## 1 Introduction  
The demand for passenger and cargo flights has grown considerably over the past few decades. Due to continuing globalization and almost unlimited travel possibilities, global air travel is expected to expand even further [1]. Pre-COVID studies of the two leading aircraft manufacturers, Airbus and Boeing, predicted that the demand for flights will increase by up to 4.5% annually, which would lead to a doubling of air traffic every 16 years [2, 3]. Although the COVID-19 pandemic led to a decrease in air travel by 75% in 2020 and therefore slowed this growth in the short term, the demand for flights has fully recovered, and a strongly positive trend is expected in the long term [4]. While this development is desired from an economic perspective, it is associated with extensive negative environmental impacts. Especially the emission of greenhouse gases (GHGs), such as carbon dioxide (CO\({}_{2}\)), is associated with undesirable and long-lasting effects on the climate due to increased global warming [5]. In 2019, the aviation sector was responsible for 2.6% of global CO\({}_{2}\) emissions and 5.9% of global anthropogenic GHG emissions. Even if fuel efficiency improvements of approximately 25% can be achievedwith each new aircraft generation, the predicted growth in flights would cause the aviation-induced GHG emissions to triple until 2050 [6]. This is particularly critical, as the impacts of emissions at high altitudes are more severe than those from ground-level emissions [7; 8]. These negative impacts are mainly due to the combustion of fossil kerosene during flight operation [9], in which about 2.5 kg of CO2 is emitted per liter of kerosene [10]. An average medium-haul aircraft, such as the Airbus A320, consumes around 2700 L of kerosene per flight hour, causing 6750 kg of CO2 emissions during this period [10]. Due to the negative impacts on the environment as well as increasing societal pressure, political regulations have been established to reduce these emissions. Various strategies have been devised in the aviation sector, such as the introduction of the \"Carbon Offsetting and Reduction Scheme for International Aviation\" (CORSIA) [11]. Notably, the European aviation industry imposed ambitious goals on themselves until 2050 based on the Flightpath 2050 strategy: reduction of CO2 emissions by 75% per passenger kilometer traveled (pkm), reduction of nitrogen oxides (NOx) emissions by 90% per pkm, and subjective noise reduction by 65% relative to a new aircraft from the base year 2000 [12; 13]. A first step toward the achievement of these reduction goals is the mandatory participation of airlines in the European Union Emission Trading System (EU ETS) for intra-European flights [14]. However, CORSIA and the EU ETS predominantly aim to provide financial compensation for the environmental impacts of air travel. Further far-reaching strategic concepts in the aviation sector are of the utmost importance to enhance sustainable aviation independent of fossil fuels. Although a continuous improvement of aviation technologies is assumed, the efficiency increase of existing technologies will not suffice to reduce harmful environmental impacts substantially [13]. Consequently, disruptive technologies or radical changes to the powertrain system are inevitable [15; 16]. To this end, the aviation industry is increasingly focusing on the use of alternative powertrain technologies in the aircraft fleet, such as fuel cell-based and battery-based concepts. Extensive structural modifications of the aircraft accompany these technological transitions. In the short term, however, they are not technologically feasible due to pending research and approval [17; 18]. Considering the long use phases as well as the long research and development cycles of aircraft and powertrain concepts of 20-30 years in total, short-to-medium-term solutions need to complement the long-term transition [16; 18].