Project Blue Condor explores contrails from direct hydrogen combustion

Aerospace Project Blue Condor Explores Contrails from Direct Hydrogen Combustion

2025-05-02 From Stefanie Eckardt | Translated by AI 4 min Reading Time

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Hydrogen-powered aircraft are intended not only to minimize CO₂ emissions but also to have a positive impact on the climate effect of contrails. However, there has been little knowledge about contrails from direct hydrogen combustion until now. In Project Blue Condor, Airbus, Perlan, and DLR have now measured the contrails of a hydrogen turbojet engine in flight.

In Project Blue Condor, the research partners conducted the first flight measurements of contrails from a hydrogen-powered turbojet engine.(Image: AV Experts) — In Project Blue Condor, the research partners conducted the first flight measurements of contrails from a hydrogen-powered turbojet engine.
(Image: AV Experts)

Contrails, which form in the upper troposphere when the air is cold and humid, have a significant impact on the climate effects of air traffic. Alternative propulsion technologies, such as hydrogen-powered aircraft, aim to reduce CO₂ emissions and optimize the climate impact of contrails. However, there have been no flight test measurements to characterize the formation, properties, and effects of contrails from hydrogen direct combustion. In the Blue Condor project, Airbus, Perlan, and the German Aerospace Center (DLR) have now measured the contrails of a hydrogen turbojet engine in flight. The three-week flight test campaign took place in December 2024 in Minden, Nevada, USA. The DLR equipped a chase aircraft with instruments for contrail and emission measurements and operated them autonomously at cruise altitude.

Approach

The heart of the Blue Condor mission is an Arcus glider. This glider was modified by Airbus and the Perlan team to accommodate a tank for gaseous hydrogen, a lubrication system, and a hydrogen-powered turbojet engine. The engine was developed by AeroDesignWorks in collaboration with RWTH Aachen. A second Arcus glider, operated by Perlan, was equipped with a conventional kerosene engine. Both gliders were simultaneously towed by a Grob Egrett, a high-altitude research aircraft from AV Experts, to an altitude of over nine kilometers—precisely the height where contrails are expected to form. There, both gliders were released from the tow, and the tow aircraft with the measuring instruments fell back behind the gliders.

The Blue Condor glider ignited the hydrogen engine. The Egrett began its chase flight in the exhaust plume, measuring emissions and contrails from the Blue Condor glider. The additional kerosene-powered glider remained in close formation with its engine ignited. To ensure comparability of the emission data, the respective chase formations were conducted immediately one after the other under the same meteorological conditions. According to the project participants, the actual measurement phase using the auxiliary engines lasted approximately five to ten minutes each time.

Explore Microphysical Properties

Out of a total of seven test flights, contrails from the hydrogen engine formed during four flights. The aim of the investigations was to measure the microphysical properties of contrails from hydrogen direct combustion under real atmospheric conditions, as explained by the DLR. An important factor for the climate impact of contrails is the initial ice crystal number as well as the size of the ice crystals that form in the exhaust plume. Emission measurements of nitrogen oxides and aerosol particle formation in the exhaust were also conducted below the contrail formation region. Engine test runs on the ground provided additional information about the emissions of the hydrogen engine at various power settings. The project partners are currently evaluating the data in detail.

Differences from Contrails from Kerosene Combustion

Contrails from hydrogen combustion engines form at higher temperatures and lower altitudes in the atmosphere compared to their kerosene combustion counterparts, due to higher water vapor emissions. Unlike conventional engines, which emit soot and volatile particles during kerosene combustion as nuclei for the ice crystals of contrails, the exhaust of a hydrogen combustion engine is ideally free of particle emissions. If the engine operates without impurities—for instance, from oil droplets—theoretically, the aerosol particles in the surrounding atmosphere serve as nucleation seeds for the ice crystals in the aircraft's exhaust plume. Model simulations suggest that due to the low concentration of ambient aerosol, potentially fewer and larger ice crystals form in the wake of the hydrogen engine. This can minimize the contrail's lifespan and its warming effect. However, a validation of these models has not yet been conducted. Comprehensive analysis of the flight tests needs to be awaited.

Modified Measurement Technology

The instruments integrated on the Egrett were supplied by the DLR Institute of Atmospheric Physics. These were used for ice crystal, aerosol, and trace gas measurements. All instruments were modified and adapted to the Egrett's cabin. Additional modifications to the aircraft included a fuselage extension to enable the integration of the nitrogen oxide measuring device "Sioux." Scattered light spectrometers and specific trace gas inlets were mounted on the landing gear. The researchers operated the instruments autonomously, collected the data via an Iridium downlink on the ground, and guided the Egrett pilot via radio to the optimal measurement positions in the contrail.

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Date: 08.12.2025

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With the fully evaluated data from Project Blue Condor, the partners are confident that it will be possible in the future to explore the climate impact of the contrails of a potentially hydrogen-powered fleet using global models. (se)