Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Rapid Interplanetary Tether Transport SystemsIAF-99-A.5.10orbital velocity of the tetherÕs center-of-mass sothat the tether tips would periodically touchdown on the Moon with zero velocity relative tothe surface (to visualize this, imagine the tetheras a spoke on a giant bicycle wheel rolling aroundthe Moon).As it rotates and orbits around the Moon, t h etether could capture payloads from Earth as theypassed perilune and then set them down on t h esurface of the Moon. Simultaneously, the tethercould pick up payloads to be returned to Earth,and later throw them down to LEO.Moravec found that the mass of the tetherwould be minimized if the tether had an armlength equal to one-sixth of the diameter of theMoon, rotating such that each of the two armstouched down on the surface of the Moon threetimes per orbit. Using data for the best materialavailable in 1978, Kevlar, which has a densityof 1.44 g/cc and a tensile strength of 2.8 GPa,Moravec found that a two-arm Skyhook with adesign safety factor of F=2 would have to massapproximately 13 times the payload mass. Eacharm of MoravecÕs tether would be 580Êkm long, fora total length of 1160Êkm, and the tether centerof-masswould orbit the Moon every 2.78 hours ina circular orbit with radius of 2,320Êkm. At thatradius, the orbital velocity is 1.45 km/s, and soMoravecÕs Skyhook would rotate with a tipvelocity of 1.45 km/s.Using MoravecÕs minimal-mass solution,however, requires not only a very long tether butalso requires that the payload have a very highvelocity relative to the Moon at its perilune.Because the lunar tether in MoravecÕs design hasan orbital velocity of 1.45 km/s and the tethertips have a velocity of 1.45 km/s relative to t h ecenter-of-mass, the payloadÕs perilune velocitywould need to be 2.9 km/s in order to match upwith the tether tip at the top of their rotation.In order to achieve this high perilune velocity,the outbound lunar transfer trajectory would haveto be a high-energy hyperbolic trajectory. Thispresented several drawbacks, the mostsignificant being that if the lunar tether failed tocapture the payload at perilune, it wouldcontinue on and leave Earth orbit on a hyperbolictrajectory. Moreover, as Hoyt and Forward 6found, a high lunar trajectory energy would alsoplace larger ∆V demands on the Earth-orbittethers, requiring two tethers in Earth orbit tokeep the system mass reasonable.Lunavator ª DesignIn order to minimize the ∆V requirementsplaced upon the Earth-orbit portion of theCislunar Tether Transport System and therebypermit the use of a single Earth-orbit tether witha reasonable mass, we have developed a methodfor a single lunar-orbit tether to capture apayload from a minimal-energy lunar transferorbit and deposit it on the tether surface withzero velocity relative to the surface.Moon-Relative Energy of a Minimum-Energy LTOA payload that starts out in LEO and isV payloadV tipV orbitalV tipV orbitalCentral FacilityCenter-of-MassCounterbalanceMassL cm,1L ω LOrbitalf 0cm,2VelocityωL 2cm,0Central Facility"Climbs" Up TetherVTip VelocityOrbital VelocityFigure 6. Method for a lunar tether to capture a payload from a minimal-energy LTO and deposit it onthe Moon with zero velocity relative to the surface.9