Ultrasonic technique resolution for solid propellant burning rate measurement

Ultrasonics has been successfully used to measure the burning rate of solid rocket propellants. When used in a closed bomb it provides data over a wide range of pressures in a single test saving time and material. But a number of questions remain regarding how burning surface roughness or a melt layer affects the ultrasonic signal and the size of the surface area probed by the signal. This study observed simulated propellant samples with known surface characteristics using ultrasonics to determine the measurement effects. It was found that imperfections measuring a minimum of between 0.13 to 0.5 in. were necessary to be clearly detectable, depending on the type of imperfection. The surface diameter "seen" by the transducers was approximately 0.4 in. Also, a simulated melt layer provided a separate echo from the underlying solid material if the melt layer had a different impedance and was at least 0.1 in. thick.