Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Rapid Interplanetary Tether Transport SystemsIAF-99-A.5.10rotating, the direction of the current must bealternated as the tether rotates to produce a netthrust on the facility. Using a simulation oftether dynamics and electrodynamics, we havemodeled reboost of a rotating tether system.Figure 5 shows the reboost of the tetherÕs orbitover one day, assuming that the tether facilityhas a power supply of 11 kW and is able to storeup power during most of its orbit and expend it a ta rate of 75 kW during the portion of the orbitwhen the tether is below 2000 km altitude. In oneday, the facility can restore roughly 20 km to itsorbitÕs semimajor axis; in roughly 85 days it couldrestore its orbit and be prepared to boost anotherpayload to the Moon. More rapid reboost could beaccomplished with a larger power supply.Dealing with Apsidal PrecessionAs noted earlier, the oblateness of the Earthwill cause the line of apsides of the tetherfacilityÕs elliptical orbit to precess. In theCislunar Tether Transport System, we can dealwith this issue in two ways. First, we can utilizetether reeling maneuvers to counteract theapsidal precession. 11 By simply reeling thetether in and out slightly once per orbit, thetether facility can exchange angular momentumbetween its rotation and its orbit, resulting inprecession or regression of the line of apsides.With proper phasing and amplitude, tetherreeling can hold the tetherÕs orbit fixed so that i tcan send payloads to the Moon once per month. 12A second method is to choose the tether orbitssuch that their precession rates are nearlyharmonic with the MoonÕs orbital rate, so thatthe line of apsides lines up with the MoonÕs nodesonce every several months. Furthermore, we canuse propellantless electrodynamic tether propulsionto Òfine-tuneÓ the precession rate, eitherby raising/lowering the orbit or by generatingthrust perpendicular to the facilityÕs velocity.In the design given above, the mass andinitial orbit of the tether facility was chosensuch that after throwing a payload to the Moon,the tether enters a lower energy elliptical orbitwhich will precess at a rate of 2.28 degrees perday. The initial, high-energy orbit has a slowerprecession rate of approximately 1.58 degrees perday. These orbits were chosen so that in the 95.6days it takes the Moon to orbit 3.5 times aroundthe Earth, the tether facility can reboost itselffrom its low-energy orbit to its high-energy orbitusing propellantless electrodynamic propulsion,and, by properly varying the reboost rate, theapsidal precession can be adjusted so that the lineof apsides will rotate exactly 180¡, lining thetether orbit up properly to boost another payloadto the Moon.System Design for Round-Trip TrafficOnce a lunar base is established and begins tosend payloads back down to LEO, the orbit of thetether system can be modified slightly to enablefrequent opportunities for round-trip travel.First, the facilityÕs orbit will be raised so that itshigh-energy orbit has a semimajor axis of12577.572 km, and an eccentricity of 0.41515. Thetether will then pick up a payload from acircular, 450 km orbit and toss it to the Moon sothat it will reach the Moon as the Moon crossesits ascending node. The facility will then drop toa lower energy orbit. At approximately the sametime, the return payload will be released by thelunar tether and begin its trajectory down to LEO.When the return payload reaches LEO, theEarth-orbit tether facility will catch it a tperigee, carry it for one orbit, and then place i tinto the 450 km initial payload orbit. Upondropping the return payload, the facility willplace itself back into the high-energy orbit. Theperigee of this orbit will precess at a rate suchthat after 4.5 lunar months (123 days) it willhave rotated 180¡, and the system will be readyto perform another payload exchange, this timeas the Moon crosses its descending node. If morefrequent round-trip traffic is desired, tetherreeling could again be used to hold theorientation of the tetherÕs orbit fixed, providingtransfer opportunities once per sidereal month.Design of a Lunavator ª Compatiblewith Minimal-Energy Lunar TransfersThe second stage of the Cislunar TetherTransport System is a lunar-orbit tether facilitythat catches the payloads sent by the Earthorbittether and deposits them on the Moon withzero velocity relative to the surface.Background: MoravecÕs Lunar SkyhookIn 1978, Moravec 8 proposed that it would bepossible to construct a tether rotating around theMoon that would periodically touch down on t h elunar surface. MoravecÕs ÒSkyhookÓ would havea massive central facility with two tether arms,each with a length equal to the facilityÕs orbitalaltitude. It would rotate in the same direction asits orbit with a tether tip velocity equal to the8