Moon & Mars Orbiting Spinning Tether Transport - Tethers Unlimited

IAF-00-S.6.04controlled very precisely by varying this gate voltage.To enable the FEAC devices to operate reliably in theLEO environment, the emitter tips must be ruggedizedto survive bombardment by the atomic oxygen andother constituents found in LEO. TUI is currently collaboratingwith NASA/MSFC, JPL, the University ofMichigan, SRI, LRI, and NRL to develop and testFEAC devices coated with carbide materials intended toprovide the necessary ruggedization.Tether DeployerThe prototype deployer for the Terminator Tether ª isshown in Figure 4. In this figure in which the tether iswound onto a spool. The TCU electronics and othercomponents are contained inside this spool. The TerminatorTether ª system will be housed inside the hostsatellite, with the bottom surface (with the RF antenna)positioned flush with the satellite's bottom or side surface.The TCU, electron emitter, and batteries are containedin a cylindrical housing that slides inside of thedeployer spool, so that during the dormant phase theelectronics will be shielded from radiation by the several-cmof the wound aluminum wire tether. When theTCU activates the deployment sequence, it triggers anejection mechanism which propells the entire TerminatorTether ª unit (except for the mounting bracket andtether anchor) down and away from the host spacecraftat a velocity of several meters per second.Device Mass and SizingTUI is currently building a prototype of the TerminatorTether ª that is sized to provide deorbit capabilityfor a 2000-3000 kg LEO spacecraft. A mass breakdownfor this prototype is given below.Tether mass: 10.0 kgShroud: 1.9 kgSpool Assembly: 4.9 kgEjection Mechanism 5.0 kgElectron Emitter: 1.2 kgTCU Electronics: 3.7 kgTether Anchor 0.06 kgTotal Tether system mass: 26.76 kgDeorbit PerformanceUsing the TetherSimª numerical simulation tool, wehave studied the potential performance of the TerminatorTether ª for deorbit of satellites from various LEOorbits. 7Figure 5 shows the time required to decreasethe altitude of a satellite to 250 km from a range ofinitial orbital altitudes and inclinations. A TerminatorTether ª massing approximately 2% of the mass of thehost spacecraft could deorbit an upper stage from a 400km, 50¡ orbit within about two weeks, or a communicationssatellite from a 850 km, 50¡ orbit within aboutthree months. Figure 6 shows data for satellite altitude,tether current, and tether libration angle from asimulation of deorbit of a small satellite from a 370km, 51¡ inclination initial orbit.Figure 4. Photo of the Terminator Tether ª deployer prototype.Deorbit Time (days)4003503002502001501005090°75°0200 400 600 800 1000 1200Altitude (km)50°25°0°1400 1600Figure 5. Time for a Terminator Tether ª with a 7.5 km, 15 kg aluminumtether and 15 kg endmass to decrease the perigee altitude ofa 1500Êkg spacecraft to 250 km. Note that deorbit time can be decreasedby using longer or more massive tethers.Host Altitude (km)400350Altitude(km)300Tether Current (A)Current(A)In-Plane Angle (deg)2500 1 2 3 4 510.550In-PlaneLibration(deg) 000 1 2 3 4 5-5035 cm43 cm0 1 2 3 4 5Figure 6. TetherSim ª results for altitude, tether current, and tetherlibration for deorbit of a microsatellite from a 370 km, 51¡ orbit.3