Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Rapid Interplanetary Tether Transport SystemsIAF-99-A.5.10IntroductionThe possibility of using rotating ÒmomentumexchangeÓtethers to pick up payloads from oneorbit and toss them into another orbit has beendiscussed conceptually numerous times over thepast several decades. 1,2,3,4 In this paper, weinvestigate the design of specific tether systemarchitectures for two important missions: first,transport between low Earth orbit (LEO) and thesurface of the Moon, and second, transport ofpayloads between LEO and low Mars orbit.Ecliptici mMoon'sOrbitλ mMoon'sEquatorialPlanei eEarth'sEquatorialPlaneThe Cislunar Tether TransportSystemA ÒCislunar Tether Transport SystemÓcomposed of one rotating momentum-exchangetether in elliptical, equatorial Earth orbit and asecond rotating tether facility in a low lunar orbitcan provide a means for repeatedly exchangingpayloads between low Earth orbit (LEO) and thesurface of the Moon, with little or no propellantexpenditure required. In 1991, Forward 5 showedthat such a system is theoretically possible froman energetics standpoint. A later study by Hoytand Forward 6 developed a first-order design forsuch a system. These previous studies, however,utilized a number of simplifying assumptionsregarding orbital and tether mechanics in theEarth-Moon system, including assumptions ofcoplanar orbits, ideal gravitational potentials,and infinite facility ballast masses. The purposeof this paper is to remove these assumptions anddevelop an architecture for such a system thattakes into account the complexities of orbitalmechanics in the Earth-Moon system.Figure 1. Conceptual illustration of the CislunarTether Transport System.To sunFigure 2. Schematic illustrating the geometry of theEarth-Moon system.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.Orbital Mechanics of the Earth-Moon SystemOrbital mechanics in cislunar space are madequite complex by the different and varyingorientations of the ecliptic plane, the EarthÕsequatorial plane, the MoonÕs orbital plane, andthe MoonÕs equatorial plane. Figure 2 attempts toillustrate these different planes. The inclinationof the EarthÕs equatorial plane (the Òobliquity ofthe eclipticÓ), is approximately 23.45¡, but variesdue to tidal forces exerted by the Sun and Moon.The angle i m between the MoonÕs equatorial planeand a plane through the MoonÕs center that isparallel to the ecliptic plane is constant, about1.58¡. The inclination of the MoonÕs orbit relativeto the ecliptic plane is also constant, about λ m =5.15¡. 7 The line of nodes of the MoonÕs orbitregresses slowly, revolving once every 18.6 years.As a result, the inclination of the MoonÕs orbitrelative to the EarthÕs equator varies between2