The CFM RISE Open Fan Engine, the Return of the Unducted Rotor, and the Twenty Percent Fuel Burn Promise That Could Define the Next Generation of Commercial Jets

CFM International’s RISE program - Revolutionary Innovation for Sustainable Engines - is developing an engine architecture that eliminates the traditional nacelle entirely, targeting more than 20 percent better fuel burn than the current LEAP engine. The design achieves this through an open fan configuration with projected bypass ratios of 20:1 or higher, compared to the LEAP’s 9:1. If RISE reaches commercial service, it will almost certainly power the next-generation narrowbody aircraft succeeding the Airbus A320neo and Boeing 737 MAX family after 2035.

CFM International and the LEAP Legacy

CFM International is the joint venture between GE Aerospace and Safran Aircraft Engines. Their LEAP engine currently powers both the A320neo and the 737 MAX, with more than 30,000 orders placed - the largest narrowbody engine backlog in aviation history. The RISE program is CFM’s announcement that the architecture following LEAP will not be an incremental refinement.

Why Bypass Ratio Is the Engine Efficiency Equation

Turbofan efficiency is fundamentally about bypass ratio: the more air that moves around the engine core rather than through it, the more efficient the engine becomes. The CFM56 engines powering the original A320 and 737 Classic had bypass ratios of roughly 5:1. The LEAP pushed that to 9:1 or higher, delivering approximately 15 percent better fuel burn. Continuing to raise bypass ratios means continuing to cut fuel costs - but there is a hard physical ceiling on how far that improvement can go.

The Nacelle Constraint That Limits Every Modern Engine

The nacelle - the round cowling that wraps the front of a jet engine - creates a diameter ceiling on bypass ratio. Increasing bypass ratio requires a larger fan, but larger fans must clear the ground and fit within existing aircraft and hangar geometry. The Boeing 737 MAX is the most visible consequence of this constraint: the LEAP fan was large enough that the engine had to be repositioned forward and upward on the wing to maintain ground clearance. That repositioning altered the aircraft’s handling characteristics and contributed to the requirement for the MCAS software. The entire episode was a downstream effect of chasing higher bypass ratio inside nacelle constraints.

The Open Fan: Removing the Diameter Ceiling

RISE proposes eliminating that constraint almost entirely. The open fan design uses two counter-rotating rows of large, swept, composite blades at the front of the engine with no surrounding casing. Without a nacelle diameter limit, engineers can optimize blade geometry purely for aerodynamic performance. Projected bypass ratios reach 20:1 or beyond, producing the claimed 20-plus percent fuel burn improvement over LEAP. On a narrowbody flying thousands of cycles per year, that figure restructures airline operating economics at a fundamental level.

The 1980s UDF: The Open Rotor’s First Chapter

The open fan concept is not new. GE flew the UDF - the Unducted Fan Demonstrator - in 1987 on a Boeing 747 testbed, testing it against an MD-80. The fuel burn numbers were remarkable. The problem was noise. Counter-rotating open blades create an acoustic interaction when the wake from the forward row strikes the rear row, producing a distinctive tone that was too loud for commercial passenger use. When oil prices dropped in the mid-1980s and fuel economics improved, there was no financial incentive to solve the noise problem. The program ended.

What Changed in Four Decades

Three developments shifted the calculus by the time CFM announced RISE at the 2021 Paris Air Show. Fuel costs and carbon regulation both rose substantially. And computational fluid dynamics advanced to the point where blade interaction acoustics can be modeled with a precision that was simply impossible in 1987. The blade designs under RISE look nothing like the UDF’s: they are scimitar-shaped, swept composite structures engineered from the beginning to manage acoustic interaction, not just optimize thrust. CFM held the most successful engine program in narrowbody history with the LEAP and had every commercial incentive to iterate conservatively. The decision to announce a clean-sheet open fan architecture suggests their internal aerodynamics and acoustics models gave them genuine confidence that the noise problem is now solvable.

The Certification Challenge: No Existing Standard Applies

The most significant unresolved engineering and regulatory issue is blade containment. When a turbofan blade fails, the nacelle contains the fragment - that containment function is a core part of the certification basis for every turbofan flying today. An open fan has no containment. The FAA and EASA will need to develop new certification standards for this architecture from scratch; no existing Advisory Circular covers an unducted turbofan at commercial scale. The blades must be designed to not shed dangerous fragments, or the surrounding airframe must survive a debris event, or both. This is a documented challenge without an established regulatory path.

Timeline: Mid-2020s Testing, Mid-2030s Service Entry

CFM is targeting a flight demonstration on a testbed aircraft in the mid-2020s. Realistic service entry for a production engine in an all-new airframe is the mid-2030s at the earliest. Both Airbus and Boeing are watching the program closely, as RISE is the near-certain powerplant for whatever narrowbody succeeds the A320neo and 737 MAX. Those successor aircraft, if they proceed on schedule, are expected to enter service between 2035 and 2040.

SAF and Hydrogen: Designing for an Uncertain Fuel Future

RISE is being designed from the outset to operate on 100 percent sustainable aviation fuel (SAF) without modification. Hydrogen combustion is built in as a future option. CFM and Airbus are separately testing hydrogen combustion on an Airbus A340 testbed as part of the Airbus ZEROe research program. An engine architecture being finalized in 2021 will operate in a fuel environment that may look significantly different by 2040, and the engineering decisions reflect that uncertainty.

What RISE Means for Commercial Aviation Economics

A 20 percent fuel burn reduction on the narrowbody fleet - the segment that carries the majority of the world’s air passengers - is not a marginal optimization. It changes route economics, fleet replacement calculus, and competitive dynamics between carriers. Airlines that delay fleet renewal into the 2040s may find themselves operating aircraft with structurally higher seat-mile costs than competitors flying RISE-powered successors. The unducted fan flew in 1987, impressed the industry, and went quiet for nearly four decades. The question CFM is betting billions to answer is whether the engineering has finally caught up with the concept.

Key Takeaways

• CFM RISE targets more than 20 percent better fuel burn than the LEAP engine, which itself burns approximately 15 percent less than the CFM56 it replaced
• The open fan eliminates the nacelle diameter constraint, enabling projected bypass ratios of 20:1 or higher versus the LEAP’s 9:1
• The concept failed in 1987 due to noise from counter-rotating blade interaction; four decades of advances in computational fluid dynamics and composite materials underpin CFM’s renewed confidence
• Blade containment certification is the primary unresolved regulatory challenge - the FAA and EASA must create new standards with no existing framework to reference
• Flight demonstration is targeted for the mid-2020s; commercial service entry is realistically the mid-2030s on a new-generation narrowbody replacing the A320neo and 737 MAX