Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

INTERPLANETARY TETHER TRANSPORT OVERVIEWGerald D. Nordley* and Robert L. Forward**AbstractTravel to and from planets may be accomplished by a system of rotating tethers inelliptical orbits about each planet. A payload is picked up near periapsis and tossed laterstill near periapsis, at a velocity sufficient to give the payload a substantial hyperbolicexcess velocity. At the destination planet, it may be caught near periapsis and released ashort time later in a bound trajectory. The system works in both directions and is reusable.Kinetic energy lost by the throwing tethers can be restored either by catching incomingpayloads, by propellantless tether propulsion methods, and/or high specific impulsepropulsion systems. In preliminary studies with simplified assumptions, tethers with tipvelocities of 3 km per second may send payloads to Mars in as little as 70 days ifaerobraking is used at Mars to dissipate excess relative velocity and the orbital phasing isfavorable. Tether-to-tether transfers without aerobraking may be accomplished in about110 to 160 days. A rotating free space tether with a payload at each end approaching aplanet may be split at periapsis, leaving one payload bound and the other getting anadditional velocity boost. Tether systems using commercially available tether materials atreasonable safety factors can be as little as 15 times the mass of the payload being handled,however mass ratios on the order of 100 provide more tether orbit stability.INTRODUCTIONThe idea of using rotating tethers to pick up and toss payloads has been in the tether literaturefor decades [1-7]. In 1991, Forward [8] combined a number of rotating tether concepts publishedby others [2,6,7] to show that three rotating tethers would suffice to move payloads from asuborbital trajectory just above the Earth's atmosphere to the surface of the Moon and back again,without any use of rockets except to get out of the Earth's atmosphere. The three tethers consistedof a rotating tether in a nearly circular Low Earth Orbit (LEO), a rotating tether in a highly EllipticalEarth Orbit (EEO), and a rotating "Lunavator" tether cartwheeling around the Moon in a circularorbit whose altitude is equal to the tether arm length, resulting in the tip of the tether touching downon the lunar surface. This concept has since been examined in detail by Hoyt and Forward [9-12].In thinking about ways to improve the performance of the system, Forward realized that muchof the gain in the three-tether system came from the EEO tether, since its center-of-mass velocity atperigee was quite high, and when the tether tip rotational velocity was added, the toss velocity washyperbolic with respect to the Earth-Moon system and could be used for interplanetary transport.Forward enlisted the aid of his co-author, who extended the back-of-the-envelope calculationswith more detailed calculations, including the post separation orbit of the tether. The Mars-EarthRapid Interplanetary Tether Transport (MERITT) System was the result [13]. Extending theprinciple of high periapsis velocity in EEO to a hyperbolic, with the negative delta v to the postseparation tether led to the idea of a rotating tether in a hyperbolic orbit leaving one payload boundand the other getting a velocity boost. This could apply, for instance, to a Jupiter-Pluto mission.Background material from [13] is reproduced in part below for overview purposes, but the wehave incoporated some new data and concepts included in pending archival papers.*Consultant, 1238 Prescott Avenue, Sunnyvale CA 94089-2334; member, AIAA; Phone: 1-408-739-4032; Email: gdnordley@aol.com**Chief Scientist, Tethers Unlimited, Inc. 8114 Pebble Court, Clinton WA 98236; associatefellow, AIAA; Phone/Fax: 1-360-579-1340; Email: TU@tethers.com; Web: www.tethers.com1