German hydrogen propulsion startup H2FLY has achieved an innovative milestone in zero-emission aviation by completing the world’s first crewed flights powered by liquid hydrogen. The flights took place at a small airport in Maribor, Slovenia, as part of Project HEAVEN (High powEr density FC System for Aerial Passenger VEhicle fueled by liquid HydrogeN), an EU-funded partnership aimed at demonstrating the feasibility of using cryogenic hydrogen in aircraft.
H2FLY’s HY4 aircraft relies solely on liquid hydrogen as fuel, utilizing a hydrogen fuel-cell powertrain for the entire flight. During a recent test flight, the HY4 remained in the air for over 3 hours, demonstrating its ability to operate with reduced emissions and longer flight times. The aircraft can stay aloft for up to eight hours when fully loaded with its 24kg hydrogen storage capacity.
Liquid hydrogen offers several advantages over gaseous hydrogen, including higher energy density, which reduces the need for large fuel tanks and allows for greater payload capacity. However, it requires cryogenic temperatures (around -253°C) for storage and refueling, adding complexity to logistics.
While the HY4 is not intended for commercial production, H2FLY plans to scale up its fuel-cell system to megawatt capacity. The H2F-175 system, developed in partnership with Deutsche Aircraft, aims to retrofit a 30-seat Dornier 328 demonstrator with hydrogen-electric fuel cells, with test flights expected by 2025. H2FLY’s CEO, Dr. Josef Kallo, envisions scaling up to power larger aircraft for up to 100 passengers in the future, potentially including entirely new aircraft designs.
H2FLY’s HY4 aircraft. (Image: H2FLY)
The success of hydrogen-powered aviation also depends on establishing refueling infrastructure. H2FLY collaborated with Air Liquide for Project HEAVEN, marking the first time that liquid hydrogen was refueled at a commercial airport. The hydrogen was transported to Maribor by truck under strict safety regulations.
Liquid hydrogen’s potential to reduce emissions and extend flight ranges is considered a significant advancement in the quest for cleaner aviation. H2FLY, founded in 2015, was acquired by Joby Aviation in 2021, a California-based company focused on developing electric vertical take-off and landing (eVTOL) vehicles for air taxi services.