Momentum Exchange Space Tethers

Momentum-Exchange/Electrodynamic-Reboost Tethers
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.

momentum exchange

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.

Tether Transport Architectures
Several research efforts have investigated conceptual designs for momentum-exchange tether systems. In 1991, Carroll proposed a tether transport facility that could pick payloads up from suborbital trajectories and provide them with a total delta-V of approximately 2.3 km/s.

Soon thereafter, Forward proposed combining this system with a second tether in elliptical Earth orbit and a third tether in orbit around the Moon to create a system for round-trip travel between suborbital Earth trajectories and the lunar surface. In 1997, Hoyt developed a preliminary design for this "LEO to Lunar Surface Tether Transport System."

In 1998, Bangham, Lorenzini, and Vestal developed a conceptual design for a two-tether system for boosting payloads from LEO to GEO. Their design proposed the use of high specific impulse electric thrusters to restore the orbit of the tether facilities after each payload boost operation. Even with the propellant mass requirements for reboost, they found that this system could be highly economically advantageous compared chemical rockets for GEO satellite deployment.

Under a Phase I NIAC effort, Hoyt and Uphoff refined the LEO to Lunar system design to account for the full three-dimensional orbital mechanics of the Earth-Moon system, proposing a "Cislunar Tether Transportation System." This architecture would use one tether in elliptical, equatorial Earth orbit to toss payloads to minimum-energy lunar transfer orbits, where a second tether, called a "Lunavator" would catch them and deliver them to the lunar surface. The total mass of the tether system, could be as small as 27 times the mass of the payloads it could transport.

Figure 2. 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.

The same NIAC effort also resulted in a preliminary design by Forward and Nordley for a "Mars-Earth Rapid Interplanetary Tether Transport (MERITT)" sys-tem capable of transporting payloads on rapid trajectories between Earth and Mars.

Momentum-exchange tethers may also provide a means for reducing the cost of Earth-to-Orbit (ETO) launches. This architecture would use a hypersonic air-plane or other reusable launch vehicle to carry a payload up to 100 km altitude at Mach 10-12, and handing it off to a large tether facility in LEO which would then pull it into orbit or toss it to either GTO or escape. This concept is called Tether Launch Assist.


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