How to probe a hydrogen contrail
By Keith Button|March 2024
Airbus’ initiative to build hydrogen-powered passengers planes is supposed to help meet the air transportation sector’s goal of achieving carbon neutrality by 2050. But no one knows what effect contrails from hydrogen combustion might have on the climate compared to burning kerosene jet fuel. Studies have shown that the contrails left by the exhaust of conventional jets have a greater warming effect than carbon dioxide emissions.
Enter Blue Condor, an experiment by Airbus and DLR, the German Aerospace Center. Scientists later this year plan to measure the composition, including the size of the ice crystals, of the contrails left by hydrogen combustion. They could then compare any warming influence to that of conventional contrails. In flight trials scheduled to begin before the end of June at 30,000 feet over the Nevada desert, a turboprop will steer into the contrails created by one glider outfitted with a hydrogen combustion engine and another with a conventional jet engine burning kerosene. On each flight, “the snorkel,” a 2-meter-long tubular appendage atop the turboprop’s fuselage, will suck in air to measure trace gases, aerosols, nitrogen oxide and water vapor, while a torpedo-shaped spectrometer under its wing measures the size and number of ice crystals in the contrails.
“There are no contrail data available from hydrogen combustion, so this is really a first, and therefore it’s extremely exciting for us,” says Tina Jurkat-Witschas, head of the DLR team that will monitor the instruments from the ground and then analyze the contrails.
Flights of the hydrogen-powered glider began in November, but for the full campaign, the teams must wait for the right humidity and temperature for contrails to form — around minus 40 degrees Celsius.
The results could tell Airbus whether the climate-friendly aircraft it wants to have ready for customers by 2035 should be powered by hydrogen-combustion jet engines, turboprops powered by hydrogen fuel cells or a hybrid approach.
Airbus later plans to test hydrogen fuel cell and combustion engines by attaching them to the fuselage of a conventionally powered A380. This aircraft wasn’t right for the contrail experiment, though, because a pristine sky is needed. Modified Arcus-J gliders were chosen because one can be easily propelled by a single engine at contrail-forming altitudes, and because the turboprop can fly close behind without turbulence.
Most of aviation’s global warming effect comes from contrails, which consist of ice crystals that form mainly around soot from kerosene combustion. Hydrogen combustion doesn’t create soot, but scientists nevertheless expect that ice crystals would be formed by water vapor in the exhaust coalescing around dust and other naturally occurring aerosol particles. The question is how big and numerous these crystals would be compared to those that form around soot, though current models suggest that the crystals will be larger and fewer.
If the crystals are large enough, they will quickly drop out of the atmosphere, and their climate warming effect will be less than crystals from kerosene combustion. However, there’s a possibility that hydrogen combustion could form contrails at lower altitudes where kerosene combustion cannot, creating more overall contrail coverage. The Blue Condor results will help DLR researchers model whether this is the case. That analysis should be publicly available about six months after the tests, Jurkat-Witschas says.
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