Passive Control of Pressure Oscillations in Solid Rocket Motors: Cold-Flow Experiments

Cold-gas experiments are used to study the pressure oscillations occurring in solid rocket motors 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 a 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. An important reduction of pressure oscillations (I order of magnitude) is obtained, both for the axial and radial flow injection configurations, by removing the cavity located around the nozzle head. However, the nozzle integration cannot be avoided and this solution then cannot be implemented on a 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 a small attenuation of the pressure oscillations; however, its design could be optimized to maximize the acoustic damping. The three-dimensional-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 with excitation. For a similar cross section, the asymmetric inhibitor (crenel-shaped) provides a reduction of pressure oscillations by a factor of 2 compared with an axisymmetric inhibitor. Therefore, the asymmetry of the inhibitor seems to be the best candidate for reducing the pressure oscillations.