World-first in-flight measurements of contrails from hydrogen propulsion
The world's first in-flight measurements of contrails from a hydrogen-powered turbojet engine have been conducted.
14 Apr, 2025. 4 minutes read
Released at nine kilometres altitude. Image from the tail camera of the Egrett as it released the Blue Condor Arcus glider from its tow. The Blue Condor, with a hydrogen engine and an optically very thin contrail, is at the front. Credit: AV Experts LLC
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www.dlr.deFormed in the upper troposphere when the air is sufficiently cold and humid, condensation trails (known as contrails) may be one of the most significant contributors to aviation's climate impact. Future technologies such as hydrogen-powered aircraft promise not only a reduction in carbon dioxide (CO2) emissions, but also a reduction in the climate impact of these human-made clouds. Until now, however, no in-flight measurements have been carried out to characterise the formation, properties and effects of contrails from direct hydrogen combustion. In the Blue Condor project, Airbus, the Perlan team and the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have, for the first time worldwide, measured contrails of a hydrogen turbojet engine in flight. The three-week flight test campaign took place in December 2024 in Minden, Nevada (USA), and used a chase plane equipped by DLR with autonomously operated instruments for measuring contrails and emissions.
"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," says 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."
Comparing hydrogen and kerosene-fuelled engines
The centrepiece of the Blue Condor mission is an Arcus glider. This aircraft was modified by the Perlan team to accommodate a gaseous hydrogen tank, a lubrication system and a hydrogen-powered turbojet engine. The engine was developed by AeroDesignWorks in collaboration with RWTH Aachen University. The Blue Condor aircraft was flown by Jim Payne, a Perlan team pilot who holds the world altitude and speed records for gliding. A second Arcus aircraft operated by the Perlan team was equipped with a conventional kerosene engine. Both gliders were towed simultaneously by a Grob Egrett high-altitude research aircraft operated by AV Experts LLC to an altitude of more than nine kilometres – high enough to enter a region where contrail formation was predicted. Both gliders were then released and the tow chase plane with the measuring instruments manoeuvred behind them.
The Blue Condor glider ignited its hydrogen engine. The Egrett then began its chase flight, entering the exhaust plume to measure the glider’s emissions and contrail properties. The kerosene-powered glider remained in close formation with its engine running. To ensure comparability of the emission data, the respective chase formations were carried out one after the other in quick succession, under the same meteorological conditions. The actual measurement phase lasted around five to ten minutes for each engine.
Of the seven test flights conducted, four resulted in the formation of contrails from the hydrogen engine. The aim of the investigations was to measure the microphysical properties of the contrails resulting from direct hydrogen combustion under real atmospheric conditions. The initial number and size of ice crystals that form in the exhaust plume play an important role in the climate impact of contrails and the contrail cirrus that evolve from them. The research team also conducted emission measurements of nitrogen oxides and possible aerosol particle formation at lower altitudes. Engine test runs on the ground provided additional information on the emissions of the hydrogen engine at various power settings. The data is currently being evaluated in detail and will be published in a scientific paper.
Contrails from direct hydrogen combustion form differently
Contrails from hydrogen combustion engines form at higher temperatures and lower altitudes than classical contrails from kerosene combustion, due to higher water vapour emissions. Unlike conventional engines, where kerosene combustion forms soot and volatile particles that act as nucleation sites for ice crystal formation, the exhaust gas from a hydrogen combustion engine is, ideally, free of particulate emissions. If the engine is operated without contaminants such as lubricating oil droplets, the aerosol particles present in the atmosphere theoretically serve as nuclei for ice crystals in the aircraft's exhaust plume. Model simulations show that the low concentration of ambient aerosols potentially leads to fewer and larger ice crystals in the wake of the hydrogen engine, which can reduce the lifetime of the contrail and its warming effect. Validation of these models will be possible following a comprehensive evaluation of the flight test results.
DLR instruments on board
The instruments integrated into the Egrett were provided by the DLR Institute of Atmospheric Physics and included a series of ice crystal, aerosol and trace gas sensors to detect trace gases such as carbon dioxide, nitrogen oxides and water vapour. All instruments were modified and adapted to the unpressurised cabin of the Egrett. "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," explains Tina Jurkat-Witschas, project lead at the Institute of Atmospheric Physics. Other modifications made to the aircraft by AV Experts included the extension of the fuselage to integrate the SIOUX nitrogen oxide sensor, while light scattering spectrometers and dedicated trace gas inlets were attached to the landing gear. The DLR researchers operated the instruments autonomously, transmitted the data via Iridium satellite downlink and guided the Egrett's pilot by radio to the optimum measurement positions in the contrail.
Evaluation of the data from the Blue Condor project will pave the way for global modelling of the climate impact of contrails from a fleet of hydrogen-powered aircraft. In the meantime, the Blue Condor project was a finalist for the prestigious Collier Trophy of the National Aeronautic Association (NAA).