Catalytic Decomposition of Nitrous Oxide for Use in Hybrid Rocket Motors

In recent years, there has been significantly increased interest in using nitrous oxide as an oxidizer in hybrid rocket motor systems. The attraction of nitrous oxide is its low relative toxicity and high vapor pressure, potentially enabling self-pressurizing systems. Its decomposition is highly exothermic with a large energy barrier, typically requiring high temperature (700°C) to achieve appreciable decomposition rates. One solution is to use a catalyst to enable decomposition at significantly lower temperatures. This Paper uses a commercially available rhodium-based catalyst in a custom test cell to decompose nitrous oxide at temperatures well below the uncatalyzed self-sustaining decomposition temperature. This Paper also demonstrates that decomposition using the tested catalyst is direct to N2 and O2 at all temperature ranges observed and is incomplete with residual N2O present at lower temperatures. Further, this Paper couples our test cell with a counterflow burner to ignite and burn hydroxyl-terminated polybutadiene fuel with the decomposed nitrous oxide and evaluate performance via fuel regression rate at a variety of conditions. The fuel autoignites when the gas exit temperature reaches 750°C, and regression rate is shown to increase with both mass flux and temperature.