A skyhook is a proposed momentum exchange tether that aims to reduce the cost of placing payloads into space. A heavy orbiting station is connected to a cable which extends down towards the upper atmosphere. Payloads, which are much lighter than the station, are hooked to the end of the cable as it passes, and are then flung into orbit by rotation of the cable around the centre of mass. The station can then be reboosted to its original altitude by electromagnetic propulsion, rocket propulsion, or by deorbiting another object equal in mass to the payload.
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 versions 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 began 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.
An engineer speculated in 1994 that the skyhook could be cost competitive with what is realistically thought to be achievable using a space elevator.
In 2000 and 2001, Boeing Phantom Works, with a grant from NASA Institute for Advanced Concepts, performed a detailed study of the engineering and commercial feasibility of various skyhook designs. They studied in detail a specific variant of this concept, called "Hypersonic Space Tether Orbital Launch System" or HASTOL. This design called for a hypersonic ramjet or scramjet aircraft to intercept a rotating hook while flying at Mach 10.
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.
By rotating the tether around the orbiting center of mass in a direction opposite to the orbital motion, the speed of the hook relative to the ground can be reduced. This reduces the required strength of the tether, and makes coupling easier.
The rotation of the tether can be made to exactly match the orbital speed (around 7–8 km/s). In this configuration, the hook would trace out a path similar to a cardioid. From the point of view of the ground, the hook would appear to descend almost vertically, come to a halt, and then ascend again. This configuration minimises aerodynamic drag, and thus allows the hook to descend deep into the atmosphere. However, according to the HASTOL study, a skyhook of this kind in Earth orbit would require a very large counterweight, on the order of 1000–2000 times the mass of the payload, and the tether would need to be mechanically reeled in after collecting each payload in order to maintain synchronization between the tether rotation and its orbit.
Phase I of Boeing's Hypersonic Airplane Space Tether Orbital Launch (HASTOL) study, published in 2000, proposed a 600 km-long tether, in an equatorial orbit at 610–700 km altitude, rotating with a tip speed of 3.5 km/s. This would give the tip a ground speed of 3.6 km/s (Mach 10), which would be matched by a hypersonic airplane carrying the payload module, with transfer at an altitude of 100 km. The tether would be made of existing commercially-available materials: mostly Spectra 2000 (a kind of ultra-high-molecular-weight polyethylene), except for the outer 20 km which would be made of heat-resistant Zylon PBO. With a nominal payload mass of 14 tonnes, the Spectra/Zylon tether would weigh 1300 tonnes, or 90 times the mass of the payload. The authors stated:
The primary message we want to leave with the Reader is: "We don't need magic materials like 'Buckminster-Fuller-carbon-nanotubes' to make the space tether facility for a HASTOL system. Existing materials will do."
The second phase of the HASTOL study, published in 2001, proposed increasing the intercept airspeed to Mach 15-17, and increasing the intercept altitude to 150 km, which would reduce the necessary tether mass by a factor of three. The higher speed would be achieved by using a reusable rocket stage instead of a purely air-breathing aircraft. The study concluded that although there are no "fundamental technical show-stoppers", substantial improvement in technology would be needed. In particular, there was concern that a bare Spectra 2000 tether would be rapidly eroded by atomic oxygen; this component was given a technology readiness level of 2.
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- See also: letter in Science 152:800, May 6, 1966.
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