Modeling and Analysis of Thermal Aspects for a Hybrid Stratospheric HAPS

High Altitude Platform Stations (HAPS) have attracted a lot of interest in recent years as they can be seen as a complement to terrestrial, aerial and satellite systems, allowing to target new applications. The applications of these vehicles cover many sectors, from defense to communication systems to Earth observation. A challenge to be addressed in the design of such hybrid aircraft is the analysis of thermal aspects in all the flight envelope and, consequently, the design of thermal management system which is of fundamental importance to ensure that the electrical components on board performs correctly throughout the flight. One of the factors that represents a difficulty for the success of the mission is the surrounding environment: HAPS fly for most of the mission at stratospheric altitudes; this implies extreme environmental conditions in terms of density, temperature and air pressure. Another aspect to take into great consideration is the presence of the Sun, a help and a hindrance in these cases: it supplies electrical power to the components through photovoltaic cells but it is also the main heating source for them. Another aspect to take into consideration is the mission profile of the HAPS: its rate of ascent associated to and the achievement of the operational altitude represents fundamental mission requirements; altitude angle, velocity vector and presence of winds are parameters that must be considered to assure the safety of the mission. This work presents, firstly, a description of the typology of components used inside the HAPS avionics bay, from a thermal management perspective. The thermo-dynamic atmospheric variables are then descripted. The knowledge of the HAPS location in terms of longitude and latitude allows for greater precision in calculating all model variables. The thermal model used and the results obtained are then presented and discussed. This model takes into account all the heat exchange modes (radiation, convection and conduction) and the exchange possibilities between the avionics bay and the external environment. The model is based on the electro-thermal analogy, working with an equivalent electrical scheme. Taking into account the type of mission in progress, it is necessary to know the value of the temperature (in particular that of the components) during the entire duration of the flight. To do so, the theory of lumped parameters is used. The results show the temperature trends of all avionics components in the bay during the entire duration of the mission, climb, cruise and final descent, ensuring that their temperatures remain within their operating temperature range to avoid interruptions or damages during the flight. The aim of this paper is to provide a valid methodology to determine the temperature conditions for both the equipment and the buoyancy gas during the design of a HAPS in order to determine the requirements on the temperature operating range for equipment and the need of a thermal management system and, eventually, to design it.