WASHINGTON, United States (AFP) — NASA has inked a deal with Lockheed Martin to develop a supersonic “X-plane” that could break the sound barrier without a sonic boom, officials said Tuesday. The $247.5 million contract allows for the design, building and testing of a plane that would make its first test flight in 2021, NASA said. The experimental plane “will cruise at 55,000 feet (16,764 meters) at a speed of about 940 mph (1,513 kph) […]