Comparative analysis of triple-combustion and dual-mode solid-fuel ramjets with varying engine lengths

Boron, characterized by its high volumetric and gravimetric energy density, is an attractive solid fuel for air-breathing propulsion, but its application in scramjets is hindered by difficult ignition and slow burnout associated with its high ignition, melting and boiling points. This study proposes a novel solid-fuel ramjet configuration, the Triple-Combustion Ramjet (TCR), inspired by dual-combustion ramjet concepts and incorporating a gas generator, a subsonic combustor and a supersonic combustor in series. Three-dimensional CFD simulations are performed for TCR and conventional Dual-Mode Ramjet (DMR) configurations with overall lengths of 1.5 m, 2.0 m and 2.5 m to compare their flow fields, combustion characteristics and overall performance. The results show that the TCR provides a more favorable high-temperature, high-pressure environment and a longer residence time for boron particles in the subsonic combustor, leading to significantly higher boron burnout levels than in the DMR. Under the present conditions, the boron combustion efficiency in the TCR exceeds that in the corresponding DMR by 40.7%, 39.0% and 36.5% for lengths of 1.5 m, 2.0 m and 2.5 m, respectively. In terms of global combustion efficiency, the TCR outperforms the DMR by 21.8%, 24.3% and 22.1% at the same three lengths. Both engines exhibit increasing specific impulse with length, but the gain is almost linear for the DMR and more gradual for the TCR. Nevertheless, the specific impulse of the TCR remains higher than that of the DMR by 75.3 s, 72.8 s and 25.4 s at lengths of 1.5 m, 2.0 m and 2.5 m, respectively. In addition, the TCR shows consistently lower total-pressure loss, with exit total pressure 34–45 kPa higher than in the corresponding DMRs.