The flight altitude determines the climate effect of hypersonic aircraft

News – 15 December 2022

Researchers from the German Aerospace Center DLR, TU Delft and Université Paris-Saclay have found that the climate effect of hypersonic aircraft flying on liquid hydrogen at 25 and 35 km altitude is at least 10 to 20 times worse than a regular subsonic plane. at an altitude of approximately 10 km on the same routes and with the same number of passengers. This is due to the accumulation of water vapor in the stratosphere. Water vapor is an important non-CO2 greenhouse gas emitted during the combustion of liquid hydrogen in an internal combustion engine. The research shows that the higher in the atmosphere you fly on liquid hydrogen, the greater the climate effect the water vapor has. The article appeared in the journal Atmospheric Chemistry and Physics and was selected as a highlight of the journal. It is available with open access: Pletzer, J., Hauglustaine, D., Cohen, Y., Jöckel, P., and Grewe, V., The climate impact of hydrogen-powered hypersonic transport, Atmos. Chem. Phys. 22, 14323-14354, 2022.

(Image: Getty. Roberto Machado Noa)

Hypersonic flight

For the aviation industry, it is – still – an interesting ‘business case’ to fly to the other side of the world within a few hours. Several aircraft manufacturers and research organizations are working on innovative new supersonic and hypersonic ‘Concordes’ that fly faster than the speed of sound. As the industry is also working hard to make aviation more sustainable, the question arises: what is the climate effect of these high-speed planes, even when they fly on liquid hydrogen, a promising sustainable jet fuel?

Researchers from the German Aerospace Center DLR, the Faculty of Aerospace Technology at Delft University of Technology and the Université Paris-Saclay investigated the climate effect of two different hypersonic aircraft designs, ZEHST and LAPCAT. Both concepts use – sustainably produced – liquid hydrogen as fuel. The plane’s speed is Mach 5 (~6000 km/h) or Mach 8 (~9500 km/h) and the cruising altitude is 25 or 35 km, i.e. in the stratosphere, far above the daily weather, while at this height approx. 18 respectively 21 Tg of water vapor is emitted per year (1 Tg = 1 Megaton).

Distribution of water vapor emissions due to combustion of liquid hydrogen fuel for potential long-range flight at hypersonic speeds (from Pletzer et al., 2022, CC-BY 4.0)

The harmful effects of water vapor

The researchers used two different climate-chemical models to examine the effect of these water vapor emissions on the atmosphere. This shows that the dry stratosphere becomes significantly more humid due to the emissions from these aircraft and that the accumulation of water vapor is significantly greater at higher altitudes. Volker Grewe, professor of climate effects of aviation at Delft University of Technology and senior researcher at DLR, is one of the authors: “These results are astonishing. At these altitudes, water vapor is subject to rapid photochemical depletion and the accumulation should be low. The models confirm this depletion, but the recombination of the loss products leads back to water vapor, and at the same time an increase in methane oxidation is observed, which also leads to the production of water vapor. These chemical processes overcompensate for the original photochemical water vapor depletion at these high altitudes. research has shown that the effect on the climate is at least 10 to 20 times greater than that of a representative subsonic aircraft.”

Non-CO2– climate effects

The research clearly shows that flying on the potentially sustainable liquid hydrogen will not make hypersonic aircraft in the stratosphere carbon neutral. This is due to non-CO2– climate effect of water vapor accumulations. The flight altitude is a decisive factor for water vapor accumulation: the higher you fly on liquid hydrogen, the greater the negative climate effect. The research was carried out within the framework of the EU projects STRATOFLY and MORE&LESS.

More information

Disclosure

The research paper is available with open access: Pletzer, J., Hauglustaine, D., Cohen, Y., Jöckel, P., and Grewe, V., The climate impact of hydrogen-powered hypersonic transport, Atmos. Chem. Phys. 22, 14323-14354, 2022.

EU programs

The research was carried out within the framework of the EU projects STRATOFLY and MORE&LESS.

Contact

Prof. Dr. Volker Grewe, Professor Climate Effects of Aviation, TU Delft and Senior Researcher at the German Aerospace Center DLR: volkser.grewe@dlr.de

Press officer Ineke Boneschansker, +31 (0) 15 278 5361, i.boneschansker@tudelft.nl.

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