In this study, multi-disciplinary design optimization of a space capsule was investigated, focusing on structural material and geometric features. Design disciplines, including geometry, aerodynamics, trajectory, heating, and structure, were considered. To reduce the computational cost, surrogate models were created using the Kriging method, and multi-disciplinary optimization was performed with the single-level All-At-Once framework and genetic algorithm. The single-objective problems included mass minimization, volumetric efficiency maximization, ballistic coefficient minimization, longitudinal static stability maximization, structural deformation minimization, internal volume maximization, structural frequency maximization, and structural buckling strength maximization. In addition, a dual-objective problem was defined to simultaneously optimize the two main objectives, namely mass and structural rigidity. These results were validated by flight testing of the native capsule. The results showed that using carbon-epoxy composite as the shell material is preferable to other materials (6061 aluminum and glass-epoxy composite) in all structural optimizations. Furthermore, increasing the thickness of the layers by 5% improved the structural rigidity, and weight reduction of up to 8% was possible without loss of mechanical performance. These results show that by choosing the correct geometry and adjusting the thickness of the layers, a more efficient design for lightweight and strong composites can be achieved. The proposed framework can be used as a practical guide in the design of future space capsules, reducing structural weight, increasing stability, and improving their overall performance.