Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tethers Unlimited, Inc.Cislunar Tether TransportTHE CISLUNAR TETHER TRANSPORT SYSTEM ARCHITECTURERobert P. HoytTethers Unlimited, Inc., Seattle, Washingtonwww.tethers.comAbstractWe describe a space systems architecture for repeatedly transporting payloads between lowEarth orbit and the surface of the moon without significant use of propellant. This architectureconsists of one rotating momentum-exchange tether in elliptical, equatorial Earth orbit and asecond rotating momentum-exchange tether in a circular low lunar orbit. The Earth-orbit tetherpicks up a payload from a circular low Earth orbit and tosses it into a minimal-energy lunartransfer orbit. When the payload arrives at the Moon, the lunar tether catches it and depositsit on the surface of the Moon. Simultaneously, the lunar tether picks up a lunar payload to besent down to the Earth orbit tether. By transporting equal masses to and from the Moon, t h eorbital energy and momentum of the system can be conserved, eliminating the need for transferpropellant. The Earth-orbit tether can also be used to send payloads to the Moon withoutreturn traffic if electrodynamic tether propulsion is used to restore its orbit in between payloadboost operations. Using currently available high-strength tether materials, this system can bebuilt with a total mass of less than 37 times the mass of the payloads it can transport. Usingnumerical simulations that incorporate the full three-dimensional orbital mechanics andtether dynamics, we have verified the feasibility of this system architecture and developedscenarios for transferring a payload from a low Earth orbit to the surface of the Moon thatrequire less than 25 m/s of thrust for trajectory targeting corrections.IntroductionUnder funding from NASAÕs Institute forAdvanced Concepts, Tethers Unlimited, Inc. hasinvestigated the feasibility of using momentumexchangetether techniques and electrodynamictether propulsion to create a modulararchitecture for transporting payloads from lowEarth orbit (LEO) to the surface of the Moon, andback, with little or no propellant consumption. 1,2A ÒCislunar Tether Transport SystemÓ would becomposed of one rotating momentum exchange/electrodynamicreboost tether inelliptical, equatorial Earth orbit and amomentum-exchange rotating tether facility in alow circular polar lunar orbit. This architecturecan repeatedly exchanging payloads betweenLEO and the surface of the Moon, with the onlypropellant requirements being for trajectorycorrections and rendezvous maneuvering.In 1991, Forward 3 showed that such a system istheoretically possible from an energetics standpoint.A later study by Hoyt and Forward 4developed a first-order design for such a system.These previous studies, however, utilized anumber of simplifying assumptions regardingorbital and tether mechanics in the Earth-Moonsystem, including assumptions of coplanar orbits,ideal gravitational potentials, and infinitefacility ballast masses. In this paper, wesummarize work done to develop an architecturefor such a system that takes into account the fullcomplexities of orbital mechanics in the Earth-Moon system. We then present a system conceptfor a Tether Boost Facility designed to boost 1000kg payloads to the Moon.The basic concept of the Cislunar TetherTransport System is to use a rotating tether inEarth orbit to pick payloads up from LEO orbitsand toss them to the Moon, where a rotatingtether in lunar orbit, called a ÒLunavator ª Ó,could catch them and deliver them to the lunarsurface. As the Lunavator ª delivers payloads tothe MoonÕs surface, it can also pick up returnpayloads, such as water or aluminum processedfrom lunar resources, and send them down to LEO.By balancing the flow of mass to and from theMoon, the orbital momentum and energy of thesystem can be conserved, eliminating the need toexpend large quantities of propellant to move thepayloads back and forth. This system isillustrated in Figure 1.Copyright © 2000 by Tethers Unlimited, Inc. Published by theSpace Frontier Foundation with Permission1