Thin film stability of melting solid fuels with application to hybrid propulsion

Abstract

In a hybrid propulsion system the oxidizer is stored as a liquid and the fuel as a solid producing a design with reduced mechanical complexity and minimimum chance of a chemical explosion. The hybrid enjoys many safety and environmental advantages over conventional systems however large hybrids have not been commercially successful. The reason is that traditional systems use polymeric fuels that evaporate too slowly, making it difficult to produce the high thrust needed for most applications. Research at Stanford University in the late 1990’s led to the development of paraffin-based fuels that burn at regression rates 3 to 4 times that of polymeric fuels. Heat transfer from the combustion zone and the action of the oxidizer flow over the melting fuel surface, leads to the formation of a thin, hydro-dynamically unstable liquid film. Entrainment of droplets from the liquid-gas interface substantially increases the rate of fuel mass transfer leading to much higher fuel regression rates than can be achieved with conventional polymeric fuels. To demonstrate these fuels, a series of scale-up tests using several oxidizers were carried out on intermediate scale motors. The data from these tests are in agreement with small scale, low pressure and low mass flux laboratory tests and confirm the high regression rate behavior of the fuels at chamber pressures and mass fluxes representative of commercial applications.