If you followed avaition in the 1980s, you probably remember GE’s GE36 Unducted Fan.
It was an F-404 derived core powering counter rotating free turbines attached to props.
With sustainability front and center on the aerospace industry agenda, plans are firming up on both sides of the Atlantic for a new wave of ambitious large-scale technology demonstrators to pave the way for ultraefficient next-generation commercial airliners.
Ranging from advanced propulsion and airframe concepts to new systems, structures and fuels, the main demonstrators will form part of the proposed Clean Aviation initiative in Europe and the next round of NASA X-plane projects in the U.S. Clean Aviation, which is expected to succeed Europe’s long-running Clean Sky program, supports the European Union’s broader Horizon Europe research and innovation framework effort for 2021-27 and will feed technology into new civil aviation projects later this decade and into the 2030s.
Open rotors, also known as unducted fans or propfans, were initially developed in the U.S. in the 1970s and 1980s amid concerns over rising fuel costs. Although two concepts—GE’s GE36 and the Pratt & Whitney/Allison 578 DX—were flight-tested, both were shelved by the early 1990s after oil prices fell. Although development of propfans continued in Russia, it was not until greenhouse gas emissions became a legislative factor in the 2000s that Western interest in the concept was revived.
In the U.S. NASA, GE and the FAA collaborated between 2009 and 2012 on wind tunnel tests of an open rotor with blades developed using modern computer-based design methods. The tests showed up to a 3% improvement in net efficiency relative to the best 1980s design, while nominally achieving a 15-17-EPNdB noise margin to Chapter 4 limits.
Around the same time, two open-rotor concepts were evaluated in Europe under the SAGE effort, with a Rolls-Royce-led team evaluating a direct-drive propulsor system while a Safran-led group developed the geared pusher CROR. The Rolls project was later rescoped to focus on lean-burn combustion, while Safran developed a CROR ground demonstrator using its M88 military engine as a gas generator.
So the direct drive free turbine is not a part of the equation this time around, which is kind of a pity.
I liked the elegance of that arrangement.