Isolating the effects of oxidizer characteristics and catalytic additives on the high-pressure exponent break of AP/HTPB-Composite propellants

Abstract

AP/HTPB-composite propellants are widely used in the field of energetics due to their burning rate controllability using a variety of formulation variables. At higher pressures, the burning rate becomes 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 catalysts on the exponent break and high-pressure burning rates of AP/HTPB-composite propellants containing a variety of AP particle sizes, distributions, and concentrations. The burning rates of seven formulations with varying AP characteristics 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. For the others, the characteristic pressure where the exponent break occurred changed with the inclusion of the catalysts, increasing it to 34.5 MPa and higher. The characteristic pressure was found to be dependent on the corresponding baseline burning rates, occurring where the additive burning rate curve intersected with its respective baseline burning rate curve. Additionally, all the burning rates fell above the AP deflagration rate curve, corroborating the existence of an AP barrier and the general theory in the literature that AP decomposition dominates in the very-high-pressure regime. 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 with emphasis on the role of mixture variables and catalytic additives on the characteristic break pressure and the post-break pressure exponent.