Direct ultrasonic measurement of solid propellant ballistics

Abstract

This article illustrates the application of an ultrasonic pulse–echo technique to determine the burning rate of a composite solid propellant as a function of pressure. An evaluation of the measurement uncertainty of the method is also presented. Unlike the more traditional strand burner techniques, where dozens of constant pressure tests are necessary, the ultrasonic technique measures the burn surface position thousands of times per second as the pressure varies. This reduces the number of tests necessary to determine the ballistic characteristics of the propellant by an order of magnitude. This work presents new methods to characterize the changing speed of sound in the propellant and quantitative estimates of the measurement uncertainty in the burning rate measurement. The results of the uncertainty analysis showed that the measurement is accurate to around 4%. The propellant samples were tested in a closed-combustion vessel, under pressurization rates of up to 15.8 MPa/s. The data obtained with the closed-combustion-vessel tests using the ultrasonic method are experimentally congruent with tests conducted in strand burners in steady-state conditions.