The return of supersonic passenger air travel is now one step closer to reality. NASA is working to build its first piloted supersonic X-plane in decades. The X-31, a Mach 1.28 jet designed to test thrust vectoring technologies achieved first flight back in 1990. The agency’s next faster-than-sound experimental plane – the Low-Boom Flight Demonstration (LBFD) aircraft – will be concerned with the physics of breaking the sound barrier. It will be used to teach scientists more about sonic booms and how to avoid them. It is the first in a series of X-planes in NASA’s New Aviation Horizons Initiative, introduced in the agency’s Fiscal Year 2017 budget.

The companies bidding to build the low-boom supersonic X-plane

In the pursuit to bring supersonic flight to commercial aviation, NASA plans to design and fly a supersonic jet that produces a low rumble when it breaks the sound barrier. "Developing, building and flight testing a quiet supersonic X-plane is the next logical step in our path to enabling the industry's decision to open supersonic travel for the flying public," said Jaiwon Shin, associate administrator for NASA's Aeronautics Research Mission. Multiple commercial aviation companies are determined to build new supersonic passenger planes in the post-Concorde era, including Spike Aerospace, Boom Supersonic, and a partnership between Aerion and Airbus. However, any successful supersonic passenger planes will be limited to breaking the sound barrier on over-ocean routes exclusively. NASA hopes its next faster-than-sound X-plane can change that.

After conducting feasibility studies and working to better understand acceptable sound levels across the country, NASA's Commercial Supersonic Technology Project asked industry teams to submit design concepts for a piloted test aircraft that can fly at supersonic speeds, creating a supersonic "heartbeat" – a soft thump rather than the disruptive boom currently associated with supersonic flight. The preliminary design for the X-plane was produced by Lockheed Martin Skunk Works, which assisted in low and high-speed wind tunnel tests with scaled models. The tests were conducted at NASA Glenn Research Center in February, according to Popular Mechanics, and produced “more favorable results than we expected,” said Peter Iosifidis, program manager at Skunk Works for the preliminary design Quiet Supersonic Transport (QueSST). Work on the QueSST is wrapping up with low-speed wind tunnel tests at NASA Langley Research Center using 15 percent scale model of the aircraft that stretches 15 feet long.

Contractors submitted bids to build the full-scale, low-boom demonstrator aircraft this September. Among the firms is Lockheed Martin, which has been working closely with NASA on the preliminary design for the aircraft. Data from the QueSST program are being sent to aerospace firms interested in building the X-plane. The other known company to submit a bid is Spike Aerospace, an upstart supersonic hopeful that wants to bring a 22-passenger Mach 1.6 quiet-boom business jet in the early 2020s. The full list of aerospace companies that submitted a bid is not available, as Popular Mechanics reports. However, Boeing and Gulfstream have notably been involved in NASA’s supersonic research programs in the past.

The design specifics of NASA’s low-boom supersonic X-plane

The current model for NASA’s quiet-boom X-plane includes a number of unique design features to dampen the noise of a sonic boom. A long nose with a flattened tip, designed to be hollow on the full-scale X-plane, is shaped to prevent shock waves from coalescing on the front of the aircraft. These pockets of air pressure at the front of the aircraft, followed by minimum pressure at the rear as the plane breaks the sound barrier. The tendency of multiple shock waves to combine before breaking the sound barrier produces the distinct double bang that aerospace engineers refer to as an “N wave” sonic boom. By keeping the shock waves separated on the airframe, the noise of a sonic boom can be reduced to a lower rumble.