Agglomeration in solid rocket propellants: novel experimental and modeling methods

Abstract

Metal agglomeration is a key factor affecting the performance of metalized solid propellant rockets since many of the mechanisms that degrade specific impulse can be ascribed to metal powder aggregation and agglomeration. Condensed combustion products are generated at or near the burning surface of the propellant and then released in the gas phase where they are shaped by the core flow viscous forces and oxidized by the reactive environment. On this basis, detailed information about the size of agglomerates as they are generated from the propellant may improve the knowledge of the core flow and, thus, the prediction of its effects (namely, two-phase flow losses). This topic entails both modeling and experimental activities, and the aim of this work is to present some recent developments achieved at the Space Propulsion Laboratory in cooperation with other international institutions. The experimental part shows the application of high-speed and high-resolution imaging of propellant combustion for automated measurement of particle size exploited by means of an ad-hoc image processing tool. The modeling part demonstrates how heterogeneity can explain the agglomeration by means of a pocket model using spatial statistics and two- (2D) or threedimensional (3D) virtual propellants. A definition of a characteristic time that fits the agglomeration data for tested propellants is suggested, and a method for predicting the agglomerate size is given. Both activities are still a matter of research but the maturity level reached so far permits the application in some practical cases.