Combustion mechanisms of wide distribution solid propellants
Abstract
The objective of this research is to describe the effects of oxidizer particle size distribution on the burning rate of solid propellants used in rocket motors. Current models over predict the burning rate of wide distribution (wide distribution denotes two oxidizer modes that have extreme differences in mean diameter) formulations by 40 to 200 percent indicating combustion mechanisms unique to this type of propellant. Four sets of AP/HTPB propellants were formulated to control the physical and chemical heterogeneities characteristic of the propellant surface using 400 and 20$\mu$ oxidizer particles. The propellants were tested at pressure levels from 0 to 2000psig. An optical, distance measurement technique was developed and used to measure the local, non-steady surface deflagration of the propellant burning surface. The method uses a laser beam, synchronous detection, and closed-loop tracking to locate the surface in the hostile combustion environment. An acoustic emission technique determined average burning rates. Combustion phenomena were also accessed using high-speed photography and scanning electron microscopy. The results indicate that the burning rate of wide distribution propellants is suppressed by the formation of a layer of liquid binder on the burning surface. High-speed motion pictures showed molten binder flowing off the burning surface during the propellant combustion. The flow increased as the oxidizer-to-fuel ratio of the pocket propellant is decreased. Examination propellant surfaces, extinguished by depressurization, showed that the oxidizer surface was partially covered with a thin binder layer during the burning. This establishes a condensation reaction over the covered portion of the oxidizer which reduces its burning rate. This phenomenon accounts for the difference in model predictions and experimental results. Because of the fuel-rich nature of the fine particle combustion, the propellants also exhibited extreme sensitivity to changes in binder composition. This indicated a shift from the dominance of the oxidizer deflagration to the dominance of the binder pyrolysis.
Degree
Ph.D.
Advisors
Osborn, Purdue University.
Subject Area
Aerospace materials
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