MIT reports NASA has the next generation Plus three (planes for 2035) project whose objective is to develop concepts for, and evaluate the potential of, quieter subsonic commercial planes that would burn 70% less fuel and emit 75% less NOx than today’s commercial planes. [Note updated to MIT link] NASA also wanted an aircraft that could take off from shorter runways
MIT, Aurora Flight Sciences Corporation and Pratt and Whitney met NASA’s challenge by developing two designs: the 180-passenger D “double bubble” series to replace the Boeing 737 class aircraft, currently used for domestic flights, and the 350 passenger H “hybrid wing body” series to replace the 777 class aircraft now used for international flights
The engineers conceived of the D series by reconfiguring the conventional tube-and-wing structure. Instead of using a single fuselage cylinder, they used two partial cylinders placed side by side to create a wider structure whose cross-section resembles two soap bubbles joined together. They also moved the engines from the usual wing-mounted locations to the rear of the fuselage.
Unlike the engines on most transport aircraft that take in the high-speed, undisturbed air flow, the D-series engines take in slower moving air that is present in the wake of the fuselage. Known as the Boundary Layer Ingestion (BLI), this technique allows the engines to use less fuel for the same amount of thrust, although the design has several practical drawbacks, such as creating more engine stress.
MIT team designed two versions: a higher technology version with 70% fuel-burn reduction, and a version that could be built with conventional aluminum and current jet technology that would burn 50% less fuel and might be more attractive as a lower risk, near-term alternative.