Regression rate studies of hybrid rocket fuel on a lab scale rocket motor

Hybrid rocket propulsion is attractive because of their potential features such as high specific impulse, stop-start capability, thrust modulation, mission abort and non-polluting characteristics. Various combustion models and hybrid combustion theories have been developed to understand the physics behind hybrid combustion. The solid fuel regression rate was considered as key constituent on the performance and development of the hybrid rocket design for future space applications. In the present study, an effort has been made to explore the local regression rate and average regression rate of a solid hybrid fuel in terms of oxidizer injection pressure and mass flux of gaseous oxidizer (GOx). A series of static tests firing were conducted with Polyvinyl Chloride (PVC) as solid fuel in a lab scale ballistic test motor. Ballistic tests were performed with different gaseous oxidizer injection pressure ranging from 150 psi to 350 psi. The effect of oxidizer injection pressure on local regression rate was evaluated using the Marxman's theoretical formulation and compared with experimental data. It was observed that the variation of regression rates along the length of the grain depended on the oxidizer injection pressure, injector design parameter, grain position in the chamber, combustion chamber pressure and oxidizer injection velocity. The local regression rate was found to depend on the length of burned fuel and varied throughout the grain axial length before achieving a constant value. The theoretical pressure dependence of average regression rate was determined using a power law and the experimental data exhibited a similar trend. The theoretically calculated pressure exponent value was found 4 % lower compare to experimentally determined one. The lower value may be attributed to increase in mass diffusion of oxygen at the flame zone and also due to increased heat transfer from the flame zone to the fuel regressing surface. From the present theoretical validation, it was found that the solid fuel regression rate depended on oxygen mass flux rate and as well as on the chemical pyrolysis at the solid fuel surface due to enhanced entrainment. The dependence of local regression rate along the axial length of burnt fuel increased quite appreciably at the beginning and then decreased due to lower oxidizer concentration.