PPT Thrust Stand

Abstract A torsional-typethrust stand has been designed and built to test Pulsed Plasma Thrusters (PPTs)in both single shot and repetitive operatingmodes. Using this stand,momentum per pulse was determinedstrictly as a function of thrust stand deflection, spring stiffness, and natural frequency. No empiricalcorrections were required. The accuracy of the method was verified using a swinging impact pendulum.Momentum transfer data between the thrust stand and the pendulum were consistent to within 1 percent.Following initial calibrations, the stand was used to test a Lincoln Experimental Satellite (LES-8/9)thruster. The LES-8/9system had a mass of approximately 7.5 kg, with a nominal thrust to weight ratioof 1.3 x 10-5.A total of 34 single shot thruster pulses were individuallymeasured. The average impulsebit per pulse was 266 12N-s,which was slightly less than the value of 300 /2 N-spublished in previousreports on this device. Repetitive pulse measurements were performed similar to ordinary steady-statethrust measurements. The thruster was operated for 30 minutes at a repetition rate of 132 pulses perminute andyielded anaveragethrustof573 12N. Using average thrust, the average impulse bit per pulsewas estimated to be 260 12N-s,which was in agreement with the single shot data. Zero drift during therepetitive pulse test wasfound to beapproximately1 percentof themeasuredthrust.A program to develop advanced PPTs for futuremission applications has been initiated. 4 Goals of thisPulsed plasma thrusters (PPTs) are electric program areto increasedeliveredtotal impulseby a factorofpropulsion devices in which the propellant is accelerated two while reducing system mass by the same factor aselectromagnetically. Pulse durations are typically on the comparedto state-of-artdevices. New propellant optionsareorder of one to tenmicroseconds. PPTs werestudiedduring also being explored.5 Direct thrust measurements will bethe 1960's and tested by the US in an orbital flight essential to the success of this program. Impulse bits onexperiment launched in 1968.1 Operational status was the order of 300 12N-sare anticipated from thrusters withreached on board NOVA navigation satellites beginning in several kilograms mass. The average thrust-to-weightratio1981.2 The NOVA system uses solid fluorinated polymer is significantlybelow that ofsteady-stateelectricpropulsionas an ablatively dispensed propellant and is fired at a (EP)devicesand aspecializedthrustbalanceisrequired.repetition rate of up to1 pulse per second. The unit has an The most sensitive PPT impulse measurementaverage power input ofabout 30 wattsand achievesspecific capability in the United States was developed by Goddardimpulse values approaching 1000 seconds. The small Space Hight Center during theearly 1970s.6 Designated asimpulse bits of each f'wingare used for very fine orbital the Micropound Extended Range Thrust Stand (MERTS),corrections of the NOVA vehicle, and the PPTis currently this device was used to measure both individual pulses andperforming this mission successfully) average repetitive thrust at levels down to 25 12N. ARecent interest in small satellites has sparked custom-builtdifferential plate capacitance system was usedrenewed interest in PPTs. Available electric power on to measure thrust stand deflections to a resolution of 2.5 xsmall satellites may be limited to less than0.01 kW, which 10.8 m. Both a null balance method and a calibratedeliminates most steady-state forms of electric propulsion displacement method were implemented. A rotary thrusterfrom consideration. PPT power throttling is managed mount was used to vector the plume at different angles insimply by reducing the pulse repetition rate and does not order to determine asymmetric side forces. Oscillation, affect performance. The simplified solid propellant damping was accomplished with an electronic feedbackdistribution arrangement of PPTs eliminates the need for circuit actingthrough an electromagnetic driver. A wirelessvalves, tanks, or precision metering components. Strict system of optical and radio links was used for thruster• temperature controls often required for pressurized fluid telemetry and control. The thrust stand also had a built-in,propellants are also eliminated. Being a relatively inert variable-position,counterweight arrangement and requiredsolid, fluorinated polymer doesnot require special handling no special adjustments to operate thrusters of very differentprotection often needed when loading toxic liquid weight or thrust levels. Calibration was performed bypropellants, sending current through an electromagnetic forcing coil.