Editing Skyhook (structure) (section)

==Rotating skyhook==

[[File:Cycloid f.gif|thumb|The rotating concept. If the orbital velocity and the tether rotation rate are synchronized, the tether tip moves in a [[cycloid]] curve. At the lowest point it is momentarily stationary with respect to the ground, where it can 'hook' a payload and swing it into orbit.]]

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&nbsp;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.<ref name=":0" /><ref name="Chen.2013" /> 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.<ref name="Boeing.2000" />

Phase I of Boeing's '''Hypersonic Airplane Space Tether Orbital Launch''' ('''HASTOL''') study, published in 2000, proposed a 600&nbsp;km-long tether, in an equatorial orbit at 610–700&nbsp;km altitude, rotating with a tip speed of 3.5&nbsp;km/s. This would give the tip a ground speed of 3.6&nbsp;km/s (Mach 10), which would be matched by a hypersonic airplane carrying the payload module, with transfer at an altitude of 100&nbsp;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&nbsp;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:

<blockquote>
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."<ref name="Boeing.2000" />
</blockquote>

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&nbsp;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.<ref name="Boeing.2001">{{cite web |url=http://www.niac.usra.edu/files/studies/final_report/391Grant.pdf |title=Hypersonic Airplane Space Tether Orbital Launch (HASTOL) Architecture Study. Phase II: Final Report. |access-date=2015-10-18}}</ref>