PPT Thrust Stand

A torsional-type thrust 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 determined strictly as a function of thrust stand deflection, spring stiffness, and natural frequency. No empirical corrections 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-8/9 system had a mass of approximately 7.5 kg, with a nominal thrust to weight ratio of 1.3 x 10-5. A total of 34 single shot thruster pulses were individually measured. The average impulse bit per pulse was 266 12N-s, which was slightly less than the value of 300 /2 N-s published in previous reports on this device. Repetitive pulse measurements were performed similar to ordinary steady-state thrust measurements. The thruster was operated for 30 minutes at a repetition rate of 132 pulses per minute and yielded an average thrustof 573 12N. Using average thrust, the average impulse bit per pulse was estimated to be 260 12N-s, which was in agreement with the single shot data. Zero drift during the repetitive pulse test was found to be approximately 1 percent of the measured thrust. A program to develop advanced PPTs for future mission applications has been initiated. 4 Goals of this Pulsed plasma thrusters (PPTs) are electric program are to increasedelivered total impulse by a factor of propulsion devices in which the propellant is accelerated two while reducing system mass by the same factor as electromagnetically. Pulse durations are typically on the compared to state-of-artdevices. New propellant options are order of one to ten microseconds. PPTs were studiedduring also being explored.5 Direct thrust measurements will be the 1960's and tested by the US in an orbital flight essential to the success of this program. Impulse bits on experiment launched in 1968.1 Operational status was the order of 300 12N-s are anticipated from thrusters with reached on board NOVA navigation satellites beginning in several kilograms mass. The average thrust-to-weight ratio 1981.2 The NOVA system uses solid fluorinated polymer is significantlybelow that of steady-state electric propulsion as an ablatively dispensed propellant and is fired at a (EP) devicesand a specialized thrustbalance is required. repetition rate of up to 1 pulse per second. The unit has an The most sensitive PPT impulse measurement average power input of about 30 wattsand achieves specific capability in the United States was developed by Goddard impulse values approaching 1000 seconds. The small Space Hight Center during the early 1970s.6 Designated as impulse bits of each f'wing are used for very fine orbital the Micropound Extended Range Thrust Stand (MERTS), corrections of the NOVA vehicle, and the PPT is currently this device was used to measure both individual pulses and performing this mission successfully) average repetitive thrust at levels down to 25 12N. A Recent interest in small satellites has sparked custom-built differential plate capacitance system was used renewed interest in PPTs. Available electric power on to measure thrust stand deflections to a resolution of 2.5 x small satellites may be limited to less than 0.01 kW, which 10.8 m. Both a null balance method and a calibrated eliminates most steady-state forms of electric propulsion displacement method were implemented. A rotary thruster from consideration. PPT power throttling is managed mount was used to vector the plume at different angles in simply 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 feedback distribution arrangement of PPTs eliminates the need for circuit acting through an electromagnetic driver. A wireless valves, 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 required solid, fluorinated polymer does not require special handling no special adjustments to operate thrusters of very different protection often needed when loading toxic liquid weight or thrust levels. Calibration was performed by propellants, sending current through an electromagnetic forcing coil. This method was used for pulsed and steady-state Pulse_tPlasmaThrusters calibration. Testing at the MIT Lincoln Laboratory was PPTsareatypeofmagnetoplasmadynamicthruster performed with a simple pendulum thrust stand.7 The in that they accelerate propellant predominantly through thrusterwas fired in synchronismwith the natural frequency electromagnetic forces. With a PPT this occurs during a of the pendulum, resulting in an observable build up of brief but powerful pulse, typically at peak powers in the oscillatory motion. Measuring the changes in amplitude MW range and for durations on the order of microseconds. allowed determination of the average value of impulse per This pulsed mode allows performance and benefits shot. associated with high power to be attained with low average A Pulsed Electrothermal Thruster (PET) was tested power systems. While early development of PPTs by GT-Devices in the mid 1980s.s Liquid water flashedto a employed gaseous propellant, a solid fluorinated polymer vapor and was heated in an arc discharge to propel the PET. rod has replaced gas in most applications. Solid propellant Water PETs could be operated in single pulse or repetitive is vaporized throughablation, eliminating the need for fast modes. The thrust stand used to test the PET consisted of a acting gas valves and flow control. linear rail arrangement passing through the vacuum tank, A PPT is prepared for fh-ingby first charging the and sealed with a flexible diaphragm. Typical impulse bit main storage capacitor. The charge rate can be determined measurements were on the order of 5 mN-s. Impulse was by the spacecraft power limitations without affecting determined by observing the instantaneous velocity change thruster performance. The electrodes can remain directly of the thrust stand as a result of thruster firing. An connected to the capacitor during the charging cycle as long inductive proximetry probe provided displacement as there is no conductive path from anode to cathode. Even information, which was then differentiated to obtain at a full charge of 1000 V the thruster is designed not to fire velocity. The system was calibrated by striking the thrust spontaneously. The discharge is commonly triggered using stand with a solid pendulum. Thrust stand response was a semiconductor spark plug. This device emits a small compared with the known momentum delivered by the quantity of free electrons when activated, and brings about pendulum, an avalanche breakdown across the main electrodes. As the A torsional thruststand was developed by Fairchild capacitor is discharged, polymer from the surface is Republic Co. in the mid 1970s for the purpose of testing vaporized and ionized into a current carrying plasma. The PPTs.9 Both individual pulse and repetitiveoperation were plasma is ejectedfrom the thruster via Lorentz forces created performed at thrust levels down to 200 //N. Leveling by the discharge. Once the discharge is extinguished the motors maintained the correct null position and a linear main capacitor may be recharged for the next pulse. variable differential transformer (LVDT) measured thrust The LES-8/9 PPT was designed to operate at a stand deflections. An oil filled fluidic damper was used to nominal average power of 30 watts, and a pulse repetition dissipate oscillations resulting from the thruster firing. A rate of about 2 Hz. There are two separate discharge ports ball calibrator was developed to provide in-situ thrust stand on each thrusterassembly which are vectored at 60 degrees calibrations. Steel ball bearings were conveyed upward and to each other. Each port has a separatepair of electrodes and allowed to accelerate down an inclined plane, where they propellant track, but both sides share the same storage impacted the thrust stand. Data from the calibration was capacitor and charging system. Each thruster port is compared directly with that of actual thruster operation to controlled independently through signal wires. The thruster quantifyperformance, was designed to deliver a nominal 300 /2N-s impulse bit Following the completion of earlier PPT from either port. It has a mass of about 7.5 kg, development programs, both the Goddard and Fairchild representingan averagethrust to weight ratio of about 1.3 x thruststands were eventually discarded. A new thrust stand 10-5. has recently been built and tested for the purpose of evaluating the Lincoln Experimental Satellite thruster (LES-8/9) and future generation PPTs. This stand employs Principleof ImpulseMeasurement_ a torsional design, similar in size to that used by Goddard and Fairchild,6,9and has been used to measure individual Impulse can be defined as the cumulative effect of thrust pulses as well as average thrust during times of an applied force over a specific time interval. It can be repetitivepulsed operation. Pivot arm movement is damped shown that the impulse needed to propel a body is simply electromechanically through a feedback loop, which can the product of its mass and velocity. According to smooth the thrust signal at pulse rates down to about 1 Hz. Newton's third law, the mutual reaction force between the This report discusses the details of the new thrust propellant and the thruster occur at the same magnitude and balance and gives examples of actual PPT performance, for the same duration of time. The impulse delivered to the The impulse bits of individual thruster pulses are presented, propellant is identical, but in opposite direction to that of and these measurements are compared to time averaged the thruster. The thruster, of mass m, recoils in response to thrust values obtained during repetitiveoperation, a dischargesuch that