Effect of Varying Secondary Swirl Injector Location on Performance of Hybrid Rocket

Hybrid rockets offer various advantages, such as safety, controllability, and eco-friendliness, compared to solid and liquid rocket propulsion. Despite these, it has drawbacks such as low regression rate, lower combustion efficiency, and change in oxidizer-to-fuel ratio with burn time. Various methods have been adopted to overcome these issues. The present study analyzes the impact of adding a secondary swirl injector, along with the conventional head-end swirl injector, on the performance of a hybrid rocket motor. Paraffin wax was used as fuel and gaseous oxygen as the oxidizer. The secondary injector was positioned at three locations: 0.25, 0.50, and 0.75 secondary swirl location/entire motor length (X/L). Results showed that the secondary swirl injector prevented the swirling flow from decaying within the combustion chamber. For the head-end swirl injection, the tangential component of the swirl decays progressively after 200 mm along the flow direction. The optimal position for the secondary injector was found at 0.50 X/L, where the regression rate increased by 33.75%, combustion efficiency improved by 13.4%, and thrust and pressure increased by 53% and 30%, respectively, compared to the head-end swirl case. It was also observed that longer chambers led to incomplete combustion due to swirl decay, while shorter chambers gave a reduction in residence time, limiting the combustion efficiency.