In-Silico Analysis of Acoustic Wave Backpropagation Through Nozzle Boundary Layers of Rocket Motors at Liftoff

Combustion instability remains a major challenge in rocket propulsion, particularly during liftoff, where acoustic wave propagation interacts with evolving boundary layers. This study presents a comprehensive in silico analysis of acoustic wave generation, reflection, and damping within the nozzle boundary layers of high-performance solid rocket motors. Simulations capture the transient development of acoustic fields from ignition through steady-state, identifying boundary layer thinning as a key contributor to wave amplification. Findings show that boundary layer thickness governs acoustic impedance mismatch: thicker layers enhance damping, while thinner layers facilitate wave amplification, leading to Sanal flow choking and localized supersonic regions within otherwise subsonic flows. The study also evaluates spike geometry, demonstrating that straight spike configurations sustain thicker boundary layers and more effectively suppress acoustic backpropagation compared to concave and convex designs. Quantitative results highlight a progressive boundary layer reduction, accompanied by an ~8% increase in chamber pressure oscillation amplitudes over time. These outcomes enhance understanding of acoustic-boundary layer interactions and support the design of more stable and efficient rocket systems through geometry and flow field optimization.