## 4 Conclusions
Part-section layouts, with tanks located under the cabin floor, do not affect passenger capacity and allow to achieve flight ranges up to 4000 km (about 65% of the reference aircraft harmonic range). For longer ranges, a full-section layout, provided with a catwalk corridor for safety reasons, is required. These solutions are designed to maximise the embarked LH\({}_{2}\) in the given fuselage cross section but leads to a reduction in passenger capacity proportional to tanks' lengths. In fact, in the range interval between 4000 and 6000 km, the number of passengers reduces from 196 to 164, against 308 of the part-section layouts. Figure 23 summarises the most relevant results in terms of transport capabilities, i.e., passenger capacity and harmonic range, compared to the reference aircraft. As shown, aiming at a high passenger capacity goal, a part-section layout is preferable, whereas the full-section layout, with the LH\({}_{2}\) mass stored in more than one group of tanks, is a better solution to cover longer flight ranges. In addition, multiple tanks allow more flexibility in dealing with aircraft balancing, stability, and control, which are not faced in this work and should be part of future investigations. Since each solution is associated to different tanks' lengths, any choice needs to be verified with respect to the actual availability of fuselage volume. Concerning open problems and possible further development paths for this research, it is worth mentioning the implementation of a flow control strategy, depending on internal pressure and capability of limiting both venting and minimum pressure problems, the integration of constraints about flight mechanics (e.g., effects of longitudinal trim and stability at each time step), the implementation of a thermodynamic model capable of taking the mutual interaction between tank installed in the same fuselage portion into account, as well as more detailed studies about the structural connection between the tanks and the fuselage, with the implementation of a higher fidelity model to estimate the aircraft empty weight. Conceptualization, V.C. and D.Z. ; Data curation, V.C., D.Z. and V.B. ; Formal analysis, V.C., D.Z. and V.B. ; Methodology, V.C., D.Z., K.A.S. and V.B. ; Software, V.C., D.Z., K.A.S., V.B. and G.P. ; Supervision, V.C. ; Validation, V.C., D.Z. and V.B. ; Visualization, V.C., D.Z. and V.B. ; Writing--original draft, V.C., D.Z. and K.A.S. ; Writing--review & editing, V.C., D.Z., K.A.S., V.B. and G.P. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The authors declare no conflict of interest. The term \(\Phi\) in Equation (25) is the \"energy derivative\" and is defined in Equation (28), according to [31]. This term is non-dimensional; however, its interpretation is easier if represented with the units of [Pa/(J/m\({}^{3}\))]. These units identify the energy derivative as the  
Figure 23: Summary of transport capabilities for some of the investigated LH\({}_{2}\) aircraft configurations compared to the reference aircraft (308 px, 5722 km), for both part-section (PS) and full-section (FS) integration layouts. pressure rise per unit energy input in a unit volume [25]. It is worth noting that the energy derivative is defined for the homogeneous fluid and not on specific phase quantities, and that is variable during the mission. The energy derivative relative to a state can be computed starting from Equation (28), considering a closed rigid tank of unit volume, with the same homogeneous fluid pressure \(P\) and density \(\rho\) (or specific volume \(v\)) of the considered state. The computation is completed according to the following the steps:  
* In the fluid reference condition (state 1), is possible to determine the quality \(X\) (Equation (A1)), deriving it from the properties of the saturated H\({}_{2}\) liquid and gaseous phases [40], since they are directly linked to the current pressure.