Safran advances open-fan engine tests

An effort to bring the first open-fan engine to the skies by 2035 took a step forward last month, with the start of a Safran-led research project.

The TAKE OFF project, short for Technology And Knowledge for European Open Fan Flight, brings together more than two dozen partners, including GE Aerospace, GKN Aerospace and Airbus. This year’s testing is to focus on verification of the design and flight test instrumentation, in preparation for a full-scale ground test of the front module in early 2027. If all goes well, that could set up the first flight demonstration of an engine prototype by the end of the decade.

Open-fan designs — also referred to as open rotor — have long been attractive to engine makers for their potential to reduce fuel usage. Because the blades are not contained by a nacelle, more air can flow through the turbine compared to today’s designs. However, there are numerous technical and regulatory challenges, from noise to safety and certification, to be addressed before the 2035 target.

“It’s a high-risk, high-reward kind of project,” said Pierre Durel, a project officer at Clean Aviation, the European Union research venture established in 2021 to help advance sustainable aviation technologies. The program last month awarded 100 million euros ($116.9 million) to TAKE OFF to move the project through to flight demonstration.

He added: “Without putting aside the challenges of its integration, in terms of propulsion efficiency it’s definitely one of the biggest opportunities.’’

Test objectives

This particular open-rotor venture dates back to 2021, when Safran and GE Aerospace announced their CFM RISE program, short for Revolutionary Innovation for Sustainable Engines. The goal is to design an engine — which would be produced by their joint engine manufacturer, CFM International — that is over 20% more fuel efficient than the current crop of engines, as well as test technologies for compatibility with alternative fuels, such as sustainable aviation fuels made from food and agricultural waste.

Under the previous testing phase, which began in 2023, engineers developed the unducted fan, high-speed booster, compact gearbox, lightweight structures and other key components for the engine, said Delphine Dijoud, deputy vice president for engineering and research and testing at Safran Aircraft Engines. So far, she said, each individual component has beaten the 20% target.

With TAKE OFF, Safran now aims to test the engine tech in conditions closer to its actual operating environment — and ultimately take to the skies for a flight test. The company is constructing a new facility to conduct a full-scale test of the open fan module, slated for next spring.

Assuming next year’s test goes well, the team will then begin preparing for ground tests and assembly in 2028 before the flight test in 2029 aboard an Airbus A380. While that test aircraft won’t amount to a perfect representation of the eventual flight conditions, Dijoud said, she expressed confidence those flights will inform the final engine design.

“When you try, you always learn — so we will learn on the aeromechanical aspects, on altitudes, on the system for example,” she said. “We will learn how to demonstrate that we are flight-worthy, and it will also feed all the design requirements for the mature product.”

After that, the fate of the design will be up to Airbus, which is assessing with CFM whether it wants to utilize an open-rotor engine for its next single-aisle jet, targeted to enter service in the second half of the next decade. This would involve yet more flight testing in conditions from taxiing to take-off to descent and landing, as well as tests on the aircraft for which it is being designed.

Should Airbus choose the open rotor, another key question is whether that will be the only option for airlines, or if they will be able to order a version of the aircraft equipped with a conventional turbine.

Historically, Airbus has preferred to offer multiple engine options, but this might not be possible with an open rotor, said Rob Stallard, a London-based aerospace analyst at Vertical Research Partners. The challenge of integrating the engine into the airframe could require designing it in a way that wouldn’t allow technicians to simply swap out the engine for a conventional alternative.

Pushing the limit

Despite all these challenges, the potential benefits of open rotor could be worth the effort, experts say.

The current crop of traditional engines is reaching the limit of efficiency gains. The bypass ratio, or the amount of air passing through the fan versus through the core of the engine, has gone from single digits to around 10:1 to 12:1 for the current crop of engines. Next-generation designs are targeting 15:1, but there’s a general industry consensus that’s as high as one can go before the engine gets so large that it negates any efficiency benefits.

Enter open fan. CFM is targeting a bypass ratio of 60:1 for RISE, significantly reducing fuel burn. That’s a particularly attractive proposition for airlines, given their need to cut emissions to meet environmental targets and with fuel prices spiraling amid the conflict in the Middle East.

Still, the hurdles to getting such an engine into service are significant, according to Vertical’s Stallard.

“It makes a lot of sense, but the problem they’ve consistently come back to is not just an open rotor on its own, it’s attached to an aircraft,” he said. “What does this do to the rest of the aircraft, in terms of the wing, the fuselage and, of course, the safety certification?”

The Safran-led team working on TAKE OFF aims to address those challenges and is tackling new areas every week, said Dijoud.

The week we spoke, engineers had just started icing tests for the fan blades. They have already carried out high-speed wind tunnel tests to look at performance and acoustics and are confident of complying with noise limits, an issue due to the vibration that emits from the large open blades. They have also “tortured” prototypes to make sure they are robust and reliable, another key concern for engine performance, she said.

“It’s a long road, and the ambition is high,” she said.

Technology advances

Recent breakthroughs in supercomputing give Safran confidence that an open fan can finally be done, Dijoud said. Higher-fidelity modeling will help ensure the design meets certification requirements, from noise to safety, she said.

“We have advanced simulation, and honestly, all those tools we didn’t have a decade ago, even when we started even a few years ago,’’ she said. “We combine that with the largest test database that we can imagine, because we have now more than 400 hours of wind tunnel tests. So with all of that on our side we can now design quieter blades, and we also understand the installation effects at the aircraft level, and for sure Airbus does as well.”

She noted the team is working closely with Airbus on the issue of aircraft integration. Airbus told me last year it was assessing putting the engines underneath the wing, which would be gulled to accommodate them, and shielding the area of the fuselage closest to the engines to minimize the risk if a blade comes off an open rotor during flight.

When I reached out for an update, Airbus spokesman Justin Dubon described the design as “promising” and said Airbus is making good progress on testing the technology in collaboration with its partners.

“The next-generation engines are still in a maturing phase and we are assessing different options for a safe, open rotor architecture aircraft integration,” he said. “This will be followed by an extensive testing and certification process to ensure that the highest safety standards are met and ensuring reliable and secure operations.’’

Until that decision is made, the work continues, Dijoud said. “We are getting ready, and we will be ready whatever the decision of the airframers is.”