The CFM RISE open-fan engine and the exposed turbofan that wants to cut jet fuel burn by twenty percent

CFM International is building an open-fan jet engine that eliminates the traditional nacelle, exposing massive fan blades to open air in pursuit of a 20% fuel efficiency gain over the current LEAP engine. Called RISE (Revolutionary Innovation for Sustainable Engines), the program represents the most radical change in commercial jet propulsion since the high-bypass turbofan replaced the turbojet. Ground testing began in late 2024, and entry into service is projected for the late 2030s on next-generation single-aisle airliners.

What Is the CFM RISE Open-Fan Engine?

CFM International — the joint venture between GE Aerospace and Safran, operating since 1974 — announced the RISE program in June 2021. The company behind the CFM56 and LEAP engine families, with more than 40,000 engines delivered, set a target that turned heads: 20% better fuel efficiency than the LEAP, the engine they had just spent a billion dollars developing.

That improvement comes from a dramatic increase in bypass ratio. In a conventional turbofan, bypass ratio measures the air flowing around the core versus through it. Higher bypass means more thrust from moving a larger mass of air more slowly, which is fundamentally more efficient. The LEAP achieves about 11:1 bypass. The RISE open fan targets 50:1 to 70:1.

The problem is that moving that much air requires a fan so large it can no longer fit inside a nacelle. At a certain diameter, the nacelle’s drag and weight start canceling out the efficiency gains. CFM’s solution: remove the nacelle entirely and let the fan blades spin in open air.

Why Did Open-Fan Engines Fail Before?

This concept is not new. GE flew an open-rotor demonstrator called the GE36 in the 1980s. The efficiency numbers were impressive, but the noise from exposed contra-rotating blades was brutal. Airlines rejected it, and the program was shelved.

Three advances have changed the equation over the past 40 years:

Blade design. Modern computational fluid dynamics enables fan blade geometries that were impossible to model in the 1980s. CFM is using carbon fiber composite blades with variable pitch, allowing the blade angle to adjust across climb, cruise, and descent for optimized noise and performance at each phase.

Materials science. The engine’s hot section now uses ceramic matrix composites instead of traditional nickel alloys. These materials are lighter, tolerate higher temperatures (exceeding 2,300 degrees Fahrenheit), and allow a smaller, more efficient core that produces the same shaft horsepower with less fuel.

Acoustic engineering. CFM and Safran claim their noise reduction technologies can bring the open-fan noise signature down to the same level as a current ducted turbofan. If validated, this eliminates the single objection that killed the concept a generation ago.

Where Does Testing Stand?

GE Aerospace began testing the RISE open-fan demonstrator at its Peebles, Ohio facility in late 2024, mounting the engine on a modified 747 test bed. According to GE, early results indicate the efficiency targets are being met. Full noise data has not been released publicly — a sign that either the numbers are still being refined or not yet where GE wants them.

CFM has committed over $2 billion to the RISE program, split equally between GE and Safran. That level of investment signals a production-intent program, not a research exercise.

How Will Open-Fan Engines Change Aircraft Design?

The RISE engine is designed for the next generation of single-aisle airliners — the eventual replacements for the 737 and A320 families, expected to enter service around 2038 to 2040.

The open-fan configuration forces significant airframe changes. The engine will likely mount farther back on the wing or at the rear fuselage to provide blade clearance for safety and noise reasons. Wing structure, landing gear placement, and load paths all change accordingly.

Ground operations present additional challenges. Unlike current turbofans where the fan sits recessed inside a nacelle, an open fan demands different ramp procedures, safety markings, and potentially redesigned ground equipment. For airlines running eight legs a day per aircraft, anything adding time to a gate turn has real cost implications.

What Are the Realistic Caveats?

The 20% fuel savings is a target, not a guarantee. The gap between a demonstrator on a test stand and a certified production engine at FL350 in minus-50-degree air is enormous. Certification always involves compromises.

The noise question remains open. Some acoustics engineers are cautiously optimistic; others consider meeting current Chapter 14 noise standards at takeoff and landing the single hardest problem in the program. Simulation results look promising, but simulation is not a residential neighborhood at the end of a runway.

The timeline stretches past a decade. Sustainable aviation fuel could become cheap enough to reduce the urgency. Airbus’s parallel hydrogen propulsion program could leapfrog the concept entirely. Or the open fan could prove out and become the industry standard for 50 years.

Why Open Fan Matters More Than Alternative Fuels — For Now

RISE represents the aviation industry’s most serious bet on evolutionary physics rather than revolutionary chemistry — improving how efficiently you burn fuel rather than changing the fuel itself. The open-fan concept carries a 40-year head start in basic research and relies on an existing supply chain. It does not require building hydrogen infrastructure at every airport on the planet.

The efficiency arc across seven decades tells the story. The original turbojet achieved roughly 35% thermal efficiency. Early turbofans pushed past 40%. The LEAP reaches about 50%. If RISE hits its targets, overall propulsive efficiency approaches 60% — evidence that the turbine engine has at least one more major generational leap ahead.

Key Takeaways

CFM’s RISE open-fan engine targets a 20% fuel efficiency gain over the LEAP by increasing bypass ratio to 50:1–70:1 and removing the nacelle entirely
Three technology breakthroughs — composite variable-pitch blades, ceramic matrix composites, and advanced acoustics — address the noise and performance problems that killed the 1980s GE36 open-rotor concept
Ground testing began in late 2024 at GE’s Peebles, Ohio facility; early efficiency results are reportedly on target, though noise data has not been publicly released
Entry into service is projected for the late 2030s, likely on next-generation 737/A320 replacements, with airframe designs that will differ significantly from current configurations
CFM has invested over $2 billion, signaling production intent — but certification compromises, noise challenges, and competing technologies like SAF and hydrogen mean the outcome is far from certain