## 1 Introduction
The primary difference between the two scenarios lies in the payload mass. The authors evaluate four propulsion technologies: solar photovoltaic arrays, hydrogen fuel cells, rechargeable batteries, turboshaft engines powered by hydrogen, and regenerative fuel cells. At that time, prototype regenerative fuel cells could deliver approximately 400 Wh/kg, with projected capabilities reaching 700 Wh/kg. According to the analysis in [9], turboshaft or turboprop engines--potentially hydrogen-fueled--remained the most favorable solution for high-altitude propulsion, as they offered a lower propulsion-system-to-total-mass ratio compared to other alternatives. Two critical design requirements are emphasized in [10]: the propulsion system must be extremely lightweight and highly reliable, and must be able to operate at temperatures as low as 217 K. An additional challenge is the requirement for operation across a wide altitude range, as the platform must be capable of reaching and maintaining very high altitudes. Although propellers remain the preferred choice for many near-space aircraft, the authors of [10] also investigate unconventional propulsion concepts, including those based on electrohydrodynamic forces, radiometric forces, and thermal transpiration. However, these emerging technologies are not yet viable, mainly due to the limited understanding of the underlying physical mechanisms and the presence of multiple unresolved engineering challenges. Historically, one of the main obstacles in the practical deployment of HAPSs has been the limited efficiency of solar cells and the relatively low energy density of battery systems. At present, HAPSs are typically powered by solar energy during daylight hours and rely on battery storage during nighttime operations. This architecture is motivated by findings reported in [11], where the authors compare various unmanned aircraft vehicle (UAV) platforms powered by different energy sources, including combustion engines, regenerative fuel cells, solar energy, and batteries. While most of the UAVs analyzed are tropospheric in nature, the Airbus Zephyr HAPS (Airbus Defence and Space, Farnborough, United Kingdom) is also included as a case study due to its reliance on solar energy for propulsion. The study concludes that, compared to combustion systems and regenerative fuel cells, solar energy combined with battery storage offers the most favorable performance in terms of flight time-to-total weight ratio, payload weight-to-flight time ratio, and flight time-to-cost ratio. These advantages have significantly contributed to the growing viability and development of HAPS technology [12]. In early studies on HAPS platforms, battery energy densities were approximately 100 Wh/kg. Subsequent technological advancements led to an increase to 250 Wh/kg [13]. In 2022, Amorius Technologies (Fremont, CA, USA) announced the commercial availability of lithium-ion battery cells with an energy density of 450 Wh/kg and a volumetric density of 1150 Wh/L [14]. Similarly, MicroLink Devices (Niles, IL, USA), a manufacturer of solar arrays for both satellite and HAPS applications, has developed high-efficiency solar sheets capable of delivering over 1.5 kW/kg, with a solar energy conversion efficiency of up to 37.75% [15]. Fuel cell technology has also progressed substantially. Since 2009, when state-of-the-art systems delivered power densities of around 400 W/kg, the technology has advanced considerably, with current systems reaching up to 1.6 kW/kg and targets as high as 2 kW/kg set by key stakeholders in the aviation sector [16]. Regardless of the energy source, most contemporary HAPS concepts--across various stages of development--employ electric propulsion systems, wherein electric motors drive lightweight, high-efficiency propellers. For low-speed applications typical of stratospheric flight, propellers continue to represent the most efficient propulsion method [17].