The CFM RISE Open Fan Engine, the Counter-Rotating Blades GE Aerospace and Safran Are Building to Replace the LEAP, and the Propulsion Math Behind a Twenty Percent Fuel Burn Reduction

CFM International is developing an engine architecture that could deliver the largest single-generation fuel efficiency improvement in commercial aviation since the high-bypass turbofan was introduced in the 1960s. The RISE program - Revolutionary Innovation for Sustainable Engines - targets 20% better fuel efficiency than the current LEAP engine, using an exposed open fan with no surrounding nacelle and a bypass ratio approaching 60:1. Entry into service is targeted for the mid-2030s.

What Is the CFM RISE Program?

CFM International is a 50-50 joint venture between GE Aerospace and Safran Aircraft Engines. If you have been on a commercial narrowbody flight in the past three decades, you have almost certainly flown behind CFM hardware. The CFM56 is the most commercially successful jet engine in history by units delivered - over 33,000 engines built - powering classic Boeing 737 variants and the original A320 family. The LEAP replaced it on the 737 MAX, A320neo family, and COMAC C919.

CFM announced the RISE program in June 2021. The stated goal is an engine that burns 20% less fuel than the LEAP - not five percent, not ten. That target number is what commands the aviation industry’s full attention.

Why Is 20% Such a Significant Target?

The LEAP itself delivers roughly 15% better fuel efficiency than the CFM56 it replaced. Achieving another 20% improvement on top of that requires rethinking the fundamental architecture of the engine, not refining an existing one.

Aviation faces compounding pressure to reduce its emissions footprint. Sustainable aviation fuel, carbon offset programs, and fleet modernization all contribute to that goal. But all of those approaches work better when the underlying engines are burning less fuel to begin with. Efficiency gains and cleaner fuels are not competing strategies - they multiply each other.

How Does an Open Fan Engine Achieve Such High Efficiency?

The core physics of a jet engine comes down to one principle: move as much air as possible rearward while accelerating any individual parcel of that air as little as possible. The ratio of air that bypasses the core to air that passes through it is called the bypass ratio, and it is one of the most important numbers in propulsion engineering.

Early turbojets had bypass ratios near zero. The first high-bypass turbofans in the 1960s and 1970s - the GE CF6, the Pratt & Whitney JT9D - reached bypass ratios of 4 or 5:1, and commercial aviation became economically viable in a way it had never been before. The CFM56 runs at approximately 6:1. The LEAP runs around 9:1. Advanced widebody engines now reach 11 or 12:1.

The problem is the nacelle. The more air you want to move, the larger the fan must be. The larger the fan, the larger the nacelle that contains it - adding drag and weight. On narrowbody aircraft, ground clearance adds a hard physical constraint that widebodies do not face. The 737 MAX already has noticeably offset, slightly flattened engine nacelles to clear the ramp. You cannot keep growing the fan diameter without eventually running out of room under the wing.

The RISE solution is to remove the nacelle entirely. With no cowling, no inlet lip, and no surrounding duct, the fan can be dramatically larger. The target bypass ratio for RISE is approximately 60:1 - sixty times as much air bypasses the core as passes through it. That number explains the 20% fuel burn figure.

Didn’t the Industry Try This Before?

This concept has direct historical precedent, and that history matters.

In the 1980s, following two oil shocks, engineers at GE and Pratt & Whitney both developed what were then called propfan engines. GE built the GE36 unducted fan demonstrator, which flew on a Boeing 707 testbed. Pratt & Whitney partnered with Allison on the 578-DX, which flew on an MD-80 testbed. Both demonstrators flew and validated the efficiency numbers their designers projected.

Both programs were cancelled.

Noise was the most visible problem. A nacelle is not just a structural component - it is an acoustic management device. It contains fan noise, shapes inlet airflow to reduce tonal components, and smooths the surface to reduce blade-passing frequency radiation. Without a nacelle, the cabin environment, airport community noise, and certification requirements all faced challenges that neither 1980s demonstrator could adequately address.

Vibration from structural coupling between exposed rotating blades and the airframe at cruise speed introduced additional engineering complexity. And then oil prices dropped through the late 1980s and into the 1990s. The economic case evaporated, and the industry returned to incrementally improving the ducted turbofan for three decades.

What Has Changed Since the 1980s?

CFM’s case for RISE rests on three substantive changes.

First, materials. The composite fan blade technology available today is fundamentally different from what existed in 1985. RISE fan blades use advanced carbon fiber architecture that enables fully three-dimensional aerodynamic shaping across the entire blade span - complex twist distributions, variable chord geometry, and surface features designed specifically to manage acoustic behavior at the blade tip. The manufacturing capability to build those blades reliably did not exist when the 1980s demonstrators were flying.

Second, computational modeling. High-fidelity computational fluid dynamics and acoustic propagation simulation at today’s resolution were not available to engineers in that era. Problems that appeared as surprises during 1980s flight test can now be identified and designed around computationally before any hardware is built. This compresses the iteration cycle and reduces the probability of late-program surprises.

Third, and most importantly, the economic and regulatory pressure is structural this time. When oil prices dropped in the late 1980s, the efficiency argument lost its urgency. That dynamic will not repeat. Carbon emissions from commercial aviation are now embedded in airline business cases, government policy frameworks across multiple jurisdictions, and public commitments extending decades into the future. The EU emissions trading scheme already covers intra-European flights. Capital markets funding aircraft and engine programs are increasingly applying sustainability criteria to those investment decisions. The pressure to deliver efficiency gains is not cyclical - it is not going away when fuel prices fluctuate.

How Is CFM Addressing the Noise Problem?

The 1980s noise problem was not simply about volume. It was about frequency content and directionality. Exposed fan blades produce tonal components at the blade-passing frequency and its harmonics that propagate in patterns fundamentally different from a nacelled engine.

CFM’s approach uses two counter-rotating rows of fan blades. The second row rotates in the opposite direction to the first, performing two functions simultaneously. It recovers energy from the swirling wake left by the first blade row, improving aerodynamic efficiency. Critically, the acoustic interaction between the two counter-rotating rows can be tuned by adjusting blade count, spacing, and relative rotational speed - pushing dominant tonal frequencies into ranges that are less objectionable both for cabin environment and community noise certification.

The aerodynamic, mechanical, and acoustic coupling between two counter-rotating blade rows operating at high speed in unducted freestream air is an extraordinarily complex system. The number of design variables is large and their interactions are nonlinear. This complexity is part of why the RISE demonstration program is as extensively phased as it is - CFM is not just proving efficiency targets. It is building the engineering knowledge base required to design a production engine with predictable behavior across the full flight envelope.

Can the RISE Engine Run on Sustainable Aviation Fuel or Hydrogen?

The RISE core is designed to be fuel-flexible. Sustainable aviation fuel delivers the same efficiency improvements in a RISE engine as in any current turbofan, which benefits operators already blending SAF into their fuel supplies.

The architecture is also being developed with an eye toward potential hydrogen combustion compatibility. This is direct hydrogen burned in a modified gas turbine combustor - not hydrogen fuel cells driving electric motors, but actual combustion of hydrogen as a fuel. Combustion products are primarily water vapor, with no carbon. The energy density of liquid hydrogen per unit of mass is roughly three times that of jet-A.

The volumetric challenge is significant. Liquid hydrogen must be stored cryogenically at approximately minus 253 degrees Celsius and takes up roughly four times the volume of an equivalent energy mass of jet-A. Airport storage and distribution infrastructure does not exist at any meaningful scale.

CFM is not committing to a hydrogen-combusting RISE engine at entry into service. What they are preserving is the architectural option - the combustor design work is being conducted with awareness of the hydrogen pathway so future variants are not precluded. It is a hedge, not a promise.

How Does RISE Compare to Competing Engine Programs?

RISE is the most radical architectural departure of the three major efficiency programs currently in development.

Pratt & Whitney is advancing its geared turbofan architecture - already in service on the A220 and A320neo family - using a gear reduction between the fan shaft and low-pressure turbine to let each component operate at its aerodynamically optimum speed. The GTF fleet has worked through significant durability issues involving powdered metal in certain turbine discs, requiring extensive inspections and removals across the operator fleet. The remediated design is now in production.

Rolls-Royce has the UltraFan demonstrator, a ducted architecture with an extremely high bypass ratio and a similar gear reduction concept. The UltraFan has run as a core demonstrator and produced impressive efficiency figures. Its path to a specific production engine on a defined new airframe is less defined at this stage.

RISE carries the most technical risk of the three. It also has the largest potential efficiency upside. That is the honest engineering trade involved in pursuing step-change performance rather than incremental improvement.

When Will the RISE Engine Enter Service, and What Aircraft Will It Power?

The RISE development timeline targets a mid-2030s entry into service, positioned to power next-generation narrowbody aircraft from Boeing and Airbus that succeed the 737 MAX and A320neo family. Both manufacturers are expected to begin formally defining those successor programs in the late 2020s. The RISE demonstrator schedule is paced precisely to give airframers validated performance and acoustic data in time for engine selection decisions on platforms that will serve airlines into the 2060s and beyond.

The integration challenge is substantial. The pylon-to-nacelle interface that defines every narrowbody aircraft flying today does not exist on a RISE-powered aircraft. Integrating an exposed fan system into a fuselage and wing at cruise Mach numbers is an engineering problem with no current production precedent. Boeing and Airbus will need to design entirely new airframes around this architecture, not simply re-engine an existing platform. Those programs require time, capital, and regulatory momentum well beyond propulsion technology readiness alone.

CFM is not conducting a paper study. The demonstration program involves ground test validation of key technology components with a progression toward a flight demonstrator to validate the full open fan architecture on an actual aircraft.

Why This Matters for Pilots and the Aviation Industry

The aircraft entering commercial service in the 2040s and 2050s will likely look and sound fundamentally different from anything flying today. An exposed counter-rotating fan with no nacelle changes the visual signature of the aircraft, the acoustic environment during takeoff and climb, and the aerodynamic integration requirements across the entire airframe.

For the broader industry, every percentage point of propulsion efficiency improvement makes the sustainable aviation fuel pathway more economically viable. Cleaner fuel goes further in a more efficient engine. The two solutions reinforce each other, and both are necessary to meet the emissions commitments that airlines, manufacturers, and regulators have embedded in long-range planning.

GE Aerospace and Safran are spending real development capital on hardware. That is a consequential bet - and the demonstrator data, by CFM’s own reporting, is validating that the acoustic and structural challenges the 1980s programs could not solve are manageable with today’s materials and modeling tools.

Key Takeaways

• The CFM RISE program, announced in June 2021, targets 20% better fuel efficiency than the LEAP engine using an open fan architecture with no surrounding nacelle
• The target bypass ratio of approximately 60:1 - compared to the LEAP’s ~9:1 - is what makes the efficiency target achievable, moving vastly more air per unit of fuel burned
• Two prior open fan demonstrators, the GE36 and P&W/Allison 578-DX, flew in the 1980s and proved the efficiency numbers before being cancelled when oil prices dropped; the difference now is that the efficiency pressure is structural, not cyclical
• Counter-rotating fan blades solve a dual problem: recovering aerodynamic energy from the first row’s wake while tuning the acoustic signature to meet modern noise standards
• Entry into service is targeted for the mid-2030s, timed to power next-generation narrowbody aircraft that Boeing and Airbus are expected to begin formally defining in the late 2020s