## 3 Results and Analysis
### Analysis of Pressure Load Characteristics of Liquid Cabin  
Figure 15 shows the pressure load curve of the bottom of the middle position of the water tank measured in Test 3. As shown in the figure, the pressure loading on the side wall of the projectile during water entry can be divided into three stages: the initial shock wave stage, the cavitation loading stage, and the cavity collapse stage. In the initial shock wave stage, the projectile perforates the front plate and then penetrates into the water, causing a huge acceleration of the previously static liquid relative to the projectile. This acceleration generates a shock wave that propagates in the water in an arc (Figure 16a). Peak value of initial pressure and cavitation load are shown in Table 7. The initial shock wave has the largest peak pressure, which is 19.1 MPa in Test 1 and 22.9 MPa in Test 3 through measurement. Since the L/D ratio and initial velocity of the projectiles in the two tests are different, the ratio of their kinetic energy is 0.821, and the ratio of the two initial shock wave peak pressures is 0.83. This indicates that under the same water tank structure and projectile shape, the initial shock wave peak pressure has a linear relationship with the kinetic energy of the projectile. The larger the kinetic energy, the larger the initial peak pressure. Then, in the cavitation loading stage, the projectile penetrates into the water at a high velocity and displaces the water, which converts the kinetic energy of the projectile into the kinetic energy of the water. A cavity is formed on the moving path of the projectile, and the water keeps squeezing the water tank due to cavity expansion, causing the cavitation load. In Test 1, the specific impulse of the initial shock wave measured on the side wall is 450 MPa\(\cdot\)us, and that of the cavitation load is 2516 MPa\(\cdot\)us; in Test 3, the specific impulse of the initial shock wave is 502 MPa\(\cdot\)us, and that of the cavitation load is 2617 MPa\(\cdot\)us. The peak pressure of the cavitation load is much smaller than the initial shock wave load. However, due to its long duration, the specific impulse of the cavitation load is about five times that of the initial shock wave. Therefore, the cavitation load is the main load causing the side wall failure of the water tank. The ratio of the specific impulse of the initial shock wave to the kinetic energy ratio of the projectile is basically the same. As the kinetic energy increases, the specific impulse of the initial shock wave increases significantly. The reason  
Figure 15: Load curve of measuring points in Test 3.  
is: the initial shock wave propagates at a speed in the water close to the speed of sound, and the initial shock wave is affected by the rarefaction waves in all directions; thus, the shock wave pressure decays quickly with a short action time, and the shock wave duration is the same. Therefore, the larger peak value of the shock wave leads to larger specific impulse, but there is little difference between the cavitation load and the specific impulse. As the kinetic energy of the projectile is continuously converted into the kinetic energy of the water, the water level rises. Under the influence of the backflow of disturbed water during cavity expansion, the air flow between the cavitation bubble and the atmosphere is cut off, and the cavity is closed at the entry hole. At this time, there are lots of gas and water vapor in the cavity, hence the formation of the cavitation bubble, which has the largest potential energy. Due to the pressure difference between the inside and outside of the bubble, its wall contracts, and the bubble gradually shrinks; in this process, the potential energy of the bubble is converted into kinetic energy, and negative pressure is generated in the water tank, which lasts for a long time, but the absolute value of the load is small; thus, it barely causes damage to the structure. With the contraction of the bubble, the gas in the bubble is compressed, leading to the increase in pressure and the formation