## 2 Overall Analysis Approach and Implemented Methods
### Mission Analyses  
#### 2.6.1 Reference Aircraft and LH\({}_{2}\) Aircraft Derivation  
The box-wing aircraft used as a baseline reference for the study of integration of hydrogen propulsion is one of the results of the European project PARSIFAL ([6; 7; 8; 9; 10]). Carried out between 2017 and 2020, PARSIFAL has been funded under the Horizon 2020 programme, with the goal of studying the adoption of box-wing aircraft in the medium-range air transport sector, assessing its operational and environmental impact. One of the objectives of PARSIFAL was to exploit the theoretical aerodynamic superiority of the box-wing lifting system to increase the number of passengers with respect to direct competitors, i.e., Airbus 320 or Boeing 737 family craft, while complying with the same airport constraints and, above all, reducing the fuel consumption per passenger-kilometre ([43]). The designed aircraft has a box-wing architecture with the following main features: a number of passengers equal to 308, 66% more than the conventional competitor; a wingspan equal to 36 metres, and compatible with ICAO category 'C' airport aprons ([44]); and a maximum reduction in block fuel per passenger-kilometre equal to 22% compared to the conventional competitor ([45]). Figure 1 presents the final configuration of the PARSIFAL box-wing aircraft, whereas its main characteristics are reported in Table 7; for further details on the study and design of the box-wing aircraft developed within the PARSIFAL project, please refer to [12; 43; 45]. Table 7, in addition to reporting the main characteristics of the box-wing aircraft used as a baseline for this LH\({}_{2}\) propulsion integration study, also reports how this integration was achieved by modifying the reference aircraft. It should be underlined that the strategy to develop the LH\({}_{2}\) aircraft used in this research is based on the retrofit of the baseline aircraft; this involves a number of constrained design choices that are described hereafter. First, the shape of the aircraft and its main structure are kept the same as that of the baseline. This implies two major effects on the overall aircraft design: firstly, the mass of the main structures of the aircraft, i.e., lifting system, fins, landing gear, and fuselage, is the same as that of the baseline aircraft. Regarding the fuselage structure, the following assumption is made in this study: at this preliminary level it is assumed that the mass of structural components removed to allow the LH\({}_{2}\) tank installation (e.g., floor beams and trusses in the case of the full-section layout) is compensated by the introduction of new structures, such as the frames needed to support the tanks; in other words, the mass balance resulting from the installation of tanks inside the fuselage is almost zero. Secondly, since the external shape of the aircraft is the same as the baseline, the aerodynamic performance is also the same as calculated for the PARSIFAL box-wing aircraft. As the LH\({}_{2}\) tanks are located inside the fuselage, no modifications to the aircraft shape are foreseen; therefore,  
Concerning the retrofit of the propulsion system, as mentioned in Section 2.1.1, the assumption made is that the turbofan engine of the retrofitted aircraft is the same as the baseline aircraft, except for the introduction of a dedicated heat exchanger. The main modifications with respect to the baseline box-wing aircraft concern the internal design of the fuselage. In fact, as already explained in Sections 2.1.2 and 2.3, the cylindrical tanks employed in this study are placed inside the fuselage; this is completed at the expense of the internal volume available to house the payload. This aspect, in addition to the demonstrated larger aerodynamic efficiency of the box-wing architecture, was decisive in the choice of the baseline aircraft to retrofit.