## 2 Overview of Pressurized Capsule Structure
### Pressurized Capsule Structure  
The pressurized capsule structure is the main component of the pressurized capsule, which maintains the sealed state of the pressurized capsule and bears the load of the pressurized capsule. Compared with the general spacecraft structure, the pressurized capsule structure not only has the basic functions of providing configuration, bearing (generally excluding internal pressure load) and equipment installation but also has the most important function of bearing internal pressure load and ensuring the sealing function of the capsule structure. Therefore, the manned pressurized capsule structure is a special pressure vessel. Generally, the ratio of the wall shell thickness t (generally 1-5 mm) of the manned pressurized capsule structure to the minimum curvature radius R (generally more than 1000 mm) of the middle surface is far less than 1, so it can be regarded as the thin-walled shell structure of the pressure vessel. In the pressure vessel design specification, the stress caused by internal pressure load on the thin-walled shell can be divided into primary stress P (divided into primary overall film stress P\({}_{\text{m}}\), primary local film stress P\({}_{\text{L}}\)), secondary stress Q and peak stress F. Figure 3 is the stress classification of a typical pressure vessel shell under internal pressure. It can be seen that the stress in the spherical region A is composed of the primary overall film stress P\({}_{\text{m}}\), the secondary stress Q and the peak stress F. The stress in the shell connection regions B and C with different shapes is composed of the primary overall film stress P\({}_{\text{L}}\), the secondary stress Q and the peak stress F. According to the different effects of various stresses on the failure of the pressure vessel structure, the specified allowable stress limit value is calculated and checked according to the corresponding strength theory. Figure 3: Stress analysis of typical parts of pressure vessel thin-walled shell structure under internal pressure load. Figure 2: EDL procedures of manned spacecraft (For example: Ballistic Entry) [8]. If the pressurized capsule structure is only subjected to internal pressure load, it can be designed as a thin-walled shell like a pressure vessel, that is, a smooth shell structure. In fact, as a part of the spacecraft, the pressurized capsule has to bear the inertial load during the launch phase. Therefore, there are very few pressurized capsules with smooth shell structures in practice. In addition to the internal pressure load, the pressurized capsule structure generally bears various loads such as axial pressure, axial tension, transverse shear force, bending moment and local concentrated force. These external loads produce complex stress on the pressurized capsule structure. On the other hand, the pressurized capsule structure needs to undergo different load conditions in different flight profiles, and the load composition and load history of each load condition are very complex. In summary, the pressurized capsule structure is a special structure that meets the requirements of both the unmanned spacecraft structure and the pressure vessel structure. Its stress state is complex, and it is very difficult to carry out stress analysis and stress classification. Therefore, the lightweight design of the pressurized capsule structure is a huge challenge. For the pressurized capsules, the internal pressure load is often one of the main loads. In order to withstand internal pressure loads, the outer surface of the pressurized capsule structure is generally designed as spherical, cylindrical, conical and other rotary shapes. According to the different connection forms of the reinforcement parts and the wall shell, the pressurized capsule structure generally adopts the smooth shell structure, the semi-monocoque shell structure (Figure 4a) and the panel structure (as shown in Figure 4b).