Effects of Key Inlet Design Parameters on the Performance of Supplementary Combustion Nozzle

Solid rocket motors (SRMs) have limited propulsion performance due to their short fuel combustion duration and low specific impulse. The supplementary combustion nozzle (SCN) injects air into the nozzle expansion section to promote secondary combustion of unburned exhaust gases, providing a novel solution to the aforementioned problems. This study fills the knowledge gap in coupled inlet-nozzle interactions using a comprehensive computational framework. The framework constructs the coupled internal and external flow fields of the SCN. It solves these fields using 2D axisymmetric compressible Navier-Stokes equations and the k-ε turbulence model. This allows for a systematic study of key inlet parameters, including outflow angle, outflow position, and mass flow rate. The obtained results demonstrate that an outflow angle of 0° provides a balance between the combustion efficiency and inlet total pressure recovery coefficient, representing the optimal configuration. A proximal inlet positioning near the throat increases the thrust gain, which results in inducing significant backpressure surges. This requires comprehensive design trade-offs taking into consideration that a peak net thrust gain occurs at an air-to-gas mass flow ratio of 1.23. After conducting a multi-parameter coupled optimization, the SCN achieves a net thrust gain of 8.18%, significantly outperforming conventional nozzles. This study provides theoretical insights for high-specific-impulse design in SRMs.