General Electric is joining with NASA to revive studies of its long-abandoned GE36 unducted fan, or "open rotor" and is simultaneously launching a next-generation CF34 technology effort as part of a pressing drive to develop families of fuel-saving engines.

GE stresses that the two-pronged move, which throws down the gauntlet to Pratt & Whitney's geared turbofan (GTF) ambitions, is part of a long-term strategy that could lead to both open-rotor and advanced conventional turbofan demonstrators in the latter part of next decade. GE and NASA have signed a Space Act agreement covering studies of the open rotor concept, which offers the potential for up to 30% lower fuel burn compared with current engines. The NG34 technology plan, aimed at regional jets, has potential for up to 20% fuel savings.

Under the cost-sharing Space Act deal with NASA, GE will refurbish all the original unducted fan (UDF) test rigs, and, with the agency, will begin a rigorous analysis of data collected during the $1.2-billion propfan program that ended almost 20 years ago. "We will then be looking at all the new technology that can be added to the system that ran back then, and at what the core would be like, the materials properties of the blades and the fan shapes," according to GE.

The thrust of the open rotor study, which is still in the stages of being refined, is to overcome noise and mechanical complexity - two major hurdles that stymied the original propfan projects. Along with the GE36, an effort that also involved NASA, these included P&W/Allison's 578-DX ultra-high bypass demonstrator. Crucially, both programs produced significant fuel savings, but with prices at 65 cents per gallon toward the end of the demo phases, this was not gauged sufficiently important to warrant further ­development.

"Together with NASA [Glenn Research Center] we hope to look at counterrotating blades, and ways to take out mechanical complexity and noise." Previous data from the GE36, which was based around an F404 fighter engine driving two sets of variable-pitch fan blades, "have all been digitized," and will be run through modern 3D analytical tools to evaluate performance. "Using these tools will tell us much more from the same data than it gave us in the late 1980s," GE states.

Juan Alonso, Fundamental Aeronautics Program Office director of NASA's Aeronautics Research Mission Directorate, confirms that GE "will dust off the UDF rigs and re-assess old blades with the latest data acquisition tools, then test new blades and noise mitigation technologies." NASA, which also has a similar long-term study pact with Pratt on GTF, sees these propulsion efforts as a key part of its subsonic fixed-wing project.

This outlined goals for three notional future generations, the first of which is a 2015 entry-into-service airliner dubbed N+1. Performance targets are 32 dB. lower cumulative noise levels versus the latest Stage 4 standards, 70% lower NOx emissions compared with CAEP/2 levels and up to 15% lower fuel burn than a Boeing 737 with CFM56 engines. "You could see an aircraft in seven to eight years with lower sweep and open rotor, with noise mitigation, that could meet our N+1 targets easily," Alonso says.

The new NASA agreement is expected to reinforce parallel GE work now underway with Snecma as part of CFM International's LEAP56 study into a next-generation CFM56 successor. The GE36 UDF data to be studied under the NASA agreement was originally digitized for the LEAP56 project when CFMI made the decision to include open rotors in the development project in 2007. CFMI plans to operate a subscale rig late this year or early next. It's "a substantial effort" that will complement the work Snecma will be doing on the European Clean Sky research program, says Ron Klapproth, Leap56 program manager.

Earlier this year, CFMI officials said selection of an engine architecture for the eventual 737 and Airbus A320 replacements would be needed by 2011, and the focus through 2010 is to explore and validate key technologies. Industry insiders say Airbus has expressed "a lot of interest" in open rotors, while Boeing has yet to show its hand.

Performance of the GE36, which first flew on a 727 testbed in 1986, was shown to be in the "high propulsive-efficiency levels of the 1990s," says GE. The engine maker predicted the operating efficiency of a production GE36-C25 envisioned for a proposed McDonnell Douglas MD-92 variant would have been in the 0.5 (total-fuel/payload-lb.) range versus around 0.8 for the CFM56-powered A320 and 1.3 for the JT8D-217-powered MD-80.