INVESTIGATION ON COMBUSTION CHARACTERISTICS OF HYDRO-REACTIVE SOLID PROPELLANTS WITH Al/Mg BLENDS

This study investigates the combustion behavior of aluminum (Al), magnesium (Mg), and Al/Mg blend-based hydro-reactive solid propellants (HRSPs), developed for underwater propulsion applications. Three different propellant formulations with varying metal additive types were examined to understand their combustion behavior. Thermochemical performance results obtained using the chemical equilibrium analysis (CEA) program indicated that aluminum contributes to a higher theoretical energy density, whereas magnesium enhances ignition sensitivity. Parametric modeling identified an optimal water-to-fuel mass ratio near 0.4, with the Al/Mg blend giving theoretical specific impulse similar to the Al-based composition, whereas the presence of Mg enhances early gas-phase reaction. Experimental calorimetry and windowed strand burner tests confirmed these synergistic roles, with blended Al/Mg systems exhibiting greater effective energy release than single-metal counterparts. The 30/10 (Al/Mg, total 40 wt.% metal) composition indicated the better combustion characteristics with a burning rate of 6.8 mm/s at 40 bar (4 MPa) pressure. Post-combustion residue analyses revealed unreacted Al, MgO, and MgAl2O4 spinel present with these formulations, highlighting large condensed-phase combustion products (CCPs) at high metal loadings. These results demonstrate that balanced Al-Mg blends can improve usable energy release and combustion stability in these propellants, to optimize the overall performance of HRSP-based propulsion systems.