## 1 Introduction  
According to ACARE ([1]) and European Commission ([2]), the main milestones of the path towards sustainable aviation are a significant reduction in CO\({}_{2}\) (\(-\)75%) and NOx (\(-\)90%) emissions by 2035, and the total decarbonisation within 2050. To achieve these goals, the second one in particular, the synergy between two or more of the following enabling technologies would be needed: ultra-efficient airframes, full or hybrid-electric propulsion, and sustainable drop-in or non-drop-in fuel, as, for example, hydrogen. The low energy density of batteries makes electric propulsion suitable for aircraft up to the regional category, whereas drop-in and non-drop-in fuel may be adopted in Short-to-Medium Range (SMR)aircraft, in particular those meeting the ICAO Aerodrome Reference Code element 2 \"C\" (e.g., Airbus 320, Boeing 737, etc.). Adopting hydrogen as fuel is among the promising ways to achieve the decarbonisation goal ([3; 4; 5]), since it has very high energy density and its combustion does not produce CO\({}_{2}\). On the other hand, embarking gaseous hydrogen is not feasible, since it would require tanks about 3000 times larger than current ones; therefore, hydrogen needs to be stored in liquid conditions by means of a proper combination of cryogenic temperature and high pressure. From the decarbonisation point of view, hydrogen is to be considered more as an energy storage than as fuel since there are no direct natural sources on Earth. The hydrogen can be obtained by processing various sources, such as hydrocarbons, biomass, and water. These processes have their own CO\({}_{2}\) emission levels, which should be considered in the total emissions balance. However, this estimation is dependent on the energetic policy and infrastructure of the nation in which the aircraft is operated; moreover, although hydrogen combustion does not release CO\({}_{2}\), it generates water and NO\({}_{\times}\), whose effects on climate change are under study. Another important point about the application of this technology is the impact on airport infrastructure, as well as the choice of generating hydrogen on-site or not. For the reasons above, an estimation of the direct and non-direct emission is omitted in this paper, which should be regarded as a feasibility study of the application of the hydrogen propulsion technology to aircraft. The storage system for liquid hydrogen, hereafter indicated as LH\({}_{2}\), is thus made of large tanks with appropriate thermal insulation, whose integration with the aircraft raises questions about the effective achievable benefits under given operational constraints, as, for example, the minimum number of passengers and flight range. This aspect opens to new research opportunities which touches the aforementioned ultra-efficient airframes, i.e., aircraft architectures, different than the conventional tube-and-wing one, that can boost the adoption of new technologies leveraging on their higher aerodynamic efficiency. In fact, since the conventional tube-and-wing architectures are mature and modifying the geometries of fuselage or wings would move the design far from the current optimal solutions, considering different architectures can facilitate the introduction of LH\({}_{2}\) propulsion technologies from the very early stages of the design. A previous example of addressing a similar research question is given by the European project PARSIFAL ([6; 7; 8; 9]), in which a box-wing aircraft designed for the SMR category (Figure 1a) has been assessed under the operational and environmental standpoints. Assuming adoption of conventional turbofan engines, results have shown a potential of reducing fuel burnt by 22% per seat-kilometres, with an impact on both CO\({}_{2}\) and non-CO\({}_{2}\) emissions, hence on short-term and long-term global warming and temperature change potential ([10]).