The Effects of Density on Burning Rates of Lab-Scale AP/HTPB Composite Solid Propellants

Within identical composite solid AP/HTPB propellant formulations, the densities of each individual propellant can greatly vary depending on their manufacturing quality. Propellants with lower densities than their theoretical maximum usually indicate the presence of air bubbles, or voids, within their propellant structure. The effects of these voids on the burning rates (BR) of propellants were investigated and thoroughly characterized by comparing the manufacturing quality and performance of propellants with (intentionally) differing densities. Baseline AP/HTPB propellants were manufactured using a custom-designed extruder that can produce high-density propellants. This custom extruder can also be modified to create low-density propellants by deliberately introducing air into the system to induce the formation of voids. Burning rate curves for low-, medium-, and high-density propellants were developed for a pressure range between 500 and 2500 psia. These results indicate that low-density propellants will have measurably higher BR than high-density propellants. Several propellants were also repeatedly tested for burning rates at 500 and at 3000 psia to determine how the scatter and deviation of BR are statistically affected by propellant density at both low and high pressures. Unsurprisingly, these results indicate that lower-density propellants will display a higher BR variance compared to higher-density propellants, but the significance of the present study is that the level of deviation in BR was quantified for these lab-scale propellants as a function of percent TMD. The results indicate that for propellants with TMD greater than 95%, there was no statistical difference in BR. In addition, the low-density propellants were shown to have statistically significant, different burning rates compared to the high-density propellants at lower pressures (< 3000 psia). This study further demonstrates that voids in solid propellant can greatly impact the performance and safety of a rocket motor, but more specifically it provides a guide for assessing the level of TMD where deviations in the BR become statistically significant.