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Dec 30, 2023

Will We Ever Fly Supersonically Over Land?

By Matthew Hutson In 1947, Chuck Yeager, the Air Force test pilot, became the

By Matthew Hutson

In 1947, Chuck Yeager, the Air Force test pilot, became the first person to break the sound barrier. He did it in a tiny, orange-colored plane called the Bell X-1—essentially, a cockpit and two wings connected to a rocket engine. Like all supersonic flyers, Yeager trailed a sonic boom behind him. The principle behind the boom is simple: sound travels through the air in the form of compression waves, so called because they occur as air gets denser and sparser; as a plane flies, the waves expand in all directions at the speed of sound. But when the plane itself exceeds that speed—at around seven hundred and seventy miles per hour at sea level, or around six hundred and sixty at cruising altitude—it catches up to the waves expanding in front of it. They begin to build up, and this single, merged wave reaches the ground all at once, creating a boom. A zone of low pressure follows—the trough of the wave—and then normal air pressure returns, creating its own sound. (Often, sonic booms go boom-boom.) It's no coincidence that sonic booms sound like thunder; thunder is a sonic boom, caused by shock waves expanding around lightning bolts. Bullets travel fast enough to cause sonic booms, as do the tails of whips. Contrary to what you might imagine, a plane causes a sonic boom not just once, when it breaks the sound barrier, but continuously for the entire time that it's supersonic. The boom sweeps over everything below it—a kind of sonic broom that is about a mile wide for every thousand feet of plane altitude.

Plans for the plane that would become the Concorde—the first commercial "supersonic transport," or S.S.T.—began in the nineteen-fifties. NASA began working on supersonic transport upon its founding, in 1958, eventually settling on a design by Boeing. But these initiatives started before sonic booms were fully understood. In a technical summary written in 1960, NASA scientists warned that "shock-wave noise pressures" might be "of sufficient intensity to damage parts of ground building structures such as windows, in addition to causing annoyance." The full extent of that annoyance, however, would take a while to gauge. Over ten months in 1961 and 1962, the Air Force and the Federal Aviation Administration (F.A.A.) ran Operation Bongo, flying B-58 bombers over St. Louis and asking citizens about the hundred and fifty or so booms the planes created; the authors concluded only that, after repeated booms, "some reaction may be expected." ("Sonic boom's a top-priority public-relations problem," an Air Force major told The New Yorker, in 1962.) A clearer picture emerged in 1964, when Operation Bongo II created more than a thousand sonic booms over Oklahoma City. People complained of interruptions to their sleep, conversations, and peace of mind, and about the occasional crack in plaster or glass. By the end, about one in four said that they could not learn to live with the noise. These studies, along with tens of thousands of claims against the Air Force for property damage—horses and turkeys had supposedly died or gone insane—led the F.A.A. to ban civil overland supersonic flight, in 1973.

There are many reasons why the Concorde, which flew for the first time in 1969, stopped flying in 2003. Among them is the fact that the service was allowed to reach supersonic speeds only over the ocean. This month, United Airlines announced plans to purchase planes from Boom Supersonic, a Denver startup that aims to produce a new generation of supersonic passenger planes. But Boom's plane, the Overture, will still boom, and so remain an overseas beast, at least at full throttle. Overland supersonic travel—J.F.K. to S.F.O. in three hours, more or less—depends upon the invention of a quieter boom.

Only in the past twenty years, with enhanced computer models of aerodynamics, has a kind of sonic thump become possible. "The basic theory for sonic-boom shaping actually existed during Concorde's development, back in the nineteen-sixties," Michael Buonanno, an air-vehicle lead at Lockheed Martin, told me. Unfortunately, he went on, "computers weren't powerful enough at the time to run the advanced simulations necessary to really dial in" the ideal shape. In 2003 and 2004, using better simulations, NASA flew the Shaped Sonic Boom Demonstrator, a Northrop Grumman F-5 with a nose job; researchers saved money by grafting a removable portion onto the underside of a preëxisting jet, calling the resulting aircraft the Pelican, because of its bulbous profile. In 2006 and 2007, NASA pursued a similar idea in partnership with Gulfstream, fitting a McDonnell Douglas F-15 with a "Quiet Spike," which protruded some twenty-four feet from its nose.

In both cases, the idea was to round off the peak of the leading compression wave, turning a sharp-edged tsunami into a more gradual swell. Planes, with their distinctive shapes, actually cause many distinct wavelets; as the wavelets approach the ground, they coalesce into the bow and tail waves that cause the booms. If you can modify the plane's shape so that the waves don't combine—by spreading them out, say, by means of an extra-long nose—then the sonic booms will be of a lower intensity. In this regard, the Pelican and the Quiet Spike were modest successes; their booms weren't quite so thunderous. In 2015, JAXA, the Japan Aerospace Exploration Agency, confirmed the basic finding with a smaller-scale project, called D-SEND. The agency dropped a sleek, twenty-six-foot unpowered glider from a balloon nineteen miles above Sweden. It reached Mach 1.39—that is, 1.39 times the speed of sound—and produced a relatively flattened wave.

NASA's current project, the X-59 QueSST (for Quiet SuperSonic Technology), aims both to explore low-boom tech and to study community response to muffled booms. "The airplane is essentially just a boom, or, in this case, a thump generator," David Richwine, NASA's deputy project manager for technology on QueSST, said. Acousticians have many measures of loudness; NASA is using perceived decibel level, or PLdB. The Concorde's boom was around a hundred and three PLdBs, roughly the loudness of nearby thunder, or a car door slamming while you’re inside the car; seventy-five PLdBs, NASA's goal for QueSST, is about an eighth as loud—the equivalent of distant thunder, or a car door slamming twenty feet away. (Like decibels or earthquakes, PLdBs are measured on a logarithmic scale.) Lockheed Martin is currently constructing the plane, which will fly over American cities in 2024. (Buonanno is the company's chief engineer on the project.)

With its pointy nose and delta wings, the one-seat X-59 resembles a mini-Concorde in some ways and differs in others. It will be a hundred feet long, with a wingspan of thirty feet, an engine centered on the tail, and more surfaces than appear necessary: horizontal stabilizers at both the bottom and top of the tail, and also on the nose. "All those are used to tune those shocks," David Richardson, the X-59's program director at Lockheed Martin, said. The team hopes to stretch the front of the boom wave from a single millisecond out to twenty or thirty. ("I’ve been at the Skunk Works for about thirty years, doing a lot of different programs," Richardson added. "This is my first unclassified program—so it's really good to be able to talk about it not only to the world but to my family.")

Ultimately, by running a sort of Operation Bongo III, the X-59 team hopes to persuade the F.A.A. to revisit its 1973 ban on supersonic transport; the agency might agree, instead, to issue certification standards for commercial S.S.T. The plane contains other technology that might translate to a commercial design. One promising feature is the eXternal Vision System, or X.V.S. The X-59 is too pointy for a cockpit canopy, so the team has equipped it with high-definition cameras and monitors; pilots will stare at screens allowing them to look "through" the plane, in a kind of augmented reality. The designers of the Concorde, which was similarly pointy, allowed its pilots to see the runway by means of an elaborate mechanism that physically bent the plane's nose downward before landing—adding great weight and expense to an already over-budget aircraft. Lockheed Martin likely wouldn't make a commercial version of the jet, but it could partner with other firms; the company predicts that a passenger version of the X-59 would be two hundred and thirty feet long, about the length of a Boeing 777, and carry around fifty people.

A few companies are already pursuing low-boom supersonic passenger planes. Gulfstream has obtained patents in the area, and a company called Spike Aerospace says that it's using "Quiet Supersonic Flight Technology" to develop an eighteen-passenger business jet with a sonic boom of seventy-five PLdBs. (Neither company replied to inquiries.)

Exosonic, a California startup, is conducting scale-model wind-tunnel tests of what would be a seventy-seat supersonic plane. Its approach is similar to NASA's: "What we do is we change the shape of the sonic-boom wave form to something that is far less audible," John Morgenstern, the head of aerodynamics and boom at Exosonic, told me. (One of Morgenstern's colleagues has described Exosonic's goal as a sonic "puff.") Last September, the company received a million-dollar military contract to explore the possibility of using the plane as an Air Force One. Morgenstern joined Exosonic in April, after working at Lockheed Martin as a designer on the X-59; in his new role, he has different variables to balance. The plane must be more than just a thump generator—its design must optimize boom intensity, passenger safety, engine noise at takeoff and landing, and fuel efficiency. (The International Council on Clean Transportation has estimated that supersonic planes will burn three to nine times as much fuel per passenger as regular ones—a good reason, as Bill McKibben wrote, earlier this month, for trying Zoom, not Boom.) Exosonic's plane will fly at Mach 1.8, which is an ideal speed for S.S.T.s: slower planes reduce flight times insufficiently, whereas faster ones require noisier engines. I asked Morgenstern if it was risky to invest in a commercial low-boom plane while overland supersonic flight was still banned. "I would say it's less risky than going out there with a plane that doesn't have that technology," he said. He sketched a scenario in which regulations change around 2028 and Exosonic begins test flights four or five years later.

In 2016, the Mercatus Center, a libertarian think tank at George Mason University, published "Make America Boom Again," a white paper arguing that, given new technology, we should bring back supersonic transport. The paper's authors, Eli Dourado and Samuel Hammond, lamented "the stagnation and regress in supersonic aviation," which had broken "a trend of rapid progress" in air travel that had begun with the Wright brothers. And yet there are reasons to believe that, even if it were allowed, domestic supersonic flight would have limited commercial appeal. Richwine, of NASA, told me that he thinks S.S.T. could cut some flight times in half. But, he said, supersonic flight wouldn't proportionately reduce over-all travel time until we fixed our infrastructure: How much better is flying from L.A.X. to J.F.K. in two or three hours if you spend twice that time in airports and traffic?

For most of the years during which the Concorde flew, a traveller could walk into an airport and straight to the gate. In 2013, Doug Robinson, a Utah newspaper columnist, recalled the speed of pre-9/11 airports: "In one of the greatest athletic feats of my life, I once arrived at the curb of the airport three minutes before my plane was scheduled to leave and sprinted up the stairs and down the concourse to the gate, making it just seconds before they closed the door to the plane," he wrote. Today, with increased security, airlines recommend that passengers arrive two hours early for domestic flights, and three hours early for international flights—about the time that supersonic speeds might save. And so there's more than one sense in which supersonic flight is a return to the past. With NASA's fancy technology, we’ll be back to where we were twenty years ago.