Direct measurement of solid propellant pressure-coupled response functions with ultrasound

The pressure-coupled response function of a series of composite non-metallized propellants was measured in a modified small-scale motor. The technique is unique in the manner in which the pressure oscillations are generated. An inert gas is injected into the nozzle throat to modulate the pressure inside the combustion chamber. The regression rate of the propellant was measured using a pulse-echo ultrasonic technique. The frequency of modulation was at 17 Hz and 70 Hz and the pressure range was from 250 psig to 2100 psig. Evaluation of the resolution of the ultrasonic technique showed that the present 12-bit data acquisition system is capable of detecting displacements of the burning surface to 1.5 μm. An analytic model of the transient pressure conditions within the combustion chamber was developed to aid in the determination of the settings needed to obtain the desired conditions in the test. Moreover, a computational study of wave propagation through media with stepwise acoustic impedance was conducted to correctly identify the material boundary in a pulse-echo measurement method. The data reduction technique employed digital signal processing techniques that allowed for the real and imaginary parts of the pressure-coupled response to be computed. A Monte Carlo technique was used to compute the uncertainty in pressure-coupled response for each test. Four propellants were specifically formulated and tested to develop and evaluate the burner. The first propellant was composed of 86% ammonium perchlorate (AP), 12% hydroxyl-terminated polybutadiene (HTPB), and 2% catalyst. The three other propellants were specifically formulated to interpret the effect of AP particle size on the characteristics of the propellant. These propellants were composed of 75% AP, 23% HTPB, and 2% additives. The first was bimodal 17/200 μm propellant whereas the second and third were mono-modal 200 μm and 17 μm propellants. The results showed that the pressure-coupled response of solid propellants is a rising function of pressure and the amplitude of response is the sum of the components' amplitude in proportion to their mass fraction.