The CFM RISE Open Fan Engine, the Unducted Rotor That Could Cut Airliner Fuel Burn by More Than Twenty Percent, and What GE Aerospace and Safran Are Building for the Post-LEAP Generation

CFM International is developing an open fan propulsion architecture under its RISE program - short for Revolutionary Innovation for Sustainable Engines - targeting more than 20% lower fuel consumption compared to the current LEAP engine. Announced in 2021, the program targets technology readiness for a mid-2030s service entry on next-generation single-aisle commercial aircraft.

What Is the CFM RISE Program?

CFM International is a 50/50 joint venture between GE Aerospace and Safran Aircraft Engines - the same partnership that produced the CFM56, which powered the original Boeing 737 and the Airbus A320 family for decades, and the LEAP engine, which currently powers the Boeing 737 MAX and A320neo family.

RISE is a research and technology program, not a product launch. Its goal is a new propulsion architecture for single-aisle commercial aircraft entering service in the mid-2030s, with a headline target of more than 20% reduction in fuel consumption versus the LEAP.

That figure is significant in context. In commercial propulsion, a two to three percent efficiency improvement typically warrants a press release. Twenty percent represents a step change in what the architecture can deliver.

What Is an Open Fan Engine?

The defining technology in RISE is the open fan - also called an open rotor or unducted fan. The concept removes the nacelle from a conventional high-bypass turbofan: the fan blades spin in open air rather than inside a housing.

The efficiency logic is straightforward. A turbofan’s fuel efficiency is closely tied to its bypass ratio - the amount of air flowing around the engine core versus through it. Higher bypass ratios produce greater efficiency. But a nacelle imposes a physical ceiling on fan diameter, and beyond a certain size, the duct itself becomes a drag penalty.

Removing the nacelle removes that constraint. An open fan can move enormous volumes of air at low velocity - exactly what thermodynamic efficiency demands - without the structural and aerodynamic penalties of an ever-larger duct.

Why Was Open Rotor Technology Abandoned in the 1980s?

The open rotor concept is not new. It emerged from the energy crisis of the 1970s, when fuel prices drove manufacturers to explore architectures that conventional turbofans couldn’t reach.

In the 1980s, GE flew a demonstrator called the GE36 on a modified Boeing 727. Pratt & Whitney and Allison flew their own unducted fan demonstrator on a McDonnell Douglas MD-80. Fuel burn results were exceptional by the standards of the era.

Then fuel prices fell. Airlines saw no business case for a new architecture when existing turbofans were cheap to operate. The programs stalled, and the concept went into the archives.

Why RISE Is Different From the 1980s Programs

The key differences are not primarily economic - they are engineering.

The 1980s open rotor blades were metallic or early composite structures with relatively simple geometry. The computational tools to simulate aerodynamic performance, vibration, and noise in detail didn’t exist. Engineers iterated with physical hardware because accurate simulation wasn’t feasible.

The RISE open fan uses advanced variable-pitch composite blades with aerodynamic geometries that couldn’t have been manufactured - or even fully modeled - forty years ago. Modern computational fluid dynamics allows thousands of blade geometries to be evaluated in simulation before any hardware is built. The materials are lighter, more precisely shaped, and structurally capable in ways early composites were not.

The result is a blade that can be optimized for efficiency and acoustic performance simultaneously - a tradeoff that proved intractable in the previous generation.

How CFM Plans to Address the Noise Problem

Noise was a significant concern with 1980s open rotor designs. Without a nacelle, fan noise propagates more directly into the environment, and cabin noise was a real issue in early testing.

CFM’s primary response is configuration and blade design. The RISE open fan uses a pusher configuration, with the fan stage positioned at the rear of the engine rather than the front. Combined with aerodynamic blade shaping, this arrangement is intended to meet ICAO Chapter 14 noise standards - currently the most stringent in force.

Acoustic testing has been a significant component of the ongoing ground test program. CFM has stated the program is on track to meet those standards.

The Hybrid-Electric Component Most Coverage Misses

RISE is not only an open fan program. The architecture includes a hybrid-electric boost system that applies electrical power during specific flight phases - particularly climb, where the core thermodynamic cycle operates least efficiently.

This matters for how the program is categorized. RISE is not an electrification play in the sense of replacing jet fuel. The goal is to burn significantly less fuel and to operate the combustion core at its optimal thermodynamic point more consistently throughout a flight profile.

The design is compatible with 100% sustainable aviation fuel (SAF) from the outset - not added as a retrofit consideration.

Real Engineering Challenges That Remain

Blade containment is the most fundamental unresolved challenge. On a conventional turbofan, the nacelle is a critical containment structure if a fan blade fails. An open fan has no such structure. The design philosophy shifts from containing a failed blade to ensuring a blade release causes no secondary damage to the aircraft. This requires a different approach to blade attachment, blade material, and the certification pathway for the entire system.

Foreign object ingestion is a related concern. A nacelle provides natural geometry that limits what debris or wildlife can reach the fan. An open fan does not. The certification requirements are more complex, and the solutions less established than for a ducted engine.

The efficiency claim also carries context worth noting. The LEAP is already one of the most efficient narrowbody engines flying. A 20%+ improvement over the LEAP is a meaningful number - but comparisons against older baseline engines can produce larger figures that sometimes appear in reporting without that context. The accurate framing is more than 20% over the current state of the art in its class.

None of these challenges are disqualifying. They are, however, the reason a program like this takes a decade to mature from concept to certification.

Current Testing and Timeline

Ground testing is active at Safran’s facility in Villaroche, France and GE Aerospace’s test facility in Peebles, Ohio. The open fan demonstrator has been running, with data collection ongoing in aerodynamics, thermodynamics, and acoustics.

Safran is leading development of the fan system, drawing on its composite blade manufacturing expertise. GE Aerospace is leading on the core and hybrid-electric integration. The partnership structure mirrors how the CFM56 and LEAP were developed.

Because the open fan architecture cannot be retrofitted to an existing narrowbody airframe, it requires a clean-sheet aircraft platform. Airbus has publicly discussed studying a next-generation narrowbody to succeed the A320neo family. Boeing will face the same decision. The RISE development cycle is calibrated to align with those airframe programs.

Why This Matters Beyond Commercial Aviation

Most general aviation pilots will not encounter this technology in their logbooks. RISE is aimed at single-aisle commercial transports, not light aircraft.

The indirect effects are real. The engineering knowledge generated by the program - computational blade design methods, composite manufacturing techniques, hybrid-electric integration - will flow into the broader propulsion ecosystem. The SAF compatibility work has implications for every aircraft category, not just narrowbodies.

More broadly, RISE represents the first serious, well-resourced attempt to revisit an open rotor architecture with tools that weren’t available the last time the question was seriously examined. The turbofan has dominated commercial aviation for sixty years because it was the right answer to the efficiency, noise, and thrust problem at the scale of commercial transport. RISE is asking whether a different architecture, built with modern materials and modern engineering tools, produces a better answer. The data so far suggests it might.

Key Takeaways

CFM RISE, announced in 2021, is a joint program between GE Aerospace and Safran targeting more than 20% fuel savings over the LEAP engine for next-generation single-aisle commercial aircraft.
The core technology is an open fan (unducted rotor) - a concept tested in the 1980s but limited by the materials and computational tools of that era; modern composite blade manufacturing and CFD make the current design viable in ways those programs were not.
The architecture includes a hybrid-electric boost system and 100% SAF compatibility as part of the base design - it is not a pure electrification program.
Key engineering challenges remain, particularly blade containment (no nacelle to catch a failed blade) and foreign object ingestion - these require new certification pathways, not just engineering solutions.
The program targets a mid-2030s service entry on a clean-sheet airframe; it cannot be retrofitted to current narrowbodies.