Numerical Validation and Assessment of Injection Pattern Effects on Regression Rate in a Small-Scale HTP-HDPE Hybrid Rocket Engine

Injector geometry strongly influences the internal ballistics of small-scale hybrid rocket engines by modifying jet development, recirculation, mixing, and heat transfer. This work investigates a small-scale HTP/HDPE hybrid rocket thruster through an experimentally validated CFD model, with particular attention paid to the effects of injector diameter on combustion chamber flow and fuel consumption. The numerical framework was first validated against firing test measurements. Using the mean grain-port regression rate and mean chamber pressure as reference parameters, deviations below 8% were obtained. This agreement indicates that the model can satisfactorily predict the experimentally observed radial regression, independently reconstructed from post-test 3D scans of the fuel grain. The validated model was subsequently applied to four injector configurations under identical operating conditions and initial grain geometry. Reducing the injector diameter substantially modified jet development, recirculation structures, propellant mixing, temperature distribution, and local regression rate profiles. Smaller diameters generally enhanced near-wall transport and increased the mean regression rate; however, the smallest injector produced a strongly fuel-rich condition and a markedly non-uniform axial regression profile. The results demonstrate that injector sizing governs both global performance and local fuel consumption. The validated numerical approach complements experimental measurements by revealing the physical mechanisms responsible for the observed behavior and supports the design of improved injection systems for small-scale hybrid rocket thrusters.