Introduction

Decades after its collapse, the Soviet Union is remembered for many things, not all of them positive. One lesser-known achievement is that on April 15, 1988, the Soviets were the first to demonstrate a successful liquid hydrogen-powered flight using a modified Tu-154 airliner. Unfortunately, this research did not continue for long, as the USSR dissolved a few years later.

In the intervening decades, interest in liquid hydrogen as an aerospace fuel waned, but has seen a resurgence since September 2023. This was triggered by a successful demonstration by the HEAVEN consortium, which flew a HY4 aircraft, modified for liquid hydrogen, that took off from Maribor Airport in Slovenia and flew for several hours. The company claimed that using liquid hydrogen doubled the aircraft's range compared to the gaseous hydrogen it had previously used.

Following this, liquid hydrogen is now firmly back in the spotlight – and for good reason. Society's increasing awareness and demand for reduced CO₂ emissions, coupled with ambitious targets set by the European Union and its member countries, highlight the urgency for transformative changes in aviation.

Estimates vary, but the sector’s 900 million tons of annual CO₂ emissions account for 2.5% of global energy-related CO2 emissions, and have grown faster in recent decades than rail, road, or shipping according to the International Energy Agency (IEA). Furthermore, emissions of nitrogen oxides (NOx), soot, and water vapor push aviation's full contribution to global warming significantly higher [1][2].

As a result, governments meeting at the 2022 International Civil Aviation Organization (ICAO) Assembly in Montreal adopted a goal of net-zero carbon emissions for international flights by 2050, and the European Union aims to become carbon neutral by 2050 with its Green Deal.

In the pursuit of a more sustainable future, the quest for efficient and versatile fuels has rightly taken center stage. Among the contenders, hydrogen in its various forms shows promise for long-term chemical energy storage, particularly in harnessing surplus renewable energy that would otherwise go untapped.

For example, renewable energies have periods when there is excess supply (eg. on very windy or sunny days). In such cases, power companies either shut down production or store this excess power in expensive battery storage centers. Green hydrogen production could aid in load management of the grid, producing hydrogen with the ‘spare’ energy from these other sources. This additional energy can then easily be delivered in gas/liquid form, wherever needed [3][4].

Hydrogen also offers benefits beyond emissions reduction, like longer flight ranges and heightened fuel efficiency (43.5 MJ/kg and 34000 MJ/M3 for aviation fuel vs. 8500MJ/M3 and 142 MJ/kg for liquid hydrogen) [2].