Research on the combustion and internal ballistic characteristics of paraffin-enhanced grains with dual-layer skeleton in hybrid rocket motors

Hybrid rocket motors have seen increasing applications in launch vehicles, upper stages, suborbital spacecrafts, and sounding rockets. 3D printing technology has been utilized to manufacture skeletons with complex geometries that support paraffin-based fuel, enhancing the regression rate. However, few studies have investigated the evolution of the burning surface at the interface of the two fuels, and considered layered skeleton technology for thrust adjustment. This study introduced a novel paraffin-enhanced grain with a dual-layer skeleton that combined the benefits of both paraffin and acrylonitrile-butadiene-styrene. Using a combination of simulation and experimental methods, the changing law of the burning surface, dual-stage performance, and advantages of the dual-layer skeleton grain were investigated. The results showed that the dual-layer skeleton forms sharp angles in the 1st stage and burning surface expansion angles in the 2nd stage, with the angle variations being determined by the regression rates of skeleton and paraffin. This grain achieved dual-stage combustion and internal ballistic performance through distinct skeleton structures in the inner and outer layers, providing high thrust in the 1st stage and progressively increasing lower thrust in the 2nd stage. At a constant oxidizer mass flow rate of 30 g/s, the dual-layer skeleton grain achieved a packing fraction of 88 %, a burn duration of 31.6 s, an initial thrust of 68.7 N, and a thrust adjustment ratio of 1.92, demonstrating enhanced performance compared to conventional grains. Our findings improved the performance of grains in hybrid rocket motors to benefit the aerospace industry.