November 8, 2008 – Vol.13 No.33
EFFICIENT, OPEN ROTOR JET ENGINE – REVISITED.
For the airlines of the world cutting fuel consumption by 30 percent would be huge. It would mean a return to profitability. It would mean a return to the skies for many travelers as ticket prices dropped. It would mean a significant cut in greenhouse gas emissions.
It may also be possible.
In the 1980’s there was another fuel crisis not unlike the one the world experienced in much of 2008. (Though on the mend now with steadily lower oil costs, pricey oil will rear its ugly head again as global economies recover, whenever that is.) In response to expensive jet fuel 25 years ago, NASA and General Electric, with partner Snecma, embarked on a research program to rethink and redesign, the jet engine for greater fuel efficiency.
The result of the program was the GE36, an open-rotor jet engine. It was successful. It was considered a breakthrough. It saved up to 30 percent, compared to similar-sized jet engines with conventional, ducted front fan systems. It was never commercialized. Fuel prices collapsed in the late 1980’s and into the 90’s. For the airlines and aircraft builders fuel consumption was no longer an issue.
The GE36 was test flown on Boeing 727 and McDonald-Douglas MD-80 aircraft, both airframes with the engines strapped to their tail end – neither are still in production. The GE 36 featured an aft-mounted, open-rotor fan system with two rows of counterrotating composite fan blades. Unlike jet engines where all the whirling parts are hidden from view, the spinning fans on the GE36 were exposed for the world to see. The GE 36 had tremendous thrust from its open fan system, thus the remarkable fuel efficiency.
Now GE Aviation and NASA are revisiting the GE36 design. While they won’t be bolting GE36 engines on an airplane any time soon, they’ll be wind tunnel testing scale models of open rotor jet engine designs, not full engine tests, in several configurations in simulated flight conditions. They’ll be using computer modeling in tests not available in the 1980’s.
The testing will begin in wind tunnel facilities at NASA's Glenn Research Center in early 2009 and continue into midyear. They’ll even use the same rigs used in the 1980’s to test the scale models.
GE has estimated the technology could be ready to enter operational service after 2017, depending on whether noise, aircraft configuration and regulatory issues can be overcome.
If the open fan design of the GE36 were to make a comeback the real test would come from passengers. Air travelers have become accustomed to not seeing whirling propellers outside their little oval windows. To the airlines propellers, are from a bygone era even though turboprops still used by many commuter operations (and by the military) offer significant fuel savings (up to a half) using similar gas turbine technology.
An adoption of open rotor jet engines would mean a return to things spinning outside the window. But the planes would be greener. They’d keep the airlines in business and travelers could return to the skies. With a little marketing, the airlines could sell the idea that green flying is better even if it involves churning fan blades outside the window. Besides, some might find it kind of cool.
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