The CFM RISE Program, the Open Fan Engine, and the Propfan Gamble That Could Power the Next Generation of Narrow-Body Airliners

CFM International’s RISE program - Revolutionary Innovation for Sustainable Engines - is developing an open fan engine architecture targeting more than 20% fuel burn improvement over the current LEAP engine. Announced in 2021, RISE represents a fundamental departure from conventional turbofan design, with a target service entry in the mid-2030s. If it reaches production, it will be the most significant change to commercial airliner propulsion since the high-bypass turbofan.

What Is the CFM RISE Program?

CFM International is a 50/50 joint venture between GE Aerospace and Safran Aircraft Engines of France, in partnership since 1974. Their CFM56 engine - the most-produced commercial jet engine in history with over 33,000 units built - powered the classic Boeing 737 and Airbus A340. Their current LEAP engine powers the 737 MAX, A320neo family, and COMAC C919, delivering roughly 15–16% fuel efficiency improvement over the CFM56.

The RISE program is CFM’s next step. The 20%-plus fuel burn improvement target over the LEAP cannot be achieved through refinement of existing architecture. It requires a fundamentally different approach to moving air.

How Does an Open Fan Engine Work?

Every jet engine generates thrust by accelerating air rearward. The fundamental thermodynamic principle: for a given thrust level, accelerating a large mass of air by a small amount is more efficient than accelerating a small mass of air by a large amount.

This is why turbofan bypass ratio matters. Early turbofans ran around 2:1 bypass ratio. The LEAP runs approximately 11:1 - eleven times more air bypasses the core than flows through it. This is why modern narrow-bodies burn a fraction of the fuel a 1960s airliner burned for equivalent payload and distance.

The limiting factor is the nacelle. A larger fan requires a larger nacelle, which creates drag and adds weight. At some point, the efficiency gain of a bigger fan is offset by the aerodynamic penalty of the larger tube surrounding it.

The open fan removes the nacelle entirely. Bypass ratios of 20, 30, or higher become achievable without dragging a shroud through the air - approaching the thermodynamic efficiency of a large propeller at jet cruise speeds.

Why Didn’t the Open Fan Concept Succeed in the 1980s?

It nearly did. In October 1986, a Boeing 727 took off from GE’s facility in Peebles, Ohio with a GE36 Unducted Fan demonstrator attached where the tail engine normally sat - eight highly swept composite blades spinning in the open air, no nacelle. The GE36 later flew on a McDonnell Douglas MD-80. The competing Pratt & Whitney–Allison 578-DX demonstrator also flew on an MD-80.

Both demonstrated meaningful efficiency advantages over contemporary turbofans. McDonnell Douglas was drawing up the MD-91 and MD-92 - production airliners designed around propfan engines. Airlines were interested.

Then jet fuel prices collapsed. From roughly 90 cents per gallon in the mid-1980s, jet fuel dropped toward 30 cents per gallon as oil market dynamics shifted. The propfan’s economic case evaporated. Programs were cancelled. Hardware went into storage.

What’s Different About the CFM RISE Design?

Three things separate the RISE open fan from its 1980s predecessors in meaningful ways.

Blade design. The composite materials and computational fluid dynamics available today didn’t exist in the 1980s. CFM can optimize blade geometries with a precision that was simply impossible then, controlling the flow physics responsible for noise and structural loads using algorithms processing millions of data points.

Single-stage architecture. The GE36 used counter-rotating blades - two rows spinning in opposite directions. Counter-rotation recovers energy left in the slipstream but adds mechanical complexity and creates challenging noise signatures from the interaction between blade rows. RISE uses a single rotating fan row with variable-pitch blades and a fixed stator row behind it. Mechanically simpler, and potentially quieter.

Variable pitch. Adjusting blade angle across the flight envelope - takeoff, climb, cruise, descent - is central to the RISE concept and a key source of its efficiency gains. It brings a capability long proven on turboprops to a jet-speed engine.

Can an Open Fan Engine Operate at Jet Cruise Speeds?

This was the core engineering problem in the 1980s and remains the central challenge now. A conventional propeller loses efficiency above roughly Mach 0.6 as blade tips approach the speed of sound, generating shock waves, noise, and structural loads.

The propfan solution is highly swept, scimitar-shaped blades. The sweep does for the blade what wing sweep does for an airfoil - it delays the point at which the effective Mach number at the blade surface exceeds 1.0. A well-designed swept open fan blade can operate efficiently at Mach 0.78 to 0.82, which covers commercial narrow-body cruise speeds.

The 1980s demonstrators proved this in principle. Modern computational tools and manufacturing capabilities allow far more precise execution of the same concept.

What Engineering Challenges Does RISE Still Face?

Noise. A conventional turbofan’s nacelle suppresses noise by design. Without a nacelle, sound propagates differently. CFM’s blade design and operating speed targets are engineered toward compliance with current and anticipated noise regulations, but this won’t be fully validated until flight test data comes in. The 1980s propfans were noisier than airlines and communities found acceptable - modern blade geometry may change that equation, but it remains unproven at full scale.

Certification. The FAA and EASA certify turbofans under frameworks that assume a contained engine - if a blade fails catastrophically, the nacelle captures the debris. An open fan has no such containment. Certifying under new provisions specific to an open-fan jet-speed engine requires extensive early engagement with regulators and significant testing time. CFM is having those conversations now, which is the right sequence - but it is a path nobody has walked at this scale before.

Airframe integration. This may be the most constraining challenge. A large open fan cannot simply mount where a conventional pod engine sits. Under-wing placement creates ground clearance problems that the 737 MAX geometry already illustrates at the edge of what’s manageable. Aft-fuselage mounting is one alternative. Either way, RISE cannot drop into an existing airframe. A new aircraft must be designed around the engine - which means the technology and the program timeline are fundamentally linked to a new aircraft program decision.

How Does RISE Compare to Other Next-Generation Propulsion Approaches?

CFM RISE sits in a competitive landscape with two other major visions for next-generation commercial propulsion.

Geared turbofan evolution. Pratt & Whitney’s GTF - which powers the Airbus A220, A320neo family, and Embraer E2 family - uses a reduction gearbox between the fan and low-pressure turbine, allowing each to run at its optimal speed. P&W has a development roadmap of continued efficiency improvements through advanced materials and combustor refinements that could narrow the gap with open fan performance without requiring a new certification approach.

Hydrogen combustion. Airbus’s ZEROe program targets hydrogen-burning aircraft by 2035. Zero tailpipe carbon, if the hydrogen is produced cleanly. The aircraft technology may be achievable on that timeline. The fuel infrastructure - airport storage, production at scale, distribution networks - almost certainly won’t be ready at scale.

RISE’s strategic position: it runs on conventional jet fuel, accepts sustainable aviation fuel directly, and can be adapted for hydrogen combustion in the core engine. The efficiency gains come from the thermodynamics of moving air, not from what’s burning. In an industry that isn’t sure which energy pathway wins, that fuel-agnosticism has genuine strategic value.

Why This Matters for Commercial Aviation Now

Everything that killed the propfan in the late 1980s has reversed. Jet fuel is expensive again. Unlike the 1980s, there is now persistent regulatory and social pressure around carbon emissions - ICAO has adopted ambitious carbon reduction targets for aviation, and airlines have made public net-zero commitments - that won’t reverse the way oil prices did in 1988.

The engineering tools have advanced beyond what the 1986 Peebles, Ohio test team could have imagined. And the organizations making the bet - CFM, GE Aerospace, and Safran - are the same ones that produced every successful commercial engine program of the last four decades.

The outstanding uncertainty is the airframe. A mid-2030s service entry implies that Boeing or Airbus needs to commit to a clean-sheet next-generation narrow-body very soon. Neither has done so. Boeing is focused on stabilizing production and certification on existing programs. Airbus is managing a backlog that stretches well over a decade on current models. Until an airframe commitment materializes, RISE remains a technology development program without a confirmed airplane to go into.

Key Takeaways

CFM RISE targets more than 20% fuel burn improvement over the LEAP engine - a gain not achievable through conventional turbofan refinement, requiring a fundamentally different architecture.
The open fan concept was demonstrated in principle in October 1986 and nearly reached production, but collapsing jet fuel prices killed it; today’s economic, regulatory, and technological conditions have reversed in almost every dimension.
RISE uses a single rotating fan with variable-pitch blades and a fixed stator, departing from the counter-rotating design of 1980s propfans - mechanically simpler and potentially quieter.
Three major challenges remain unresolved: noise compliance at scale, certification of an uncontained blade failure scenario, and airframe integration - all solvable in principle, none solved yet.
RISE is fuel-agnostic - compatible with conventional jet fuel, SAF, and adaptable for hydrogen - a strategic advantage when no single energy pathway for aviation has been decided.
A confirmed mid-2030s service entry requires a new aircraft program commitment from Boeing or Airbus in the near term; that decision has not been made.