A model of teflon ablation in a pulsed plasma thruster

Abstract

The computational code MACH2 was used to simulate the plasma flow in cableguns and a pulsed plasma thruster (PPT-4, developed at the University of Illinois at Urbana-Champaign) and the computed results were compared to the experimental operation of the devices. The capabilities of MACH2 were extended by incorporating a new radiation model as well as new thermodynamic equation of state tables for plasmas containing either Teflon or a mixture of 39% Teflon and 61% copper (by mass).In developing the tables, the thermodynamic variables at low temperatures were calculated using the NASA Equilibrium Code (CEA). The high temperature calculations were performed by iterating the multicomponent Saha equation. Cubic polynomials were then used to blend the high and low temperature calculations together over the intermediate temperature regions. The Saha-calculated specific heats contained large numerical fluctuations and were filtered using a (1,l)-fold trimmed mean filter. The data was also smoothed over suitable temperature intervals using cubic polynomials.Using spectroscopic data gathered in the cablegun experiments at UTSI, a preliminary analysis was performed to obtain a rough estimate of plasma temperature.In the MACH2 simulations, the plasma absorption coefficient and the propellant absorptivity were adjusted to give the correct values for plasma velocity and ablated mass. The resulting impulse bits for the cableguns were 6.89e-04 NOS for a plasma consisting of only Teflon and 1.84e-03 N.s for a plasma containing 39% Teflon and 61% copper (by mass) The corresponding specific impulses are 1254 s and 695 s, respectively. The PPT-4 thruster produced a total impulse of 2.03e-04 NOS and an average specific impulse of 521 s. Although providing less impulse per shot, the PF'T-4 thruster produced a greater impulse per unit energy stored in the capacitor. Engineering, Tel AvivUniversily, P. 0. B. 39040, Tel Aviv 69978. ISRAEL A kinetic model of the propellant ablation in a pulsed plasma thruster is developed. The model takes into account the non-free nature of oblation due to the presence of a high-density discharge plasma. The kinetic approach namely bi-modal distribution function was used to determine thc parameters at the interface between the kinetic Knudscn layer and the hydrodynamic layer. By coupling the kinetic layer solution with the hydrodynamic non-equilibrium layer model an ablation rate was calculated. The model allows the calculation of ablation rate as a function of propellant surface temperature, plasma density and tcmpcrature, and in combination with a plasma discharge model [I] self-consistently describes thc electrical discharge in an electrothermal pulsed plasmathruster. An example of the calculated ablation rate as a function of plasma bulk density and Teflon surface temperature is shown.

Device and plume model of an electrothermal pulsed plasma thruster

“…the plasma parameter distribution along the cavity and a more sophisticated ablation mode!. The present model of the capillary discharge employs a recently developed ablation model 12 . These changes allow us to provide more accurate boundary conditions for the piume simulation.…”