The CFM RISE open fan engine and the unducted future that could cut jet fuel burn by twenty percent

The CFM RISE open fan engine, developed jointly by GE Aerospace and Safran through their CFM International partnership, aims to reduce fuel consumption by 20% compared to current LEAP engines by removing the nacelle and pushing effective bypass ratios to approximately 70:1. If successful, this technology would power the next generation of narrowbody airliners entering service in the 2035–2040 timeframe and represent the largest single efficiency gain in commercial jet propulsion in decades.

What Is the CFM RISE Program?

RISE stands for Revolutionary Innovation for Sustainable Engines. It is a joint development program between GE Aerospace and Safran, operating through CFM International — the same 50/50 partnership that produced the CFM56, arguably the most successful jet engine in history, and the LEAP, which powers both the Airbus A320neo family and the Boeing 737 MAX.

The core concept: remove the nacelle (the housing around the fan) and let the fan blades operate in open air, exposed like a propeller, while retaining the gas turbine core behind them. The result is an open fan, also called an unducted fan or ultra-high-bypass-ratio engine — a hybrid that looks like a cross between a turboprop and a turbofan.

Why Does Removing the Nacelle Matter?

In a modern turbofan, most thrust comes not from hot exhaust but from the fan moving a large mass of cold air around the core. This cold-air bypass is where fuel efficiency lives. The higher the bypass ratio (cold air moved relative to hot core flow), the better the fuel economy.

Current LEAP engines run a bypass ratio of about 11:1. The Pratt & Whitney GTF geared turbofan pushes 12:1. Going higher requires a bigger fan, which demands a bigger nacelle, which creates more aerodynamic drag — erasing the efficiency gains.

The nacelle becomes the limiting factor. CFM’s solution is to eliminate it entirely, allowing the fan to grow unconstrained and pushing the effective bypass ratio to roughly 70:1. The result is dramatically more air moved per unit of fuel burned.

How Much Fuel Would the RISE Engine Save?

CFM’s publicly stated target is a 20% reduction in fuel consumption compared to current LEAP engines, which themselves cut fuel burn by 15% over the CFM56. The compounding effect across global narrowbody fleets would be enormous — potentially eliminating tens of millions of tons of CO2 annually.

GE Aerospace ran the RISE open fan demonstrator on a test stand for the first time in 2024, and early results reportedly tracked with their efficiency models. The advanced compact core, designed to run hotter and more efficiently than anything in the current fleet, has been tested separately. The plan is to integrate the open fan with this core and conduct flight tests on a modified test bed aircraft within the next few years.

Why Did Open Fan Engines Fail in the 1980s?

This is not a new concept. During the oil crisis era, both GE and Pratt & Whitney tested open fan engines extensively. GE built the GE36 Unducted Fan, which flew on a Boeing 727 test bed. The technology worked and the fuel savings were real.

It was shelved for two reasons:

Oil prices dropped. The economic pressure disappeared, and airlines returned to conventional turbofans.
Noise. Open fan blades spinning at high speed with no nacelle to contain acoustic energy were unacceptably loud — both on the ramp and inside the cabin. Passengers would not have tolerated it.

The propfan concept went into the archives, and conventional turbofans improved incrementally for the next four decades.

What Has Changed Since Then?

Four critical advances make the RISE program viable where the 1980s propfan was not.

Computational fluid dynamics. Engineers in 1986 designed blades with slide rules and wind tunnels. Today, CFM models airflow across blades in three dimensions, down to the millimeter, simulating thousands of operating conditions before cutting metal. RISE fan blades are sculpted, swept composite structures shaped by algorithms optimizing simultaneously for thrust, efficiency, and noise.

Advanced materials. RISE fan blades use carbon fiber composites with woven 3D architecture — lighter and stronger than anything available in the 1980s. Lighter blades enable lower tip speeds for equivalent thrust, and lower tip speeds mean less noise.

Counter-rotating architecture. The RISE demonstrator uses two sets of blades spinning in opposite directions. Counter-rotation cancels swirl losses that plagued single-rotation propfans and changes the acoustic signature. Interaction tones between blade rows can be engineered to interfere destructively, using physics to quiet the engine.

Permanently higher fuel costs. Between carbon pricing in Europe, sustainable aviation fuel mandates, and structurally elevated fuel costs since the pandemic, the economic conditions that killed the propfan in 1988 no longer exist. A 20% fuel burn reduction is not optional — it is a strategic necessity for airlines.

What Are the Major Engineering Challenges?

Installation geometry. An open fan cannot bolt onto an existing wing the way a LEAP does. The fan diameter is too large for conventional under-wing mounting on a low-wing narrowbody. The engine likely needs to be mounted on the aft fuselage or over the wing, both of which require fundamental airframe redesign. RISE does not retrofit into existing aircraft — it requires a clean-sheet airplane designed around it.

Certification. No open fan engine has ever been certified for commercial passenger service. The FAA and EASA will need to develop or adapt standards for blade containment, bird strike, icing, and foreign object damage on exposed fan blades. A ducted engine uses the nacelle as a containment ring if a blade fails. An open fan has no such safety net, so CFM must prove their blade design, materials, and retention system can handle every failure mode independently.

Airframer timelines. Airbus and Boeing must each decide when to launch the next-generation narrowbody — the successor to the A320neo and 737 MAX. Current estimates place a program launch in the late 2020s or early 2030s, with entry into service around 2035–2040. CFM can have the engine ready, but without a new airframe to mount it on, the technology has no platform.

Passenger perception. Exposed fan blades look like propellers to passengers, and propellers carry associations with older, slower aircraft. Airlines will need to manage this perception carefully when marketing open fan aircraft.

How Does the RISE Compare to Pratt & Whitney’s Approach?

Pratt & Whitney, through parent company RTX, is pursuing an alternative path: an advanced geared turbofan that pushes bypass ratios higher while keeping the fan ducted inside a nacelle. Their argument is that most of the open fan’s efficiency gains can be captured without the noise, installation, and certification complications.

Both approaches may prove viable for different missions. The open fan could dominate on long, thin routes where fuel burn is the primary cost driver. The advanced geared turbofan may win on shorter sectors where noise restrictions at urban airports are the binding constraint.

Why This Matters for Aviation’s Climate Problem

Aviation accounts for roughly 2–3% of global CO2 emissions, and that share is growing as other sectors decarbonize faster. Sustainable aviation fuel helps but remains 2–4 times more expensive than conventional jet fuel with severely limited supply. Electric and hydrogen propulsion work for small aircraft on short routes but cannot scale to a 180-seat narrowbody flying 3,000 nautical miles.

For the mission that carries the vast majority of the world’s passengers, the solution is a better combustion engine burning less fuel per seat mile. That is precisely what CFM RISE targets — the largest single lever in commercial aviation’s carbon equation.

Key Takeaways

CFM RISE aims to cut fuel burn by 20% over current LEAP engines by using an open fan (unducted) architecture that pushes bypass ratios to ~70:1
The technology is not new — 1980s propfan programs proved the concept but were killed by falling oil prices and unresolved noise problems
Advances in CFD, composite materials, and counter-rotating blade design have addressed the technical barriers that stopped earlier programs
A clean-sheet narrowbody aircraft is required — the engine cannot retrofit onto existing A320neo or 737 MAX platforms, with new airframes expected in the 2035–2040 timeframe
The competitive landscape includes Pratt & Whitney’s advanced geared turbofan, which may offer a less disruptive path to similar (though likely smaller) efficiency gains