Synergistic Enhancement in Mechanical and Ballistic Properties of Composite Propellant for Solid Propulsion Systems

Abstract

Performance of a solid propulsion system is based on reliable working of the propellant stored in the system. The performance of composite propellant is a strong function of its mechanical and ballistic properties. There has been a constant endeavor to improve the mechanical properties of the composites to meet demands for performance in various propulsion applications in more extreme conditions. These properties were mostly investigated separately, out of contact with the ballistic behavior. In the present work, mechanical properties of the Al/AP/HTPB conventional composites have been investigated by (1) varying particles size distribution, (2) modification in polymer matrix, (3) strengthening the bond between the polymer matrix and oxidizer, and correlated to their ballistic properties. The composites with high solid loadings were fabricated by mixing the ingredients in a planetary mixer. The composites were formed by casting followed by curing at elevated temperature for several days. Theoretical work comprises of sensitivity analysis of various losses incurred in non-ideal propulsion. This work leaded towards enhancing system performance by adopting the approach of minimizing the energy losses. The effect of ablation loss on the system performance has also been analyzed, for which a separate model has been proposed. First, the mechanical properties were determined by varying particle size distribution of inorganic crystalline oxidizer and aluminum particles. A strong influence of particle size distribution on mechanical properties was observed, exhibiting an inverse relationship between tensile strength and ultimate elongation for variation in quantity of fine particles. Similarly, burning rate measured using strand burner at various pressure levels revealed a linear relationship with increasing ratio of fine particles. Secondly, to improve the mechanical properties of the traditional composites, short chain diols were incorporated in the matrix systems as chain extenders (CE), which confer enhanced microphase segregation commensurate with short chain hard block reinforcements. The stress-strain analysis revealed a manifold improvement both in tensile strength and elongation with chain extenders as compared to traditional composite. The maximum tensile strength was obtained with butane diol (BDO) and maximum elongation was achieved with hexane diol (HDO). Further, SEM investigation of fractured specimens in tensile test revealed that the particles of the composites fabricated with short chain diols were securely embedded in the polymer matrix as opposed to the conventional composites. Optimum results were achieved with BDO. Normally, bonding agent (BA) is used in the conventional composites to strengthen the bond between the polyurethane matrix and oxidizer. Therefore, influence of BA on mechanical properties of the composites was determined and compared with BDO incorporation. The composite without both additives failed against the applied load to any considerable extent. Maximum tensile strength and ultimate elongation were achieved by introducing BDO in the conventional composite (composition containing BA). However, it was observed that BDO has greater effect on mechanical performance in comparison with BA. The influence of chain extender (CE) and bonding agent (BA) was also investigated for ballistic properties. Results indicated a relationship between the two properties. Better mechanical properties in modified composite containing both of the additives also contributed in improving the ballistic behaviour. SEM analysis of the condensed combustion residues obtained from the propellant burnt in open atmosphere revealed a considerable decrease in the agglomerate size in the modified composite. This effect was reflected as higher specific impulse in this composite, determined from thrust-time profile obtained in static bed test. Combustion products of the composites without bonding agent contained significant amount of the unburned metal (Al) clumps which resulted in reduction of combustion efficiency of the composite, ultimately reducing the performance of rocket motor. Pressure-time profile for the modified composite manifested a stable and consistent burning at the tail end in tubular geometry as opposed to the conventional system that exhibits a susceptible and unstable tail-off. However, unstable behavior at the end burning was more pronounced for composite without either the CE or BA. The research has been reported in international peer reviewed journals; (i) Zahid Mehmood, Muhammad Bilal Khan, Tanveer Abbas and Nadeem Ahmad: “Influence of Solid Particle Size on Burning and Mechanical Properties of AP/Al/HTPB Composites”,