An unexpected driving effect of solid, inert particles on combustion instabilities of a solid rocket motor subject to parietal vortex shedding was shown by experiments and simulations. Droplet size proved to be the key parameter, as in damping cases. In general, the accurate simulation of polydisperse sprays in unsteady gaseous flows is a major issue for solid rocket motor optimization. Yet solving for unsteady two-phase flows with high accuracy on the droplet sizes is a challenge for both modeling and scientific computing. The EulerianMulti-Fluid method is a good candidate for capturing polydispersion with industrial-oriented codes. A two size moment method has been recently extended to coalescence in CEDRE, a multi-physics platform used for solid propellant combustion research and industrial computations. The present work aims at hammering the impact of coalescence, which occurs in the combustion chamber of aluminized propellants. For this purpose, it furthers simulations performed in the case of particle driven instabilities, but assumes liquid particles. The slight size distribution shift due to coalescence is shown to have strong effect on the flow and particularly on instability levels. Finally, three points are made obvious on this case of hydrodynamic and acoustic instability coupling : the sensitivity of computational material to modeling and numerical methods, the necessity to account for coalescence in solid propellant combustion and the ability of the CEDRE code to do so.