Several metallized solid rocket propellants, AP/Metal/HTPB in the ratio 68/18/14, were experimentally analyzed at the Space Propulsion Laboratory of Politecnico di Milano. Effects of the metals (micrometric and nanometric Al, B, Mg, and a variety of dual metals) on the performance of the propellant were studied and contrasted to a conventional micrometric aluminum (30 μm average grain size) taken as reference. It is shown that the propellant microstructure plays a fundamental role in controlling the critical aggregation/agglomeration phenomena occurring below and near the burning surface. Two specific effects of microstructure in terms of steady burning rate and average agglomerate size are illustrated.