Study on the negative thrust of the underwater solid rocket engines

To simulate the working process of the underwater solid rocket engines, a coupling model between the process of gas generation and the process of gaseous jets injected into water was proposed. The burning rate of the solid propellant was considered in this coupling model and the User Defined Functions (UDF) were used to define the boundary condition of the nozzle inlet. The numerical method is validated by the experimental data. The results show that the negative thrust appears when the engines work in the water. The reason is as follows: at the initial stage of the propulsion process, the high compressed gas jets into water to form a supersonic "gas bag" behind the nozzle exit, causing the pressure in this area to drop suddenly. Furthermore, the inertia effect of the water causes the pressure in the "gas bag" to keep dropping, creating the negative thrust due to the pressure difference between the rear and front parts of the engine.