AIAA SPACE 2009 Conference & Exposition AIAA-2009-6734 14-17 September 2009, Pasadena, California Optimizing Electrodynamic Tether System Performance Keith R P. Fuhrhop * Northrop Grumman, Redondo Beach, CA., 90278 and Brian E. Gilchrist † University of Michigan, Ann Arbor, MI., 48109 A time-averaged electrodynamic tether (EDT) system simulation tool has been developed and used to conduct studies of tether performance under varying conditions. The studies included evaluating passive end-body electron collection and active ion emission approaches, a comparison of active electron emission technologies (hollow-cathode, electron field emission, hot cathode), adjustment of bare conductor versus insulated tether lengths, boosting and de-boosting conditions, and other various system element configurations. The study results indicate that in many cases bare tether anodes provide optimal electron collection. In addition, it was shown that while hollow cathodes may be the best active electron emission technique, field emitter arrays result in less than 1% difference in average system thrusting and use no consumables. This is based on the assumption that multi-amp field emitter arrays can be ultimately fabricated and qualified for space. Three case-studies were performed in order to better understand the trades for performance optimization. The cases were: (1) orbit maintenance of the International Space Station; (2) the use of an EDT system for reboost and deorbit of NASA’s GLAST spacecraft; and, (3) operation of the Momentum Exchange Electrodynamic Reboost (MXER) system. From evaluation of these cases, a recommended design “algorithm” is proposed. Case (1) is presented in this paper in its entirety, and cases (2) and (3) are briefly described. Nomenclature A = constant in Richardson Eq. [A/cm 2 ] A e = area of emitter [m 2 ] Į,ȕ = Parker Murphy correction factor [ ] B = constant in Fowler Nordheim equation [A/V 2 ] B North = magnetic flux density in north direction [T] C = constant in Fowler Nordheim equation [V] D = distance across sheath [m] dl = unit distance [m] dF = force per unit distance [N] e,q = electric charge [C] İ o = permittivity constant [F/m] Ș = thermionic cathode efficiency (~0.97) F = electric field in [V/m] I CL = space charge limited current [A] I omle, ,I omli = electron/ion Orbital Motion Limited Current [A] I ram = Ram Current [A] I the , I thi = electron / ion Thermal Current [A] I t = current in the tether [A] J = current density [A/m 2 ] k = Boltzmann's constant in [J/K] m e = mass of electron [kg] n e , n i = electron / ion density [particles/m 3 ] ɮ = work function of element in [eV] ɮ o = Intermediate Potential for Parker Murphy [V] ȡ = perveance [pervs] r b = radius of emitter [m] r s = Radius of plasma sheath [m] SA2d = 2-d Surface Area [m 2 ] T = temperature [K] T e ,T i = electron / ion Temperature [eV] T o = energy of emitted electrons [eV] V = plasma sheath gap potential [V] V p = Plasma Potential [V] V emf = electro motive force [V] ǻV tc = potential across the thermionic cathode [V] v orb = orbital velocity wrt. local plasma [m/s] Ȧ ce , Ȧ ce = electron / ion cyclotron frequency [Hz] I. Introduction lectrodynamic * tethers (EDTs) † are ‡ being considered as a propellantless propulsion technology for spacecraft in low Earth orbit. An orbiting tether system naturally orients along the local vertical due to gravity. Current flowing along the 1 * Systems Engineer, Aerospace Systems, One Space Park / R5-2281B, AIAA Senior Member † Full Professor, Electrical Engineering and Atmospheric, Oceanic and Space Sciences, 1301 Beal Ave. / 2240 EECS, AIAA Associate Fellow E American Institute of Aeronautics and Astronautics AIAA SPACE 2009 Conference & Exposition 14 - 17 September 2009, Pasadena, California AIAA 2009-6734 Copyright © 2009 by Keith R. P. Fuhrhop. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.