The CFM RISE Program, the Open Fan Engine, and the Physics of Removing the Nacelle

CFM International’s RISE program - Revolutionary Innovation for Sustainable Engines - proposes removing the nacelle from commercial jet engines and replacing it with large, variable-pitch open fan blades spinning in free air. Announced publicly in June 2021, the program targets more than 20% better fuel burn compared to today’s LEAP engines and is designed from the ground up for sustainable aviation fuel and potential hydrogen combustion. Entry into service is targeted for the mid-2030s.

Who Is CFM International?

CFM International is a 50-50 joint venture between GE Aerospace and Safran Aircraft Engines. If you’ve flown a Boeing 737 or Airbus A320neo in recent decades, you’ve flown behind one of their engines. The company has produced over 35,000 engines. Their CFM56 series defined commercial aviation for forty years. Their LEAP engine - Leading Edge Aviation Propulsion - powers the current 737 MAX and A320neo family.

CFM is not a startup making promises. Their track record is the backbone of narrowbody commercial propulsion worldwide, which is precisely why this program deserves serious attention.

The Physics of Bypass Ratio - and Why the Nacelle Becomes a Problem

Jet engine efficiency is driven primarily by bypass ratio: the ratio of air flowing around the hot combustion core versus air flowing through it. Early turbojets had a bypass ratio near zero. Adding a large front fan changed everything - most air bypasses the core entirely, accelerates over a much wider area, and produces thrust far more efficiently.

The progression tells the story. The CFM56 engines on the original 737 had a bypass ratio of roughly 5–6:1. The LEAP engines on the MAX raised that to approximately 9:1. Every step up in bypass ratio brought better fuel efficiency and lower noise.

The problem is geometry. A higher bypass ratio means a larger fan, and a larger fan means a larger nacelle. At some point the nacelle simply doesn’t fit under the wing. The 737 MAX already shows this constraint - its LEAP engines were repositioned further forward and angled upward because the nacelle couldn’t hang lower without striking the runway.

The logical conclusion: what happens if you remove the nacelle entirely?

What an Open Fan Engine Actually Is

An open fan engine is exactly what it sounds like. Picture a conventional turbofan, then remove the outer casing. What remains is a set of large, highly swept composite fan blades rotating in free air, with the core - compressor stages, combustion section, turbines - exposed behind them.

The blades are significantly larger in diameter than a ducted fan of equivalent thrust because they’re unconstrained by a cowling. From the outside, the visual effect is striking: not smooth pods, but something closer to large propellers mounted on a jet core. That comparison is aerodynamically apt.

RISE uses a single row of variable-pitch composite fan blades, not the contra-rotating dual rows of older propfan designs from the 1980s. The variable-pitch mechanism is critical - it allows blade angle to be optimized for noise during approach and climb, then shifted for peak efficiency at cruise.

The 1980s Propfan: History Repeating, Better Prepared

Open fan propulsion is not a new idea. During the fuel crisis of the 1980s, NASA and major engine manufacturers developed what they called propfans or unducted fans - contra-rotating open rotor designs that demonstrated fuel burns 25–30% better than contemporary turbofans. NASA flew test hardware on a modified MD-80 demonstrator. The numbers were compelling.

Then oil prices dropped, and noise problems - which were real - made the designs politically difficult for airport communities. The programs were shelved.

RISE is picking up that thread, but the engineering landscape has changed substantially. Advanced composite materials, computational acoustic modeling, and additive manufacturing techniques that simply didn’t exist in the 1980s are all now part of the development toolkit.

The Efficiency Case

The RISE program’s efficiency target is more than 20% better fuel burn compared to the LEAP engines in current service. To contextualize that number: the jump from the CFM56 to the LEAP represented approximately 15% improvement. RISE is claiming it can clear that bar by a meaningful margin.

The program is also designed from inception to run on sustainable aviation fuel at any blend ratio, with a roadmap that includes hydrogen combustion compatibility. Hydrogen eliminates carbon from the combustion equation entirely, but requires an engine architecture built for it from the start. You cannot retrofit there from a conventional turbofan design.

Commercial aviation accounts for roughly 2–3% of global CO₂ emissions, a share that grows with travel demand as the global middle class expands. Fuel represents 20–30% of airline operating costs. A 20% fuel burn improvement applied across thousands of narrowbody aircraft over decades is not a marginal efficiency gain - it is a structural shift in both emissions and economics.

The Engineering Challenges That Remain Unsolved

The noise problem is real, and the honest assessment is that open fan engines will be louder than equivalent ducted turbofans. Without a nacelle acting as an acoustic suppressor, sound radiates much more freely. Blade-passing frequency tones, inlet distortion interaction noise, and broadband cruise noise are all harder to manage in the open.

Variable-pitch blades help - angle can be optimized during approach and low-altitude operations when community noise matters most. Composite blade materials can be tuned for specific acoustic properties. But whether the overall noise signature can meet current airport community noise certification standards is not yet fully resolved. Engine noise standards have adapted before, but those conversations between regulators, airlines, and airport communities take time.

Blade containment presents a second challenge without a clear precedent in current regulations. On ducted turbofans, the nacelle is a certified containment structure - engineered to catch debris from a blade liberation event. The FAA requires demonstrated blade containment capability for turbine engine certification. With no nacelle, this requirement must be addressed through extraordinary blade reliability standards, design approaches that prevent liberation at relevant energy levels, or structural shielding integrated into the airframe. The FAA has indicated openness to performance-based certification rather than prescriptive nacelle requirements, but this is genuinely new regulatory territory.

Bird ingestion and ice also behave differently on an open fan. A nacelle shapes inlet airflow to reduce bird strike probability and manages ingested ice before it reaches sensitive engine components. Open fan blades are more directly exposed to the environment. CFM has active birdstrike testing programs underway, but the certification basis for ingestion events on open fan designs is still being written.

These are engineering challenges, not engineering impossibilities. But each one requires hard technical work and regulatory negotiation before type certification.

The Timeline and Its Dependencies

The RISE program timeline, as currently published:

June 2021: Public announcement
2023–2024: Subscale open fan demonstrator testing to validate aerodynamic and acoustic models
Mid-2020s: Full-scale ground demonstration
Before end of this decade: Full demonstrator flight testing
Mid-2030s: Engine certification and entry into service

That’s approximately a decade from public announcement to airline service - consistent with the LEAP program’s own development timeline, and arguably optimistic given that RISE represents a significantly more ambitious architectural change.

The commercial dependencies matter as much as the technical timeline. The 737 MAX and A320neo are not candidates for RISE engines. Their airframes were designed around existing engine pylon geometry and ground clearance. Open fan engines, with their larger blade diameter, require a new aircraft designed from the start to accommodate them.

RISE is positioned for the next-generation narrowbody - an aircraft Boeing and Airbus have studied extensively but neither has formally launched. An entry into service in the mid-2030s assumes a new aircraft program launches in the near term and that the airframe is designed from scratch around the open fan architecture. That is a meaningful set of dependencies.

Why This Architecture Matters Beyond the Efficiency Numbers

The transition from the turbojet to the high-bypass turbofan in the 1960s and 1970s was exactly this kind of fundamental rethink. It disrupted manufacturing, required new materials, and demanded new certification frameworks. It also defined commercial aviation propulsion for the next six decades.

The CFM56 and LEAP are recognizably the same category of machine - incrementally better, but operating on the same core premise about what a jet engine looks like. RISE questions that premise.

The nacelle has been so fundamental to what a jet engine looks like that it stopped being questioned. CFM International has decided to question it, with multi-billion-dollar development budgets, decades of propulsion experience, and the full weight of the GE Aerospace and Safran engineering organizations behind the bet.

Key Takeaways

CFM RISE targets >20% better fuel burn vs. current LEAP engines by eliminating the nacelle and using large open composite fan blades - a larger gain than the ~15% improvement the LEAP delivered over the CFM56
The concept draws on 1980s propfan research that demonstrated 25–30% efficiency advantages but was shelved due to noise problems and falling fuel prices; modern materials and modeling have substantially changed the engineering landscape
The two largest unresolved challenges are noise certification (open fans are inherently louder than ducted designs) and blade containment (no nacelle means no FAA-standard containment structure, requiring new certification approaches)
Entry into service is targeted for the mid-2030s, contingent on full-scale demonstration, a new narrowbody aircraft program, and airframe designs built from scratch around the open fan architecture
The efficiency prize - applied to thousands of narrowbodies over decades - represents a structural reduction in both commercial aviation’s carbon footprint and airlines’ fuel costs, which consume 20–30% of operating budgets