Vortex Flow Analysis of a Large Segmented Solid Rocket Motor

It is not uncommon to find pressure oscillations in large size segmented solid rocket motors. During the static test of segmented solid rocket motor (SRM-3), unanticipated pressure oscillations were seen after some of the propellant has burnt and the oscillations sustained for certain duration of time. The purpose of this paper is to present the results of the analysis carried out on the experimental data using commercial CFD software 'FLUENT' and compare the results with experimental data to find out the cause of the pressure oscillations. Inhibitors are provided for the full web thickness of propellant at the segment joint interfaces to prevent end-burning of propellant at the joints. The char and erosion rate of these inhibitions are lower than the burning rate of the propellant. As burning of the propellant progresses, annular inhibition wall starts projecting above the burning surface of the propellant. These obstacles (inhibition) are acting as wall and sheared gas flow occurs in the rocket motor. CFD analysis was carried out on a quadrilateral mesh for the geometry of SRM-3 at the time of 32 s after ignition when the pressure oscillations peaked. It has been found that the vortex shedding frequency obtained by CFD analysis closely matches with the frequency of pressure oscillations occurring during static testing. Thus, it has been revealed that the obstacle vortex shedding generated by the compartmentalization of the combustion chamber by the protrusion of inhibition at the segment joints was the cause of the pressure oscillations in SRM-3.