For 50 years, jet engines became more efficient by making the fan larger and moving more air around the core. But keep increasing bypass ratio and eventually the nacelle itself becomes a penalty, larger, heavier and draggier. CFM International’s RISE program is betting the next leap requires removing it. Its Open Fan uses a roughly 4-metre exposed rotor with swept, variable-pitch blades, targeting 110-130 kN of thrust and Mach 0.8 ( 850 km/h ) cruise while cutting fuel burn by more than 20% versus today’s most efficient engines. Making that possible requires two opposite material problems to be solved simultaneously. The exposed blades are over 1.6 metres long. Safran is developing lightweight 3D-woven carbon-fibre composite blades because every kilogram removed at that radius reduces enormous centrifugal loads. Their sweep also helps manage the transonic aerodynamics encountered at jetliner speeds. Behind them sits an increasingly compact core. CFM is developing new compressors, combustors, turbine cooling, heat-resistant alloys and ceramic-matrix-composite technology to extract greater thermal efficiency while keeping the hot section durable. Next-generation high-pressure-turbine airfoils have already accumulated over 3,000 endurance cycles and 2,000 dust-ingestion cycles that have been ongoing now for 4-5 years. Then comes the harder problem, making an exposed rotor survive airline life. Bird strikes, hail, icing, vibration, fatigue, blade retention, noise and interaction with the aircraft itself all become certification problems. More than 175 blade ingestion and endurance tests are already complete with more ongoing. Open rotors actually flew in the 1980s. Noise, counter-rotating complexity and weak crude oil prices, helped kill them. Modern composites, computational aeroacoustic, variable-pitch controls and decades of hot-section advances have changed the equation and made these advances possible. Launched in 2021, RISE now involves over 2,000 engineers and roughly 500 test campaigns. Flight demonstration is planned for late this decade, with commercial technology targeted around 2035. If CFM succeeds, aviation’s next great jet engine may look remarkably like the propeller the jet age replaced.