A skyhook is a proposed momentum exchange tether concept that aims to reduce the cost of placing payloads into space.
Skyhooks are often confused with an Earth surface to geostationary orbit space elevator, but they are different. A skyhook is much shorter than a space elevator and does not reach down to the surface of the Earth. A skyhook requires a suborbital launch vehicle to get to the bottom end of the skyhook, a space elevator does not.
Its promoter speculated in 1994 that the skyhook concept 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, the rotating skyhook is in fact deemed "not engineeringly feasible using presently available materials".
Skyhooks come in two types: rotating and non-rotating. While no skyhook has been built so far, there have been a number of flight experiments exploring various aspects of the space tether concept in general.
Types of skyhooks
A non-rotating skyhook is a vertically oriented, gravity gradient stabilized, tether whose lower endpoint does not reach the surface of the planet it is orbiting. As a result it appears to be hanging from the sky, hence the name skyhook. The idea of using a tidal stabilized tether for downward looking Earth observation satellites was first proposed by the Italian scientist Giuseppe Colombo.
The idea of using a non-rotating skyhook as part of a space transportation system where 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, was first proposed by E. Sarmont in 1990, and expanded on in a second paper published in 1994. Other scientists and engineers have also investigated and added to the concept.
The non-rotating skyhook is not a space elevator. The non-rotating skyhook is a much shorter version of the Earth surface to geostationary orbit Space Elevator that does not reach down to the surface of the Earth and is much lighter in mass. It works by starting from a relatively low altitude orbit and hanging a cable down to just above the Earth's atmosphere. Since the lower end of the cable is moving at less than orbital velocity for its altitude, a launch vehicle flying to the bottom of the skyhook can carry a larger payload than it could carry on its own. This type of skyhook can start out as short as 200 km and grow to over 4,000 km in length using a bootstrap method that takes advantage of the reduction in launch costs that come with each increase in tether length. When the cable is long enough, single-stage to skyhook flight with a reusable launch vehicle becomes possible.
In the case of the 200 km overall length, 150 km working length, non-rotating skyhook, the lower endpoint of the cable is moving at 96.67% of orbital velocity for its altitude. As the length of the cable increases, the mass of the cable increases, the required speed of the arriving spacecraft decreases, and the cost of flying to the skyhook decreases. Once the working length of the lower half of the non-rotating skyhook reaches 1,047 km, the lower endpoint of the cable is 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 be in orbit around the Earth, with a tip speed equal to its orbital speed (around 7–8 km/s). The tip rotates down, and as it does so, it moves in the direction of Earth's rotation, enters the atmosphere at low speed and picks up a payload from the ground (or the atmosphere). It then carries it up into space. The skyhook acts as a momentum exchange tether. If it is used to lift many payloads into orbit its own orbit will degrade. If it catches fast moving 'rocks' on the high end of the skyhook, their kinetic energy would help lift the skyhook into higher orbits.
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 yet.
A form of hard-structure subsonic skyhook was constructed during the events of Jack McDevitt's novel Deepsix.
- In the anime Bubblegum Crisis: Tokyo 2040, the three main protagonists arrive at the series' climactic battle with Galatea in Earth orbit by commandeering a skyhook transit system.
- Turn-A Gundam, anime series, depicts an ancient hypersonic skyhook which has been maintained operationally by nanomachines over thousands of years. An ancient mass driver is also used for transporting space-vessels from earth's surface to the skyhook.
- In the Star Wars expanded universe, skyhooks are common above Coruscant. They are frequently private retreats owned by corporations or wealthy individuals.
- In the LucasArts video game Star Wars: The Force Unleashed a skyhook is being constructed on the planet Kashyyyk.
- The planet of Tara K. Harper's Grey Ones series features a number of skyhook stations. The tethers are apparently no longer functioning, but large terminal structures still exist.
- A skyhook figures prominently in Arthur C. Clarke's posthumous novel The Last Theorem, which he co-wrote with Frederik Pohl. The novel describes the skyhook as a means of interplanetary travel rather than simply a means to reach orbit. It is used as a means of transport by athletes and delegates to the "first-ever lunar Olympics".
- Skyhook construction is a central theme in the science fiction novel The Barsoom Project, the second book in the Dream Park series, by Larry Niven and Steven Barnes. The destructive potential of a falling skyhook is also explored, and the potential for this to be exploited by terrorists.
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