Modeling and Empiricizing the Decomposition of HAN-Based Monopropellant and Associated Catalysts, Phase II
ABSTRACT: The decomposition of hydroxylammonium nitrate (HAN)-based monopropellants is a complex process that involves scores of unimolecular and bimolecular reaction steps and dozens of species. Chemical reactions between these species and the catalyst, as well as thermal effects, can all have a profound impact on the life of the catalyst. In Phase I, Ultramet performed rigorous heat, mass, and momentum balances to develop a model that describes the transient temperature and chemical composition profiles that result when AF-M315E monopropellant is decomposed in a packed bed of either catalytic or non-catalytic material. The model predictions, which agree well with empirical data, were used to predict the iridium recession rate throughout the bed. In Phase II, Ultramet will use ab initio data to better model the decomposition pathways of hydroxyethylhydrazinium nitrate (HEHN) (as reliable data already exist for HAN) via finite rate kinetics, the overall model for the ignition system will be refined, and key pieces of empirical data will be obtained for anchoring the model and verifying its predictions. As was the case in the Phase I model, the effects of chamber pressure, bed loading, propellant composition, bed preheat temperature, and the nature of the bed packing (catalytic or non-catalytic) will all be taken into account. Each of these variables will be investigated empirically to isolate their effect and to anchor that specific element of the model. The result will be a model that can predict engine performance over an extremely wide parameter space. Equally as important, it will also predict the thermochemical environment at any location within the bed as a function of time, thus providing key information on the various environments that the ignition system materials, the bed plate, and the chamber itself will need to endure. 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 trial-and-error 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.
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