Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tethers Unlimited, Inc.Cislunar Tether TransportSystem DesignFigure 5 illustrates the system concept designfor the Tether Boost Facility. The Tether BoostFacility is composed of a Control Station, atapered high-strength tether, and a GrappleAssembly. In addition, a Payload AccommodationAssembly (PAA) will be attached to thepayload to provide maneuvering and guidance forrendezvous. This PAA will initially be anexpendable unit incurring recurring costs, but onceround-trip traffic is established the PAAÕs couldbe re-fueled and reused for return payloads.The tether facility is sized to be deployed witha single launch of a Delta-IV-H or comparablevehicle. Note that the system mass given inTable 1 is not the minimum possible system mass;a lighter system mass could be designed for asystem optimized for boosting payloads to LTO.This system mass was chosen to utilize the fullcapability of the Delta-IV-H vehicle, and tooptimize the system for boosting larger satellitesto geostationary transfer orbits. As Figure 5shows, the 3490 kg Delta upper stage will beretained for use as ballast mass. The controlstation includes an array of solar panels whichswivel to track the sun as the tether facilityrotates. In this design, we have chosen to placethe control station at the end of the tether,rather than at the center of mass of the facility.This choice was made for several reasons:because it minimizes the dynamical complexity,because it requires only one tether deployer, andbecause the center of mass of the system shiftswhen the payload is captured and released.Electrodynamic Reboost of the Tether OrbitAfter boosting the payload, the tether facilitywill be left in a lower energy elliptical orbit. Torestore the orbit, the tether system must increasethe perigee velocity by 125 m/s, and increase thefacilityÕs orbital energy by 29 GJ. Because thetether is rotating, the direction of the currentmust be alternated as the tether rotates toproduce a net thrust on the facility. Using asimulation of tether dynamics and electrodynamics,we have modeled reboost of a rotatingtether system and found that the electrodynamicthrusting efficiency is approximately 33ʵN/W,averaged over the perigee thrust period (shownin Figure 6). The tether facility will be able tocollect solar power over approximately 80% of itsorbital period. To reboost the orbit within 30days, the facility will need a solar panel able tocollect approximately 50 kW, and the tetherfacility will expend the collected energy at arate of 200 kW during the perigee passes.Dealing with Apsidal PrecessionIn order to deliver the payload to the Moon,the tether facility in equatorial Earth orbit musttoss the payload out to a point near where theMoon will cross the EarthÕs equatorial plane.Thus the tetherÕs perigee must be lined up on theopposite side of the Earth from that point. Theoblateness of the Earth, however, will cause theline of apsides of the tether facilityÕs ellipticalorbit to precess. In the Cislunar Tether TransportSystem, we can deal with this issue in threeways.First, we can use propellantless electrodynamictether propulsion to change or oppose theoblateness-induced precession, either by raising/lowering the orbit or by generating thrustperpendicular to the facilityÕs velocity.Second, we can utilize tether reeling maneuversto counteract the apsidal precession. 7 By reelingthe tether in and out a small percentage of itstotal length once per orbit, the tether facility canexchange angular momentum between its rotationand its orbit, resulting in precession or regressionof the line of apsides. With proper phasing andamplitude, tether reeling can hold the tetherÕsorbit fixed so that it can send payloads to theFigure 6. The HEFT Boost FacilityÕs initial orbit. Thered lines indicate the bounds of the perigee portion ofthe orbit where electrodynamic thrusting is effective.6