## 5 Research Progress on Key Technologies of IPPCS
### Research on the Design Method of IPPCS  
Since the design space of the IPPCS is the global space of the capsule, the structural optimization method plays a more significant role in the design. However, due to the high coupling degree of structural characteristics of the IPPCS, it is not easy or even impossible to carry out global optimization directly for all structural parameters. In order to reduce the difficulty of optimization, following the principle of bottom-up, according to the type of key structural features, the IPPCS design procedure is deconstructed into four steps of shell cross-section, basic stiffener configuration, transition zone stiffener configuration and local structure, and optimized design is carried out at each step. Based on the above ideas, the design process of the IPPCS is proposed, as shown in Figure 12.  
(1) Design of shell cross-section [19; 21]  
Based on the outline shape of the pressurized capsule, the two-dimensional axisymmetric shape cross-section is taken firstly. Then, the preliminary design of the shell cross-section is carried out. Finally, the section parameters optimization under internal pressure load is carried out, and the shell cross-section of the pressurized capsule structure is determined. Figure 11: (**a**) Evolution of the plastic dissipation and (**b**) experimental testing of the shakedown behavior. (2) Design of basic stiffener configuration of pressurized capsule [17; 18; 21; 22; 37; 38; 39; 40]  
According to the two-dimensional results obtained in the previous step, a three-dimensional finite element model of the pressurized capsule shell structure is established. On this basis, the optimization design of the basic configuration is carried out, and the design scheme of the stiffened configuration is optimized. However, the pressurized capsule structure has the characteristics of non-straight generatrix and variable curvature section, which leads to the difficulty of parametric modeling of stiffeners. At the same time, there are too many stiffener parameters in the pressurized capsule, and the single analysis takes a long time, resulting in insufficient efficiency of structural optimization design. Aiming at the difficulty of parametric modeling, [PERSON] et al. [38] proposed a data-driven complex surface modeling method. By training the mapping relationship between the simple plane and the complex surface solid domain, the simple plane stiffened shell mesh is mapped to generate a complex curved stiffened shell. Compared with the traditional modeling method, the modeling accuracy and efficiency are greatly improved. On this basis, [PERSON] et al. [18] combined the Nonuniform Rational B-Splines (NURBS) method to establish a variable thickness integral stiffened shell modeling method, which can consider the variable thickness modeling and optimization of ribs and skins, and fully tap the potential of lightweight structure. Compared with the traditional curve design, NURBS [39; 41] is the best representative form for the curve and surface. Since the forming curve is smooth and the curvature can be kept continuous after local modification, the NURBS curve can fit a variety of complex shapes. The method is verified by the example of the pressurized capsule in the project. Aiming at the problem of insufficient optimization ability of the pressurized capsule structure, an analysis method based on Bloch wave acceleration is proposed [42]. This method performs buckling analysis on a part of the rotating body structure and simulates the overall buckling analysis results of the structure by setting boundary conditions. Compared with the overall structure's analysis, the analysis efficiency is greatly improved. At the same time, many data-driven methods have been proposed to apply to structural optimization design so as to improve its optimization ability and efficiency. Among them, [PERSON] et al. [40] optimized the parameters of the pressurized capsule structure based on the multi-objective Covariance Matrix Adaptation Evolution Strategy (CMA-ES) method.