## 2 Materials and Methods
### Aircraft Design and Operating Costs  
The overall aircraft design is conducted by integrating several disciplinary tools into a workflow which is built in RCE [33]. The data schema CPACS [34; 35] is applied to allow a seamless communication between the disciplines. The core component is the handbook based tool openAD [36]. A more detailed description of the workflow can be found in [37]. The results of the high fidelity turbofan engine performance simulations, together with the emission characteristics, are integrated in engine performance decks which can be interpreted by the mission performance tool. This assures a clean interface and sophisticated results of the aircraft's behaviour at any possible flight state in the missions. As Mach number and maximum altitude trades are conducted, the methodologies have to be capable to include the necessary dependencies with the correct sensitivities. Consequently, a minimum set of input parameters has to be used which incorporates the low-speed aerodynamics and performance calculations.  
For calibration and validation purposes, the Boeing 767-300 [38] is recalculated and the top level aircraft requirements (TLAR) are derived, see Table 3. The B767 is used as a reference, as it's entire family, B767-200, B767-300, and B767-400, perfectly matches the promising future market segment of 200-300 passengers with its typical two class layouts leading to 216 to 296 available seats [38]. The design range of 3900 NM also fits between the short-range single aisle aircraft with design ranges of less than 3000 NM and modern long-range aircraft such as the Airbus A350 and the Boeing B787 with design ranges of more than 8000 NM. Furthermore, the twin aisle layout is used to cover the upper end in terms of range and available seats of the mid-range market segment, whereas the lower end could potentially be covered by a single aisle concept similar to an Airbus A321 neo. The focus of this study is on the upper end of the mid-range market segment as this also allows a potential substitution of some long-range flights. Additionally, the twin aisle concept reduces the boarding and de-boarding times compared to a long single-aisle layout which is in most cases at the critical path of turnaround schedules.  
The main challenge of liquid hydrogen powered aircraft is the storage and distribution. This is why more advanced methods are applied, including a dynamical thermodynamic calculation of the tank behaviour along the trajectories. A detailed description of LH2 tank design methods can be found in [39]. Correlations by Brewer [40] are applied to estimate the mass of the storage subsystems as well as the distribution lines.  
Even though this study focuses on the climate impact reduction potentials of synthetic kerosene and green liquid hydrogen, operating costs are calculated only to capture the impact of forcing the aircraft to fly lower and slower. The operating costs are calculated by applying the methods described in [41; 42] which is the extended TU Berlin method [43; 44]. The direct operating costs consist of air-traffic-control, ground handling and landing fees, maintenance costs, crew costs, energy costs, and capital expenditures (CAPEX). The recurring costs (RC), which are typically the main portion of the capital expenditures (CAPEX), are calculated by [45] where the production costs of each aircraft part is calculated separately. This method is extended by a LH2 tank price correlation extracted from [46]. For the unconstrained LH2 concept D261+H2, a price of 6.2 million USD is calculated for the two storage tanks and subsystems. The energy carrier production costs are taken from [42] which provides data for comparable boundary conditions. For power to liquid kerosene, 42.8 USD/GJ, and for LH2 34.8 USD/GJ is applied, both based on averaged electricity costs of 12.5 USD/GJ. This are the important assumptions concerning the difference between synthetic kerosene and liquid hydrogen. For further details of this method, see [41; 43; 44].