5.5.1. Load-Bearing Structure Carrying the Ablative TPS

The dynamic pressure load acts on the component mainly via bending. Thus, the most promising lightweight design concept is that of a sandwich structure. To maximize the load bearing capability to mass ratio, the sandwich is designed with an aluminium honeycomb core between CFRP face sheets. As schematised in Figure 7, the bending load due to pressure acts bi-directionally on the component, i.e., bending it around the longitudinal (*x*-)axis and the circumferential (*y*-)axis. While an increase of the aluminium core thickness increases the stiffness of the structure, stresses in the face sheets are increased as well. Therefore, X-shaped aluminium reinforcements are introduced on the inner face sheet of the ADD against bending deformations, as shown in Figure 16.

**Figure 16.** Structural design of the load-bearing composite sandwich structure carrying the ablative TPS with reinforcements in form of an X-shaped frame and beams along the outer edges.

The layup and dimensions of the structure were determined in an iterative design process using geometrically non-linear static analyses in Abaqus® 2020 on a mesh consisting of a linear shell and beam elements. Typical material data for unidirectional (UD) T700 prepreg material and aluminium honeycomb are used (see Appendix B). Due to the optimised ablative TPS, virtually no thermal loads act on the load-bearing structure

underneath. Therefore, only dynamic pressure loads (see Figure 8b) are considered. In the laminate, the number and orientation of the UD layers was iterated, as well as the cross-section of the X-shaped reinforcements and the frame along the ADD's perimeter.

The obtained design has face sheet laminates with a thickness of 1.75 mm each and a core thickness of 50 mm, resulting in a mass of the structure of 35 kg. For the evaluation of stresses in the composite material, the Tsai–Wu failure criterion [41] is utilised with the goal of maintaining the criterion in all layers below 1.0. For sake of brevity, only the failure criterion values in the most critical composite layer with the highest failure criterion overall are reported (see Figure 17). As the maximum value of 0.92 occurs at border of the ideally stiff boundary condition, stresses in the real component are assumed to be lower than calculated here.

This preliminary structural design study was performed to obtain a benchmark design whose mass can then be compared to the lattice core and corrugated core ITPS solutions. It can be concluded that it was feasible to find a lightweight design for the load-bearing structure when subjected to dynamic pressure loads. The reported mass of 35 kg is that of the load-bearing structure only. The mass of the PICA TPS layer amounts to 32 kg per ADD, thus resulting in an overall mass of 67 kg (neglecting bonding, attachment points, inserts, etc.).

**Figure 17.** Tsai–Wu failure criterion in the critical composite layer of the load-bearing structure.
