When Rolls-Royce and easyJet announced that they had run a modern aero-engine on 100% hydrogen, the headlines wrote themselves: the age of zero-carbon flight had arrived. The reality is more complicated — and more interesting. Aviation Explained looks at what it would actually take to fly the world’s airliners on hydrogen, and why a fuel so promising on paper remains so stubbornly difficult in practice.
Why hydrogen is so tempting
The appeal is simple chemistry: burn hydrogen and the exhaust is water, not carbon dioxide. At the point of use, a hydrogen aircraft emits no carbon at all. For an industry that accounts for a few per cent of global emissions and has very few easy ways to decarbonise, that is an extraordinarily attractive proposition. Hydrogen is also remarkably energy-dense by weight — kilogram for kilogram it holds roughly three times the energy of jet kerosene. On paper, it looks like the perfect fuel.
Two very different approaches
There are two ways to turn hydrogen into thrust. The first is simply to burn it in a modified gas-turbine engine, much as kerosene is burned today; this is the route Rolls-Royce and easyJet have been testing, and its great advantage is that it builds on a century of familiar engine architecture and suits larger aircraft. The second is to feed hydrogen through fuel cells that generate electricity to drive propellers or fans. Fuel cells are cleaner still and very efficient, but they are heavy, which for now makes them better suited to smaller, regional aircraft than to airliners.
The problem is the tank, not the fuel
The catch is volume. For all that it is light, hydrogen is extraordinarily bulky: a given quantity holds far less energy per litre than kerosene does. To carry enough of it, an aircraft must either compress the gas to very high pressure or — more practically for an airliner — chill it to a liquid at around minus 250 degrees Celsius and store it in large, heavily insulated cryogenic tanks. Those tanks cannot be tucked into thin wings the way kerosene is; they are bulky and heavy, and they force a fundamental redesign of the airframe. There is a further subtlety. Hydrogen is only as clean as the method used to make it: ‘green’ hydrogen, produced by splitting water with renewable electricity, is genuinely low-carbon, whereas the ‘grey’ hydrogen that dominates today is made from natural gas and releases substantial carbon dioxide in the process. And even clean-burning hydrogen still leaves water vapour high in the atmosphere, whose contrails exert a warming effect of their own.
Where it realistically fits
Taken together, these constraints point to a phased future rather than an overnight revolution. Hydrogen’s combination of light weight and awkward bulk makes it best suited, at least at first, to short and regional routes, where tank size matters less and the ground networks are simpler; manufacturers including Airbus have studied hydrogen-powered concepts along precisely these lines. Long-haul flying, where every cubic metre is precious, is likely to depend for years yet on sustainable aviation fuels and conventional kerosene. The Rolls-Royce and easyJet test does not mean hydrogen airliners are imminent — but it removes one large question mark, by demonstrating that a modern engine can run on the fuel at all.
Think about it
- The article distinguishes ‘green’ hydrogen from ‘grey’. Why does that distinction matter so much when judging whether a ‘zero-emission’ flight is genuinely clean?
- Hydrogen demands both a redesigned aircraft and new airport infrastructure. Which obstacle do you think is harder to overcome, and why?
- If hydrogen suits short-haul routes first, what might that mean for the kinds of airlines and airports that adopt it earliest?
- Given the storage problem, do you find hydrogen or sustainable aviation fuel the more convincing path to lower-carbon flying?
CEFR Level C1 / ICAO Level 6
The test that prompted all this is worth reading in full: Rolls-Royce and easyJet ran a jet engine on 100% hydrogen.