Moravec investigated whether it was possible to design a rotovator for the Earth and other planets. Unfortunately, his analysis showed that unless very strong tether materials can be found, the mass ratio of a rotovator for the Earth or other large planet becomes too high to be practical to build. This is unfortunate, because otherwise, it would be possible to build tether transport systems that would allow travel between the surface of the Earth and the surface of any solid body in the solar system without requiring fuel, provided the mass flow in toward the Sun slightly exceeds the mass flow outward. Moravec did find that rotovators were feasible for the Moon and other small airless bodies. Rotovators could be designed that would touch down from one to n times per orbit, but the tether mass was minimum for a rotovator that had a total length one-third the diameter of the body, was in an orbit with an altitude of one-sixth the diameter of the body, and rotated so that it touched down six times per orbit. In a later, unpublished paper, Moravec designed a rotovator made of the Dupont fiber Kevlar for use around the moon.
Recent calculations indicate that with current materials such as Spectra 2000, with a tensile strength of 3.25 GPa and a density of 0.97 g/cc, or
By using tether reels and small thrusters on the "grapple" structure at the tip of the tether that attaches to the payload, the time for depositing the payload on the surface and picking up a new payload can be increased to many minutes. The payload would be "flown" in early by letting out cable from the tip reels and using a combination of rockets and lunar gravity to get to the surface and land earlier than the tip would normally arrive. As the payload sits on the ground and the tether descends, the grapple reels would reel in the excess cable. A well designed cable reeling system would not abruptly relax all tension in the cable as the payload touched the lunar surface, but would maintain most of the payload weight by cable tension so as to minimize transients in the main tether. After the nominal touchdown time has passed, the payload can remain on the ground for an additional time by merely releasing cable as the main tether starts to pull away. After the payload transfer has been safely completed, the rate of unreeling of cable would be decreased, and the payload lifted from the surface.Â
The concept of combining momentum-exchange tether principles with electrodynamic tether propulsion techniques to create a capability for transporting payloads from low Earth orbit, without using propellant, was originated in the late 1980's by Dr. Robert Hoyt of TUI. In a "Momentum-Exchange/Electrodynamic-Reboost (MXER) tether system, a long, thin, high-strength cable is deployed in orbit and set into rotation around a massive central body. If the tether facility is placed in an elliptical orbit and its rotation is timed so that the tether will be oriented vertically below the central body and swinging backwards when the facility reaches perigee, then a grapple assembly located at the tether tip can rendezvous with and acquire a payload moving in a lower orbit, as illustrated below.
Half a rotation later, the tether will release the payload, tossing it into a higher energy orbit. This concept is termed a momentum-exchange tether because when the tether picks up and throws the payload, it transfers some of its orbital energy and momentum to the payload. Because the MXER tether facility's orbit drops when it boosts the payload, it's orbital energy must be restored if it is to boost additional payloads. The tether facility's orbit can be restored without consuming propellant by reboosting with electrodynamic tether propulsion.
The Cislunar Tether Transport System. (1) A payload is launched into a LEO holding orbit; (2) A Tether Boost Facility in elliptical, equatorial Earth orbit picks up the payload (3) and tosses it (4) into a lunar transfer trajectory. When it nears the Moon, (5), a Lunavator Tether (6) captures it and delivers it to the lunar surface.