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■ In its search for jet fuel replacements, Safran is also investigating the direct use of hydrogen in engines. This is a particularly attractive solution because the only theoretical product of hydrogen combustion is water. However, this would still require a major leap in technology.
■ Usinghydrogeninfuelcells,asinvehicles,isanother beneficial path, but for the moment this solution is limited to small aircraft: a system generating the 10 to 20 MW needed to power an A320-class airplane would still be much too heavy. So for this type of application, we would have to focus on using hydrogen as a fuel for a gas turbine. But, once again, this entails tremendous challenges in terms of the aircraft design and the entire fuel system, since it requires the storage of liquid hydrogen, meaning very low temperatures (-253°C, as for launch vehicles using this cryogenic fuel). At the same time, there are specific restrictions related to the use of hydrogen (volume and volatility), and a complete supply chain would have to be developed to meet these special conditions, also using green energy sources to provide the hydrogen. Furthermore, hydrogen combustion raises further questions in terms of operability, mechanical resistance and limiting NOx releases.
■ These concepts are currently being studied by the French Civil Aviation Research Council, CORAC. Through Europe’s Clean Aviation research program. Safran is supporting its customers and actively working on these concepts based on a gradual investment policy keyed to the maturation of the enabling technologies. They would likely first be applied to regional aircraft, then subsequently to larger, longer- range aircraft in the single-aisle jet class. Safran has already initiated several joint research programs in this area: with Airbus and ArianeGroup to study hydrogen distribution methods and its use for propulsion (HyPERION project, through the French aviation support plan); with Airbus and the French aerospace research agency ONERA on the availability of “green” hydrogen; and also on the impact of the specific contrails produced by these new fuels, since burning hydrogen naturally produces more water vapor than jet fuel. In addition, Safran is conducting research into high-power-density fuel cells, which offer propulsion potential for smaller aircraft (commuters, and eventually small regional aircraft).
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At the end of the 1980s, the Soviet Union developed the Tupolev Tu-155, a derivative of the Tu-154B. This experimental jetliner was powered by three jet engines, one designed
to operate with either liquid hydrogen or liquefied natural gas (LNG). However, the test program was abandoned after the fall of the USSR.

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