Effects of Grain Geometry and Vibration on the Suppression of Combustion Instability in a Solid Rocket Motor

A comprehensive numerical model for internal ballistic simulation under dynamic flow, combustion and structural vibration conditions is used to investigate the effectiveness of grain port area transitions of a reference solid rocket motor as a means for suppression of axial combustion instability symptoms. Modification of the propellant grain geometry is one of several traditional means for suppressing symptoms in actual motors. With respect to these symptoms, individual transient simulation runs show the evolution of the axial pressure wave and associated DC shift for the given grain geometry, as initiated by a given pressure disturbance. Limit pressure wave magnitudes are collected for a number of simulation runs for different grain area transition positions, steepness and aspect ratios, and mapped on an attenuation trend chart. Effects of acceleration, through structural vibration of the propellant surface, on the combustion process are investigated, and their influence on the effectiveness of grain area transitions is examined. With or without acceleration/vibrations effects included, the numerical results produced in this study confirm the significant ability of a grain area transition to suppress combustion instability symptoms.