## 3 Propulsion Systems for HAPSs
### Combustion Propulsion  
Combustion propulsion was the first type of propulsion system investigated for achieving stratospheric flight. Early stratospheric aircraft employed piston engines equipped with large radiators to cool the system, due to the low-density environment where heat convection efficiency is poor. The Boeing Condor, with an endurance of 56 h at approximately 20 km altitude, utilized a liquid-cooled, two-stage turbocharged spark-ignition engine. The Aurora Flight Sciences Perseus A UAV, designed to operate for several hours at an altitude of around 25 km, used a spark-ignition engine and carried liquid oxygen to support combustion. The Grob Strato 2C, designed for operations at 24 km altitude with an eight-hour endurance, employed the same propulsion system as the Boeing Condor but featured an additional turbocharging stage. Subsequently, turboprop and turboshaft engines have been investigated. In [8], a propulsion system capable of powering an aircraft to an altitude of approximately 30 km for several hours was analyzed. The described system is a turboprop engine using fuel and oxidizer carried onboard. Specifically, a stoichiometric mixture of methanol and hydrogen peroxide, diluted with water to maintain acceptable turbine inlet temperatures, was considered as the propellant. Compared to piston engines, this system offers the advantage of eliminating the need for a radiator, as all waste heat is expelled through the exhaust gases without requiring convective heat transfer to the atmosphere. Preliminary analyses demonstrated the capability of this system to operate effectively at high stratospheric altitudes, achieving flight endurance of several hours.  
Recently, propulsion systems utilizing hydrogen have gained attention. Historically, hydrogen has received limited consideration in the literature for propulsion systems due to safety concerns arising from its high flammability and storage complexity. Additionally, hydrogen's low energy density per unit volume necessitates large storage volumes. However, hydrogen offers several advantages in the HAPS domain, particularly for hybrid buoyant aircraft [53]. These aircraft, characterized by large hulls filled with low-density gases, can accommodate the storage of large hydrogen volumes without issue. Furthermore, although hydrogen's volumetric energy density is lower than that of fuels such as kerosene, its gravimetric energy density is substantially higher. This implies that less fuel mass is required to deliver the same energy for a mission, making hydrogen especially suitable for applications where weight reduction is critical. Moreover, combustion of hydrogen with air produces water vapor, leading to reduced atmospheric pollution compared to conventional fuels. An example of a HAPS employing a liquid hydrogen-fueled propulsion system is AeroVironment's Global Observer, designed to operate for several days at an altitude of 20 km. Although the project was discontinued years ago, a prototype successfully completed test flights. Currently, due to significant advances in batteries, solar cells, and fuel cells, hydrogen-based propulsion systems receive comparatively less attention.