World-first flight test reveals how hydrogen engines form contrails

World-first flight test reveals how hydrogen engines form contrails
by Robert Schreiber
Berlin, Germany (SPX) Apr 15, 2025

In a groundbreaking campaign, researchers have captured the first-ever in-flight measurements of contrails produced by hydrogen-powered aircraft, marking a pivotal advancement in assessing the climate impact of next-generation aviation propulsion.

The German Aerospace Center (DLR), in collaboration with Airbus and the Perlan Project, conducted the Blue Condor project in December 2024 over Minden, Nevada. Over three weeks, a DLR-equipped chase plane trailed gliders fitted with hydrogen and kerosene engines, collecting direct emission and contrail data to compare both propulsion types under real atmospheric conditions.

"These world-first measurements of condensation trails from a hydrogen-powered aircraft are a major milestone in our quest to gain a complete understanding of the climate compatibility of hydrogen propulsion in aviation," said Markus Fischer, DLR's Divisional Board Member for Aeronautics. "We developed the initial conceptual ideas and theoretical foundations for such an experiment a few years ago in a DLR-funded junior research group. This is just one example of how DLR is laying the groundwork for climate-compatible flight - strengthening the economy and preparing society for the future. We're very pleased that, in collaboration with Airbus and the Perlan team, we have now successfully conducted this technically challenging flight experiment."

Central to the mission was a modified Arcus glider, outfitted by the Perlan team with a gaseous hydrogen tank, lubrication system, and hydrogen-powered turbojet engine, developed in partnership with AeroDesignWorks and RWTH Aachen University. Veteran pilot Jim Payne, known for gliding world records, flew the hydrogen-powered glider, while a second Arcus, powered by kerosene, offered a direct comparison. Both were simultaneously towed to over nine kilometers altitude by a Grob Egrett aircraft from AV Experts LLC, enabling contrail formation.

Each glider initiated engine runs in turn, while the Egrett aircraft maneuvered into their exhaust plumes to conduct measurements. The chase plane, carrying DLR's suite of autonomous sensors, sampled emissions and contrail particles for 5 to 10 minutes per engine type under identical meteorological conditions. Out of seven test flights, four successfully generated contrails from the hydrogen engine, yielding valuable microphysical and chemical data.

Researchers focused on quantifying ice crystal properties, nitrogen oxides, and potential aerosol formation. Ground-based engine tests complemented the airborne campaign by characterizing hydrogen exhaust under varying power settings. Full data analysis is underway, with peer-reviewed publication expected to follow.

Initial observations indicate that hydrogen-fueled contrails behave differently than those from kerosene combustion. Due to higher water vapor output and absence of soot particles, hydrogen contrails tend to form at warmer temperatures and lower altitudes. Lacking traditional nucleation sites, these trails rely on ambient aerosols, potentially resulting in fewer but larger ice crystals, which could limit their persistence and radiative forcing. Once fully evaluated, the data will validate existing climate models predicting reduced warming effects from hydrogen contrails.

DLR outfitted the Egrett with custom instrumentation adapted for high-altitude, unpressurized operation. Ice crystal sensors, aerosol detectors, and trace gas analyzers were installed across the aircraft, including a mast above the propeller arc and landing gear-mounted spectrometers. Tina Jurkat-Witschas, who led the project at DLR's Institute of Atmospheric Physics, explained, "We measured trace gases and aerosols from a long mast at the top of the aircraft, to avoid any interference from the propeller and the Egrett's own exhaust gases."

Further aircraft modifications supported the SIOUX nitrogen oxide sensor and remote data transmission via Iridium satellite. DLR staff operated the systems autonomously and guided the chase maneuvers via radio. The resulting dataset is expected to feed into global climate models assessing the atmospheric impact of hydrogen-powered aviation.

The Blue Condor initiative, which garnered recognition as a finalist for the NAA's Collier Trophy, represents a milestone in sustainable aviation and sets the stage for further exploration of hydrogen's environmental footprint in flight.