Activated Aluminum Powders for Space Propulsion

Aluminum powders are commonly used in solid propellants to enhance the performance of space propulsion systems. During combustion, a fraction of the fuel metal particles, which emerge from the bulk, tends to merge into aggregates. These structures eventually leave the combustion surface in the shape of partially molten agglomerates which can reach the size of hundreds of microns. These condensed combustion products partake in nozzle expansion and hinder the delivered specific impulse of the rocket unit. The enhancement of original particle reactivity improves combustion quality and may reduce sensibly agglomerate size and relevant losses. More reactive aluminum fuel can be obtained by activation of micron-sized powders, without resorting to the use of nano-metals. One of the methods consists of a chemical treatment with a processing solution which alters the standard oxide layer at the surface of the particles. Such modifications grant lower ignition temperature and faster propellant burning rates but deplete a fraction of the active metal content, as a result of the chemical reaction. The present paper compares the features of three batches of aluminum particles which were treated with fluorine-based activating solutions of different concentrations. The batches were supplied in the frame of HISP FP7 European Project. The characterization focused on physical, chemical and thermal properties, looking at the reactivity of the samples and at the alterations introduced by the chemical processing. Finally, activated aluminum batches were tested in lab-scale propellants, monitoring the variation of ballistic properties with respect to a reference formulation.