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.

The aircraft’s wing, horizontal tail, and vertical fin with a small trim surface are also designed to prevent the coalescing of shock waves, but on the rear of the aircraft. The “gull wing”, with bends in the wing surface, helps spread the sound waves of a sonic boom over a wider area below the aircraft, while small canards on the front assist with aircraft control. NASA is aiming at a sonic boom noise of 70-75 perceived decibel level (PLdB), significantly lower than the 105 PLdB that the Concorde made. 

The length of the nose and cockpit design, which is embedded in the fuselage, both limit the pilot’s field of view significantly. To fly the aircraft safely, NASA is developing an external vision system (XVS) with a high-definition camera in front of the cockpit, a lower-resolution camera under the aircraft for approach and landing, and a 4K ultra-high-definition display in the cockpit with overlaid heads up display icons, similar to the HUD system in a fighter jet.

NASA is using an off-the-shelf GE F414 afterburning turbofan to power the low-boom demonstrator, the same engine used by the F/A-18 Super Hornet. Embedded in the tail as a “dorsal engine”, the turbofan is outfitted with a diverterless supersonic inlet, which prevents air close to the airframe, known as boundary layer airflow, from entering the intake. Lockheed plans to conduct tests on the inlet design in the coming months as its facilities in Fort Worth, Texas. In addition, vertical surfaces known as vortex generators sit just behind the cockpit and camera housing to divert the disturbed airflow over these surfaces away from the engine. 

To keep program costs low, NASA is planning to use a number of existing aircraft parts to design the LBFD X-plane. In addition to the F414 engine, NASA will repurpose the rear cockpit of a T-38 to serve as the supersonic X-plane’s cockpit, including canopy and ejection seat. The jet, about the weight of an F-16, will use F-16 landing gear and brakes for high-speed landings with touchdown expected at 180 knots, or 207 mph.

Once the full-scale aircraft is complete, NASA will begin flight tests over the deserts of Edwards Air Force Base where so many X-planes have left a legacy of pushing the limits of aircraft capability, dating all the way back to the era of Chuck Yeager. After supersonic flight tests to about Mach 1.4 are completed, NASA plans to fly the aircraft over residential communities to gauge public response to the noise. After the survey, the information could be presented to lawmakers in an effort to loosen supersonic flight restrictions over land. As of now, NASA is targeting 2021 for first flight of the single-engine, single-pilot jet, as Popular Mechanics reports.