Transient flow characteristics and performance of a solid rocket motor igniting in deep water

The startup transient of a solid rocket motor igniting across an extended depth range of 0-300 m is numerically studied. A sophisticated, multi-domain integrated model of propellant transient burning, nozzle separation flow, and underwater supersonic gas jet is established. The unstable behavior of the jet boundary, the oscillation characteristics of the nozzle separation flow, and the internal ballistics and thrust performance of the motor are carefully analyzed. In addition, a parametric study is conducted for different operating water depths, closure-opening pressure differences, and designed chamber equilibrium pressures. The results show that the jet behavior near the nozzle in deep water is characterized by alternating bulging and pinch-off. The reciprocating oscillations of the separation shock lead to pressure fluctuations and gas-vapor-liquid mixed flow in the divergent section. Furthermore, the local high pressure and re-entrant jet generated by jet pinch-off downstream of the throat are likely to cause unchoked flow. The intermittent unchoking in the throat causes the chamber pressure to deviate from the sea-level value, presenting a zigzag variation. This effect becomes more significant as the nozzle over-expansion intensifies. As water depth increases, the amplitude and frequency of thrust oscillations increase, while the average thrust decreases. At a depth of 300 m, the average thrust during the startup transient for different motors drops to approximately 42.8%-47.1% of that at sea level.