H2Fly Chases Hydrogen Propulsion Power Density Boost

Hydrogen propulsion pioneer H2Fly is stepping up efforts to bring a fuel cell-based powertrain to market. The German company’s CEO, Ralph Müller, told AIN that a hydrogen-electric propulsion system must have high gravimetric power density, strong efficiency, and low maintenance costs to be competitive.

According to market studies, including work by Eytan Adler and Joaquim Martins published in Progress in Aerospace Sciences in January 2025, fuel cell systems will need to reach a power density of at least two kilowatts per kilogram power-to-weight ratio to be competitive. This is the industry benchmark that H2Fly says it is tracking.

By the end of this year, H2Fly’s Stuttgart-based engineering team aims to have a third-generation testbed in trial operations. Over the course of the second half of 2026, it aims to have built the 350-kilowatt system, which H2Fly will demonstrate as part of the German Balis 2.0 project, and to deliver fuel cells to other manufacturers as a standalone revenue source.

“There is an addressable market if we can reach two kilowatts per kilogram,” Müller said during the recent Aero Friedrichshafen show. “Depending on the configuration and aircraft mission, we can expect a time between overhaul above 8,000 hours.”

Clearing Hydrogen Hurdles

H2Fly is also focused on having a detailed assessment of the overall operating costs for hydrogen-powered aircraft. “We need to reduce the hurdles for aircraft manufacturers and airports to dealing with hydrogen, so that they can focus on the aircraft,” Müller added.

At this point, H2Fly sees growing market interest in propulsion systems for new aircraft designs, and the company believes it could support designs capable of operating at ranges of 200 to 900 nm and speeds up to 300 knots. Along with several other companies, including ZeroAvia, H2Fly is also willing to convert existing regional airliners to hydrogen propulsion.

As it works to complete its third-generation propulsion system, H2Fly has taken more direct control of components. It has brought the manufacturing of key systems, such as power electronics, in-house.

Müller, who was appointed CEO in 2024, explained H2Fly is using a breadboard system—a reusable, solderless platform—to build and test multiple components. He predicted the company might have a sufficiently robust system to start the next phase of flight testing in 2027.

H2Fly has conducted two previous rounds of flight tests with earlier iterations of its propulsion system. In September 2023, the company made the first piloted flight of a hydrogen-powered aircraft using its HY4 demonstrator. The following year, an example of the four-passenger eVTOL model being developed by its parent company, Joby Aviation, was converted to run on H2Fly’s propulsion system.