| Gelled hypergolic propellants offer potential safety and performance improvements over conventional liquid and solid rockets. Fundamental knowledge of these systems is limited, including the essential understanding of droplet combustion. This study examines gelled monomethylhydrazine (MMH) droplets burning in an environment of nitrogen tetroxide (NTO) diluted with argon, nitrogen, and helium, maintained at a constant pressure. Of particular interest are the effects of diluents on ignition and on the dual flame structure (the inner flame representing a decomposition front, and the outer flame representing a reaction with the oxidizer) that is a feature of combusting hydrazine fuels. Hypergolic ignition was found to occur at 80% Ar, 78% N2, and 74% He by molar concentration. As diluent concentrations increased, the flame histories progressed through ranges in which they demonstrated a double flame only, a double flame fading to a single flame, a single-to-double-to-single flame, and finally a single flame. The mean flame front distances did not reveal any conclusive trends as concentration increased, although for nitrogen, the inner flame moved toward the outer flame. Jetting, caused by the gelled nature of the droplet, introduced a large amount of variation into the flame front data. |
