Flow-field evolution and transient load characteristics during separation in an integrated solid rocket–ramjet combined nozzle

This study addresses the transient separation of a combined nozzle in an integrated solid rocket ramjet (ISRR) subjected to strong coupling among structural displacement, compressible flow, and unsteady aerodynamic loading. An experimental–numerical coupled framework was developed. A two-dimensional axisymmetric model incorporating dynamic mesh techniques and fluid–structure interaction (FSI) was established and validated against ground-test measurements, with deviations in axial separation velocity and displacement maintained within ≤15.1%.The results indicate that the separation process follows a three-stage evolution pattern characterized by pressure attenuation, control response, and fluid–structure destabilization. At the instant of detachment, nozzle motion induces a localized choking phenomenon, generating a pressure perturbation with a peak magnitude of 0.783 MPa. These observations suggest that separation cannot be adequately described as a quasi-static pressure-difference-driven event. Instead, it arises from flow destabilization and structural reconfiguration governed by dynamically evolving boundary constraints. Furthermore, asymmetric pressure redistribution caused by internal–external flow interaction during the late separation stage is identified as the primary source of potential lateral loading. From a multiphysics dynamical perspective, this work reconstructs the theoretical framework of transient nozzle separation and provides mechanistic guidance for separation control optimization and high-fidelity simulation improvement.