Pulsed Plasma Thruster Systems for Spacecraft Attitude Control

Pulsed Plasma Thrusters (PPTs) are finding renewed user appeal due to the growth in small satellite applications. PPTs are especially well suited to small satellite applications because they are simple, low-mass, and high Isp propulsion systems. The solid Teflon fuel allows for a self-contained, inert and stable propellant system. With a power draw of only 0.1 to 150 W and a very small (50 - 800 µNs) impulse bit, PPT technology makes it possible to consider a revolutionary attitude control system (ACS) concept providing stabilization and pointing accuracies previously obtainable only with reaction wheels, with reduced mass and power requirements. NASA Lewis Research Center (LeRC) and Olin Aerospace Company (OAC) are working together to develop an advanced PPT system with twice the total impulse capability and half the mass of the previous best PPT system. The two key factors to accomplish these goals are: 1 ) significantly improving thrust efficiency - the ratio of thrust power to input electrical power and 2) improving the energy density and life of the energy storage capacitor. Typically, PPTs provide relatively low efficiency, with the LES 8/9 PPT delivering a little more than 7 percent. OAC has tested a matrix of configuration parameters with improvement in the efficiency by a factor of 1.5 to 2.0. To achieve the LeRC goals, the capacitor must be capable of 20 million pulses at an energy level of 40 J, ideally with a mass of no more than 1 kg. LeRC and OAC have embarked upon a two-step process to demonstrate the capacitor technology, with benchtop testing at OAC and integrated PPT/capacitor life testing at LeRC to be conducted in the development phase. The program provides for design, fabrication and qualification of a flight PPT, which is then slated to fly as an orbit raising demonstration aboard the Air Force Phillips Lab MightySat II.1 in early 1999. A second unit, configured for ACS functions, is planned for flight on the NASA New Millennium EO-1 spacecraft in mid-1999. With a light, high performance PPT in development for flight applications, it becomes possible to consider replacement of momentum wheels with PPTs. Typical momentum wheel attitude control systems consume 10's of W power and weigh 0.1 kg per kg of spacecraft weight, including the momentum desaturation devices. Mission analysis to be presented shows the PPT to be very competitive with these systems, with the advantages of lower cost, lower mass, extension of ACS capability to very small (nano) satellites, and simplicity in replacing both the wheels and the desaturation devices.