Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

Tether Boost Facility Design AIAA 2000-3866Figure 7. LEFT: Tether Boost Facility initial orbit (yellow ellipse) and payload initial orbit (red circle). RIGHT:Tether Facility orbit after payload boost (inner yellow ellipse) and Payload GTO (red outer ellipse).Simulation of Electrodynamic ReboostAs the Tether Boost Facility catches and tosses apayload into GTO, its orbit drops, as illustrated inFigure 7. The apogee drops 2340 km, and the perigeedrops 16 km. To restore the orbit, the tether systemmust increase the facilityÕs orbital energy by 54 GJ, andit will do so by performing electrodynamic thrustingwhile the tether is within the dense portion of the ionospherenear the perigee of its orbit. Because the tether isrotating, the direction of the current must be alternatedas the tether rotates to produce a net thrust on the facility.Using the TetherSim ª program, we have modeledreboost of a rotating tether system to investigate theefficiency of the reboost, and to develop methods forcontrolling the electrodynamic thrust to achieve thedesired final orbit.Method:To study the performance of electrodynamic reboostof the tether facility, TetherSim ª was used to simulatereboosting of the orbit of the Tether Boost Facilitydescribed in Table 1 over a period of two days. TetherSim ªis a numerical simulation tool that includesmodels for tether dynamics, orbital mechanics, electrodynamics,thermal behavior, geopotential, geomagneticfield, ionospheric density variations, neutral gas densityvariations, and other relevant physics.In the simulations, thrusting was performed when thetether facilityÕs altitude was under 2000 km. The electrodynamictether system had hollow-cathode plasmacontactors at both ends of the conducting tether, so thatit could carry current in both directions. The thrustingwas performed at a maximum power of 450 kW. TheControl Station contained a 150 kW solar power supply,a 8500 A¥hr (120 V) battery system. Peak tethercurrent levels were limited to 20 A, with typical currentsvarying between 15 and 20 A. In addition, thrustingwas performed only when the tether was within ¹/4of vertical.ResultsReboost SimulationsFigure 8 shows the orbit semimajor axis, and Figure9 shows the orbit eccentricity during the two days ofboosting simulated. The semimajor axis increases at52 km/day. Note that if the electrodynamic boost systemadds energy to the orbit at a constant rate, the rateof semimajor axis increase will accelerate due to theinverse relation between orbital energy and semimajoraxis. The eccentricity increases at 0.0034/day. Notethat the eccentricity change rate will also vary duringreboost. Figure 10 shows the apogee altitude increase.Thrust Efficiency:The thrust efficiency is shown in Figure 12. Thegraph shows that the thrust efficiency varies cyclicallyduring each day; this variation is due to the fact thatthe Earth, and its magnetic field, are rotating inside thefacilityÕs orbit, and thus the angle between the geomagneticfieldÕs axis and the orbit plane varies once perday. In addition, not readily apparent on this timescale,the thrust efficiency varies with altitude and withthe angle of the tether relative to local vertical. Overthis one day period, the average thrust efficiency is 40µN/W (thrust efficiency calculated using the power inputto the electrodynamic tether).Reboost Time:Since the rate of semimajor axis increase varies duringthe reboost operation, the best way to estimate the timeneeded to reboost the orbit is to assume that the rate atwhich the orbital energy of the system is increased isrelatively constant during the reboost period. To reboostthe orbit from 391x6105 km to 407x8445 km,the electrodynamic system must restore 54 GJ of energyto the tether facilityÕs orbit. In the 2-day simulation,the electrodynamic thrusting restored the facilityÕsorbital energy at a rate of 2.7 GJ/day, as illustrated inFigure 11.7