Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tethers Unlimited, Inc.Cislunar Tether Transporteffects of lunar perturbations to the tetherÕs orbit.Thus tether reeling may provide a means ofstabilizing the orbit of a polar Lunavator ªwithout requiring propellant expenditure. Thistether reeling, however, would add additionalcomplexity to the system.Cislunar System SimulationsTether System ModelingIn order to verify the design of the orbitaldynamics of the Cislunar Tether TransportSystem, we have developed a numericalsimulation called ÒTetherSimÓ that includes:• The 3D orbital mechanics of the tethers andpayloads in the Earth-Moon system, includingthe effects of Earth oblateness, using Runge-Kutta integration of CowellÕs method.• Modeling of the dynamical behavior of thetethers, using a bead-and-spring model similarto that developed by Kim and Vadali. 11• Modeling of the electrodynamic interaction ofthe Earth-orbit tether with the ionosphere.Using this simulation tool, we have developed ascenario for transferring a payload from a circularlow-LEO orbit to the surface of the Moon usingthe tether system designs outlined above. Wehave found that for an average transfer scenario,mid-course trajectory corrections of approximately25 m/s are necessary to target theReel tether inagainst low tidal forceExtend tether underhigh tidal forceFigure 10. Schematic of tether reeling maneuver toreduce orbital eccentricity.payload into the desired polar lunar trajectory toenable rendezvous with the Lunavator ª . Asimulation of a transfer from LEO to the surface ofthe Moon can be viewed at www.tethers.com.Targeting the Lunar TransferIn addition to the modeling conducted withTetherSim ª , we have also conducted a study ofthe Earth-Moon transfer to verify that thepayload can be targeted to arrive at the Moon inthe proper plane to rendezvous with theLunavator ª . This study was performed with theMAESTRO code, 12 which includes the effects ofluni-solar perturbations as well as the oblatenessof the Earth. In this work we studied targeting toboth equatorial and polar lunar trajectories.We have found that by varying the energy ofthe translunar trajectory and adjusting theargument of perigee, it is possible to target thepayload to rendezvous with a polar orbitLunavator ª with a wide range of ascending nodepositions of the Lunavator ª orbit. Oursimulations indicate that the viable nodalpositions ranges at least ±10¡ from the normal tothe Earth-Moon line.Comparison to Rocket TransportTravelling from LEO to the surface of the Moonand back requires a total ∆V of more than10Êkm/s. To perform this mission using storablechemical rockets, which have an exhaustvelocity of roughly 3.5 km/s, the standard rocketequation requires that a rocket system consume apropellant mass equal to 16 times the mass of thepayload for each mission. The Cislunar TetherTransport System would require an on-orbit massof less than 37 times the payload mass, but i twould be able to transport many payloads. Inpractice, the tether system will require somepropellant for trajectory corrections andrendezvous maneuvers, but the total ∆V for thesemaneuvers will likely be less than 100 m/s. Thusa simple comparison of rocket propellant mass totether system mass indicates that the fullyreusable tether transport system could providesignificant launch mass savings after only a fewround trips. Although the development anddeployment costs associated with a tether systemwould present a larger up-front expense than anexisting rocket-based system, for frequent, highvolumeround trip traffic to the Moon, a tethersystem could achieve large reductions intransportation costs by eliminating the need to10