Several Novel Applications of Microwave Interferometry in the Measurement of Solid Rocket Propellant Regression Rates
Abstract
When characterizing a new solid propellant, one of the most important steps in determining its usefulness is discovering how the burning rate changes in response to changes in pressure. While there are many dynamic methods for directly measuring the regression rate of a burning propellant sample, few of them are capable of being used in typical harsh motor conditions: high pressures, high temperatures, and in an environment comprised of propellant exhaust products. This paper describes and evaluates the use of two custom-built microwave interferometers, one operating at 35 GHz and the other operating at 94 GHz, in several different configurations for the measurement of propellant regression rates. Four different configurations of interferometer and waveguide are presented and contrasted, with example results of experiments included. A polytetrafluoroethylene (PTFE) waveguide, utilized in previous works for explosives detonation velocity characterization, was used to directly couple interferometer signal with a burning propellant strand. This PTFE coupling is shown to be applicable to pressure vessel studies by simply using a cable feedthrough. In this configuration, signal quality is high but signal amplitude is low, especially when the waveguide is encased by support structures. A novel PTFE truncated cone waveguide expander is presented which performs three tasks: expanding the microwave signal such that an oversized (relative to signal wavelength) strand may be examined via microwave interferometry, functioning as a weak antenna that can observe phenomena through interstitial material without picking up significant amounts of environmental reflection, and acting as a sealing surface for pressure vessel experiments. Additionally, the use of a more-standard hollow-core waveguide and high-gain antenna is displayed, highlighting the increased signal strength but the larger number of spurious reflections in the signal. This study shows, through various experiments using the aforementioned configurations, the capability of microwave interferometry to quickly characterize a full propellant burning rate curve using a single dynamic-pressure test with 40g of propellant in a 2.5cm diameter propellant strand. Several novel combinations of mechanical configuration and propellant composition are shown that may guide future studies into the use microwave interferometry for solid propellant regression rate analysis.
Degree
M.Sc.
Advisors
Son, Purdue University.
Subject Area
Atmospheric sciences|Mathematics|Optics
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