With the X-59, NASA and Lockheed Martin want to do the seemingly impossible -- build a jet that flies faster than the speed of sound, without the explosive boom.

The X-59 quiet supersonic airplane.

NASA/Lockheed Martin

In a windowless hangar in the California high desert, the final touches are coming together on an aircraft that could reshape aviation. A needle-nosed airplane that looks more like a futuristic sketch from 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, this is the supersonic airplane of the future. And when it takes to the skies, NASA and Lockheed are hoping you won't even notice it flying by.

I'm at the Armstrong Flight Research Center, just outside of Lancaster, California, to see the X-59 QueSST (short for Quiet SuperSonic Technology) -- a demonstrator aircraft designed to fly faster than the speed of sound without generating an explosive sonic boom. 

A traditional supersonic aircraft can create a sonic boom in excess of 100 decibels when it flies, a sharp sound louder than a fireworks display. It was this disruptive sound that led the Federal Aviation Administration to ban commercial supersonic flight over land in 1973. 

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," says 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."

When the X-59 takes to the skies, the goal is to generate a "sonic thump" and by doing so, convince regulators like the FAA that a 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. If NASA's X-59 QueSST program succeeds, 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. 

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. 

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A mock-up showing the X-59 taking off.

NASA/Lockheed Martin

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," says 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 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. 

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.

At Armstrong, NASA is testing 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 -- he's here to show me how the system works. 

Larson has spent the morning doing a routine test flight in one of NASA's F-15s. Now he's back in the air conditioning from the 114-degree heat outside, where he was putting 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," says Larson. "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

The real test of NASA and Lockheed Martin's efforts will come when the X-59 begins testing. In 2022, Lockheed will conduct initial flight tests to make sure everything is working as expected and the plane is flying with the right speed and altitude. Then Lockheed hands the keys over to NASA, which will begin "acoustic validation" testing in 2023 to ensure the sonic boom has been satisfactorily scaled back to a sonic thump.

This second stage of testing will involve sending 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," says Cliatt, theacoustic 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

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. 

Then comes the third stage of testing in 2024 when the X-59 will be flown over a handful of cities and towns across the United States to gauge the community response. 

At the end of all these rounds of testing, NASA will 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. But if NASA can prove that supersonic planes can fly without the boom, it could open up a whole new world of aviation.

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 says. 

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Getting up close to the build of the X-59 at Lockheed Martin's Skunk Works.

The blink of an eye 

In the hangar at Lockheed Martin Skunk Works, I get a sense of the scale of the X-59 build as it goes through the final stages. The aircraft feels 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, is delighted to show me around. He takes us up around the scaffolding at the top of the plane to point out the electronics being installed by the engineering crew. He hands me a hard hat and takes me underneath the body of the plane to show the sensors that will feed data back to the XVS. He lets 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 will eventually be.

The build is getting close to completion and the team is gearing up for the first flight. 

For NASA's Larry Cliatt, it's been a long road to get this far. 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."