## 3 Propulsion Systems for HAPSs
### Electric Propulsion
Following shutdown, without the cooling effect of propwash, the motor temperature rises, leading to overheating. Depending on the mission profile, the most demanding condition for the motor may not be cruise but take-off. According to [61; 62], power losses during take-off can exceed those during cruise, making heat dissipation critical in this phase and representing the greatest challenge for the thermal management system. An alternative approach proposed in the literature involves integrating heat exchangers within the motor design. Designing such heat exchangers for these applications is challenging due to nu merous constraints, including requirements for lightweight construction, compact volume, high mechanical strength to support active motor components, low thermal resistance between heat source and sink, and effective integration of thermal management within the propulsion system. In [61,62], two innovative heat exchangers with different surface areas, designed to be placed inside the stator inner bore of electric motors for HAPSs, were developed. These motors are intended to operate in environmental conditions ranging from 45 \({}^{\circ}\)C to \(-85\)\({}^{\circ}\)C and pressures from 101 kPa to 5.5 kPa, corresponding to sea level and 20 km altitude, respectively. To meet the stringent requirements described above, the authors employed additive manufacturing techniques using powder bed selective laser melting, with an aluminum alloy (AISi10 Mg) as the powder material. Although this manufacturing method is unconventional for heat exchangers, it proved capable of fulfilling the design criteria. This technique enabled the authors to create heat exchangers featuring a very high active surface area-to-volume ratio and internal structures that would be unachievable with traditional manufacturing methods. Propulsion device refers to elements like propellers and fans that, driven by the engine shaft, generate thrust. A detailed analysis of propellers for HAPSs is presented in the following chapters. An initial feasibility assessment of a HAPS, equipped with an electric propulsion system that exploits both solar and fuel cells to power an electric motor, has been conducted in [57]. The feasibility assessment would provide the viability for operating a HAPS, with also the goal of pointing out any limitations and constraints. The analysis takes in consideration the environmental model of the US cost and the state of the art of power and propulsion technologies (exploiting conservatives \"off-the-shelf\" values for most of the components exploited in the system) to determine the capabilities and limitations of a HAPS. The results of the analysis state that a HAPS equipped with this type of propulsion system could operate on West US cost continuously, using the technology that was available on market at the time in [57]. On East US coast, where the winds are stronger, the operation was possible during the whole year except in winter months. Therefore, at the time of [57], further technological advancements of the propulsion systems were necessary to support operations during the winter months. Nowadays this kind of propulsion is the most considered for HAPSs due to recent remarkable developments in the field of renewable energy sources [13,14].