Oscillatory combustion of aluminized composite propellants

The unsteady combustion of aluminized composite propellants (Al/AP/HTPB) was studied using laser recoil and other measurements. It was found that propellants with fine aluminum (<15 μm) and high aluminum loading (>15%) produced large aluminum agglomerates and a strong low-frequency (<100 Hz) oscillatory combustion response with burning rate oscillations at the same frequency, and in phase with departure of large aluminum agglomerates at the propellant surface. Coaser aluminum propellants produced only moderate-sized agglomerates and minimal oscillatory combustion response. The frequency of the burning rate oscillations increased (approximately linearly) with increasing mean burning rate and aluminum loading and decreased with increasing aluminum ingredient particle size. The oscillatory burning response appears to be related to periodic accumulation, agglomeration, ignition, and departure of alumimum at the burning surface of the propellant. Both the oscillatory burning and agglomerate particle size data are consistent with Gany and Caveny's model of aluminum accumulation in a mobile surface melt layer. These observations confirm that aluminum can have a significantly stronger influence on the combustion of AP/HTPB propellants than that indicated by the normally small changes observed in macroscopically steady burning rate or the mean burning rate during macroscopically unsteady (oscillatory) combustion. The proposed mechanism linking aluminum behavior and propellant burning rate is that of cyclic heat sink and source effects associated with the periodic aluminum accumulation and departure.