| dc.description.abstract | At higher pressures, the burning rates of AP/HTPB-composite propellants become less controllable as it experiences a transition regime often referred to as an “exponent break” typically between 20.7 and 34.5 MPa. The pressure exponent drastically increases to values greater than 1, making the burning rate extremely sensitive to pressure fluctuations. This study systematically evaluated the effects of AP characteristics, micron-aluminum, and catalytic additives on the exponent break and high-pressure burning rates of AP/HTPB-composite propellants. A total of sixteen formulations containing varying AP characteristics (particle size, concentration, and distribution) and either Mach I iron oxide or titania nanoparticles or in-situ titania as a catalyst were evaluated between pressures of 6.89 MPa and 68.9 MPa. All formulations with the exception of two, 46.0-µm AP with Mach I titania and 138.9-µm AP with 0.50% in-situ titania, showed an exponent break. Decreasing the AP particle size decreased both the characteristic pressure where the exponent break occurred (or P*) and the pressure exponent after the break. AP size distribution also affected P*, whereas changes in AP concentration did not. The inclusion of aluminum lowered P* compared to the non-aluminized formulations. Additionally, increasing the aluminum concentration appeared to lower the post-break pressure exponent. For the formulations containing catalytic additives, the characteristic pressure increased and was dependent on the corresponding baseline burning rates, occurring where the additive burning rate curve intersected with its respective baseline burning rate curve. These burning rate results along with the Thomas et al. composite propellant burning model were subsequently used to evaluate existing exponent break mechanisms in the literature. The existence of an AP barrier and the general theory in the literature that AP decomposition dominates in the very-high-pressure regime were corroborated. Additionally, it was shown that both crack propagation and formation in AP crystals and series-burning, proposed by Irwin et al. and Cole, respectively, are promising as components of the exponent break mechanism. This study adds new data to the severely limited database for very-high-pressure, AP-based, composite propellant burning rates in the open literature and provides one of the first fundamental studies on the exponent break feature. | en |
