusable launch vehicle would become possible.
In the case of the 200 km overall length, 150 km working length, non-rotating skyhook, the lower endpoint of the cable would be moving at 96.67% of orbital velocity for its altitude. A longer cable with a greater mass would mean that the speed of an arriving spacecraft could be decreased, thus lowering costs. Once the working length of the lower half of the non-rotating skyhook reaches 1,047 km, the lower endpoint of the cable would be moving at 80% of orbital velocity for its altitude.
The cost cannot be calculated until a new fiber that is at least 20 times stronger than anything currently in existence is developed. A 2000 Boeing report concluded that "in general, the non-spinning tether skyhook concept does not look competitive with the rotating tether concepts."
A rotating tether is a type of cable that would orbit the Earth with a tip speed equal to its orbital speed (around 7–8 km/s). The tip would rotate down, moving in the direction of Earth's rotation. It would enter the atmosphere at low speed and pick up a payload from the ground or the atmosphere. It then carries it up into space.
However, a Boeing study in 2000 assessed that "Trying to lower the tether tip speed to 4.0 km/s (13 000 ft/s or Mach 13) would require a skyhook tether mass greater than 200 times the payload mass." The Boeing team stated that "If the mass of the tether alone started to exceed 200 times the mass of the payload, then that was an indication the particular scenario being considered was not engineeringly feasible using presently available materials". As of 2013, a tether material with the required strength has not been developed.
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