Modeling the Decomposition of HAN-Based Monopropellant and Associated Catalysts

The decomposition of hydroxylammonium nitrate (HAN)-based monopropellants is a complex process that involves numerous unimolecular and bimolecular reaction steps. During the decomposition sequence, numerous intermediate reaction products form and are then later consumed. Chemical reactions between the catalyst and these species, the unreacted propellant ingredients, and the final combustion products can all have a profound impact on the life of the catalyst. In addition to the chemical effects on the catalytically active material, thermal effects are also important. Numerous investigators have attempted to model the decomposition of HAN-based monopropellants, but very few have attempted to understand the fundamental reaction sequences. In this project, Ultramet will apply rigorous mass, energy, and momentum balances to experimental data and combine the results with known heat capacities and heats of reaction. The results, which will enable the extents of the individual reactions to be calculated, will be incorporated into a one-dimensional model of a catalyst bed in a thruster. The net result will be a model capable of predicting both the temperature and the chemical composition of the partially decomposed propellant at any axial location within the catalyst bed. With known temperatures and compositions throughout the catalyst bed, the recession rate of iridium, the catalytically active material, will then be modeled. The resulting temperature profile in the bed can also be used to model the sintering behavior of the catalyst support. BENEFIT: Being able to predict the temperature and chemical composition of HAN-based monopropellant decomposition products at any location in a thruster will enable catalysts, combustion chambers, and bed plates to all be designed more rigorously and rely less on empiricism. This will greatly accelerate the implementation of HAN-based thrusters in both military and commercial applications. Military applications include reaction control systems for launch vehicles, attitude control systems and main engines for satellites, and divert and attitude control systems for missiles and missile defense systems. Commercial applications include similar launch vehicle and satellite applications, as well as main engines and attitude control systems for interplanetary spacecraft and sample return missions.