Theoretical performance analysis of Al-Mg-B alloy fuels in three propulsion systems

Metal fuels feature high energy density and excellent environmental adaptability. However, aluminum, magnesium, and boron exhibits great discrepancies in their energy performance, combustion behavior and density, these metals are frequently alloyed to achieve superior comprehensive performance. Based on the engineering background of multi-mode cross-medium combined motor, thermodynamic calculation were performed to analyze the theoretical specific impulse of Al-Mg-B alloy fuels under typical design conditions. The fuels were matched to air-breathing ramjets, water-breathing ramjets, and AP-based solid rocket motors (ammonium perchlorate as the oxidizer). The contour maps illustrating the variation of theoretical specific impulse with the mass fractures of the three metallic components were generated and plotted on ternary phase diagrams. The results show that for air-breathing and water-breathing ramjets, boron is the most prominent component affecting theoretical specific impulse, followed by aluminum and then magnesium. Both propulsion systems can use the same alloy fuel. At identical oxidizer-to-fuel ratio, compositional adjustments of Al-Mg-B alloy fuels lead to maximum specific impulse difference of roughly 120 N·s/kg, equivalent to around 6% of the nominal theoretical specific impulse. This research provides a reference for formulation optimization of metal-loaded propellants used in multi-modal cross-media combined motors.