## 2 Ejection Seats  
If not accurately managed, the ejection of a seat can be a serious physical risk for the pilot. However, the ejection of the seat may be the only chance to survive when critical aircraft failures occur.  
The use of ejection seats, which became popular during the Second World War, has the aim to eject out of the aircraft the pilots, in case of critical flight difficulties, allowing a safe landing by opening a parachute.  
Whilst the principal goal of the ejection seat mechanism is to guarantee the security of the pilot, it involves a challenging sequence of operations, which may expose the pilot body to huge loads. However, this risk is considered acceptable if compared to a sure death of the pilots in critical circumstances.  
Modern aircrafts adopt a fourth-generation ejection seat system that includes a rocket propulsion systems and sensors to monitor the aircraft's altitude and speed, which are important factors for the ejection. An example of a modern ejection seat [31] is shown in Figure 1.  
Among the various injuries that the ejection can cause to pilots, the spinal compression due to the axial pulse is the most common and dangerous. Therefore, the management of axial accelerations acting on the pilot during ejection become a main concern. In modern seats, the ejection thrust is strictly controlled to ensure a safe ejection.  
As illustrated in Figure 2, according to military standards [32; 33; 34] the ejection axial acceleration is schematized by means of a trapezoidal shape in the plane acceleration vs. time.  
To provide a quantitative read of Figure 2, the ejection motion parameter ranges are listed in Table 1.  
The first step of the ejection is the pre-ejection. This phase describes the time interval between the instant when a dangerous situation occurs and the beginning of the ejection. In some flight conditions, for example during landing and take-off, this time interval is very short and the pilot does not have enough time to prepare the aircraft for an optimal and completely safe ejection. The pilot has the time to adapt the aircraft for an optimum ejection by, for example, reducing the flight speed for in flight conditions, such as while in cruise.  
Figure 1: Fourth-generation k-39 ejection seat. (**a**) Front view; (**b**) rear view.  
Figure 2: Acceleration shape (**a**) ejection phases and (**b**) according to military standards.  
The second ejection step is known as \"primary acceleration\". In this process, first by activating an explosive charge and then with the aid of rockets, the pilot-seat system is accelerated out of the cockpit with an acceleration ranging from 12 g to 30 g. This is the ejection phase where the pilot is most exposed to injuries. The seat, during the primary acceleration phase, actually exerts on the aircraft structure an inertial force due to this acceleration. Therefore, a reaction of the aircraft structures is applied to the seat and, hence, to the pilot.  
After this phase, the pilot-seat system is subjected to an approximately constant acceleration due to the aerodynamic resistance in relation to the altitude variation. This phase is schematized in Figure 2 with the name \"Forces of windblast\" and can last a variable amount of time depending on the ejection conditions.  
The last phase is the parachute descent and landing. Since the impact between the seat-pilot system and the ground occurs at about 6 g [35], significant injuries can also occur during landing. Moreover, injuries can, potentially, also arise due to the shock of opening the parachute.  
This work investigates the effectiveness of an energy absorbing system composed of five shock absorbers designed to take into account the capabilities of additive manufacturing in order to alleviate the loads acting on the pilot during the initial explosion that produces the upward acceleration of 12 g.