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  • NASA Takes Closer Look at MIT's Double-Bubble
    Posted by Graham Warwick 4:53 PM on Apr 22, 2011

    MIT has been funded to take a closer look at its D8 "double-bubble" configuration, which has emerged as a promising concept from NASA's N+3 studies of airliners that could enter service around 2035.

    The D8 is a modified tube-and-wing configuration that combines a lifting-body fuselage with embedded engines and a slatless, nearly-unswept natural laminar-flow wing to reduce fuel consumption, emissions and noise. A reduced cruise speed of Mach 0.72 and optimised cruise altitude of 45,000ft are also part of the equation.

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    Images: MIT

    MIT's study concluded
    the basic three-engine, 180-seat D8 configuration could, with minimal technology insertion, meet NASA's 2025-timeframe N+2 goals of reducing noise by 42dB below Stage 4, NOx emissions by 75% below CAEP 6 and fuel burn by 40% from today's CFM56-powered 737-800 (MIT calculated the D8-1's fuel burn would be 50% less).

    With advanced technology, the D8 could meet three of NASA's four tougher 2030-timeframe N+3 goals. Fuel burn for the composite-fuselage D8-5 is almost 71% less than the 737's, NOx emissions more than 87% below CAEP 6 and field length for a 3,000nm mission is 5,000ft - less than two-thirds that of the 737. Noise is 60dB below Stage 4, just missing NASA's goal of 71dB.

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    A big part of the magic is in the location of the engines. They are mounted at the back, where they can ingest the entire boundary layer over the upper fuselage. This reduces drag and allows the engines to be smaller. Their noise is also shielded by the lower fusleage where it extends aft and by the outboard vertical fins.

    Now NASA has awarded MIT a follow-on, three-year, $4.6 million N+3 contract to study two key aspects of the D8 configuration: propulsion-airframe integration, and particularly boundary-layer ingestion; and high-efficiency, small-core engines that allow higher bypass ratios for lower noise and fuel burn without increasing the overall size of the nacelles.

    The D8 gets its "double-bubble" monicker from the fuselage cross-section, which is essentially two cylinders siamesed together lengthwise. This and the upswept nose enable the fuselage to generate three times the lift of a conventional tube and allow a smaller-area wing. One of the things NASA wants a closer look at in the windtunnel is the "pi-tail", because of concerns the top-mounted horizontal stabilzer could reflect engine noise to the ground.

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    Tags: awt, NASA, aerodynamics

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