Supersonic Flight Didn’t End With Concorde. It’s (Quietly) Coming Back.

The only passenger jet to fly faster than the speed of sound—the Concorde—made its last flight in 2003. Bob van der Linden, a curator at the National Air and Space Museum, was a passenger on that final flight. About 30 minutes in, the jet hit Mach 2, or twice the speed of sound and about 1,350 miles an hour, without a bump, he wrote in a story for Air & Space magazine a year later. He noted that the windows felt warm—from the heat of their passage—while the sky outside looked "a stunning dark purple."  

Concorde flew its first passengers in 1976, cutting flight times between New York and London in half. But its noise, especially its loud sonic boom, was so intense that aviation agencies restricted it to flights over the sea. The jets’ massive fuel consumption partially drove the high price of a seat—several times that of a first-class ticket. But customers didn’t want to pay, and most Concorde flights were half-empty. Operators finally pulled the plug after the disastrous Air France crash in 2000 and a sharp downturn in air travel after 9/11. Many analysts thought that the end of Concorde also meant the end of the era of supersonic passenger travel.

But more than twenty years later, dueling projects from NASA and private aerospace startup called Boom Supersonic are keeping the dream alive. Earlier this year, the space agency’s X-59 experimental jet broke the sound barrier for the first time. Meanwhile, Boom has already conducted several successful supersonic flights of a test jet called XB-1, or "Baby Boom." Both projects aim to overcome the same noise and fuel issues as Concorde, but if successful, they could transform air travel by slashing travel times.

"I want there to be a future where we have to teach our kids that we didn't just always fly supersonic," says Boom’s CEO Blake Scholl. His company is aiming for the first commercial flight of its Overture aircraft in 2030. The full-size aircraft will be roughly three times the size of the XB-1, and the company is due to begin testing its crucial jets in a matter of weeks.  

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Opening supersonic routes over land will require the elimination of the sonic boom. Concorde's jets rattled windows in England, and people on the U.S. East Coast heard the "double thunderclap" of its passage even when indoors.

The physics seem inescapable: A sonic boom is the shockwave caused when an object exceeds the speed of sound in air. But Boom has developed a strategy it calls "Boomless Cruise": Over land and at high altitudes, their aircraft would slow from its usual Mach 1.7 speed to between Mach 1.1 and 1.3. When the air temperature is low enough (a data point that will be monitored from the Overture cockpit), layers of the atmosphere change the direction of sound waves from a jet, resulting in a boom that's inaudible on the ground. The technology has been successfully tested on Boom's XB-1, which has now exceeded supersonic speed six times—without audible sonic booms.

NASA is testing a different approach through a mission called Quesst, for Quiet Supersonic Technology. Instead of trying to reflect sound upwards, as Boom does, its X-59 experimental jet—built by Lockheed Martin—uses an elongated nose and other radical geometries to reduce the sonic boom as much as possible. Unfortunately, its nose is so long that a pilot can't see over it and must fly with cameras and a screen. Witnesses at the June test flight described the noise of the shockwave as a "quiet thump," rather than the thunder of Concorde.

Scholl argues that Boom's idea is better: "Our 'Boomless Cruise' technology… makes the sonic boom disappear entirely compared to X-59, which seeks to attenuate a still-audible but quieter boom," he says.

But aerospace engineer Nicole Viola of Italy's Polytechnic University of Turin thinks that NASA is a technical step ahead. While the unmitigated sonic boom of Concorde was a "first generation," Boom's Overture represents a "second generation" of supersonic aircraft that seeks to reduce the sonic boom by working around it, while the X-59 represents a "third generation," which seeks to reduce the sonic boom at its source, she says. The NASA approach would make higher speeds possible, and the sound of a sonic boom might be reduced to the level of a car door closing across the street.

Engineer and scientist Bernd Liebhardt, who studies passenger supersonics at the German Aerospace Center in Hamburg, fears the design techniques from the X-59 won't be enough to reduce "superbooms"—the intense sonic booms caused by acceleration and maneuvering. "The X-59 campaign should not be considered a concluding effort, but rather a first step towards a resilient solution," he says.

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Quieter supersonic jets would mean vast in improvements in travel time. New York to London by Concorde, for example, took three and a half hours, and Boom estimates that route would take about four hours on its future jets. But that speed will come with a major environmental tradeoff: lower fuel efficiency.

Boom has also designed ultra-efficient “Symphony” engines to get more thrust from jet fuel with lower emissions than Concorde. The fuel savings mean that seats from New York to London on Overture could be a quarter of the price on an equivalent Concorde seat, or about the same as business class today, Scholl says. The company recently announced that their Symphony design would be installed as turbines at natural gas power plants—a big endorsement.

Boom and other operators of supersonic passenger aircraft still face an environmental battle: Despite advances in engine efficiency, supersonic passenger jets will still use a lot more fuel than regular jets on the same routes—up to nine times as much. Supersonic jets would inevitably result in an increase in carbon emissions by a similar factor; Boom plans to offset this by using aviation fuel from sustainable sources, such as fuel recovered from industrial and urban waste.

Boom isn’t the only company to try to bring supersonics back, but other efforts have collapsed or been sidelined. Once completed, the company’s full-sized Overture jet, a sleek delta-winged dart, will be hold between 64 and 80 passengers (Concorde was usually equipped for up to 100 passengers.)

Boom is banking on their supersonic fleet getting extensively utilized, potentially resulting in lower costs and cheaper tickets: "Supersonic flight can be affordable for tens of millions of passengers," Scholl says. "Our ultimate goal is to make it available to everyone."

But some industry experts think Boom's 2030 start date is overambitious, not least considering the bureaucracy involved—safety tests, certification for commercial use, and more. "This will take years, even if there are no technical problems with a brand-new aircraft with brand-new engines and systems," says van der Linden.

Aerospace engineer Iain Boyd of the University of Colorado Boulder agrees: "While Boom is continuing to make progress, they have not yet flown any passengers at supersonic speed," he says. "There is a tremendous amount of qualification and safety work to be done."

Even with the hurdles facing supersonic passenger jets, some want to push the throttle even further. There's now talk of "hypersonic" passenger jets, which would fly at more than five times the speed of sound and cut the New York to London time to under two hours; sub-orbital rocket flights for passengers, which have also been proposed, could reduce that time even further. One proponent, an aerospace company called Hermeus, plans a 20-seat hypersonic jet called Halcyon that would fly at over 3800 miles an hour, traveling from New York to London in about 90 minutes.

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But many experts think hypersonic flight will not be feasible for a very long time. "We first need to see some kind of safe and repeated super-highspeed cargo transportation before we can even think about hauling humans," Liebhardt says. "Aviation has tremendously high safety standards that have been built over a whole century, and it doesn't look like they are up for debate."

Boyd adds that passenger rocket flights will likely happen first. "Hypersonic flight is even more challenging from a technology and human safety perspective than supersonic flight," he says, while safe rockets flights for space tourists are already happening: "I predict we will not see a hypersonic aircraft for many decades."

Van der Linden notes that such superfast aircraft and rocket flights face an additional problem: Their destination might be closed when they arrive, which wasn't so much of a worry before. A morning sub-orbital rocket flight New York, for example, would take less than an hour to get to Tokyo but would arrive there shortly before midnight. He thinks this issue has been insufficiently discussed: "What good is traveling to a distant city for a pressing meeting when you arrive too late or too early to conduct business?"