Backpressure buildup mechanisms induced by nozzleless booster residual propellant during stage transition of an integrated solid rocket ramjet

Residual propellant remaining in a nozzleless booster introduces additional mass supply and heat release during stage transition of an integrated solid rocket ramjet (ISRR), while the mechanisms governing its influence on transient backpressure development remain unclear. The present study investigates the physical mechanisms of backpressure buildup induced by residual propellant from a tandem dual-burning-rate nozzleless booster. Results show that the booster operating process is governed by the coupled effects of propellant gas generation, dynamic thermodynamic-throat evolution, and chamber-volume variation. During stage transition, residual propellant modifies the afterburner flow and combustion processes through the combined effects of residual-propellant mass addition, sustained release of fuel-rich species, particle oxidation, and aft-chamber flow confinement, which collectively accelerate pressure accumulation and govern transient backpressure evolution. When both high- and low-burning-rate residual propellants remain present (Case b), a larger reacting region extends downstream and rapidly establishes a high-temperature, high-pressure region, producing the strongest backpressure response. After depletion of the high-burning-rate residue (Case c), the reacting region contracts toward the aft chamber, resulting in a substantially weaker pressure response. Compared with the no-residual condition (Case a), the maximum and stabilized dimensionless backpressures increase by factors of 9.15 and 10.80 in Case b, and 3.67 and 4.45 in Case c, respectively. Comparison with the reference anti-backpressure value reported for a geometrically similar inlet under the same flight condition suggests that residual-propellant combustion may substantially reduce the available inlet-starting margin under comparable operating conditions. These results identify high-burning-rate residual propellant as the primary contributor to transient backpressure buildup and provide new physical insight into the coupled evolution of mass addition, combustion, and pressure-wave development during ISRR stage transition, offering guidance for transition-timing design and residual-propellant management.