Direct hydrogen combustion in a jet turbine

There are two basic aircraft designs that use hydrogen as an energy carrier; hydrogen can either be used to operate a fuel cell to produce electricity that drives electric motors or used as combustion fuel to power a jet engine. A third "hybrid" design incorporates fuel cells to drive electric motors and combusts hydrogen directly in a jet engine. None of these options produces direct CO2 emissions during the operation of the aircraft (although the upstream emissions depend on the technologies used to produce and transport the hydrogen). Combustion of hydrogen produces NOx (as a consequence of interactions with nitrogen in the atmosphere) and water vapour, both of which can contribute to global warming.
The low volumetric energy density of pressurised hydrogen, about one-eighth that of jet kerosene even at pressures of around 700 bar, means that cryogenic hydrogen (hydrogen cooled to below -250°C such that it becomes a liquid and increases its energy density to around one quarter that of kerosene) is likely more practical for longer-distance flights and is preferred by existing programmes. This calls for new aircraft design with fuel tanks potentially located in the fuselage rather than in wings to meet flying range requirements, which would reduce the space available for passengers.
Aircraft designs that use hydrogen as a direct combustion fuel are currently not being tested in aircraft operations, although tests of jet engines that combust hydrogen have been conducted, and Airbus is targeting a test of hydrogen combustion in its aircraft in the near future.
Hydrogen aircraft are at an early development stage and commercial application in small regional jets is only expected in the 2030s, at the earliest. An early application of hydrogen in aircraft will be in auxiliary power units (APU) for non-propulsion applications such as lighting, HVAC, or cabin pressurisation.