Breakdown in ablative pulsed plasma thrusters (APPTs) must be studied in order to design new types of APPTs and measure particular parameters. In this paper, we studied a parallel-plate ablative pulsed plasma thruster that used a coaxial semiconductor spark plug. By operating the APPT about 500 times with various capacitor voltages and electrode gaps, we measured and analyzed the voltage of the spark plug, the voltage between the electrodes, and the discharge current. These experiments revealed a time delay (∼1–10 μs) between spark plug ignition and capacitor discharge, which may affect the performance of high-pulsing-rate (>10 kHz) and double-discharge APPTs, and the measurements of some of the APPT parameters. The delay time decreased as the capacitor voltage increased, and it increased with an increasing electrode gap and increasing number of ignitions. We explain our results through a simple theoretical analysis.
Velocity of ablation vapor near the surface of heated compound-materials strongly affects the kinetic layer parameters modeled and manifested in the Knudsen layer. This paper discussed overlooked physics and clarified inaccuracies in the expression of velocity at the outer boundary of the kinetic layer induced by discharge plasma. The changes of average molecular mass coupling with discharge current on mass and momentum conservation equations in plasma layer were considered when modifying the expression of this boundary velocity. Our assessment of these effects indicated that velocity of ablation vapor showed a downtrend as the ratio of average molecular mass at inner and outer boundaries of plasma layer increased, which plays a decisive role in reducing the ablation rate. Compared with single species fluid model, the modified model that applies to the pyrolysis of heated compound-materials showed 56% drop in Teflon's ablation rate when plasmas were fully ionized.
Accelerated aging tests under pre‐strain were conducted on HTPB‐based composite solid propellant with the goal of investigating the effect of pre‐strain aging on its damage properties. A statistical damage constitutive model based on continuum damage theory and statistical strength theory was established. The aging damage coefficient, making aging process of propellant equivalent to a form of damage, was introduced to correct the damage variable. Experimental results show that theoretical model has good agreement with experimental results and can accurately describe the mechanical behavior of propellant during pre‐strain aging. Further analysis indicated that the damage effects caused by pre‐strain can be identified from the equation of the aging damage coefficient. Aging time influences both tensile strength and shape characteristics of the stress‐strain curve of propellant in the damage stage, while pre‐strain only decreased the tensile strength. The strain damage threshold value decreased linearly over the aging period and with increasing pre‐strain level during the aging process.
To illuminate the thermal transfer mechanism of devices adopting polytetrafluoroethylene (PTFE) as ablation materials, the thermal radiation properties of PTFE plasma are calculated and discussed based on local thermodynamic equilibrium (LTE) and optical thin assumptions. It is clarified that line radiation is the dominant mechanism of PTFE plasma. The emission coefficient shows an opposite trend for both wavelength regions divided by 550 nm at a temperature above 15 000 K. The emission coefficient increases with increasing temperature and pressure. Furthermore, it has a good log linear relation with pressure. Equivalent emissivity varies complexly with temperature, and has a critical point between 20 000 K to 25 000 K. The equivalent cross points of the average ionic valence and radiation property are about 10 000 K and 15 000 K for fully single ionization.
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