A Review of Liquid Hydrogen Aircraft and Propulsion Technologies⁠↗

The awareness of a sustainable/green economy and long-term energy security has given rise to substantial global momentum in the search for alternative energy sources. The carbon-free nature of green hydrogen production and use, and the versatility of both hydrogen production sources, as well as its end-use applications, have resulted in an unprecedented focus on hydrogen. In 2022, 26 countries have set national hydrogen strategies which forecast 145–190 GW total green hydrogen production capacity by 2030

The global aviation industry accounts for approximately 12 % of transport sector carbon dioxide ( CO2) emissions

CO2 emissions are expected to double by 2050 even with an optimistic 2 % fuel efficiency improvement per annum. The results show that the adoption of both sustainable aviation fuel (SAF), by 2025, and a revolutionary green fuel, by 2030, is required to achieve the net-zero target.

Its energy content is 2.8 times higher than kerosene allowing a corresponding reduction in the amount of fuel per mission [12]. However, hydrogen is four times less dense even when stored cryogenically in liquid form, requiring approximately four times larger storage volume than kerosene. Cryogenic storage is preferred for aerospace due to its lower storage pressure and higher density, hence the ability to carry a higher total mass of fuel on board [9]. The cryogenic conditions add design and integration complexity in storage, distribution, and fuel conditioning, but also create an opportunity to integrate this into the aircraft thermal management system achieving aircraft-level fuel burn benefits. Using hydrogen requires significant adaptation of the aircraft systems but offers carbon-free emission.

According to the German Aerospace Centre (DLR), the contrails cirrus cloud formation, nitrous oxides ( NOx) and soot emissions at altitude are far more damaging with almost twice the effect for global warming compared to CO2 emissions [32]. Hydrogen combustion offers 50 %–70 % lower nitrogen oxides NOx compared to kerosene/SAF fuelled gas turbines whereas the use of fuel cells offers zero NOx

Due to a more complex and less efficient production process of SAF and a carbon tax imposed on kerosene, hydrogen is expected to reach a cost-competitive point by the early 2030s [9]. Hence, hydrogen could be a long-term solution from economic and emissions perspectives, excluding the effect of contrails.

The 1970s oil price shock sparked large-scale hydrogen feasibility projects for civil aviation.

The global consensus on climate change, and major efforts to convene capabilities and resources since 2020, have given rise to the green fuel momentum and a push for a credible alternative fuel and energy vector. Investments in new technologies tend to be driven primarily by predicted economic returns over the short to medium term. Nevertheless, the landscape has shifted, and commercial entities are forced to adapt and take a longer-term view, including making strategic investments, to remain relevant and within stringent regulatory constraints around emissions and environmental impact over the product lifecycle.

Interestingly, all studies concluded that a conventional tube and wing (CTW) aircraft with an evolutionary enhancement of fuselage, wings, and other components as the most promising configuration at the early stage.

All studies agree that a single tank at the rear fuselage bulkhead covering the entire cross-section (cylindrical) provides the highest gravimetric and volumetric efficiency [46,113] (FlyZero [127,129], Airbus [143] and Embraer [103] concepts). LOCKHEED 1970s [46] adopted two tanks, one front (with sidewalk passage for cockpit access) and one behind the passenger cabin. However, recent structural evaluation demonstrated a ∼10 % decrease in gravimetric efficiency due to the sidewalk [139].