By 2035, airplanes designed to be friendlier to the environment should be ready for takeoff. This 180-passenger “double bubble,” or D series, aircraft is one of two designs that a team led by MIT’s Department of Aeronautics and Astronautics presented to NASA in May as part of the agency’s research program to identify technologies that are more ecologically sound. Instead of using a single fuselage cylinder, the engineers 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 wings to the rear of the fuselage, which reduces the amount of fuel that the engines need to produce a given amount of thrust. The plane would burn 70 percent less fuel than today’s commercial planes and reduce nitrogen oxide emissions by 75 percent. It would also be quieter. In addition, the MIT team designed a 350-passenger “hybrid body,” or H series, to replace the 777-class aircraft now used for international flights. The H series uses much of the same technology as the D series and features triangular wings on a wide fuselage.