Mode transition dynamics and stability criteria of a dual-inlet integrated solid rocket ramjet under combustor residual effects

Abstract

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.