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Supersonic Travel Without the Sonic Boom: Inside NASA's X-59 Plane

CNET 12/6/2022 Claire Reilly
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For decades, flying faster than the speed of sound has meant speeding across the skies in an aircraft that creates a powerful sonic boom -- a huge noise that travels down to the ground below like a crack of thunder. 

But imagine being able to travel across the world at over 1,000 miles an hour, without that off-putting and startling noise. Suddenly a new world of consumer travel and aviation would open up -- something that has not been possible for decades. 

NASA wants to make this dream a reality. At the Armstrong Flight Research Center, just outside of Lancaster, California, the space agency is working on the X-59 QueSST (short for Quiet SuperSonic Technology) airplane -- a demonstrator aircraft designed to fly faster than the speed of sound generating nothing more than a "sonic thump,"

Traditional supersonic aircraft can create a sonic boom in excess of 100 decibels during flight -- a problem that led the US Federal Aviation Administration to ban commercial supersonic flight over land in 1973. 

A mock-up showing the X-59 taking off. NASA/Lockheed Martin © Provided by CNET A mock-up showing the X-59 taking off. NASA/Lockheed Martin

But the X-59 has been shaped to minimize the shock waves that cause a sonic boom midflight, reducing its sound at ground level to 75 decibels. According to NASA, that's about as loud as a car door slamming down the street. 

To design this "low-boom" aircraft, NASA and Lockheed Martin returned to the basic principles of aerodynamics. The result is an airplane that is both incredibly advanced and elegantly simple. 

"Those principles of physics, of aerodynamics, have been around since the beginning of time," said Lockheed Martin's X-59 program director, David Richardson. "This is what Mother Nature wants to see. Just like birds are perfectly designed, this airplane is being perfectly designed to fly supersonic as quiet as it can."

In a windowless hangar in the California high desert, workers have been assembling the X-59 and putting it through the paces for its first test flight. Up close, the needle-nosed airplane looked like it was pulled from the pages of a 1950s sci-fi comic -- all sweeping lines and unbroken curves, a narrow cockpit concealed in the center. Designed and built by NASA and Lockheed Martin, it's billed as the supersonic airplane of the future. 

The goal is to convince regulators like the FAA that the ban on supersonic passenger travel over land can be overturned. That change could open the door to a future where supersonic travel is no longer just for fighter pilots, and flying faster than the speed of sound may be possible again for the first time since the Concorde was retired in 2003.

The science of sound

To understand how a sonic boom works, you need to know a little something about the basic physics of sound. 

Sound is essentially a wave of compressed air -- imagine it like a pulse in a slinky, moving from point A to point B at a speed of roughly 340 meters per second. When a plane flies through the air, it pushes air out in front of it, creating those compression waves. 

But when a plane flies at supersonic speeds (above Mach 1), it's traveling faster than those waves of compressed air can move out of the way. As a result, the plane generates shock waves that travel down to the ground where they are perceived as a sonic boom. 

When a plane flies, it pushes waves of compressed air out in all directions. When it flies at supersonic speeds (faster than the speed of sound), those waves coalesce and produce a shock wave that is heard on the ground as a sonic boom. Amy Kim/CNET © Provided by CNET When a plane flies, it pushes waves of compressed air out in all directions. When it flies at supersonic speeds (faster than the speed of sound), those waves coalesce and produce a shock wave that is heard on the ground as a sonic boom. Amy Kim/CNET

Any big variation in shape on the body of the plane, like the cockpit jutting up at the front or the tail sticking up at the back of the plane, can produce a shockwave. To minimize the shockwaves that travel down to the ground, you need to change the shape of the plane and make it far more streamlined, smoothing out the variations in shape and spreading them out across a much longer body. 

That's what NASA and Lockheed have done with the X-59. The plane is 99 feet, 7 inches long, but only carries one passenger; at over 30 feet long, the nose takes up roughly one-third of the plane and leads seamlessly to the swept-back wings and a single engine at the rear. 

According to Larry Cliatt, NASA's acoustic testing technical lead for the X-59, all those features combine to make sure the shockwaves being produced midair are "well behaved."

"We want to keep [the shock waves] parallel and separated from each other so they don't combine into a loud sonic boom," said Cliatt. "So we're dragging out those volume changes, making them very gradual across the entire body of the airplane."

A new way of flying

The X-59 is so long and streamlined that its cockpit has no forward-facing window.

Instead, the pilot uses an External Vision System (XVS) created by NASA to fly the plane. The XVS uses two cameras above and below the aircraft to create a real-time view of the front of the plane shown on an HD screen. But the XVS also acts as a head-up display, or HUD, showing data such as altitude, airspeed and flight path. 

At Armstrong, NASA has tested that XVS in its X-59 flight simulator. NASA test pilot Nils Larson will be one of the pilots who eventually flies the X-59 using the XVS and he showed me how the system works. 

NASA test pilot Nils Larson in the X-59 flight simulator, using the same External Vision System that will be used in the final X-59 aircraft. John Kim/CNET © Provided by CNET NASA test pilot Nils Larson in the X-59 flight simulator, using the same External Vision System that will be used in the final X-59 aircraft. John Kim/CNET

After spending the morning doing a routine test flight in one of NASA's F-15s, Larson stepped in from the 114-degree heat outside, back into the air conditioning, and put the flight sim through its paces. For Larson, the experience of flying with a cockpit window and using the XVS display isn't all that different. 

The benefit comes with combining the real-world view from the cameras with the kind of data you see on a monochromatic head-up display in a fighter jet. The XVS lets pilots see flashing warnings or colored text over the horizon, things they wouldn't ordinarily see through a cockpit window. 

"You use it just like you would any other window," Larson said. "But because it's a display, it actually gives us more capability than you might have if it was just a window."

The sonic thump

A Lockheed Martin technician works on a 11.5% scale model of the X-59's forebody during wind tunnel testing. Lockheed Martin © Provided by CNET A Lockheed Martin technician works on a 11.5% scale model of the X-59's forebody during wind tunnel testing. Lockheed Martin

Throughout 2022, Lockheed and NASA have been conducting initial checks on the X-59, but the real test of the aircraft comes with the first flight. That happens in 2023 when in what's known as the "acoustic validation" phase, when NASA will fly the X-59 to ensure the sonic boom has been satisfactorily scaled back to a sonic thump.

NASA will send the X-59 up with an F-15 fighter jet that will act as a chase plane, measuring the shockwaves being produced by the aircraft midflight. And perhaps most impressive of all, NASA will capture images of the shockwaves -- a process that's known as schlieren photography. 

Photographing a plane moving faster than the speed of sound is no easy feat.

"The X-59 has to eclipse the sun because we use the sun as a backdrop," said Cliatt, the acoustic testing lead. "All of that has to happen perfectly. It's like threading a needle to get that gorgeous image."

Schlieren images like this one can capture the shockwaves coming off supersonic aircraft midflight. NASA © Provided by CNET Schlieren images like this one can capture the shockwaves coming off supersonic aircraft midflight. NASA

But the big decider will be the sound on the ground. In the acoustic testing phase, NASA will set up an array of microphones across a 30-mile-long stretch of the Mojave Desert in California to measure the sonic thump and make sure it's as quiet as intended. 

After the X-59's big sound check comes the third stage of testing in 2025, when the aircraft will be flown over a handful of cities and towns across the United States to gauge the community response. NASA will then submit its data to regulators with the goal of changing the restrictions around supersonic flight.  

After all, back in the '70s when the Concorde started flying and the FAA introduced its ban on commercial supersonic flight over land, noise was the problem. That limited Concorde to trans-Atlantic flights and ultimately sounded the death knell for the company. But if NASA can prove that supersonic planes can fly without the boom, it could open up supersonic travel to a new generation.

The X-59 could pave the way for private companies and airlines to reintroduce supersonic flights to everyday passengers, all across the world. According to Lockheed Martin's David Richardson, flights for the general public could come as soon as 2035. And they'll be a game changer.

"You don't just see this demand from high-end consumers, you see this from everybody -- everybody would like to 'get there' faster," he said.

Getting up close to the build of the X-59 at Lockheed Martin's Skunk Works. Logan Moy/CNET © Provided by CNET Getting up close to the build of the X-59 at Lockheed Martin's Skunk Works. Logan Moy/CNET

The blink of an eye 

Visiting the hangar of Lockheed Martin Skunk Works, I got a sense of the scale of the X-59 build. The aircraft looks more like a giant dart than a plane, with those swept-back wings and the nose that stretches out for yards and yards. 

Richardson, who has hitherto worked on highly-classified projects for Lockheed, was my guide for the day, showing me around the scaffolding at the top of the plane to point out the electronics being installed by the engineering crew. He handed me a hard hat and took me underneath the body of the plane to show the sensors that will feed data back to the XVS. He let me pop up in the cavity where the landing gear will go and gaze out through the skeleton of the plane, looking out where the engine would later be installed.

For NASA's Larry Cliatt, the X-59 program has involved years of designing, testing and building that will all lead to one moment of truth during that first test flight. 

"We're going to have a lot of people staring at data, waiting to see the very first sonic thump from the X-59 to make sure all of our work has paid off," says Cliatt. "You know, it's going to happen in the blink of an eye. A sonic boom is 200 milliseconds long. And that's what all of this is about -- 200 milliseconds."

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