Thermochemical and transport processes in pulsed plasma microthrusters: A two-temperature analysis

The pulsed plasma thruster (PPT) is a device which has been in use for decades as a spacecraft propulsion system. Traditionally the fuel used is a polymer such as polytetrafluoroethylene (PTFE) also known as Teflon, or polyvinyl chloride (PVC). Teflon is used as fuel for the majority of cases and it is the material studied in this research. The use of this polymer fuel, which results in very complex flow chemistry, mixed with the complexity of the arc physics, which may lead to severe thermal nonequilibrium, makes the thruster very difficult to analyze both theoretically and computationally. The primary goal of this research is to provide accurate thermochemical and transport properties of PTFE suitable for use in the flow regimes experienced in the PPT to aid in the theoretical and computational modeling of these complex thrusters.; A preliminary analysis is performed to judge the equilibrium state and flow characteristics of a typical thruster. The results give a range of Knudsen numbers of 10−3 to 10−1 indicating noncontinuum effects may be important in some operating regions. An examination of the ratios of characteristic times for ionization and thermal equilibration to characteristic flow times indicate that the thruster may progress through varying degrees of local thermodynamic equilibrium (LTE). The ratio for ionization reactions varies from 10−3 to 10−6 and for thermal equilibration varies from 10−5 to 10 −3 indicating that the thruster is, for the most part, in chemical equilibrium but may have an electron temperature that is elevated above the heavy particle temperature—a two-temperature LTE situation.; The chemical composition, thermodynamic, and transport properties of gaseous Teflon are calculated using a two-temperature LTE analysis in which the electron temperature may be significantly different from the heavy particle temperature. The results obtained here represent a significant improvement over the previously used equation-of-state, even for the single-temperature LTE case. The results for the transport properties show that the viscosity, thermal conductivity, and electrical conductivity may exhibit severe non-monotonic behavior.