## 3 Propulsion Systems for HAPSs
### Hybrid Propulsion  
Hybrid propulsion for HAPSs refers to propulsion systems that combine more propulsion technology in the same aircraft to reduce the drawbacks related to each propulsion system type. A total electric aircraft could remain in flight for months, but it necessitates high aspect ratios and huge wingspans; this is due to the high specific weight of electric storage devices for night flight. An aircraft that exploits combustion engines, instead, could carry more payload mass, but it is unable to remain in flight for long time. In [9], an investigation was conducted on the combination of various power sources and propulsion systems currently employed in HAPSs. Specifically, the authors first analyzed the performance of electric motors powered by solar photovoltaic arrays, hydrogen fuel cells, regenerative fuel cells, and rechargeable batteries, as well as turboshaft and turboprop engines fueled by hydrogen. Subsequently, they evaluated the performance of hybrid propulsion systems. Among the hybrid propulsion systems studied in [9] are configurations that use photovoltaic cells to power the electric motor during daytime flight, while relying on rechargeable batteries, fuel cells, regenerative fuel cells (which generate hydrogen from water during the day by utilizing excess solar power, and consume water and power during the night), or hydrogen-fueled turboshafts or turboprops for night time operation. At that time, prototype regenerative fuel cells were capable of delivering 400 Wh/kg, with expectations to reach 700 Wh/kg. The analysis conducted by the authors in [9], which considered the requirements for a flight at 20 km altitude with a target endurance of one month, showed that the propulsion system mass represented approximately 10% of the total airship mass when using photovoltaic cells that provide energy only during daylight. This ratio increased to 14-24% for regenerative fuel cells, 21-36% for batteries, and 30-50% for polymer electrolyte membrane (PEM) fuel cells. The mass of the engine itself was not included in these estimates, but its impact was approximately 2.1%. At that time, the turboshaft or turboprop engine--potentially fueled by hydrogen--remained the preferred solution for high-altitude propulsion, offering a propulsion system mass to total mass ratio of about 28.5%. Instead, for longer missions, combining an electric motor powered by solar cells during hours of daylight and a turboprop/turboshaft propulsion system exploited during hours of darkness could be better. This hybrid solution leads to important mass savings compared to an electric propulsion system and a longer endurance compared to a turboprop/turboshaft propulsion system. It is evident that, for long endurance (weeks or months), solar power during the day should be exploited; a long endurance flight will be impossible using consumable sources of energy. However, when going further away from the equator and particularly for arctic region flights, exploiting hydrogen-fueled turboshafts (or turboprops) both for day- and night-flight is mandatory. Other feasibility studies on hybrid propulsion systems combining electric motors and combustion engines have been conducted in [54] with the aim of leveraging the advantages of both propulsion types, resulting in a more reasonably sized vehicle at the expense of some flight endurance. Additionally, as interface methods between the electric and combustion engines, the use of a clutch--which allows each engine to independently power the propeller--and the subsequent use of the combustion engine as an electricity generator to supply power to the electric motor alongside solar cells have been investigated. The feasibility assessment results indicate that a reasonably sized vehicle suitable for long-endurance (several days) stratospheric flight can be achieved by employing a hybrid propulsion system combining electric and combustion engines with a clutch. However, due to power losses, using the combustion engine to drive the electric motor during nighttime was found to be less effective.