Review and assessment of the ammonium perchlorate chemistry in AP/HTPB composite propellant gas-phase chemical kinetics mechanisms

Abstract

Physical and chemical processes of ammonium perchlorate and hydroxyl-terminated polybutadiene (AP/HTPB) composite propellant combustion have been studied for several decades, and more than 50 years of model development can be reported. Computational methods focus on the heterogeneous aspects—the solid-phase and its decomposition—whereas AP self-deflagration and burning characteristics should be seen as a multi-step, physiochemical process. There has been a lack of systematic studies on the gas-phase chemical kinetics mechanisms for AP combustion, with emphasis on the starting gas-phase species NH₃ and HClO₄. Only three recent detailed gas-phase mechanisms with sufficient detail in terms of the number of chemical reactions and number of species are currently available in the literature prior to 2023, and simulations are carried out within the present review to assess the state of their current performance and to highlight potential knowledge gaps that should be filled. Given the importance and prevalence of AP in modern propellants, it is surprising that the chemical kinetics of AP combustion are very much understudied. The authors highlight the fact that the few existing AP mechanisms have never been fully vetted against an applicable database of experimental results, certainly not in the manner that mechanisms are typically validated within the combustion science community for fuels such as hydrogen and various hydrocarbons. This review does not put forward such a mechanism, but rather 1) brings to light the limitations of current AP kinetics mechanisms in predicting some limited, available kinetics data, and 2) underlines the need for additional, fundamental data that can be used to calibrate an AP kinetics model. A limited gas-phase experimental database was identified from currently available sources for two main compound families: ammonia (NH₃) and perchloric acid (HClO₄). The decomposition of AP is initiated by NH₄ClO₄ → NH₃ + HClO₄ and leads to these two rather complex molecules that differ strongly in their nature and consequently in their reaction schemes for combustion processes. On the one hand, existing measurements of ignition delay times, laminar flame speeds, and speciation were collected for NH₃, N₂O, and NO₂, and on the other hand, a similar albeit much smaller body of experimental results was assembled for HClO₄, ClO₂, and Cl₂. These global kinetics data were used to evaluate modern AP/HTPB propellant models. We observe that there is much room for improvement regarding models' performance. Significant improvements in our ability to model the gas-phase chemical kinetics of AP combustion can be made by taking advantage of recent developments in ammonia oxidation chemistry modeling. However, additional, fundamental data are needed before similar strengthening of the perchlorate-related chemical kinetics as well as for cross-system reactions involving both N- and Cl-based species can be made.