Experimental techniques for characterization of particles in plumes of sub-scale solid rocket motors

An innovative collection methodology based on a recently developed supersonic probe enabled the collection of alumina particulate from the exhaust plume of a sub-scale solid propellant rocket motor and the comparison with quench collection bomb analysis of propellant incipient agglomeration. Laser diffraction, scanning electron microscopy, and X-ray spectroscopic methods were used to determine particle size, morphology, crystalline nature, and elemental composition. A significant reduction of the particle size occurred across the rocket nozzle. The size distribution resulting from the expansion was monomodal and centered around 2μm to 3μm. Propellants containing lower aluminum mass fraction led to number-based distribution in the sub-micrometric region while, for higher metal loading, particle distributions were sensibly shifted towards larger size in the same rocket operative conditions. Similarly, the size was identified to be weakly dependent on chamber pressure, with an increase of the former as the latter decreased. The Hermsen correlation supported and verified the experimental analysis. The majority of the particles was composed by γ-alumina phase, had a typical size lower than 3μm, and was characterized by smooth surfaces. Occurrences of spitting and collision-to-coalescence phenomena were identified and analyzed.