CHARACTERISTICS OF PRESSURE OSCILLATIONS IN LARGE SEGMENTED SOLID ROCKET MOTORS

The internal motor pressure oscillations measured during three different solid rocket motor static tests (S139, S200, and SS1) are analyzed. From the literature, the nature of these oscillations is found to be similar to the ones caused by vortex shedding at inhibitors and acoustic feedback resulting from the impingement of the vortices on other inhibitors or on the nozzle. The frequencies of the pressure oscillations vary around the motor's fundamental acoustic mode and generally agree with a simple empirical relationship commonly used to describe acoustic feedback. This flow-acoustic coupling typically occurs when an integer number of vortices lies between the segment inhibitor and the nozzle. In the present work, two different spectral analyses are applied to static test data to understand the characteristics of these oscillations, which vary with time. Although the amplitude of these oscillations is well depicted by the conventional fast Fourier transform algorithm, it is unable to produce high-resolution results for the identification of continuous frequency variation, as described in the acoustic feedback model. Hence, the maximum entropy method is used for a better description of this phenomenon, aiding in the understanding of different parameters that influence the flow-acoustic coupling in solid rocket motors.