Very-High-Pressure Burning Rates of Aluminized and Nonaluminized AP/HTPB-Composite Propellants

To further characterize the roles of aluminum and ammonium perchlorate (AP) characteristics in ammonium perchlorate/R45-M hydroxyl-terminated polybutadiene (AP/HTPB)–composite propellants, aluminized and nonaluminized formulations were tested at very high pressures, up to 68.9 MPa (10,000 psi). A total of nine formulations were examined: seven nonaluminized with varying AP distributions, concentrations, and average particle sizes, and two aluminized with varying aluminum concentrations. All nine propellants showed an exponent or “slope” break above 20 MPa (2900 psi) and a postbreak pressure exponent greater than one. Decreasing the AP particle size decreased both the characteristic pressure where the exponent break occurred (or ) and the pressure exponent after the break. AP distribution also affected , whereas changes in AP concentration did not. The inclusion of aluminum lowered compared with the nonaluminized formulations. Additionally, increasing the aluminum concentration appeared to lower the postbreak pressure exponent. The present study provides one of the first fundamental, systematic studies on the exponent break feature of AP/HTPB-composite propellants and adds new data to the limited database available in the open literature on the AP-based, composite propellant burning rate exponent break at very high pressures, with emphasis on the effect of mixture variables and Al on the characteristic break pressure and the postbreak pressure exponent.