The characterization of the two-phase flow in a heterogeneous solid-propellant rocket chamber and nozzle is crucial in ballistic/performance predictions, aeroacoustic studies, erosion analyses, slag accumulation rate estimates, predictions of thermal loads, plume analyses etc. In particular, aluminum particles initially embedded within the binder and injected in the flow during the burning of the grain, undergo mechanical and chemical interactions with the flow itself, and constitute a principal component of the condensed phase. In the present work the attention has been focused on the numerical simulation of the internal fluid dynamics of solid rocket motors, with particular attention on multiphase features in the flow, related to the presence of aluminum particles as primary ingredient in the propellant. Several simulation have been performed, both on very simple geometries (rockets with cylindrical grain) and on a real motor (Zefiro 9A SRM). The results show that the thermo-fluid dynamics features inside the chamber are strongly related to the behaviour of aluminum combustion and are very sensitive to initial dimensions of aluminum particles.