## 1 Introduction  
The United Nations' International Civil Aviation Organization (ICAO) [1], the European Union [2], and the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) [3] project aviation emissions to account for 25% of the global carbon budget by 2050, up from the current 2-3% [4]. Therefore, new sustainable methods of powering aircraft are needed to ensure a different reality by 2050. Another driving factor is the electric power demand increase in modern aircraft, which affects aircraft range and endurance. [PERSON] [5] deduced the incoming trend of the electrical power demands on modern aircraft as the power demands keep increasing due to onboard systems being electrified, for instance, the environmental control systems and hydraulic actuators. Therefore, the electric power fraction, i.e., the ratio of electric power demand to total power demand (propulsive and electrical) of the aircraft, at any point on the flight profile, is increasing. For commercial aircraft, the power fraction is less than 5%, with the Boeing 787 being the highest power fraction of 4%. Future commercial aircraft are expected to have power fractions exceeding 25%. Currently, sensor-laden unmanned aerial vehicles (UAVs) have power fractions up to 20%, for instance, the MQ-9 and future versions of the Global Hawk RQ-4 will reach 15% power fractions. This increase in electric power demand necessitates higher gas turbine (GT) fuel consumption to fulfill the electrical power requirements within each flight cycle. However,this heightened demand comes at a trade-off: it limits the aircraft's range and endurance capabilities. Different alternative methods to meet this increase in electric power fractions have been proposed, the most notable amongst them being battery-powered aircraft [6], fuel cell-battery [7,8], and fuel cell-powered aircraft [9]. Amongst the fuel cell options, the solid oxide fuel cell (SOFC) has been the subject of many studies recently. Such studies have analyzed how it can be utilized in the transporting sector, including marine [10], road [11] and aviation transport. Therefore, motivated by the need to improve aero-engine range and endurance and reduce emissions, an aircraft SOFC/GT hybrid system was designed and analyzed. The SOFC/GT hybrid system has been proven to show great potential amongst power systems utilizing SOFCs as one of the contenders replacing conventional aeroengines. The SOFC/GT combines the advantages of both systems where the electrical efficiencies of SOFC/GT can be as high as 70% [12]. One of the main advantages of SOFCs over other types of fuel cells is their fuel redundancy. They can utilize a wide range of hydrocarbon fuels, natural gas, hydrogen, carbon monoxide, and other renewable fuels, making them an attractive option [13]. This proves to be a major advantage in freight logistics. SOFCs have demonstrated reliability, and good durability in their various applications. SOFCs can also be replaced much more readily at an airport. The SOFC/GT systems have been noted to reduce fuel burn by 5-7% [5], reduce fuel costs by over 50% [14] and reduce emissions by up to 70%. Quiet vibration-free operation of SOFCs also eliminates the noise that is greatly associated with aircraft and conventional power systems. As an electric power source, it proves most useful in replacing mechanical generators and auxiliary power units (APUs), which generally have fuel first pass through the Brayton cycle for mechanical power before converting it to electrical power. This fuel-to-electric power conversion in fuel cells is direct and more efficient, up to 50-60% in systems without heat recovery cycles. There are also shortcomings in utilizing SOFC systems in aircraft, and this is mainly due to the power density and power-to-weight ratio of the current existing SOFCs. [PERSON] et al. [14] note that, in 2005, the power density of an SOFC stack used to be 0.25 kW/kg, which has since improved to 0.9 kW/kg.