Flow Separation Behavior in Highly Overexpanded Nozzle Operation during Solid Rocket Motors Tail-off Phase

During the static fire test of the third qualification motor of the Zefiro 40 - the solid rocket motor used as the second stage of the Vega C launcher - a peculiar phenomenon was observed during the tail-off phase, with no impact on motor performance. Although the pressure measurements followed the predicted behavior, the thrust profile exhibited a sudden jump. Due to the regular trend of internal pressure, the phenomenon was attributed to the divergent section of the nozzle. A computational fluid dynamics analysis identified the interaction between flow separation and the internal geometry of the post-burn nozzle as the cause of the thrust jump.