Numerical and experimental study on combustion instability of a dual-burning-rate solid rocket motor

Aiming at the issue of multi-frequency pressure oscillations occurring at the end of the boost phase in a dual-burning-rate solid rocket motor with a large aspect ratio. This study comprehensively analyzes experimental data, acoustic modes of the combustion chamber, and vortex shedding characteristics, thereby clarifying the mechanism of pressure oscillations and elucidating the regulatory effects of the parameter matching between high and low burning rate propellants, and nozzle structure on pressure oscillations. The results demonstrate that the nonlinear acoustic-vortex coupling in the combustion chamber induces multi-frequency pressure oscillations; an increase in combustion temperature differences between high/low burning rate propellants exacerbates combustion instability, and multi-frequency oscillations are more readily excited when the combustion temperature of the low-burning-rate propellant exceeds that of the high-burning-rate propellant; the growing burning rate disparity between high and low burning rate propellants drives the evolution of large-scale vortices into small-scale vortex clusters, resulting in a spectral shift of pressure oscillations from multi-frequency to single-frequency; an increase in the axial length fraction of low burning rate propellant induces a transition of the dominant frequency in pressure oscillations from fundamental frequency to higher frequency; reducing the nozzle inlet diameter or the convergent half-angle can enhance the energy dissipation of vortex structures, thereby suppressing oscillation amplitudes and mitigating the intensity of acoustic-vortex coupling. Based on the aforementioned findings, a multi-parameter collaborative optimization strategy for the pressure oscillation suppression is proposed, and its feasibility is verified via a full-scale motor test. This work provides theoretical insights and engineering guidance for resolving the pressure oscillation in the solid rocket motors with a dual burning rate.