Tether system for exchanging payloads between low Earth orbit and the lunar surface

Abstract

Systems composed of several rotating tethers may provide an economic means of exchanging payloads between low Earth orbit and lunar bases with little or no propellant required. The underlying concept is to use long rotating tethers to throw payloads to the Moon and to catch return payloads sent from the Moon. By transporting equal masses to and from the Moon, the total energy and momentum of the system can be conserved. Because the mass of a rotating tether increases dramatically with the AV it can impart to a payload, splitting the lunar transfer boost operation up into two or more stages is necessary to reduce the required tether mass to reasonable levels. This work develops analytical methods for calculating designs for staged tether systems capable of repeatedly exchanging payloads between low Earth orbits and bases on the lunar surface. By properly choosing the design of the system, the total Earth-orbit mass required for a tether system to throw payloads to the Moon, using currently available materials, can be less than ten times the payload mass. Nomenclature a semimajor axis e ellipse eccentricity E orbital energy L tether arm length r radius rp perigee radius V velocity (0 angular velocity Y phase angle flight path angle lie Earth's gravitational parameter = GMe [im Moon's gravitational parameter = GMm subscripts: ma apogee »p perigee TLA's: LEO Low-Earth Orbit IPO Initial Payload Orbit EEO Elliptical Earth Orbit PEO Payload Elliptical Orbit LLO Low-Lunar Orbit LTO Lunar Transfer Orbit SOI Gravitational Sphere of Influence TTF Transport Facility Introduction In his paper Tether Transport from LEO to the Lunar Surface, Forward showed that it is conceptually possible to construct a system of rotating tethers in Low-Earth-Orbit (LEO), Copyright © 1997 by Tethers Unlimited. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Elliptical Earth Orbit (EEO), and Low-Lunar Orbit (LLO) which can lift payloads from LEO to the lunar surface while simultaneously dropping lunar resources down to LEO, without requiring any propellant. In reality, such a LEO-Lunar transport system will require some propellant for orbital maintenance of the tether facilities and for performing modifications to payload orbital-trajectories. In this paper, we discuss the design of such a tether transport system and develop a method for planning orbital parameters for the system that will minimize the propellant mass required for payload propulsion. In designing a tether transport system, minimizing the total mass of the system is a primary design driver. The two largest contributions to the total mass of the system are the tether mass required to support the loads on the rotating tethers and the facility ballast mass necessary to prevent de-orbit of the tether after a payload boost operation. The tether mass can be minimized by staging the lunar transfer orbit insertion into two or more AV operations. The necessary facility mass can be minimized by proper choice of orbital parameters of the system. LEO-EEO Staging While it is possible to design a single tether in LEO to throw payloads to the Moon, the large AV requirements make a single tether for this purpose very massive. However, the scaling of tether mass with AV makes it possible to greatly