Hybrid rocket propulsion is a promising propulsion system which presents appealing features such as safety, low cost and environment friendliness. On the other hand certain issues like low regression rates, mixture ratio shifting, mixing inefficiencies, etc. restrict its use as a preferred propulsion system. The present investigation presents an approach for improvement of one of the most important among those issues, namely low fuel regression rate.

Fuel regression rate can be increased by increasing oxidizer flow speed over the burning fuel surfaces. This is because flow over the burning surface creates shear stress which facilitates fuel and oxidizer mixing. One method of improving shear stress and thus regression rate is to induce an oxidizer vortex into the combustion chamber.

The stepped fuel grain configuration is one of the mean to create vortex combustion field in a port. To generate this flow, oxidizer is injected axially in the fuel grain and vortex created inside the grain due to different size of cylindrical port size. A series of static rocket firing were conducted to investigate the solid-fuel regression rate behavior and combustion characteristic of stepped solid fuel grain hybrid rocket motor. Results from five tests of laboratory scale stepped grain hybrid rocket motor using polyvinyl chloride with gaseous oxygen at different oxidizer injections pressures i.e. 16.872, 21.092, 28.122, 42.184, and 48.511 (kgf/cm2) respectively and three tests of normal cylindrical grain using same composition at oxidizer injection pressure i.e. 21.092,28.122 and 42.184 (kgf/cm2) have been presented here.

The local regression rate of normal cylindrical grain has been observed maximum at the leading edge of the grain, it decreases sharply with in a short distance from the leading edge and after that it remains more or less same in the most of the length part except at the end of grain. The similar patterns of variation of local regression rate were found at all oxidizer injection pressure except that the local regression rate of hybrid fuel grain is found to increase with increase in oxidizer injection pressure.

The local regression rates of cylindrical stepped fuel grain was found similar to normal cylindrical grain as regression rate being highest at the head end but after that the uneven regression of grain surface was found till nozzle end. The higher port I.D of grain segment has lower regression rate compare to lower port I.D grain segment. The similar patterns of variation of local regression rate were found at all oxidizer injection pressure and the local regression rate of hybrid fuel grain is found to increase with increase in oxidizer injection pressure. It is found that the average regression rate in case of stepped grain is higher compared to cylindrical port grain.