Effects of AlH₃ particle size and loading on the combustion and agglomeration of solid propellants

Owing to its hydrogen storage capacity, aluminum hydride (AlH₃) has been proposed as a potential fuel additive in solid rocket propellants to achieve a higher specific impulse and cooler combustion. In this experimental investigation, we examine the effects of AlH₃ loading and particle size on the thermal decomposition, ignition, agglomeration and combustion of high-energy solid propellants at pressures up to 9.1 MPa. We use a variety of diagnostics, including thermogravimetry, differential scanning calorimetry, energy dispersive X-ray spectroscopy, scanning electron microscopy and so on. The results show that adding AlH₃ can enhance the thermal decomposition of the propellants. As the loading of fine-grained AlH₃ increases, the combustion intensity drops, but the ignition delay time drops as well (by 60 %). When the AlH₃ particle size increases, the ignition delay time rises, but it remains shorter than that of a baseline propellant containing Al instead of AlH₃. Although AlH₃ is found to lower the gas-phase flame temperature and temperature gradient, it enhances the burning rate and lowers the pressure exponent from 0.58 to 0.50. Fine-grained AlH₃ is also found to shrink the near-burning surface agglomerates and condensed combustion products, increasing the combustion efficiency from 81.9 % to 94.7 %. However, coarse-grained AlH₃ shows the opposite effect, decreasing the combustion efficiency to 63.3 %. Overall, this study sheds new light on the influence of AlH₃ addition on high-energy propellants, facilitating the application of this promising fuel additive in solid rocket motors.