| A theoretical effort supported by suitable experiments was initiated to investigate the dominant physical processes and extract meaningful insights that can lead to optimization of the ablation fed pulsed plasma thruster (APPT) operating in the 60-J stored energy range. Initially, an analytic model was developed capturing general trends of propellant mass utilization and impulse-bit within one-dimensional channel flow. The model indicated that the propellant mass flow rate from the solid propellant, polytetrauoroethelyne (PTFE or Teflon), during discharge correlates quadratically to the discharge current and is also directly proportional to the ratio of inter-electrode spacing to channel width. More rigorous computations were subsequently conducted using the well-established magnetohydrodynamics (MHD) code, MACH2. The code was upgraded with a comprehensive thermochemical model for PTFE as well as a new ablative boundary condition based on energy flux balance at the exposed surface of the PTFE propellant. The new two-dimensional, time-dependent MHD model was validated by comparisons to experimental data using a new high-vacuum facility designed, constructed, and operated at Arizona State University and a facility located at NASA Glenn Research Center.;Simulations of two unique prototype configurations operating at 20, 40, and 60 J, in addition to a traditionally configured APPT operating between 40-140 J, predicted values for ablated mass that captured the trends and magnitudes for both thrusters, while predictions for impulse-bit were in agreement for the prototype but overestimated the values of the traditional thruster. The prototype thruster utilized housing that prevented any losses in the direction normal to the computational plane modeled by the MACH2 code, while the traditional configuration was subject to such losses suggesting that such propellant confinement will be beneficial to the performance of the latter. Both experimental and computational results demonstrated that the prototype APPTs consume, on average, three times the propellant of the traditionally configured APPT while generating half the impulse-bit. Additionally, radiative transfer to the surroundings was demonstrated to be a dominant energy loss mechanism, reducing the predicted directed kinetic energy by up to 90%. Insights gained from the theoretical computations were used to propose modifications which could lead to an optimized prototype configuration. |
