Correlating the Effects of AP Particle Size and Concentration on AP/HTPB Composite Propellant Burning Rates

The burning rates of IPDI-cured AP/HTPB propellants with a standard curing ratio were analyzed. It is well known that the particle size of AP as well as their concentration greatly affects the burning rate, yet there exists a lack of archival databases for a wide range of particle sizes, concentrations and pressures. Testing conditions for this study included an average AP size range of 20 µm – 400 µm with four intermediate steps, a pressure range of 500 - 2,250 psi with multiple intermediate steps, and lastly a concentration range of 70% to 85% AP by mass with three intermediate steps. As expected, decreasing the AP size increased the burning rate for sizes above about 45 µm and increasing the concentration of AP led to an increase in burning rate. The results for the smaller sizes (20 and 45 µm) converged to similar burning rates, indicating that the mixture is nearing a premixed burning limit. All propellants were manufactured and tested in house at Texas A&M using the same experimental techniques and testing procedures used in the past by previous authors. Power law burning rate expressions were obtained for each mixture. Most importantly, an empirical correlation describing the burning rates over the entire range of propellants tested was created by analyzing the trends seen in the pressure coefficient and exponent of each propellant formulation. The correlation was built off of multiple data sets. These data sets include data taken by the author at TAMU, data taken by Kohga, King, and Foster. The correlation was applied and compared to multiple data sets and produced good agreement with an R-squared value of 0.968. The advantage of this correlation over other theoretical models is primarily its simplicity and accuracy. Its form mirrors the well-known power law which makes it easily adoptable as a potential replacement. The correlation also has a large potential for future modification and adjustment. In the future, these same formulations should be tested between the 2,250 to 5,000 psi ranges where the role of AP becomes crucially important. Catalytic effects of additives should also be included as additional terms in the pressure coefficient and exponent as well as the effect that initial propellant temperature has on burning rate