Modeling of N2O Decomposition Events

This paper addresses the safety issues associated with the oxidizer nitrous oxide (N2O) with emphasis on propulsion systems. Even though N2O is a widely used energetic material, the number of decomposition related accidents are quite limited due to its abnormally slow decomposition kinetics. However hazards do exist especially in propulsion systems where large quantities of N2O are stored at room temperature in thin walled vessels. Moreover the closely coupled combustion chamber is a significant source for ignition which does not naturally exist in other applications. A detailed kinetics model for the N2O decomposition process is presented. It is shown that a simplified single step first order kinetics model accurately captures the decomposition process at pressures larger than 40 atm. With use of the kinetics data, it has been shown that, at the same pressure and temperature, the N2O decomposition rate is six orders of magnitude slower than the decomposition of hydrogen peroxide (H2O2), making it a much safer propellant. Models for homogenous and local thermal ignition are also presented. It is shown that the estimated minimum ignition energy for pure N2O is approximately 450 mJ which is three orders of magnitude larger than the ignition energy for a stoichiometric CH4/air mixture. Small concentrations of diluents (i.e. N2, O2 or He) further increase the ignition energy making the mixture extremely difficult to ignite at dilution levels higher than 30%. The results of a model developed to predict the pressure rise in a closed vessel subject to decomposition is presented to demonstrate the significant hazard that exists in the N2O tank. The model predicts a 20 fold increase in pressure over a time period of many seconds for tanks that are in the range of 1-3 meters in length. Finally, a list of safety related recommendations unique to N2O operations have been included. The general conclusion is that despite its potential decomposition hazard, if handled properly, N2O is one of the safest oxidizers being used in rocket propulsion systems.

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