RECENT ADVANCES IN HYBRID PROPULSION

The idea of the hybrid rocket is to store the oxidizer as a liquid and the fuel as a solid, producing a design that minimizes the chance of a chemical explosion. While the hybrid enjoys many safety and environmental advantages over conventional systems, large hybrids have not been commercially viable. 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 and Space Propulsion Group (SPG) has led to the development of paraffin-based fuels that burn at regression rates 3-4 times that of polymeric fuels. Under the action of the oxidizer flow, the new fuels form a thin, hydrodynamically unstable liquid layer on the melting surface of the fuel. Entrainment of droplets from the liquid-gas interface can substantially increase the rate of fuel mass transfer, leading to a much higher surface regression rate than can be achieved with a conventional fuel. To demonstrate the use of these fuels, a series of scale-up tests using several oxidizers has been carried out on intermediate-scale motors. The data from these tests are in agreement with small-scale, lowpressure, 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. Recently, SPG has developed a new class of oxidizers based on refrigerated mixtures of N2O and oxygen. The mixtures combine the high vapor pressure of dissolved oxygen with the high density of refrigerated N2O to produce a self-pressurizing oxidizer with high density and good performance. The combination of these technologies leads to a hybrid rocket design with reduced system size and mass.