Tests carried out on behalf of engineering firm Rolls-Royce and short-haul airline Easyjet have found that 100% hydrogen can be combusted in an aircraft engine that is producing maximum thrust — enough to ensure the plane’s take-off, the duo announced today (Monday).

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The tests, carried out on Rolls-Royce’s Pearl 700 engine at the German Aerospace Centre Deutsches Zentrum für Luft-und Raumfahrt (DLR) in Cologne, centred on combusting H2 safely and efficiently in conditions that represent maximum take-off thrust.

Combusting 100% hydrogen under such conditions presents a “significant engineering challenge” because it burns hotter and more rapidly than kerosene, Rolls-Royce said.

Researchers at DLR, working with the UK’s Loughborough University, have developed advanced fuel spray nozzles to control the flame position, while also integrating a new system that progressively mixes air with the hydrogen to control the fuel’s reactivity.

The tests showed that operability and emissions were both “in line with expectations”, Rolls-Royce reported.

“This is an incredible achievement in a short space of time. Controlling the combustion process is one of the key technology challenges the industry faces in making hydrogen a real aviation fuel of the future,” said Grazia Vittadini, chief technology officer at Rolls-Royce. “We have achieved that, and it makes us eager to keep moving forward.”

But critics have warned that hydrogen will be difficult and expensive to incorporate into airline operation, on account of its poor round-trip efficiency and the complexity of storing and loading the fuel.

No modern operator has yet carried out a flight-test on a combustion engine running on 100% hydrogen, although several have carried out tests on aircraft running on hydrogen fuel cells — the most recent being Hy2Fly’s three-hour flight.

But Rolls-Royce and Easyjet have committed to developing a hydrogen-powered combustion engine compatible with a range of aircraft by the mid-2030s, a timeline that finds it in close competition with Airbus’s commitment to roll out its first commercial-scale H2-powered aircraft by 2035.

Last year, the duo tested one of Rolls-Royce’s AE2100 engines with a 100% green hydrogen mix at a UK-government-owned airfield in Boscombe Down.

“These recent tests mean the combustion element of the hydrogen programme is now well understood, while work continues on systems to deliver the fuel to the engine and integrate those systems with an engine,” Rolls-Royce said in a statement.