The CFM RISE Open Fan Engine, the Propfan Concept That Failed in the Eighties, and Why GE Aerospace and Safran Are Betting It Works This Time

GE Aerospace and Safran are developing an open fan jet engine that could deliver a 20% fuel efficiency improvement over current-generation turbofans, with commercial entry into service projected between 2035 and 2037. The program, called RISE (Revolutionary Innovation for Sustainable Engines), marks the most significant departure from conventional turbofan architecture in roughly sixty years. It also revives a concept that failed publicly once before.

What Is the CFM RISE Program?

RISE is the next-generation engine program from CFM International, the 50/50 joint venture between GE Aerospace (United States) and Safran Aircraft Engines (France) that has operated since 1974. CFM has produced over 35,000 engines. The CFM56 is the most successful commercial jet engine in history by unit count. Their current LEAP engine, which entered service in 2016 powering the Boeing 737 MAX and Airbus A320neo family, set the benchmark for narrowbody efficiency.

RISE is what comes after LEAP.

The program was announced publicly in 2021. Demonstrator hardware testing began in the mid-2020s. The target entry into service is the mid-2030s, with 2035 to 2037 being the range CFM has cited publicly.

Why Remove the Nacelle?

Every commercial jet flying today uses a ducted fan - a large fan enclosed within a nacelle. The nacelle shapes airflow, acts as an acoustic shroud, and contains fan blades if one separates. It also adds weight and drag, and it limits how large the fan diameter can grow before ground clearance becomes a problem.

Fan diameter matters because of a fundamental principle: propulsive efficiency improves as bypass ratio increases. Moving a large volume of air slowly is more efficient than accelerating a small volume to very high velocity. Following that physics to its logical conclusion means removing the nacelle entirely, exposing the fan blades to open air, and making them far larger. That is the open fan concept.

The 1980s Failure: What Went Wrong With the UDF

This is not a new idea. In 1987 and 1988, GE flew a design called the UDF (Unducted Fan) on a modified Boeing 727 and a McDonnell Douglas MD-80. The concept used two counter-rotating rows of blades with no surrounding nacelle. Fuel efficiency results were extraordinary for the era.

The program went nowhere.

The dominant failure mode was noise. Without an acoustic shroud, the open blades produced sound levels unacceptable for commercial service. In a counter-rotating design, the wake from the forward rotor interacts with the aft rotor blades in a phenomenon called blade-vortex interaction noise - a loud, tonal signature that passengers found objectionable. When oil prices collapsed in the late 1980s and jet fuel became cheap, the efficiency argument evaporated alongside any remaining commercial interest.

Why CFM Believes 2025 Is Different From 1987

CFM’s bet is that four decades of advances in materials science, computational fluid dynamics, and aeroacoustic research change the fundamental equation.

The RISE open fan uses large-diameter composite blades designed with computational tools that did not exist in 1987. Modern CFD can model blade wake acoustics at a fidelity that makes it possible to identify and design around noise problems in simulation - before anything spins on a test stand. Thousands of blade profiles and pitch angles can be evaluated computationally before cutting a single piece of hardware.

Variable pitch is a critical technical difference from the UDF. RISE blades are designed to adjust their pitch angle in flight based on speed, altitude, and operating condition. During approach and community noise segments of flight, pitch can be set to reduce blade tip Mach number - one of the primary drivers of open rotor noise. At cruise, blades optimize for efficiency. The UDF design lacked this tool entirely.

RISE also uses a single-rotation open fan, not the counter-rotating dual-rotor architecture of the old UDF. Single rotation sacrifices some peak efficiency but eliminates the blade-vortex interaction between two rotor stages - the acoustic mechanism that made the UDF commercially untenable. The noise problem becomes difficult but tractable, rather than potentially fundamental.

The Hybrid Electric Architecture

RISE is not purely an open fan engine. It integrates an electric motor into the powertrain - not for battery-powered propulsion, but for power management within the system.

The hybrid element allows the core turbine to operate at an optimal point across a wider range of conditions, smooths power transients, and enables the combustion core to remain offline during taxi operations. That last capability has direct implications for airport ground-level emissions and terminal area noise.

The architecture is also designed with hydrogen combustion as a viable future pathway. Near-term testing uses sustainable aviation fuel (SAF). But as the EU and national regulators pursue hydrogen as a long-range decarbonization pathway for aviation, an engine platform that can accommodate that transition has significant strategic value.

Timeline and Certification Challenges

The 2035 to 2037 entry into service timeline is long, but it is realistic given the program’s complexity.

For comparison, the LEAP program began in earnest around 2008 and entered service in 2016 - an eight-year arc for an evolutionary design. RISE is substantially more complex. An open fan architecture has never been certified for commercial service at this thrust class. The hybrid electric integration requires regulatory frameworks that the FAA and EASA are still developing. Variable pitch composite blades in this power class represent new certification territory.

CFM has not announced a launch customer, which is not unusual at this stage. The obvious candidates are whatever next-generation narrowbodies Boeing and Airbus develop - successors to the 737 MAX and A320 family respectively. The 737’s basic airframe lineage traces to the 1960s, and the MAX is widely regarded as the final iteration of that architecture.

Competitive Context

RISE is not the only next-generation architecture in development. Pratt & Whitney (now part of RTX) is pursuing its own next-generation design. Rolls-Royce is developing the UltraFan, which uses a geared power system to achieve extremely high bypass ratio within a ducted configuration - a different technical path toward similar efficiency targets.

Which architecture powers the next narrowbody generation will shape which industrial ecosystem dominates commercial aviation propulsion for the following 30 to 40 years. CFM is an American-French venture. Sovereign governments have direct stakes in this outcome, which means the investment commitments behind RISE are durable.

Why This Matters for Pilots

For commercial pilots whose careers extend into the late 2030s and beyond, the probability of a type rating on an open fan aircraft is real. Different sound signature, different systems architecture, and operating limitations with no precedent in current training curricula.

For general aviation, the direct application is further out. But the materials science, aeroacoustic modeling tools, and hybrid electric architectures being developed for RISE will migrate into smaller engine programs over the decade that follows. What gets solved for a commercial-class open fan engine shapes what becomes available to the turboprop and piston world.

The open fan concept failed in the 1980s because of noise and cheap oil. Neither of those conditions holds today.

Key Takeaways

CFM RISE targets a 20% fuel efficiency improvement over current-generation engines, with entry into service projected for 2035–2037
The open fan concept was tried in 1987–1988 with GE’s UDF and abandoned due to noise and collapsing oil prices; modern CFD, composite materials, and variable pitch blades are the core reasons CFM believes the equation has changed
RISE uses a single-rotation open fan rather than counter-rotating rotors, eliminating the primary acoustic failure mode of the original UDF design
The hybrid electric architecture enables power management, ground-operation emissions reduction, and future compatibility with hydrogen combustion
Competing programs from Pratt & Whitney and Rolls-Royce (UltraFan) mean the next narrowbody engine race is genuinely contested - with national industrial stakes on multiple continents