The transient mechanism of dual-boundary dynamic opening in the inlet during stage transition of an integral solid rocket ramjet (ISRR) remains insufficiently understood. To address this issue, a combined approach involving numerical simulations and free-jet experiments was employed. A parametric model describing the time-sequenced opening of inlet and outlet cover was established. The influences of sequence and progression of opening and flight conditions on transient flow evolution and inlet stability were systematically examined. It is found that when the inlet is opened first, a “dead cavity” tends to form inside the inlet, which subsequently triggers pronounced pressure oscillations. Under baseline conditions, the peak outlet pressure reaches approximately 0.90 MPa, with a dominant frequency of about 66.7 Hz. Conversely, when the outlet is opened first, the cavity-induced oscillation is effectively suppressed; however, a transient “flow choking” overpressure and a delayed establishment of the flow field are observed. The discrepancies between simulations and experiments for key pressure characteristics under two representative opening modes are maintained within 5%, confirming the robustness of the proposed methodology. Further analysis reveals that increasing the Mach number markedly intensifies flow instability and reduces the stability margin, whereas higher flight altitudes help attenuate cavity oscillations. A strong coupling between the opening rate and temporal sequence is also identified. Specifically, for inlet-first scenarios, a slower inlet opening combined with a rapid outlet opening is preferable, while for outlet-first cases, rapid opening on both sides yields better performance. On this basis, a “stability window map” defined by the temporal difference (Δt) and opening duration (Topen) is proposed. This map delineates the distributions of stable, transitional, and hazardous regimes under varying conditions, which may offer a quantitative reference for adaptive control strategies in the ISRR stage of transition. Interestingly, these findings suggest that slight timing adjustments could substantially reshape the transient flow behavior. Notably, the introduction of the dual-boundary temporally coordinated forcing leads to flow responses within the inlet that exhibits pronounced path dependence and non-uniqueness. Such behavior deviates from the conventional understanding established under the single-boundary frameworks, where transient mode-transition processes were typically assumed to be uniquely determined. More importantly, these findings offer a renewed physical interpretation of inlet mode-transition dynamics, thereby providing both quantitative support and practical guidance for the adaptive design of ISRR transition control strategies. In particular, the results suggest that incorporating multi-boundary temporal effects could significantly enhance the robustness and flexibility of the control-law formulation.

A transient numerical framework incorporating dynamic mesh techniques was developed to simulate the launch process. On this basis, a thermal–fluid–structural multi-physics coupling paradigm was proposed to interpret the evolution of the flow field and the associated load response throughout the entire firing sequence. The results show that flow development follows a multi-stage dynamic pattern, comprising gas-impact filling, gap-jet formation, and subsequent free-jet expansion. A pronounced spatially heterogeneous phase lag was observed in the pressure–Mach number response. This phenomenon arises from a mismatch among the characteristic time scales of pressure-wave propagation, flow inertia, and shock–boundary-layer interaction. Quantitative analysis further indicates that the spatial superposition of high-temperature zones, high-Mach regions, and elevated-pressure areas activates a thermal–fluid–structural positive-feedback loop that drives the local peak temperature to approximately 2.5 × 10³ K. The temperature response lags behind the pressure maximum by approximately 30–50 ms, reflecting the governing time scale of thermal inertia. In addition, vortical structures near the tube base account for nearly 15% of the total thrust. These findings provide a theoretical foundation for predicting transient peak loads in concentric cylindrical systems and for optimizing instantaneous thermal protection strategies.

The transition process of a dual-inlet integrated solid rocket ramjet (ISRR) remains insufficiently understood, particularly regarding the role of combustor residues. To address this issue, a combined approach integrating free-jet experiments and three-dimensional unsteady full-flowpath simulations was employed. The effects of residual-induced gas release on transition dynamics and stability were systematically investigated. The results indicate that the transition can be categorized into four stages: post-boost exhaust, cavity oscillation, inlet startup with combustor filling, and combustion establishment. During the outlet-closed phase, a self-sustained oscillation at ∼59.8 Hz was observed, which was associated with shock reflection–incoming flow coupling. Once combustor residues were introduced, the system response shifted from a rapid monotonic establishment to a delayed process accompanied by pronounced unsteadiness. The peak combustor exit pressure increased by ∼89%, while a low-frequency oscillation (∼2 Hz) emerged near the throat, triggering inlet unstart–restart behavior. Moreover, the presence of residues markedly altered local equivalence ratio distribution and intensified heat-release non-uniformity, thereby promoting transition delay and low-frequency instability. Based on comparative analysis, a transition stability criterion incorporating residue effects was proposed, revealing the coupled influence of equivalence ratio distribution, heat-release fluctuation, and pressurization rate on system stability. These findings provide theoretical guidance for transition sequencing, residue management, and engineering design of hypersonic propulsion systems and extend current understanding of dual-inlet ISRR transition mechanisms.