Over the last few years, much of the hubbub about hydrogen in the advanced air mobility sector has revolved around hydrogen fuel cells, which could power eVTOL aircraft and small regional airplanes without producing harmful emissions. But hydrogen combustion technology has been picking up steam, too, with several companies preparing test campaigns with hydrogen-burning engines in 2025.

Manufacturers of traditional fossil fuel-burning engines—including Rolls-Royce, Pratt & Whitney, and Safran—are betting on liquid hydrogen as the most effective means of powering large passenger-carrying airliners more sustainably and helping the industry achieve net-zero carbon emissions by 2050.

Although hydrogen burns cleaner than jet fuel, hydrogen engines are not totally emissions-free. However, they could enable longer flights with larger aircraft than those that use hydrogen fuel cells, thanks to the high energy density and compact nature of liquid hydrogen.

Stored in a cryogenic tank onboard an aircraft, liquid hydrogen can fuel traditional turbine engines with some modifications. It’s compatible with existing technologies. On the other hand, hydrogen fuel cells present a more complex engineering challenge. They use hydrogen to generate electricity that powers electric motors. Hydrogen fuel cell powertrains also have a lower power-to-weight ratio due to the weight of the system’s components, so it doesn’t scale well for large aircraft.

Airbus, which wants to introduce a hydrogen-powered passenger airliner by 2035, is mulling over several configurations for the propulsion system and airframe, including both hydrogen fuel cells and hydrogen combustion engines. The first jet engine Airbus plans to test with hydrogen fuel will be a GE Passport on an A380, which is slated for a first flight in 2026. CFM International, a joint venture between GE Aerospace and Safran Aircraft Engines, is overseeing the engine modification work. 

Meanwhile, Safran is also developing hydrogen-powered turbine engines for light aircraft in the general aviation sector. In January 2024, Safran and its partner Turbotech completed the first successful ground test of a hydrogen-fueled gas turbine engine for France’s BeautHyFuel project. Those tests used gaseous hydrogen in TurboTech’s small TP-R90 engine. “Hydrogen is easier to handle in this form, especially on a small engine,” explained Adrien Cahuzac, a system architect for Safran’s energy and propulsion division.

Whereas hydrogen gas can power both combustion engines and fuel cells, storing it requires large, heavy pressurized tanks. Cryogenic liquid hydrogen has a higher energy density and is less bulky to store, making it more suitable for aviation applications, although keeping the hydrogen cold enough to remain in a liquid state presents another challenge.

Using hydrogen gas, Safran was able to test engine parameters such as flow regulation strategies and engine shutdown limits, Cahuzac explained. “On the long term, the goal is therefore to store hydrogen in liquid form to carry larger quantities and fly long distances.”

Pratt & Whitney Advances Turboprop Tests

This year, Pratt & Whitney Canada is preparing to demonstrate hydrogen combustion technology on a PW127XT turboprop engine—also starting with gaseous hydrogen—for a Canadian government-backed project called Hydrogen Advanced Design Engine Study (HyADES).

Canada’s Initiative for Sustainable Aviation Technology (INSAT) has already awarded funding for the first phase of the HyADES project, which will involve ground tests of the engine’s fuel nozzle and combustor rig running on gaseous hydrogen, the engine manufacturer announced in November. If everything goes according to plan, P&WC could receive additional INSAT funding for subsequent ground tests on the whole engine.

Separately, P&WC is working on another INSAT-supported project called Turbine Engine Advanced Materials for Efficiency (TEAME), which will study novel materials for hot section components of gas turbine engines that could help to improve their thermal efficiency, reduce fuel consumption, and curb emissions.

“This collaborative project with INSAT enables us to develop key technologies for future hydrogen-powered aircraft and complements our wider efforts to advance aviation sustainability through a range of pathways, including continued improvements to engine efficiency, hybrid-electric propulsion, and compatibility with sustainable aviation fuel,” said Edward Hoskin, vice president of engineering at P&WC.

P&WC originally introduced the PW127XT engine in 2021 as the new standard powerplant for ATR 42 and ATR 72 regional turboprops. Deutsche Aircraft has also opted to use a pair of the engines for its D328eco regional airliner, a derivative of the Dornier D328 turboprop that could someday fly on hydrogen power—although initially, it will run on sustainable aviation fuel.

Deutsche Aircraft began building the first D328eco test aircraft in mid-2024, and the company is targeting 2027 for service entry of the SAF-powered model. It has yet to announce a timeline for the hydrogen-powered variant, but it has already begun developing the concept through studies conducted in partnership with Univeral Hydrogen and H2Fly, a German company specializing in hydrogen propulsion technologies.

Joby Aviation, which owns H2Fly, has also been experimenting with the integration of a hydrogen fuel cell powertrain in its four-passenger eVTOL aircraft. In July, Joby and H2Fly flew an eVTOL prototype with a hydrogen fuel cell powertrain. According to Joby, the hydrogen-electric demonstrator aircraft flew 523 miles—more than five times the range of Joby’s standard battery-powered JAS4-1 eVTOL aircraft.

Rolls-Royce Puts Hydrogen in Pearl Engines

Rolls-Royce is also making inroads with hydrogen-powered business aircraft engines amid the shutdown of its electric propulsion business unit, which the company officially confirmed in November.

In September, the engine manufacturer broke ground on a new test stand at NASA’s Stennis Space Center in southern Mississippi, where the company plans to test full-scale gas turbine engines on hydrogen power. It will use this test rig to demonstrate 100% hydrogen propulsion in ground tests with a modified Pearl 15 turbofan engine, which it originally developed for Bombardier’s Global 5500 and 6500 models.

Rolls-Royce has already run a Pearl 700 turbofan engine (which powers the Gulfstream G700 and G800) on pure hydrogen gas, achieving maximum take-off thrust during ground testing in 2023. This followed earlier ground tests in 2022 with a Rolls-Royce AE 2100 A turboprop engine running on “green” hydrogen, or hydrogen that is sustainably produced by splitting water molecules via electrolysis.

These efforts are all part of longer-term work Rolls-Royce is conducting in partnership with EasyJet to introduce a liquid hydrogen combustion engine for narrowbody airliners such as the Airbus A320 family in the mid-2030s. Rolls-Royce and EasyJet are also part of the Hydrogen in Aviation Alliance, a consortium of U.K.-based companies working to accelerate the adoption of hydrogen for zero-carbon aviation.

GKN Aerospace, another member of the U.K. consortium and an EasyJet partner, is advancing both hydrogen combustion and fuel cell technologies with its H2Jet and H2Gear programs, respectively. The company is also working on a cryogenic hydrogen storage system that it says could apply to either form of propulsion.

In July, GKN launched another hydrogen initiative called H2FlyGHT, which aims to demonstrate a 2-megawatt cryogenic hydrogen-electric propulsion system that could someday power smaller regional airliners. The £44 million project will integrate hydrogen fuel cell power generation with thermal management solutions, including cryogenic power distribution and advanced cryogenic drive systems.

On the hydrogen fuel cell front, several companies are already integrating hydrogen-electric powertrains in eVTOL and other small electric aircraft. In California, Joby and Alaka’i Technologies are experimenting with hydrogen in their respective eVTOL aircraft. Meanwhile, London-based Lyte Aviation is trying to use hydrogen fuel cells to power a 40-seat VTOL aircraft.

ZeroAvia, which is developing hydrogen-electric powertrain conversion kits for aircraft retrofits, expects to receive its first supplemental type certificate for the 600-kilowatt ZA600 motor in 2025. The ZA600 is intended for regional commuter airplanes and has been tested on a Dornier 228. ZeroAvia also is working on a 2-megawatt powerplant that could power a larger aircraft carrying up to 80 or 90 passengers.

Even helicopters could soon be flying on hydrogen fuel cells, with plans underway to introduce hydrogen-powered variants of Robinson’s R44 and R66 helicopters.