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27 July 2022

Rolls Royce UltraFan plus SAF are stepping stones to hydrogen

By Bogdan Avramuta

Rolls Royce told us this week it is in the final build stages for the largest aero-engine technology demonstrator, a native SAF engine that will be just about ready by which time it will need to be replaced with hydrogen.

UltraFan will have ceramic composite components and a novel architectures and a better combustion system, and will squeeze 25% more effort out of SAF that existing engines. But is it worth developing at all?

A new fan system, claiming the most powerful gearbox in aerospace will result in a higher bypass ratio (the key measure of how combustion efficient works in a fan engine) and in turn will generate 40% less NOx with 35% less noise all made to work off on SAF.

The engine will provide a test bed for any technologies Rolls Royce plans to integrate in multiple designs of different scales. It will have a thrust range from 20,000lbs to 100,000lbs, and so can test both narrow and widebody passenger engines. In the short-term future, current generation engines can make use of the UltraFan’s improved fuel efficiency by being retrofitted with its features once they are tested in a £90 million testing facility in Derby, UK, later this year.

But the question of ultimately whether it is worth bothering to develop SAF specific engines is because the UltraFan will only be commercially available in the 2030s. By that time could hydrogen be in full swing? In our last issue we noted that Airbus is building zero-emission hydrogen passenger aircraft to go into service by 2035. Innovative start-ups like ZeroAvia are already making progress in the short haul market with a multitude of orders for hydrogen power trains. Boeing turns out to have been working on a hydrogen cryotank since 2015.

The direction of the industry is clear then. Hydrogen is the thing that everybody is putting their money on, yet the next generation of gas turbine engines is being designed right now to run on SAF in the next decade. And not only by Rolls Royce. Airbus is working with CFM International, a 50/50 joint venture between GE Aviation and Safran Aircraft Engines, on an open fan engine that will be tested in the late 2020s. The open fan design hasn’t been a popular choice in the past because of its much higher noise level and increased risk of fuselage damage due to the blades not being contained in a typical nacelle, but has been brought back in the spotlight just in time for SAF. This new design will reduce in-flight emissions by around 20%.

We would suggest that the industry is not ready to commit to a straight jump to hydrogen or even hybrid electric systems, but instead will rest on SAF for the best part of the 2030s. It will then slowly phase in those technologies after they have proven their worth on smaller short haul aircraft. This decision is about keeping options open and because hydrogen needs time to mature as a usable technology at scale. As an alternative, the market leaders are trying to identify the technology that will allow them to cut down emissions further if hydrogen is not market ready come 2035.