Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. REPORT DATE (DD-MM-YYYY)10-08-2011 REPORT TYPE Conference Paper DATES COVERED (From -To) TITLE AND SUBTITLE 5a. CONTRACT NUMBER A Performance Comparison of Xenon and Krypton Propellant on an SPT-100 Hall SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)Air Force Research Laboratory (AFMC) AFRL/RZS SPONSOR/MONITOR'S Pollux Drive NUMBER(S)Edwards AFB CA 93524-7048 AFRL-RZ-ED-TP-2011-342 DISTRIBUTION / AVAILABILITY STATEMENTApproved for public release; distribution unlimited (PA #11558). SUPPLEMENTARY NOTESFor presentation at the 32 nd International Electric Propulsion Conference, Wiesbaden, Germany, 11-15 Sep 2011. ABSTRACTThe use of krypton as an alternative to xenon for Hall thruster propellant is an interesting option for satellite system designers due to its lower cost. However, this cost-savings comes at the expense of thrust efficiency. Reduction in efficiency can be caused by energy losses from Joule heating, radiation, and the ionization process as well as degradation of plume quality from an increase in velocity distribution spread (most often from an increase in multiply charged ion populations) and geometric beam divergence.1 In order to quantify this performance reduction for the case of the flight model SPT-100 HET (1.35 kW), an ongoing series of experimental measurements is being conducted to measure how various thruster efficiency terms change with propellant and operating condition. This study will combine thrust measurements with plume data from electrostatic probes. This paper presents the results of performance measurements made using an inverted pendulum thrust stand. Krypton operating conditions were tested over a large range of operating powers from 800 W to 3.9 kW. Analysis of how performance is impacted by propellant and operating condition is presented. A simple mission analysis was done based on these measurements to evaluate the practicality of krypton propellant for an SPT-100 subsystem using krypton propellant for north-south station keeping (NSSK) for a typical communications spacecraft in geosynchronous orbit.. SUBJECT TERMS

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Space Systems/Loral has implemented the SPT-100 Hall effect thruster onto geostationary spacecraft for primary north-south station keeping, allowing substantial reductions in onboard propellant mass. Consideration of the spacecraft-thruster interactions is necessary when implementing electric propulsion thrusters onto communications satellites. Impingement of high-energy xenon ions on the spacecraft solar arrays and other large surfaces causes torques on the spacecraft that must be accounted for and controlled by the spacecraft control system. Space Systems/ Loral has developed and used a model to predict the impingement torques on the spacecraft, allowing proper design of the spacecraft control system. The impingement model is based on experimental current-density and ion-energy data taken during ground testing. Accommodation-coefficient values for the surfaces being investigated were based on values taken from open literature. Predictions from the plume model were compared with flight control-system data from two Russian-manufactured spacecraft. Modifications to the model were made for current density, based on correlation with the flight torque data. This paper will compare predictions based on this model with flight data from the Space Systems/Loral-manufactured Galaxy 28 and Thaicom 4 geosynchronous commercial spacecraft. Both spacecraft are equipped with an SPT-100 subsystem to perform north-south station keeping.Nomenclature C i = curve-fit coefficient e = elementary charge, 1:6 10 19 C F = force, N f " = current-density distribution function g = gravity constant at sea level, 9:8 m=s 2 j = current density, mA=cm 2 k i = curve-fit coefficient ke i = curve-fit coefficient m = mass, kg p = normal momentum flux, N=m 2 R = distance from the thruster exit plane to a point in space, m R cm = distance from the thruster to the spacecraft center of mass, m T = torque, N m u = current-weighted velocity, m=s v = ion velocity, m=s " = ion energy, eV = angle from the thruster centerline, deg n = normal momentum accommodation coefficient t = tangential momentum accommodation coefficient 0 = normal momentum accommodation coefficient, 0 = tangential momentum flux, N=m 2 = incident angle with respect to surface normal, deg

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