The CFM RISE open-fan engine and the twenty percent fuel burn revolution coming to airliners by twenty thirty-five

CFM International’s RISE (Revolutionary Innovation for Sustainable Engines) program aims to deliver a 20% fuel burn improvement over the current LEAP engine by removing the traditional nacelle and adopting an open-fan architecture. A joint effort between GE Aerospace and Safran, the program targets entry into service around 2035 and full compatibility with 100% sustainable aviation fuel. If successful, it would represent the largest single leap in commercial engine efficiency since the high-bypass turbofan.

What Is an Open-Fan Engine?

An open-fan engine—sometimes called an unducted fan or ultra-high-bypass engine—removes the enclosed duct and nacelle surrounding the fan blades. Instead, large, sweeping carbon fiber composite blades spin in open air, moving a massive volume of air at lower velocity.

This is not a turboprop. It is a fundamentally different architecture that pushes the high-bypass turbofan concept to its logical extreme. Without a nacelle, the engine achieves an effectively infinite bypass ratio, which is thermodynamically more efficient for producing thrust.

Why Remove the Nacelle?

The nacelle on a conventional turbofan does real aerodynamic work: managing airflow, containing bypass air, and providing acoustic treatment. But it also adds significant weight and drag. As engines have grown larger to increase bypass ratio—the GE9X on the Boeing 777X has a fan diameter of 134 inches—nacelles have become enormous. At a certain point, making the nacelle bigger to move more air produces diminishing returns because the nacelle itself creates too much drag.

The open-fan design sidesteps this tradeoff entirely.

How Does CFM’s Design Work?

The RISE engine uses two rows of counter-rotating fan blades. A single row would produce significant swirl in the exhaust, wasting energy. The second row, spinning in the opposite direction, recovers that swirl energy and straightens the airflow, delivering much higher propulsive efficiency.

The blades are built from third-generation carbon fiber composites—lighter, stronger, and shaped with a complex three-dimensional twist optimized by computational fluid dynamics models that simply did not exist when previous engine generations were designed.

Didn’t GE Try This in the 1980s?

Yes. GE tested the GE36 Unducted Fan on a Boeing 727 testbed in 1986. It worked. It was efficient. Airlines rejected it because it was too loud.

An open fan without a nacelle has no acoustic liner to absorb sound. The 1980s version was unacceptably noisy on the ramp, on approach, and in surrounding communities. In an era of tightening noise regulations, the concept was shelved.

What Has Changed Since Then?

Three critical advances make the open-fan concept viable today:

Blade design. Modern computational tools allow engineers to optimize blade geometry—shape, sweep, spacing, tip speed—to push noise into frequency ranges that attenuate faster in atmosphere. CFM claims the RISE engine can meet current Chapter 14 noise standards, the same standards today’s quietest turbofans meet.

Materials. Carbon fiber composites absorb and dampen vibration differently than the metal blades of the 1980s. They can be made thinner and lighter, with internal structures engineered to reduce specific acoustic modes.

Counter-rotating aeroacoustics. The interaction between wake patterns from the first and second blade rows creates interference patterns. With careful blade count and spacing, destructive interference cancels certain noise frequencies. GE and Safran have been running advanced acoustic modeling on this for years.

CFM conducted a full-scale open-fan ground test in 2023 at GE’s facility in Peebles, Ohio, using a complete engine demonstrator. Initial acoustic results reportedly met internal targets. Ground testing differs from flight and certification testing, but the trajectory is encouraging.

What Are the Major Engineering Challenges?

Blade containment is the most critical. On a conventional turbofan, if a fan blade separates, the nacelle catches it. On an open-fan engine, nothing stands between those spinning blades and the fuselage, wing, fuel tanks, and hydraulic lines.

CFM is pursuing two approaches. First, composite blades tend to delaminate and shed material in smaller, lower-energy pieces rather than fracturing into rigid chunks like metal blades. Second, a partial shield—not a full nacelle but a structural ring or fuselage-side armor—could handle worst-case scenarios.

Airframe integration poses another challenge. Under-wing mounting places large spinning blades close to the fuselage. Aft-fuselage mounting avoids that proximity but changes the aircraft’s center of gravity and structural loads. Airbus has studied both configurations. Boeing has been less public about its plans, but both airframers are engaged because the aircraft this engine will power—replacements for the A320neo and 737 MAX families—does not yet exist.

What Does 20% Fuel Savings Actually Mean?

The current LEAP-1A on the A320neo burns roughly 2,400 pounds of fuel per hour at cruise per engine. A 20% reduction drops that to approximately 1,900 pounds per hour.

On a typical three-hour domestic flight with two engines, that saves about 3,000 pounds of fuel—roughly $1,500 per flight at current prices. An airline operating that aircraft six times daily saves approximately $3.3 million per year per aircraft. For a fleet of 200 aircraft, annual savings reach $600 million.

That math is why this program commands attention despite being a decade from service. A 10% improvement might justify incremental upgrades. At 20%, the case for an entirely new airframe becomes overwhelming.

What About Sustainability?

CFM has committed to making RISE compatible with 100% sustainable aviation fuel from day one. Current engines are certified for up to 50% SAF blends. Running pure SAF in an optimized open-fan engine could reduce lifecycle carbon emissions by more than 80% compared to a 2010-baseline engine on conventional jet fuel—approaching what the aviation industry has pledged for its 2050 net-zero targets.

There is also a longer-term hydrogen combustion possibility. Safran has been investing in hydrogen technology, and the RISE architecture’s modular gas generator core could theoretically be adapted to burn hydrogen. That is a 2045–2050 proposition at earliest, but the architecture does not preclude it.

What Are the Realistic Caveats?

Timeline risk. Entry into service in 2035 requires an aircraft program launch around 2028 or 2029—only two to three years away. As of now, CFM has completed ground testing but has not begun flight testing, which is expected in the 2026–2027 timeframe. The schedule is tight.

Performance uncertainty. The 20% figure is a design target, not a certified result. Installation losses, aerodynamic penalties from mounting on an actual airframe, and operational compromises may reduce the final number to 17–18%. Still excellent, but not guaranteed at 20.

Maintenance differences. Exposed fan blades face foreign object damage, bird strikes, hail, and ice without nacelle protection. The maintenance philosophy will be fundamentally different from what airlines operate today.

Passenger perception. Open-fan engines look different, sound different, and may produce different cabin vibration characteristics. History suggests passengers adapt quickly to technologies that reduce operating costs—those savings eventually flow into ticket prices—but the transition will require deliberate communication.

Who Else Is Competing?

Pratt & Whitney (RTX) is developing a next-generation engine that may include a geared open-fan configuration, combining their PW1000G geared turbofan expertise with open-fan aerodynamics. Rolls-Royce is pursuing the UltraFan, a geared turbofan with a bypass ratio of approximately 15:1 inside an enclosed nacelle—a more evolutionary approach that pushes conventional architecture to its limits.

The market will likely support multiple solutions. But if CFM hits its targets, the open-fan approach offers the most headroom for improvement because it represents the most radical departure from current design constraints.

Why This Matters for Pilots and the Industry

The RISE program represents a genuine inflection point. For sixty years, commercial propulsion has been variations on the same architecture: a turbine core driving an enclosed fan. Open fan breaks that paradigm. If the engineering delivers on its promise, every airliner entering service in 2040 will look fundamentally different from anything flying today. The engine decision will drive airframe design, airline economics, airport infrastructure, and emissions trajectories for decades.

Key Takeaways

• CFM’s RISE open-fan engine targets a 20% fuel burn reduction over the LEAP, with entry into service around 2035 on next-generation single-aisle aircraft
• Counter-rotating carbon fiber blades and modern acoustic engineering aim to solve the noise problems that killed GE’s 1980s open-fan attempt
• Fleet-wide fuel savings could reach $600 million annually for a large operator, making the business case for new airframes compelling
• Full SAF compatibility and potential hydrogen adaptability position the architecture for long-term decarbonization goals
• Significant risks remain in timeline, blade containment certification, airframe integration, and the gap between design targets and real-world performance