## 1 Introduction
To use light composites for shock absorbers and overcome the composite limitation due to their fragile behaviour, hybrid shock absorbers have been proposed in recent years [21; 22; 23; 24; 25; 26]. The idea behind this new class of shock absorbers is to exploit the plasticizing capability of metals combined with the lightweight and high strength-to-weight and stiffness-to-weight ratios offered by composite materials. Examples of hybrid shock absorbers are presented in [21], where the authors explored the effects of winding angles and thicknesses on the crashworthiness characteristics of carbon fibre reinforced plastics (CFRP) tube and aluminium/CFRP hybrid tube molded by the filament winding technique by performing quasi-static crushing mechanical tests. In general, in [22], it is demonstrated how the impact resistance of an aircraft can be improved by adopting, in specific key locations, hybrid shock absorbers. To improve the energy absorption performance of hybrid shock absorbers, in [26], the authors, exploited the capabilities offered by the Additive Manufacturing technique [27; 28; 29; 30] and introduced a new additive manufactured shock absorber concept. They presented a hybrid absorber panel composed of a lattice internal core produced by means of metal additive manufacturing and externally covered with CFRP composite layers. By adopting this structural concept, they succeeded in developing a shock absorber able to offer, compared to solutions produced with standard technological processes, comparable or even better performance in terms of energy absorption with a mass reduction of about 42% with respect to standard shock absorber sandwich panels. Moreover, thanks to the capabilities of additive manufacturing, they were able to design panels with a total thickness of 6 mm, hence, easily integrable in existing structures such as floors. This work aims to step up the new concept of shock absorber introduced in [26], by proposing a new shock absorber panel system applied to a specific ejection seat test case. In particular, based on the model presented in [26], a system of five shock absorber panels has been designed, which, in consideration of its small overall dimensions and high energy absorption performance, has been applied as shock absorber to reduce the acceleration peaks produced during the primary acceleration phase in ejection of the seats adopted for military aircrafts. To reduce the number of DoF, differently from what was exposed in [26], a formulation with one-dimensional beam elements has been selected for the metallic lattice core. Hence, a sensitivity analysis on the lattice core discretization has been carried out, by comparing the results with the 3D models introduced in [26]. In Section 2, a description of the loads arising during the ejection phase of modern ejection seats equipped on military aircraft is provided. In this section, the most critical phase, in terms of acceleration, during the ejection of the seat is determined according to the military standard trapezoidal shape of the peak force vs. time curve. In Section 3, the numerical model of the developed shock absorber system is introduced together with its possible integration with a modern ejection seat. This chapter also provides details of the sensitivity analysis of the mesh adopted for the mathematical model. Finally, in Section 4, the results obtained from the performed numerical analysis are presented and discussed.