Investigating the influence of water depth and expansion state on the pressure oscillation characteristics of solid rocket motors: An experimental study

Solid rocket motors play an important role in underwater propulsion systems and trans-media vehicles. In this paper, the experimental study has been conducted on the ignition and operation process of a solid rocket motor under deep‐water conditions. A variable‐water‐depth test system and a multi‐point pressure measurement scheme for the nozzle is established to simulate the ignition start‐up and operating characteristics of the motor at different water depths. Two experimental solid rocket motors are designed under different expansion states and the pressure oscillation characteristics of the motor and the separation flow in the divergent section are measured. The results show that the intensity of pressure oscillations increases with water depth, reaching a maximum at a depth of 330 m. Intermittent unchoked flow in a large expansion ratio nozzle under deep-water conditions is experimentally observed for the first time. This phenomenon causes the combustion chamber pressure to deviate from its sea‐level value and exhibit sawtooth‐shaped oscillations. There exists a critical depth for the occurrence of intermittent unchoked flow at the throat, which determines whether the combustion chamber pressure is affected by unstable jet behavior downstream of the nozzle throat. As the water depth increases, the flow separation line shifts closer to the throat. A lower equilibrium pressure in the combustion chamber results in a separation line position closer to the throat. At the depth of 330 m, the separation line moved upstream by 50.54 mm in high‐pressure motor and 54.96 mm in low‐pressure motor.