Critical Ignition Criteria for Monomethylhydrazine and Red Fuming Nitric Acid

Understanding the parameters governing ignition is crucial for safe rocket engine design, and while hypergolic propellants have been used for several decades, there is still a lack of data on the early condensed-phase reactions for many systems. The ignition and combustion of monomethylhydrazine (MMH) and red fuming nitric acid (RFNA) have been investigated in an unlike doublet impinging jet apparatus. Variation of the jet diameter and total propellant mass flow rate at constant oxidizer-to-fuel and jet momentum ratio allowed for investigation of critical ignition thresholds. Ignition probability was determined for each condition and classified as good ignition, a transition region, or failed ignition. Conditions resulting in reactive stream separation are compared with literature data obtained with hydrazine/nitrogen tetroxide. Two critical MMH/RFNA ignition thresholds were identified: a residence time of 0.2–0.4 ms and a nondimensional sheet growth rate of 2.75–5.5 s−1. Two power law correlations were identified allowing for prediction of an MMH/RFNA sheet growth rate given a residence time or Reynolds number. Thermal ignition theory was adapted to simulate the experiment, and selection of model parameters resulted in a trend that supports the experimental data and provides insight into the condensed-phase reaction rates. The results from this work are intended to provide an experimental data set for validation of MMH/RFNA combustion models and to guide the selection of injection conditions for impinging jet systems.