Ultrasonic Burn Rate Testing of Bi-Plateau Propellants Under Decreasing Pressure Conditions

TLDR: In this article, the authors used a closed combustion bomb to measure the burning rate of a bi-plateau composite propellant under a decreasing pressurization rate, using a pulse-echo ultrasonic technique.

Abstract: This paper will present the design of an experiment to measure the burn rate of a bi-plateau composite propellant under a decreasing pressurization rate. It is hoped that by investigating the burning rate characteristics of a bi-plateau propellant under these conditions, some new insights might be gained into the mechanism causing the plateau effect. A bi-plateau propellant is a propellant that has a high and lowpressure region where the burning rate remains constant as the pressure is increased. These propellants are well suited to facilitate propellant energy management and may be less prone to acoustic combustion instabilities. The testing was conducted using a closed combustion bomb utilizing a pulse-echo ultrasonic technique to measure the burning rate. Use of the ultrasonic method allows the burning rate of a propellant to be measured over a large range of pressures during a single test. To measure the burning rate under a decreasing pressure scenario, a novel testing method was developed to release pressure at a controlled rate during the test firing. This is accomplished by use of a custom burst disk and nozzle. A simple numerical model was developed to determine a nozzle throat diameter that would produce a decrease of pressure that would be a mirror image of the pressure increase during the closed portion of the test. Several samples of MURI #4 composite propellant were tested in this manner. Although burning rate data was measured for each of these tests, the data collected to date is not enough to draw any significant conclusions.