Investigation into the Hypergolic Ignition Process Initiated by Low Weber Number Collisions

Abstract

The processes leading to the hypergolic ignition of monomethyl hydrazine and red fuming nitric acid have been the subject of several studies. Whereas drop test experiments provide the scale necessary for detailed observations, they typically rely on the impact of a fuel droplet moving at its terminal velocity and an oxidizer pool contained within a crucible. By controlling the kinetic energy and the size of the fuel droplet, effectively varying the Weber number of the fuel droplet, precise amounts of reactants can be made to react at a defined location, thus, increasing the repeatability of the experiment. With an unrestrained oxidizer pool, the reaction can be made to resemble that occurring in a combustion chamber. In this study, the collisions of fuel droplets, limited to a Weber number below 12, with unrestrained red fuming nitric acid pools within a dry nitrogen environment were observed. The postprocessing of high-speed visible and infrared movies revealed that, under most conditions, explosion events precede the ignition of the propellants. The explosions result in vigorous secondary atomization, creating micron-sized drops that evaporate rapidly and serve as ignition sources. The presence and behavior of these micron-sized drops is believed to play a critical role in ignition.