## 3 Propulsion Systems for HAPSs
### Future Propulsion Concepts  
Observing all the HAPS projects present in the literature, it can be seen that they are all designed to operate in the lowest part of the stratosphere. In order to operate at higher altitudes (more than 25-30 km), new types of vehicles with entirely new propulsion systems are required. Probably, propulsion systems to operate at these altitudes will not exploit propellers to generate thrust, unless more innovative materials are developed;propellers, in fact, are already characterized by large diameters for many proposed HAPSs and will scale larger with the increase in altitude. A promising propulsion technology, based on electric power, seems to be atmospheric ion propulsion. In [64], an initial feasibility assessment on this kind of propulsion was successfully carried, demonstrating its feasibility. Additionally, in [10], an overview of advanced concepts for high altitude propulsion is provided. According to the authors, the main propulsion systems candidate for these applications could be the electro-hydrodynamics propulsion systems, the radiometric force propulsion systems, and a thermal transpiration-based propulsion system. Electro-hydrodynamics propulsion systems generate thrust by ionizing and accelerating air flow through a high potential difference between two electrodes. They are possible candidates for stratospheric loitering applications for their light weight and simplicity. However, this kind of propulsion is characterized by a very low thrust efficiency. Despite studies being conducted to enhance its efficiency, there are still several concerns regarding the devices that must be addressed. Concerning the radiometric force, it is a force that arises when there is a temperature difference between a hot side and a cold side of a plate or vane that is immersed in a rarefied gas. Numerous theories suggest that relatively high forces per unit area could be produced, enough to support a high-altitude craft. A possible application is shown in [65], in which the temperature gradient, exploited to drive this force, generated by continuous beaming of microwave energy from a ground station impacting a lightweight carbon fiber sail, produces a sufficient lift force to maintain the sail in flight. The main drawbacks of this technology are that this force became very small at higher or lower altitudes than the designed altitude and the extremely high cost of maintaining a constant beam power to generate lift. In addition, nowadays, this mechanism is still not fully understood; a better comprehension of the physical mechanism of radiometric forces generation is necessary to assess the true potential of these forces for high altitude flight applications. The latter mechanism, thermal transpiration, is a physical phenomenon where a temperature gradient across a tube containing a rarefied gas results in gas motion. The gas flow moves from the colder side of the tube to the hotter part. The main advantage is that, instead of converting solar power into electrical energy and then to propulsive force, solar beams can directly power the system. The main limitations of this kind of propulsion are the low energy efficiencies, despite numerous efforts being made to improve it, and the too low thrust level for these applications. An additional propulsion system, based on breakthrough physics, described in [10], relies on antimatter. The antimatter annihilation mechanism has the highest energy density, and less than one milligram of this material could be sufficient to operate for months. However, nowadays, the low production rates and the energy efficiency of the current production process make the use of such technology impracticable. All the advanced propulsion concepts for HAPSs, just described, face substantial technological hurdles, necessitating significant advancements for their feasibility.