The Open Fan, CFM's RISE Program, and the Engine With No Cowling That Wants to Cut Airline Fuel Burn by Twenty Percent

SUMMARY: CFM’s RISE program is developing an open fan jet engine that ditches the cowling to cut airline fuel burn and CO2 by at least 20 percent.

The open fan is a next-generation jet engine design that removes the cowling around the fan entirely, allowing a much larger fan to move far more air far more efficiently. Developed under CFM International’s RISE program, it targets a fuel burn and carbon dioxide reduction of at least 20 percent compared to today’s best engines. If it works, it could power the aircraft that replace the Boeing 737 and Airbus A320 in the mid-2030s.

What Is an Open Fan Engine?

To understand the open fan, you first have to understand what a jet engine is actually for. It isn’t a machine for going fast. It’s a machine for moving air. And the most fuel-efficient way to make thrust is to take a very large amount of air and speed it up just a little - not a small amount of air sped up a lot.

Engineers measure this with bypass ratio: the amount of air routed around the hot core versus the air sent through it. Old military-style turbojets had a bypass ratio of essentially zero - everything went through the fire. A 1960s airliner engine ran around 1:1. The engines under a modern Boeing 737 or Airbus A320 run about 11 or 12 to 1 - roughly eleven pounds of cool air bypassing the core for every one pound going through it.

Every generation, the fan up front has grown, because a bigger fan moves more air more gently. That’s why new airliner engines look so much fatter than the skinny tubes of the 1970s.

Why Take the Cowling Off?

There’s a limit to this trend. The cowling - the smooth duct wrapped around the fan - does real work. It shapes the airflow, keeps things quieter, and critically, it contains a fan blade if one ever lets go. But it’s also heavy and it creates drag. The bigger the fan gets, the bigger, heavier, and draggier the duct becomes.

At some point the duct costs more than the extra fan size buys you. Engineers call this the ducted fan “running out of runway.”

The open fan asks a radical question: what if you remove the cowling entirely? Picture a huge fan mounted at the back of the engine, spinning in open air - fewer blades, but big, wide, scimitar-shaped blades curved like a Turkish sword. Behind them sits a row of stationary vanes that straighten the swirling air back into useful thrust. No duct. Just blades in the breeze.

Remove the cowling and let the fan grow, and the bypass ratio doesn’t climb to 12:1. It jumps to something like 70:1. That’s not an incremental step - it’s a different neighborhood.

How Much Fuel Does the Open Fan Save?

The number everyone is chasing is 20 percent. The RISE program targets a fuel burn and CO2 reduction of at least 20 percent versus today’s best engines - and today’s best engines are already about 15 percent more efficient than the ones they replaced a decade ago.

In an industry that fights hard for a single percentage point, stacking another fifth on top of that is enormous. That’s the reward that justifies taking the cowling off.

Who Is Building the Open Fan?

The program is called RISE, short for Revolutionary Innovation for Sustainable Engines. It’s run by CFM International, a 50-50 joint venture between GE Aerospace in the United States and Safran in France.

You’ve almost certainly flown behind their engines. CFM’s current engine, the LEAP, hangs under the wing of essentially every new narrow-body airliner in the world - the 737 MAX and the A320neo included. This isn’t a startup with a slide deck. It’s the incumbent that dominates the single-aisle engine market betting on tearing up its own recipe.

CFM announced RISE in June 2021. The company has said it wants to ground test and fly the open fan before the end of this decade, aiming for an engine that could enter service in the mid-2030s - right when Airbus and Boeing are expected to replace the A320 and 737. Whatever engine wins that next single-aisle airplane will be built by the tens of thousands and fly for forty years.

Hasn’t This Been Tried Before?

Yes - and that history matters. In the early 1980s, after fuel prices spiked twice in a decade, GE built the UDF (unducted fan) and flew it on a Boeing 727 and later an MD-80. It used two rows of open blades spinning in opposite directions, right out in the wind. It worked, delivering the same 20-plus percent fuel savings everyone chases today.

Then it died, for two reasons. First, the price of oil collapsed in the mid-1980s, and suddenly nobody cared about saving fuel. Second, it was loud - those counter-rotating blades chopped through each other’s wakes and produced a droning noise that got into the cabin and wouldn’t leave.

So the honest question about RISE isn’t “will it save fuel.” We’ve known open rotors save fuel for forty years. The honest questions are the ones that killed it the first time.

What Are the Real Drawbacks?

Noise. This is the ghost of the old UDF. The modern design fights it with a key change: instead of two rows of spinning blades, RISE uses one row of rotating blades followed by a row of stationary vanes. One rotating stage, not two, which eliminates a whole family of noise - and the blade shapes are computer-optimized in ways impossible in 1986. But there’s no cowling to line with sound-absorbing material, so meeting modern airport noise rules remains a genuine open question, not a solved one.

Blade containment. On a normal engine, if a fan blade fails and lets go, the steel-and-composite cowling is designed to catch it - and proving that is one of the most brutal certification tests in aviation. On an open fan, there is no catcher’s mitt. A released blade becomes a free projectile. So the safety case shifts entirely: you have to prove the blade essentially cannot fail in the first place and design the airframe to protect the cabin and flight controls if it does. Modern composite blades are extraordinarily strong and can be built to fail gracefully, but it’s a harder promise to make - and regulators will demand proof of every word.

Where it mounts. These fans are too big to hang under the low-riding wing of a narrow-body without dragging on the runway. That pushes the engines toward the rear of the fuselage, which changes the entire airplane - weight and balance, cabin noise, wing shape, fuel placement. The engine and airframe must be designed together as one system. That’s exactly why Airbus signed on to flight test the open fan on one of its A380 test aircraft, putting engine maker and airframe maker in the same room from the start.

Why the Open Fan Might Finally Succeed

The open fan isn’t a gamble on a new idea. It’s a gamble that the world has changed enough to let an old, correct idea survive. The physics was always right: move more air, more gently, and you win. What killed the concept in 1986 wasn’t the physics - it was cheap oil and a noise problem engineers didn’t yet have the computing power to solve.

Both of those conditions are different now. Fuel and carbon are permanent line items on every airline’s books, and the tools to design a quiet blade now sit on an engineer’s desk. The design also happily burns sustainable aviation fuel and is built with hybrid-electric assistance in the core.

Whether that’s enough will be settled this decade - on a test stand, then bolted to the side of the largest airliner ever built. And unlike most future-of-flight stories, this one isn’t a startup hoping to exist in ten years. It’s the company that already owns the wing deciding the safe path forward is the radical one.

Key Takeaways

• The open fan removes the engine cowling so a much larger fan can move more air, pushing the bypass ratio from about 12:1 today toward roughly 70:1.
• CFM International’s RISE program - a joint venture of GE Aerospace and Safran, announced in June 2021 - targets a fuel and CO2 reduction of at least 20 percent, with service entry aimed at the mid-2030s.
• The concept was proven decades ago by GE’s UDF, flown in the early 1980s, but was killed by collapsing oil prices and unsolvable-at-the-time cabin noise.
• The three big challenges are noise (no cowling to muffle it), blade containment (no mitt to catch a failed blade), and rear-fuselage mounting that forces a whole-airplane redesign.
• Airbus is flight-testing the open fan on an A380, because the engine and airframe must be developed together as a single system.