Gelled MMH Hypergolic Droplet Investigation

Abstract

Gelled propellants are promising candidates for certain future rocket applications, offering potential improvements in performance and/or safety over conventional liquid and solid systems. In particular, gelled hypergolic propellants can eliminate some handling difficulties by reducing leakage hazard. Before such systems can be developed, however, a fundamental understanding of spray behavior and combustion in a rocket combustion chamber is required. To this end, an experimental study of vaporization and combustion processes of single droplets is underway. This paper discusses the combustion behavior of monomethyl hydrazine (MMH) droplets gelled with both hydroxypropyl cellulose (HPC) and fumed silica in an environment of gaseous nitrogen dioxide. Within the present study, an experimental system was built to investigate the combustion of hypergolic propellant droplets. This system allows studies with a NTO oxidizer and MMH fuels in a range of pressures and temperatures while maintaining strict safety measures. All MMH/HPC gels showed the typical swelling and jetting that are typical to hydrocarbon gels with an organic gelling agent. Burning rates were measured for droplet diameters from 1.8 to 3.2 mm and two ambient pressures of 1.72 and 2.89 bar. It was found that the droplet burning rate is dependent on the droplet diameter similar to liquid MMH. Over the investigated pressure range, no dependence of burning rate on pressure was found. The MMH/Silica gels showed a different combustion mechanism with the formation of a rigid silica structure, micro-explosions, and up to a 50% reduction in droplet volume during the combustion. At the end of the combustion process MMH/Silica gels broke down probably because of a lack of liquid bonding material. Combustion mechanisms were speculated for both gels.