The CFM RISE open fan engine and the radical bet to strip the nacelle off the future of flight

The CFM RISE program represents the most significant change in commercial jet engine design since the high-bypass turbofan. By removing the nacelle entirely and exposing advanced composite fan blades to open air, CFM International believes it can achieve a 20% reduction in fuel consumption and CO2 emissions compared to the current-generation LEAP engine — already considered best in class. The engine targets service entry in the late 2030s on next-generation narrowbody aircraft.

Who Is CFM International and Why Does Their Bet Matter?

CFM International is the joint venture between GE Aerospace (United States) and Safran Aircraft Engines (France). Their track record is unmatched in commercial aviation. The CFM56 powered Boeing 737 and Airbus A320 families for decades. The LEAP engine now sits under every 737 MAX and A320neo in service. Combined, CFM has delivered more than 45,000 engines.

When a company with that installed base announces a fundamental rethink of engine architecture, the industry pays attention. RISE — Revolutionary Innovation for Sustainable Engines — was announced in 2021 with targets that are measured not against some legacy baseline but against the LEAP engine CFM sells today.

Why the Bypass Ratio Trend Line Hits a Wall

The history of the turbofan is a story about one number: bypass ratio, the ratio of air flowing around the engine core versus through it. Early turbojets had a bypass ratio near zero. The JT9D on the original 747 ran about 5:1. The LEAP engine today runs roughly 11:1. The GE9X on the 777X has a fan diameter of 134 inches — over 11 feet — with a bypass ratio around 10:1.

Every generation, the fan gets bigger relative to the core, and efficiency improves. But the nacelle — the smooth aerodynamic housing — grows with it, adding weight and drag. Push bypass ratios to 30:1 or beyond, and the nacelle itself starts consuming the efficiency gains. The engineering hits a ceiling.

The open fan concept eliminates the ceiling by eliminating the nacelle.

What Does an Open Fan Engine Actually Look Like?

Instead of a fan enclosed in a cylindrical housing, the RISE engine features exposed rotor blades — visually closer to a propeller but engineered with the sophistication of modern turbofan blading. These are thin, swept, precisely shaped carbon fiber composite blades spinning in open air, driven by an advanced turbine core derived from LEAP technology. No cowling. No nacelle. Just blades and air.

The 1980s Experiment That Failed — and Why Now Is Different

This concept has been tried before. GE tested the GE36 Unducted Fan in 1986, flight-testing it on a modified Boeing 727. The fuel efficiency was impressive. GE shelved it anyway for two reasons: fuel was cheap, and the engine was loud. Airlines refused to trade cabin comfort for savings that barely mattered at a dollar-per-gallon jet fuel.

Four decades later, every variable has flipped:

• Fuel prices have risen dramatically and remain volatile
• The European Union mandates aggressive aviation carbon reduction
• ICAO’s CORSIA scheme creates binding offset requirements
• Airlines have made net-zero pledges for 2050
• Sustainable aviation fuel remains expensive and scarce

The fastest path to emissions reduction is burning less fuel, and an open fan architecture offers the single largest available lever.

How CFM Says It Solved the Noise Problem

Modern computational fluid dynamics (CFD) enables blade geometries that were impossible to calculate in the 1980s. The RISE blades use precise sweep angles and twist distributions that manage shockwaves and tip vortices — the primary noise sources in the old design. CFM reports ground-test noise levels comparable to current ducted turbofans, close enough to meet existing community noise standards at airports.

Three Technologies Stacking to Hit 20%

The RISE program is not just about removing the nacelle. Three advances converge:

Open fan architecture delivers a step change in propulsive efficiency by enabling ultra-high bypass ratios without nacelle weight and drag penalties.

Hybrid electric integration embeds a small electric motor-generator in the engine core. During descent, the system harvests energy. During takeoff and climb, it supplements core thrust. The result is optimization across the entire flight envelope rather than a single design cruise point.

Third-generation ceramic matrix composites withstand temperatures that would melt conventional nickel superalloys, allowing the core to run hotter and extract more work per pound of fuel. Combined with additive manufacturing of complex internal cooling passages, thermodynamic efficiency improves independently of the fan architecture.

Stack all three and the 20% improvement becomes achievable.

What Are the Real Engineering Challenges?

Aircraft integration is the most significant hurdle. An open fan has a much larger diameter than a ducted engine of equivalent thrust. The current 737 already sits low enough that the LEAP nacelle has a flattened bottom for ground clearance. An open fan likely requires a new wing design, higher wing position, or entirely different airframe. CFM has stated clearly that RISE is not a drop-in replacement — it is designed for next-generation narrowbody aircraft expected from Airbus and Boeing in the late 2030s.

Maintenance and foreign object damage present certification challenges. Ducted fans are shielded by the nacelle from birds, ice, hail, and runway debris. Exposed composite blades must demonstrate equivalent safety. The FAA and EASA will require extensive proof of blade durability at high rotational speeds across all operating conditions.

Competition is intense. Pratt & Whitney is advancing geared turbofan evolution and hybrid electric concepts. Rolls-Royce’s UltraFan demonstrator — a ducted design with a very high bypass ratio and power gearbox — ran for the first time in late 2023, targeting 25% improvement over the Trent 700. Multiple paths lead to the same destination, and the open fan approach is not guaranteed to win.

Where Does the Program Stand Today?

The 20% target is backed by systematic component testing, not theoretical modeling. Since 2022, CFM has tested the open fan rig at its facility in Villaroche, France, validated composite blade designs in wind tunnel and ground test environments, and tested the hybrid electric system independently. In 2025, integration of these systems began for a full-scale demonstrator. The program is reportedly on schedule — unusual enough in aerospace to be noteworthy.

Realistic service entry depends on new aircraft programs from Airbus or Boeing, putting the timeline in the 2035–2040 window.

Why This Matters Beyond the Airlines

Narrowbody aircraft account for roughly 60% of all commercial aviation fuel burn worldwide — hundreds of millions of tons of CO2 annually. A 20% reduction across that fleet, achieved without new fuels, new infrastructure, or new operating procedures, would be the single highest-impact emissions change available to the industry.

The technology also cascades. Composite blade manufacturing, ceramic matrix composites, and hybrid electric integration will filter into smaller engines, business aviation, and eventually general aviation — just as LEAP manufacturing techniques already influence turboprop component production.

How Does RISE Compare to Other Decarbonization Paths?

The open fan is the most promising near-term revolution in aviation propulsion:

• Battery-electric remains limited by energy density — unsuitable for narrowbody range and payload
• Hydrogen requires entirely new fuel infrastructure at every airport
• Sustainable aviation fuel is a useful bridge but constrained by cost and supply

The open fan runs on existing fuel, works within existing airport infrastructure, and delivers a step-change efficiency gain no other single technology can match. It requires patience, a new airframe, and industry commitment to a design that looks unfamiliar — but the physics and the testing data support the concept.

Key Takeaways

• CFM RISE targets a 20% fuel and CO2 reduction versus the current LEAP engine by combining open fan architecture, hybrid electric systems, and advanced ceramic composites
• The open fan concept removes the nacelle, enabling ultra-high bypass ratios (30:1+) that ducted designs cannot achieve without self-defeating weight and drag
• GE tested and shelved this concept in the 1980s; modern CFD, composite materials, and changed economics have reopened the door
• Service entry depends on next-generation narrowbody aircraft from Airbus or Boeing, realistically targeting the 2035–2040 timeframe
• Narrowbody jets burn ~60% of global commercial aviation fuel, making this the highest-leverage propulsion improvement available to the industry