# Analysis of the Propulsion Performance and Internal Flow Field of an Underwater Launcher
## 3 Numerical Analysis
### Gas-Curtain Evolution Characteristics  
The influence of jet structure on drag reduction effect and interior ballistics characteristics is mainly reflected in the evolution characteristics of the gas-curtain. Therefore, the evolution characteristics of the gas-curtain before projectile start-up are analyzed. Moreover, the gas-curtain can be separated into three stages according to its evolution process, namely, the Taylor cavity expansion stage (the first stage), the gas-curtain formation stage (the second stage), and the gas-curtain steady growth stage (the third stage). Figure 6 corresponds to the expansion process of the gas-curtain before projectile A starts to move. Similarly, Figures 7 and 8, respectively, correspond to those of projectiles B and C. It should be noted here that in order to obtain the best propulsion performance under the corresponding jet structure, the charge burning rate used is slightly different, while other initial loading conditions are the same. It can be found in Figures 6 and 7 that the evolution process of the gas-curtain is similar when projectiles A and B are used. In the first stage, the gas passes directly through the central nozzle, forming a bubble cavity at the outlet of the nozzle. The Taylor cavity progressively grows as gas is continuously injected, and the head of the cavity remains relatively smooth. Although the jet tail begins necking, the droplets have not been sucked into the cavity. In the following stage, the continuous injection of gas and the increase in injection pressure bring a strong Taylor-Helmholtz instability effect, resulting in great changes in gas-liquid entrainment and Taylor cavity shape. The specific manifestations are as follows: the surface of the Taylor cavity becomes extremely irregular, many droplets are entrained in the cavity, and the necking phenomenon of jet tail is more obvious. It is at this stage that the gas-curtain is initially formed, but there is still water near the projectile surface and within the gas-curtain. A depression arises at the head of the gas-curtain at the same time. The chamber gas continues to inject into the gas-curtain along the jet structure inside the projectile in the third stage. The water at the projectile head and in the gas-curtain gradually travels downstream due to the turbulence in gas-liquid mixing. Simultaneously, the gas-curtain keeps growing near the muzzle. It is clear from Figure 8 that the sidewall jet firstly forms a small cylindrical Taylor cavity between the projectile and the barrel at 2 ms. Then, the sidewall jet expands axially and radially at the same time. At 5 ms, the four sidewall jets gradually begin to contact each other, and axial displacement of the Taylor cavity also increases rapidly. Until 6.5 ms, the sidewall jets converge near the projectile head, so it can be determined that the gas-curtain is initially formed. During the third stage (13.0-30.3 ms), with the injection of gas, the gas-curtain continues to grow, but there is still a gap at the gas-curtain head. With the entrainment of gas and liquid, the gap decreases gradually. Comparing Figures 6-8, it can be found that the gas-curtain formation process of projectile C is quite different from that of a projectile with a central nozzle. The primary cause is that the jet structure of projectile C makes the process of gas entering the barrel from the combustion chamber more complex. That is, after flowing into the projectile through the central nozzle, the gas firstly turns 90 degrees to enter the side nozzle and then 90 degrees again to get into the sidewall grooves. Finally, the sidewall gas jets converge in the barrel, forming a gas-curtain immediately. At the same time, it is found that a projectile with sidewall grooves is more conducive to draining off water near the projectile head than a projectile with a central nozzle. Figure 6: The gas-curtain evolution process (projectile A). (**a**) The first stage; (**b**) the second stage; (**c**) the third stage. Figure 7: The gas-curtain evolution process (projectile B).