The CFM RISE Open Fan Engine, the Return of the Propfan, and Why GE and Safran Think They Can Finally Solve the Problem That Beat Them in the Nineteen Eighties

CFM International announced its RISE (Revolutionary Innovation for Sustainable Engines) program in June 2021, targeting a fuel efficiency improvement of more than 20 percent over today’s LEAP engines by removing the engine nacelle entirely and exposing the fan blades to open air. If development and certification milestones hold, the engine could enter commercial service in the mid-2030s and power the next generation of narrow-body airliners. It is the most significant propulsion concept to reach active development in commercial aviation in roughly three decades.

What Is the CFM RISE Open Fan Architecture?

A conventional turbofan engine encloses its front fan inside a nacelle - the cylindrical housing visible under the wing. The nacelle directs airflow, contains failed blades, and dampens noise. Modern high-bypass turbofans now carry nacelles nearly as wide as the fuselage they hang beneath.

RISE removes that nacelle entirely. The fan blades are exposed to open air, longer than any conventional turbofan fan, and spin at much lower RPM to compensate for operating without a duct. According to GE Aerospace and Safran, eliminating the nacelle removes its weight and drag penalty, allows fan diameter to grow beyond runway clearance constraints, and unlocks a thermodynamic efficiency gain that no nacelled engine of comparable size can match.

CFM International is a 50/50 joint venture between GE Aerospace and Safran Aircraft Engines of France, founded in 1974. Their CFM56 engine family is the best-selling commercial jet engine in history. Their LEAP engine - which powers the Boeing 737 MAX and Airbus A320neo, with a fan diameter of roughly 6.5 feet - sets the current efficiency benchmark. The transition from the CFM56 to the LEAP represented approximately 15 percent improvement in fuel burn. RISE targets more than 20 percent over the LEAP - a larger gain than the last generational step.

Why the 1980s Open Rotor Programs Were Cancelled

The open fan concept is not new. In the 1980s, GE and NASA flew an open rotor demonstrator called the Unducted Fan (UDF). Pratt & Whitney flew a competing design, the 578-DX. Both programs produced strong efficiency data. Both were cancelled before commercial certification.

Three factors ended them.

Fuel economics. The price pressure of the 1970s had eased by the late 1980s. Airlines had no compelling economic reason to pursue expensive new propulsion technology when conventional engines remained adequate.

Cabin noise. An exposed open rotor generates substantially more interior noise than a ducted fan. The blade designs of that era could not close the gap to acceptable passenger comfort levels.

Blade containment. A turbofan nacelle is engineered to capture a fan blade if it separates. An open fan has no nacelle. Certification authorities in the 1980s found no clean regulatory path to resolving this, and the programs ended before one was developed.

The physics were not wrong. The surrounding conditions were not right.

Why CFM Believes the Conditions Have Shifted

CFM’s case for RISE is not that the 1980s engineers were wrong - it is that three external conditions have changed enough to make the program viable where the UDF was not.

Carbon economics are now structurally different. Airlines face mandatory emissions reductions and escalating regulatory pressure, particularly across Europe where sustainable aviation fuel mandates are taking effect. A 20-percent fuel burn reduction is no longer a preference - it is a competitive survival tool.

Materials science and design tools have advanced dramatically. The blades CFM is developing for RISE are swept, scimitar-shaped composite structures. Computational fluid dynamics tools now allow engineers to model blade acoustics at a resolution that was impossible in 1987. Safran has run open rotor demonstrators at facilities in France, and published acoustic test data shows noise levels approaching ICAO Stage 4 and Stage 5 certification standards - substantially better than anything the UDF program achieved.

Certification thinking has evolved. Regulatory engineers and authorities have developed probabilistic failure analysis approaches that reframe the blade containment question in ways the 1980s framework did not permit.

None of this means the problems are solved. But they are genuinely different problems than they were 35 years ago.

The Full RISE Architecture: Open Fan Plus Hybrid-Electric Core

RISE is not only an open fan. It integrates a hybrid-electric system directly into the engine core - designed into the thermodynamic cycle from the beginning, not added as a retrofit.

The electrical system extracts power from the core more efficiently during cruise and redistributes it to handle high-demand phases like takeoff. This allows the thermal core to operate closer to its optimal efficiency point across more of the flight envelope. From a flight crew perspective, the energy management optimization is handled by the engine control system; the practical operational implications will depend on how aircraft systems are ultimately integrated.

The architecture is also being designed for eventual hydrogen combustor compatibility. Hydrogen combustion is not part of the initial engine design, but the core is being built so an upgrade does not require a full redesign - a hedge against where long-range fuel policy may go.

The Timeline and Its Dependencies

Entry into service in the mid-2030s depends on more than engineering. Ground testing is currently underway at GE Aerospace facilities in the United States. Flight testing of the open fan demonstrator is targeted for the second half of the 2020s, and Airbus has been in discussions about providing an aircraft platform for that phase.

The dominant external variable is the narrow-body replacement cycle. The Boeing 737 and Airbus A320 families are the workhorses of the global commercial fleet. Both manufacturers will need to decide in the coming years whether the next-generation narrow-body gets a new airframe, an evolved existing platform, or simply a re-engined current design. That decision directly determines how much of RISE’s thermodynamic potential can be unlocked. An open fan on an airframe not optimized around it captures less efficiency than one designed for it from scratch.

What Certification Will Require

The FAA and EASA will need to develop new standards specific to the open fan architecture. These include blade failure probability frameworks, noise exposure limits during approach and rejected takeoff, and potentially runway clearance requirements for aircraft with exposed blades operating in crosswind conditions.

No open fan aircraft has ever reached commercial certification. The pathway is not blocked - it simply has never been fully worked through the regulatory system. This is a program milestone requiring sustained regulatory collaboration alongside engineering development, and it represents one of the more variable elements in the mid-2030s timeline.

Why This Matters for Commercial Aviation

The narrow-body replacement decision will be the most consequential propulsion program choice in commercial aviation in a generation. Whatever engine powers that next platform shapes fuel costs, emissions trajectories, and airline economics for decades.

For pilots flying current 737 MAX and A320neo variants, RISE is a mid-to-long-range transition. The existing fleet and its LEAP-powered successors remain the operational standard through the foreseeable near term. For anyone tracking where the industry moves over the next 20 years, this program is the most significant propulsion development to follow closely.

The 1980s open rotor programs did not fail because the physics were wrong. They failed because the market had no urgency and the adjacent technologies had not matured. The physics still work. The materials, design tools, and regulatory frameworks have advanced considerably. The market pressure now exists that did not exist then. That is a meaningfully different set of starting conditions.

Key Takeaways

CFM RISE targets more than 20 percent better fuel efficiency than the LEAP engine - larger than the approximately 15 percent gain the LEAP achieved over the CFM56.
The open fan removes the nacelle entirely, allowing longer blades, lower RPM operation, and thermodynamic gains no equivalently sized ducted engine can match.
The 1980s UDF and 578-DX programs were cancelled due to cheap fuel, cabin noise, and unsolved blade containment certification - not flawed physics.
RISE integrates hybrid-electric capability into the core thermodynamic cycle from inception and is designed to accommodate a future hydrogen combustor without a full redesign.
Mid-2030s entry into service depends on development schedule, FAA/EASA certification of the open fan architecture, and Boeing and Airbus program decisions on the next narrow-body generation.