Reducing Aluminum Agglomeration in Composite Propellants with Gallium and Indium Additives

Mechanically processed aluminum powders containing small amounts of gallium or indium reduce the volume-mean diameter of molten agglomerations in an ammonium-perchlorate composite propellant by as much as 60%. In cases where very low concentrations of gallium or indium are used (3–5 wt %), increases in propellant burning rate were observed as well. Higher concentrations of gallium or indium (10–16 wt %) maintained smaller agglomeration sizes but did not alter the burning rate of the propellant. A mechanism explaining the tradeoff between burning rate and agglomeration potential was proposed: gallium and indium reduce the onset temperature of an aluminum particle, leading to quicker ignition, shorter surface residence time, and less time to agglomerate. However, high concentrations of gallium or indium significantly reduce flame temperatures, thereby leading to reduced heat feedback and lower burning rates. This work also discovered that high concentrations of gallium/indium reduce the maximum theoretical specific impulse of a motor by several seconds in some cases.