The aviation engine manufacturer Safran is looking at an open rotor engine, which was looked at, and largely abandoned in the 1980s because of issues with noise and airframe integration issues:
Safran Aircraft Engines now has clearly plotted the technological trajectory a counter-rotating open-rotor (CROR) engine can be part of, somewhere between an ultra-high-bypass-ratio (UHBR) turbofan and a boundary-layer-ingestion (BLI) configuration. Despite wavering interest from the rest of the industry, the France-based company believes its ground demonstrator here in Istres is proving the architecture is certifiable in terms of both safety and noise. It says it would be an efficient powerplant for the 2030-35 generation of narrowbody aircraft.
The CROR concept has to be evaluated long before a commercial program is launched. It would be a greater breakthrough than the UHBR, a geared turbofan with a bypass ratio of 15. The latter could be ready in the 2025-30 time frame and is Airbus’ priority. In Safran’s view, the CROR, with its bypass ratio of 30, would be next. That would proceed an engine designed for BLI, in 2040-45.
Compared to the CFM Leap’s fuel burn, the UHBR and the CROR would be 5-10% and 15% better, respectively. Being unducted, a CROR can have a greater bypass ratio, and therefore a lower fuel burn. At Mach 0.75, the CROR would require a minor concession in speed.
A major challenge for an unducted engine can be found in acoustics. A witness to GE36 testing in the 1980s remembers its “dreadful” noise. And, since May, no journalist has been allowed to see and listen to an actual CROR run in Istres.
Nevertheless, Safran says the problem has been solved. The pair of propellers has been aerodynamically optimized, with thin blade profiles and complex shapes. They meet the current Chapter 14 standard, according to wind tunnel-trial results. The noise level is well below that of a turboprop, Bonini adds.
The engine is in generally form remarkably similar to the GE/Snecma engine from 30 years ago.
Both are using a fighter engine in the 20,000 lb thrust class as gas generators, and in both cases the props are being driven by counter rotating free turbines without a transmission.
It’s a bit of 80s technology that never seemed to find its way into production.
Maybe this time.