Improving the agglomeration, combustion, and injection characteristics of aluminum–water propellants using graphene

Al–H₂O propellants hold promise as environmentally benign alternatives to conventional solid propellants, releasing substantially lower amounts of harmful gases (e.g., CO_x and NO_x) but suffering from suboptimal combustion efficiencies and the substantial formation of residual byproducts. Herein, graphene is integrated into Al–H₂O propellants to improve their combustion and injection properties, ammonium perchlorate (AP) was introduced as a combustion promoter. The influence of graphene content (0–1.25 wt%) on these properties is explored using thermogravimetric analysis, differential scanning calorimetry, laser ignition tests, and laboratory-scale solid rocket motor firing experiments. As the graphene content increased from 0 to 1.25 wt%, the flame luminosity during propellant combustion intensified, and the combustion rate improved from 1.27 mm s⁻¹ to 2.12 mm s⁻¹. Concurrently, an increased number of aluminum particles ignited, while the agglomeration phenomenon was notably mitigated. Consequently, the average particle diameter of combustion residues decreased from 362.2 μm to 208.1 μm, leading to an enhancement of combustion efficiency from 90.23% to 95.32%. Moreover, the propellant injection efficiency rose from 31.33% to 51.72%, primarily attributed to the accelerated gas-phase release during combustion. Powder ignition experiments demonstrated that the increased combustion rate resulted from graphene-induced promotion of AP decomposition and aluminum oxidation. Numerical calculations further revealed that the elevated combustion rate significantly reduced the melt-layer thickness, thus effectively suppressing aluminum agglomeration. The obtained insights into the optimization and reformulation of Al–H₂O propellants are expected to help enhance their performance in practical applications.