Motor and plume particle size measurements in solid propellant micromotors

Abstract

Particle size distributions were measured in the chamber, nozzle, and plume of a subscale solid propellant rocket motor. A significant reduction in the mean size of the aluminum/aluminum oxide particles occurred within the motor chamber. The mass fraction of small particles (<2 ft) at the nozzle entrance was less than 10%. Also, most particles were smaller than 50 ft, although a few as large as 85 ft were present. In the converging and throat portions of the nozzle it appeared that particle breakup dominated over collision coalescence. Collision coalescence was observed to occur more dominantly in the supersonic nozzle flow. At the nozzle exit the particle mean size was usually less than 2.5 ft. The particle size distributions were bimodal or trimodal, with the larger particles concentrated near the plume centerline. Because of the short nozzle residence times it is not known whether or not these results are also applicable to full-scale motors. The mean size of the small A12O3 particles in the plume edges was less than 0.5 ft in diam, with an index of refraction of 1.64 ± 0.04 (apparently y-Al2O3), independent of propellant composition, motor operating conditions and nozzle geometry.