The New Supersonic Boom
On 9 April 1945, less than a month before the end of hostilities in Europe, a young Luftwaffe pilot named Hans Guido Mutke put his jet-propelled Messerschmitt Me 262 fighter-bomber into a steep dive, intending to come to the aid of a fellow airman below. As the Messerschmitt accelerated downward, the plane began to shake violently, and the controls became unresponsive. Mutke managed to regain control and lived to describe the incident, in which he later laid claim to having exceeded the speed of sound, a controversial but plausible assertion.
This and similar episodes during and after World War II led some to believe that aircraft would have great difficulty ever "breaking the sound barrier"-a phrase that led to a popular misconception that there is some kind of brick wall in the sky that a plane must pierce to fly at supersonic speeds.
Piloting the bullet-shaped Bell X-1 rocket plane in 1947, Chuck Yeager became the first person to exceed the speed of sound while in horizontal flight.Everett Collection/Alamy
The aircraft that unquestionably tore down that metaphorical wall was the Bell X-1, a bullet-shaped experimental rocket-plane. In October of 1947, test pilot Chuck Yeager coaxed his bright orange X-1 to a speed that slightly exceeded that of sound while the plane was in horizontal flight, although the U.S. Air Force didn't officially announce the feat until the following year.
Since then, jets have been regularly exceeding Mach 1-shorthand for the speed of sound in the surrounding air. Even the Northrop T-38 Talon jet trainer, introduced in 1959, could do so. And some military jets can fly much faster. The SR-71 Blackbird reconnaissance aircraft, which first flew in the 1960s, can travel at better than Mach 3.
Although military aircraft were breaking the sound barrier daily during the 1950s and '60s, commercial passenger flights during this time remained limited to subsonic speeds. That situation didn't change until early in 1976, with the first scheduled flights of the French-British Concorde supersonic airliner, which could reach Mach 2. The Soviet Union's Tupolev TU-144, which could fly just as fast and had been used to transport mail and freight the previous year, began carrying passengers in 1977.
It would have been reasonable to project that we'd all be zooming around the globe at supersonic speeds by now. But, of course, we're not.
At the time, it would have been reasonable to project that we'd all be zooming around the globe at supersonic speeds by now. But, of course, we're not. The Concorde last flew nearly two decades ago. Today's airliners travel no faster than their counterparts of 60 years ago-indeed, they tend to fly somewhat slower to reduce fuel costs.
Now, several aircraft manufacturers and NASA are intent on ushering in a new era of supersonic commercial aviation. They're preparing prototypes for flight and they've got designs for full-blown airliners capable of carrying scores of passengers. And this time, their biggest challenge probably won't be the sonic booms, which backers insist they can adequately address. The main obstacles will be regulatory and, especially, environmental: Supersonic airliners could be hugely more polluting than their subsonic counterparts.
Are we nevertheless on the cusp of a new, golden age of high-speed commercial aviation? Will people soon be jetting across the Pacific in three hours? To answer those questions requires a deeper understanding of what went on, and what went wrong, during that first push to develop supersonic airliners more than a half century ago.
The Concorde, shown here at the start of a test flight in 1970, was particularly noisy, both during takeoff and when exceeding the speed of sound, which subjected people below to the loud double bang of its sonic boom.AP
In 1956, nine years after Yeager's history-making flight, the U.K. government established a Supersonic Transport Advisory Committee, which began discussions with international partners about building a supersonic airliner. And in 1962 the French and British governments forged an agreement to cooperate in the development of what soon became known as the Concorde. The sleek delta-winged airliner made its first supersonic test flight in 1969.
Although the United States chose not to participate in the development of the Concorde, in 1963 President John F. Kennedy announced plans to develop a U.S. supersonic airliner. Shortly afterward, the federal government issued a contract to Boeing, which had prevailed over Lockheed and others in a design competition, to develop such a plane.
Meanwhile, environmentalists were voicing concern-about how noisy such aircraft are taking off, about the possibility that their high-altitude emissions would erode the ozone layer, and about how disruptive the sonic booms would be. The last of these issues was perhaps the most vexing, prompting the U.S. Federal Aviation Administration to mount various exercises to gauge how the public would react to sonic booms.
The most extensive such experiment took place over Oklahoma City in 1964. For months, supersonic aircraft flew over the city, eight times a day, seven days a week, at unpredictable times but always during daylight hours. Dominic Maglieri, an expert on sonic booms whose career began in the early 1950s, recalls the results of those months-long tests.
"It looked as though people were kind of acclimating to it," says Maglieri. "But as it went on that changed-considerably: Pretty soon they were getting thousands of calls and complaints." Some of that negative feedback included demands for compensation, says Maglieri, including one from the owner of a palatial home who claimed that a sonic boom had cracked his marble floors.
The 1964 Oklahoma City tests involved more than 1,000 flights, which sparked more than 15,000 complaints, as documented in a 1971 report prepared by the National Bureau of Standards.U.S. Environmental Protection Agency
Clearly, nobody would accept stone-fracturing sonic booms. Those objections added to the concerns environmentalists were raising about the ozone layer-a scenario seemingly justified a few years later by MIT researchers, who concluded that a future fleet of 500 supersonic airliners would deplete the ozone layer by 16 percent.
Despite strong support from the FAA, the airline industry, and aerospace companies, the U.S. Senate ceased funding the development of a supersonic airliner in 1971. Two years later, the FAA banned supersonic flight over land, a prohibition that remains to this day.
The Concorde went on to serve various destinations, including some in the United States, flying at supersonic speeds only over water. That continued until 2003, when British Airways and Air France retired their fleets, together amounting to just 12 aircraft. (Fourteen production aircraft were manufactured, but one was scrapped in 1994 and another crashed in 2000.)
While the Concorde successfully overcame the technical hurdles standing in the way of supersonic passenger service, it succumbed to economics: The cost of fuel and maintenance was especially high for these planes. A new generation of aeronautical engineers and entrepreneurs are, however, keen to once again take on the technical, environmental, and economic challenges.
It's perhaps unsurprising that the 21st-century push for supersonic travel is being led by newcomers rather than established manufacturers. The best-funded of this group is Denver-based Boom Technology (which also goes by the trade name Boom Supersonic).
This artist's rendering shows Boom Technology's future Overture airliner, which will be able to carry as many as 88 people.Boom Supersonic
In 2016, while it was still in Y Combinator's startup incubation program, Boom got a big shot in the arm from the Virgin Group, which offered engineering support and optioned the first 10 of Boom's airliners. (More recently, Virgin Galactic has been designing a supersonic airliner of its own.) Virgin's interest in this sphere shouldn't be surprising: 13 years earlier, the group's founder Sir Richard Branson attempted, unsuccessfully, to purchase the seven Concorde airliners British Airways was retiring, for use by Virgin Atlantic.
Boom went on to garner more than US $150 million from various venture funds and Japan Airlines. It has used that money to build a one-third scale prototype, called the XB-1, of an airliner that will be able to carry as many as 88 passengers. The company expects commercial flights of the larger plane, which it calls Overture, to begin in 2029.
What these aircraft manufacturers are contending is that their eventual customers are going to be willing to pay to prevent net carbon emissions.
Boom is emphasizing its plans to mitigate the environmental impacts that inevitably arise with supersonic flight. Testifying to a House subcommittee on aviation this past April, Boom's CEO, Blake Scholl, noted that, "sustainable aviation fuels, or SAF, are key to Overture sustainability, and we are designing Overture from the ground up to run on 100 percent SAF, enabling net-zero-carbon flight." In preparation, Boom has investigated the use of biofuels in the engines of its XB-1 demonstrator, and it has partnered with Prometheus Fuels, which will provide the XB-1 with jet fuel synthesized using carbon extracted from the atmosphere using renewable energy.
Boom has stated that its plane will go supersonic only over water. Even so, the company is "shaping the aircraft optimally for sonic-boom reduction," according to its website. In a similar vein, another startup, Boston-based Spike Aerospace, is stressing that its planned S-512 supersonic business jet is "aerodynamically designed to offer proprietary Quiet Supersonic Flight Technology. This will enable it to operate at its full cruising speed of Mach 1.6 (1,100 miles per hour) without producing a loud, disturbing sonic boom on the ground." Ditto for California-based Exosonic, which claims that the supersonic airliner it has on the drawing board "will create a softer thump on the ground that will be quieter than typical traffic."
This artist's rendering depicts NASA's X-59 low-noise demonstrator aircraft, now being constructed by Lockheed Martin.Lockheed Martin
This is exactly the strategy that NASA is exploring with an experimental aircraft called the X-59 QueSST, that name being a contraction of sorts of "quiet supersonic technology." Lockheed-Martin Corp. is right now constructing the X-59 at its famed Skunk Works facility in Palmdale, Calif.
"I used to joke that the airplane looked like an F 16 on steroids," says David Richwine, NASA's deputy project manager for technology on the X-59. "It's a long airplane-I think it's around 97 feet long." Richwine explains that adding length is one of the ways to "manage the sonic-boom signature," which is an engineer's way of saying to make the sound less jarring.
How successful NASA is in doing so will be tested as soon as 2024, when the X-59 is flown over a small set of U.S. cities to gauge the public's reactions to what Richwine expects to be a "sonic thump." Assuming this campaign takes place on schedule, it'll be 60 years after the FAA's Oklahoma City tests. Get your marble floors ready.
Interestingly, the company that was working the hardest to reduce the sonic-boom effects from a supersonic jet it was developing, Aerion Corp., now appears to be going out of business. The company, based in Reno, Nev., was founded by billionaire Robert Bass in 2003.
Aerion's initial foray into commercial supersonic commercial aircraft was to be a 12-passenger business jet, the AS2, designed to have a top speed of Mach 1.4. The company was exploring the possibility of flying the AS2 in a fashion that would allow it to travel at supersonic speeds over land without subjecting the people below to a sonic boom. "Boomless Cruise" was Aerion's name for the technology.
Although we won't get to see it in action with Aerion's AS2, another supersonic hopeful might yet pursue this intriguing strategy, which merits a brief description.
The phenomenon of Mach cutoff requires that the air near the ground be warmer and that the plane fly not too much faster than the speed of sound. Its sonic boom would then travel downward at a shallow angle and be refracted sufficiently to stay away from the ground [left]. A plane moving faster would create a sonic boom that travels downward at an angle that is too steep to be refracted away from the ground [right].David Schneider
The key concept is a phenomenon known as Mach cutoff, the physics of which is straightforward. When a plane flies at supersonic speeds, it outpaces the sound waves it creates. Those sounds pile up, causing a shock wave to form. That boom-inducing shock wave travels away at an angle that depends on how fast the plane is moving relative to the speed of sound. For a jet traveling at many times the speed of sound, the boom propagates at a steep angle from the flight path. For one traveling just barely faster than the speed of sound, the boom propagates at a shallow angle.
That second situation is important here because of another bit of relevant physics: The speed of sound in air depends on temperature. At altitude, where the air is colder, sound travels more slowly than it does in the warmer air near the ground. This phenomenon causes sound waves to refract (bend) as they travel downward, just as light waves refract when moving between water and air or glass and air.
Because of such refraction, sounds traveling downward at a sufficiently shallow angle can be bent upward enough never to impinge on the ground. Similar physics accounts for the mirages you might see when shallowly inclined rays of light are bent upward by the air just above hot asphalt, which gives them the appearance of having reflected off a puddle.
So if an aircraft is flown not too much faster than the speed of sound, in air that is sufficiently warmer near the surface, the sonic boom it creates, loud as it might be, will never reach the ground. You can have supersonic flight without the boom.
Society will have to weigh the environmental consequences of supersonic transport against the time-savings it would offer a relatively select few travelers.
The compromise is that the plane can't travel much faster than the speed of sound-Mach 1.1 or 1.2, tops. That isn't a big improvement over something like the Cessna's Citation X business jet, which can travel at Mach 0.94. Exploiting the Mach cutoff phenomenon commercially would also require the FAA to relax its prohibition on supersonic flight over land, which it may never do.
The companies working hard now to bring commercial supersonic flight back understand that they have to address sonic-boom noise, one way or another. And the farthest along, Boom Technology, is also taking pains to explain how its planes can be flown with fuels that won't add to the enormous amounts of carbon that commercial aviation is already spewing into the air.
"There are a couple of problems with that logic," says Dan Rutherford, who is aviation and shipping program director for the International Council on Clean Transportation. "First of all, once the plane is out the door, there's very little control that a manufacturer has over what fuel is used." What these aircraft manufacturers are contending is that their eventual customers are going to be willing to pay to prevent net carbon emissions. "The planes themselves are not going to be fuel efficient," says Rutherford. He and two colleagues estimated in 2018 that a commercial supersonic airliner like the one Boom is designing would likely use five to seven times as much fuel per passenger-kilometer as a comparable subsonic aircraft.
Rutherford further notes that biomass-derived jet fuels are at least three or four times as expensive as conventional jet fuel and that synthetic jet fuel made from carbon extracted from the atmosphere will be more expensive still. Combine those higher fuel costs with the higher fuel consumption and "you start to have such high operating costs for those planes that it is very difficult to see them succeed in the market," he says.
This past June, United Airlines announced its intention to purchase 15 Overture airliners from Boom Technology. They will presumably resemble this artist's rendering after they go into service.Boom Supersonic
But Michael Leskinen, vice president of corporate development for United Airlines, which in early June announced plans to purchase 15 of Boom's Overture airliners, explained to IEEE Spectrum, "We'll be working to introduce and supply the market with more and more sustainable aviation fuel, and our hope is that with more supply, we'll be able to drive that cost of fuel down as well." Still, it's easy to imagine that the economic pressures would be such that, even if United sticks to using sustainable fuels, other operators would end up flying the aircraft with conventional jet fuel, boosting carbon emissions from air travel by five or more times per passenger-kilometer flown.
But it gets worse, according to Rutherford. "If you look at the other emissions from supersonics that also warm the planet-these are the nitrogen oxides, the particulate matter, and the water vapor for supersonics operating in the stratosphere-those could be even worse for the climate, on the order of 20 times or more just because the pollution stays up in the atmosphere so much longer."
Rutherford admits that the science of these noncarbon effects is less certain than it is for CO2. But as was true for concerns about the ozone layer back in the 1960s, proponents of supersonic commercial aviation need to consider the deleterious effects of all the pollutants these planes create and their extended residence times at the altitudes these planes fly. Will they actually do that?
"We're committed to being 100 percent green," Leskinen says. "That's across the spectrum of impacts that our aircraft have. And that will be no different for Overture than it is for any other aircraft we choose to operate." It's a grand promise, but even if United can keep to it, it's a promise that the company is making for 2050, not for 2029 when the Overture will be introduced.
Larger society will have to weigh the likely environmental consequences of supersonic transport against the time savings this futuristic mode of transportation would offer a select few travelers. There are, of course, many ways this could play out over the coming decades, perhaps with different nations adopting different policies. What seems certain, however, is that Adam Smith's invisible hand will exert considerable influence, just as it did for earlier supersonic wonders: the Concorde and the space shuttle. In the end, both proved technological dead ends simply because they cost more to operate than their services were worth.