The CFM RISE open fan engine and the unducted propeller GE and Safran are betting the next thirty years on

The CFM RISE program is an open fan jet engine—a massive, unducted set of curved blades spinning in open air—that GE and Safran are targeting for a 20% reduction in fuel burn compared to today’s best engines. It revives an idea first flown in the 1980s that worked but was shelved, and its entry into service is targeted for the mid-2030s, around 2035. The central challenge isn’t the physics, which is proven; it’s making the engine quiet enough and safe enough without a fan containment case.

What Is the CFM RISE Open Fan Engine?

RISE stands for Revolutionary Innovation for Sustainable Engines. It’s being developed by CFM International, the 50/50 joint venture between General Electric in the United States and Safran in France. CFM builds the most common jet engines on the planet—if you’ve flown on a Boeing 737 or Airbus A320, you’ve almost certainly flown behind one.

The design is what engineers call an open fan, sometimes called an unducted fan or open rotor. The concept: take the big fan at the front of a turbofan, make it much bigger, then throw away the metal duct, cowling, and nacelle that normally wrap around it. The blades spin out in the open air, looking less like a jet engine and more like a propeller.

When the two companies that power thousands of narrowbody airliners start sketching what comes after the current generation, the whole industry pays attention.

Why Remove the Duct? It Comes Down to Bypass Ratio

The whole argument rests on one number engine designers obsess over: bypass ratio.

In a modern turbofan, only a small fraction of air goes through the hot core where fuel burns. The vast majority gets pushed around the core by the big fan up front. That bypassed air produces most of the thrust—and moving a large amount of air slowly is far more efficient than moving a small amount of air fast. The bypass ratio is the air going around the core versus the air going through it.

A 737 engine today has a bypass ratio of roughly 5 or 6 to 1. The newest geared turbofans push that toward 12 to 1. Every efficiency gain wants a bigger fan, because a bigger fan moves more air more gently.

But there’s a wall. A bigger fan needs a bigger duct, which is heavier and drags more surface area through the air. Eventually the duct costs more than the bigger fan returns. The open fan answer is to remove the duct entirely so the fan can grow enormous.

The RISE demonstrator blades are roughly 12 to 13 feet across, giving an effective bypass ratio above 70 to 1—more than ten times what hangs off a narrowbody today. That math produces the headline target: 20% lower fuel burn and 20% less CO2. In an industry that celebrates single-digit gains, 20% reshapes route maps and balance sheets.

Why Did the Open Fan Fail Before?

This exact idea had its moment in the 1980s. With oil shocks driving fuel fears, GE built a demonstrator called the GE36 Unducted Fan and flew it on a Boeing 727 and an MD-80 in 1986 and 1987. It worked and cut fuel burn dramatically. Then fuel got cheap, airlines lost interest, and the program sat in a drawer for thirty years.

But fuel prices weren’t the only reason it stalled. Real engineering problems remained—the same ones CFM must solve today.

Problem 1: Noise

A duct does double duty as a muffler, its inner walls lined with acoustic material that absorbs sound. Remove the duct and you have open blades broadcasting straight into the world. The 1980s demonstrators were loud. With airport noise rules tightening every year, an engine that can’t meet the limits is a non-starter no matter how little fuel it sips. CFM says advanced blade shapes and careful aerodynamics get them under the rules—but that claim has to survive a real flight test, not a slide.

Problem 2: Blade-Out Safety

This is the question certification authorities ask first: what happens if a blade comes off?

In a conventional engine, the case around the fan is built to contain a failed blade. The fan blade-out test detonates a blade at full power, and the engine must catch the shrapnel inside the case. That containment is part of why your engine is safe enough to fly over an ocean.

An open fan has no case. A liberated blade is a 12-foot spear leaving at hundreds of miles an hour, and depending on engine placement, the fuselage and passengers could be in the firing line. You cannot build a containment ring around a fan with no duct, so the safety argument shifts entirely: engineers must prove the blade essentially cannot fail in the first place.

That’s where the materials come in. The RISE blades are woven carbon fiber composite, built by Safran using a three-dimensional weaving process where fibers run through the thickness of the blade, not just along it. The goal is a blade light enough to spin huge and tough enough to shrug off a bird strike. The FAA and its European counterpart will demand extraordinary evidence here—arguably the single hardest thing standing between RISE and your boarding pass.

Problem 3: Airframe Integration

A fan this big has a clearance problem. You can’t tuck it under a low wing the way engines hang today—the blades would be too close to the ground. So Airbus and Boeing may have to rethink engine mounting, possibly at the rear of the fuselage or over the wing. That means a different-looking airplane, with the wing and engine designed together from a blank sheet.

What Progress Has CFM Actually Made?

CFM has been running RISE since 2021, and this is not just paper and models. The company has a partnership with Airbus to mount an open fan demonstrator on an A380 flying testbed and fly it as a flying laboratory around 2027—real blades, real airflow, real noise measurements over real ground. That’s the moment the concept either becomes an airplane or it doesn’t.

There’s also a parallel overhaul of the engine’s core, with new high-temperature ceramic materials and a compact design. CFM is studying hybrid-electric assist and the ability to burn 100% sustainable aviation fuel or hydrogen down the road. RISE is really a basket of technologies; the open fan is just the part you can see from the terminal window.

When Will the RISE Engine Enter Service?

Entry into service is targeted for the mid-2030s, around 2035. That’s not a delay—it’s been the plan from the start, and it reflects an honest truth: a genuinely new engine architecture that has to rewrite the safety case from scratch takes the better part of fifteen years from concept to paying passenger. Anyone promising a clean-sheet engine three years out is selling something.

Why This Matters for Pilots

The physics of the open fan is not in doubt—the 1980s proved the fuel savings are real. What was never solved then, and must be solved now, is making it quiet enough and proving it safe enough without a containment case. CFM is betting that thirty years of progress in composite blades, computer modeling, and manufacturing have finally caught up to an idea that was always too good and too dangerous at once.

The 2027 flight tests will tell us a great deal. If it works, the engine on the airliner your kids fly will look strange and beautiful—a little like a propeller—and it will sip fuel in a way nothing in the sky does today.

Key Takeaways

• CFM RISE is an open fan (unducted) engine from the GE–Safran joint venture, targeting a 20% cut in fuel burn and CO2 versus today’s best engines.
• Removing the duct lets the fan grow to 12–13 feet across, pushing the effective bypass ratio above 70 to 1—more than ten times a current narrowbody.
• The architecture was flown successfully in the 1980s (the GE36) but shelved when fuel got cheap and noise and safety problems went unsolved.
• The three big hurdles today are noise, blade-out safety (no containment case is possible, so blades must be proven not to fail), and airframe integration.
• An Airbus A380 flight-test demonstrator is planned around 2027, with entry into service targeted for about 2035.