Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

IAF-00-S.6.04Figure 7. Schematic of the µPET Propulsion System boosting a small satellite.µPET Propulsion SystemCurrently, economic, programatic, and technologyissues are driving NASA, DoD and other organizationsto reduce their dependence on single, large satellites andinstead develop systems of several or many small, inexpensivesatellites. Many of the potential applicationsof these small satellites will require the satellites tochange orbits frequently and rapidly or to hold at altitudefor long periods of time. Because many of thesesatellites will operate in low-LEO orbits, stationkeepingpropulsion to counteract atmospheric drag will imposelarge total ÆV requirements on the satelliteÕs propulsionsystem. These microsatellites, however, willbe very power- and weight- limited, so there is a needfor small propulsion systems able to provide both rapidorbit transfer capability and high specific impulse operation.Electrodynamic tether propulsion can provide bothrapid orbit transfer and effectively infinite specific impulseoperation. A small, lightweight, and inexpensivetether system can provide propulsion capabilities fororbital transfer, inclination changes, and stationkeeping.Because it uses electrodynamic forces to provide thrustto the satellite, it will not consume propellant, and thusit can enable small satellites to stationkeep indefinitely,even in low LEO orbits where aerodynamic drag wouldotherwise impose prohibitive ÆV requirements. TUI hasrecently begun the development of a "MicrosatellitePropellantless Electrodynamic Tether (µPET) PropulsionSystem". The µPET Propulsion System concept isillustrated in Figure 7.PerformanceNumerical simulation of the µPET system indicatethat its thrust efficiency is competitive with other smallpropulsion systems such as micro-Hall and micro-Ionthrusters. Because electrodynamic thrusting utilizesinteractions with the geomagnetic field, the thrust efficiencyof the µPET varies with inclination and altitude.In an equatorial orbit at 370 km altitude, the thrustefficiency is approximately 69 µN/W. At 51¡ inclination,it is approximately 50 µN/W, while at 70¡ itdrops to 31 µN/W. Figure 8 shows TetherSim ª simulationresults for operation of a µPET on a microsatellitea 370 km, 51¡ orbit at several input power levels.These simulations indicate that a small electrodynamictether system can provide significant propulsive capabilityfor small satellites with very low power requirements.4