## 3 Propulsion Systems for HAPSs
### Electric Propulsion  
Electric propulsion is one of the most commonly investigated alternatives for HAPSs in the literature due to its advantageous properties. In general, a HAPS equipped with a properly designed pure electric propulsion system can operate for extended periods in the stratosphere (on the order of months), compared to vehicles employing combustion or hybrid propulsion systems, albeit with a significant reduction in payload mass. An electric propulsion system (EPS) can be divided into three main components: the energy device, the drive device, and the propulsion device. The energy device typically consists of photovoltaic arrays (solar cells) or fuel cells. Photovoltaic arrays convert the incoming solar energy into electrical energy with an efficiency of approximately 10-20%, supplying power to the engines, payload, and, when in excess during daylight hours, recharging batteries or regenerative fuel cells for use during night flights. An innovative storage method related to excess daytime solar energy collected by solar cells is presented in [54]. This method involves converting the surplus power into gravitational potential energy; specifically, the excess power during daylight hours is used to climb to a higher altitude. During the night, the HAPS descends to its initial altitude without engine use. The analysis in [54] indicates that this ascent and descent maneuver can result in mass savings compared to storing excess power in batteries and regenerative fuel cells or simply rejecting the surplus energy. Therefore, this approach may be preferable if maintaining a constant altitude is not a mission requirement. Fuel cells electrochemically combine two elements, commonly hydrogen and oxygen, to produce electricity with high efficiency (up to approximately 50%), outperforming combustion processes limited by Carnot efficiency. Regenerative fuel cells (RFCs), which can operate both as fuel cells and as electrolysers to store energy, are of particular interest for high-altitude long-endurance applications. If other types of hydrogen-oxygen fuel cells are used, an electrolyser may be required to replenish hydrogen and oxygen in storage tanks by splitting the water produced during fuel cell operation; this electrolyser can be powered by energy from solar arrays. In many HAPS configurations, fuel cells are commonly used in conjunction with solar cells to recharge fuel tanks. Previous studies have assessed the suitability of using fuel cells alone as energy devices for HALE platforms [55]. In particular, phosphoric acid and polymer electrolyte fuel cells, which utilize hydrogen and external air compressed by a turbo-compressor, have been considered as potential energy sources. To drive the turbine, the working fluid exiting the fuel cell is heated further through a burner and a burner preheater, thereby increasing its energy content. The results of the analysis presented in [55] indicate lower specific fuel and air consumption compared to conventional aircraft propulsion systems. However, the high specific heat rejection, which necessitates large heat exchangers, combined with the substantial volume required for hydrogen storage, render this approach non-viable as an energy source for HALE aircraft. The drive device comprises the electric motor, inverter, and controller. Electric motors are generally classified into two categories: AC and DC motors, each with its own advantages and disadvantages. DC motors operate on direct current and, for these applications, can be further divided into brushed and brushless types based on their operating principles. A primary advantage of DC motors is compatibility with power sources such as solar arrays and fuel cells, which produce DC power, thereby eliminating the need for a DC-to-AC converter (inverter). Additional benefits include ease of torque-speed control, precision, power density, versatility, and quiet operation.