Comparison of Different Passive Control Solutions for Reducing SRM Pressure Oscillations Using Cold Flow Experiments

Abstract

Cold gas experiments are used to study the pressure oscillations occurring in solid rocket motors (SRM) and the performance of different ways of passive control of these oscillations. Previous studies stated that flow-acoustic coupling is mainly observed for nozzles including cavity. The nozzle geometry has an effect on the pressure oscillations through a coupling between the acoustic fluctuations induced by the cavity volume and the vortices traveling in front of the cavity entrance. The most important reduction of pressure oscillations is obtained by removing the cavity located around the nozzle head. It has been firstly proved that removing the cavity located around the nozzle head is a very good solution both for the axial and radial flow injection configurations, with a reduction factor up to 10. However, the nozzle integration cannot be avoided and this solution can then not be implemented on real flight. A permeable membrane (with holes to allow the combustion gas to pass through) placed in front of the cavity allows a reduction by a factor 1.5. The Helmholtz resonator shows small attenuation of the pressure oscillations; however, its design can be optimized in order to maximize the acoustic damping. The 3D-shaped inhibitors show a good attenuation of the pressure fluctuations, especially when the opening cross-section is increased. This increase results in a shift of the Mach number associated to excitation. For a similar cross-section, the asymmetric inhibitor (crenel-shaped) provides a net reduction of 48% compared to an axisymmetric inhibitor. So, the asymmetry of the inhibitor provides a promising way of reducing the pressure oscillations.