'Green' Plane of the Future Might Have Two Cabins

An artist's rendering of MIT's D series, or double bubble airplane that could replace the Boeing 737 in 25 years. CREDIT: MIT/Aurora Flight Sciences View full size image

An MIT-led team has drawn up a green airplane concept that could use 70 percent less fuel than current airliners.

The drastic new design does away with the standard single cylinder for a fuselage we are all used to and instead goes with a double-barreled approach. Two partial cylinders are placed side-by-side to create a wider structure, and a cross section would resemble two soap bubbles joined together.

Dubbed the "double bubble," or more formally known as the D series, the unorthodox design is intended to replace the 180-passenger Boeing 737 relied on for domestic flights.

Another related MIT concept, the H "hybrid wing body" series, envisions replacing the 350 passenger Boeing 777 class aircraft now used for international flights with a wide, triangular-shaped body that creates forward lift, eliminating the need for a tail to balance the aircraft.

Both ideas were presented to NASA last month as part of a $2.1 million research contract to develop environmental and performance concepts that will help guide the agency’s aeronautics research over the next 25 years.

Although automobiles have undergone extensive design changes over the last half-century in becoming more fuel-efficient, the traditional "tube-and-wing" structure of an aircraft’s wings and fuselage have not. "Aircraft silhouettes have basically remained the same over the past 50 years," said Ed Greitzer, professor of aeronautics and astronautics at MIT and principal investigator on the project.

Designing an airplane that could meet NASA's aggressive environmental criteria while accounting for an expected doubling in air traffic by 2035 would require "a radical change," said Greitzer. 

Flight of the double bubble

Accordingly, Greitzer and his team wholly reconfigured the classic tube-and-wing structure for the D series. The jetliner features longer, skinnier wings and a smaller tail than today's craft that further reduce the drag and amount of fuel that the plane burns.

The engineers 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.

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.

The D design mitigates some of these problems by traveling about 10 percent slower than a 737. However, some additional incurred travel time could be recovered on the ground given the D series' wider size, which allows for quicker loading and unloading.

The hybrid wing body approach

The H series utilizes much of the same technology as the D series, though a larger design is needed for this plane to carry more passengers over longer distances.

These bigger dimensions, however, allow engineers to explore different propulsion architectures for the plane, such as a distributed system of multiple smaller engines.

Awaiting clearance for takeoff

Carl Burleson, the director of the Federal Aviation Agency’s Office of Environment and Energy, said that in addition to its "really good environmental performance," the D series is impressive because its bubble design is similar enough to the tube-and-wing structure of current planes that it should be easier to integrate into airport infrastructure than more outside-the-box designs.

"You have to think about how an airport structure can support it," Burleson said. "For some other designs, you could have to fundamentally reshape the gates at airports because the planes are configured so differently."

The MIT team expects to hear from NASA within the next several months about whether it has been selected for the second phase of the program, which will provide additional funds to one or two of the four subsonic teams in 2011 to research and develop the technologies identified during the first phase. (The other subsonic teams are from Boeing, GE Aviation and Northrop Grumman.)

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