Low Boom, High Pollution? NASA Readies for Supersonic Test Flight

At the end of the last century, a wealthy few lived the dream of flying faster than the speed of sound, crossing the ocean in champagne-and-caviar style in a needle-nosed jet called the Concorde.

But commercial supersonic aviation’s 27-year run sputtered to a stop in the 21st century as fuel, safety and maintenance costs soared at the same time passenger interest waned amid the economic and political turmoil of a post-9/11 world.

By then, a slew of environmental hazards—the outsized carbon emissions from the fuel-thirsty aircraft and damage to the ozone layer as well as window-rattling noise pollution on the ground—made the Concorde seem like a dinosaur of the skies. 

Now, the U.S. space agency NASA is working to revive the dream. It is preparing to break the sound barrier again—with an experimental “low-boom” aircraft that it says could usher in a new generation of ultra-fast commercial air travel. The agency told Congress it is aiming for the first test flight by October of its “Quesst” mission (a play on “SST,” supersonic transport).

With new private companies already working to commercialize supersonic aviation, NASA officials say it is fitting for the U.S. government to tackle some of the challenges. The project, they say, is in line with the agency’s mission of creating a future for air transportation that is “transformative, global and sustainable.”

“As you can imagine, the ability to get somewhere twice as fast is certainly transformative for global aviation,” said Peter Coen, NASA’s Quesst mission integration manager. He said the agency’s vision is for supersonic transportation that is environmentally sustainable and available to the traveling public at large, not just the wealthy.

“There is no middle space where supersonic aircraft succeed and they aren’t a major polluter—that future basically doesn’t exist,”

But critics say the expensive program, a partnership with Lockheed Martin—now expected to cost $840 million, 80 percent over its initial budget—is pouring resources into a quest that the agency’s own research shows would worsen the damaging climate impact of global aviation. They believe it is diverting attention from research needed to cut the industry’s carbon emissions. Instead of tackling aviation’s biggest sustainability problem, one of the nation’s premier science agencies—known as a world leader in climate research—is spending more money on supersonic than on any other “X-plane,” as NASA’s experimental aircraft are called.

“There is no middle space where supersonic aircraft succeed and they aren’t a major polluter—that future basically doesn’t exist,” said Dan Rutherford, senior director of research for the International Council on Clean Transportation (ICCT), a nonprofit think tank. “Either they succeed, and really harm the environment, or they’re not going to succeed. And NASA’s work is clearly aimed at opening up option one.”

Lobster in the Sky, Shuddering on the Ground

U.S. Air Force test pilot Chuck Yeager showed it was possible to break the sound barrier in a rocket-powered flight over the California desert in 1947. But it took decades to bring commercial supersonic service to what would turn out to be a small segment of the general public.

Many airlines showed initial interest in supersonics, but dropped out after 1973. That year, after much debate, the Federal Aviation Administration (FAA) banned supersonic flight over land because of sonic booms, the thunderous noises that supersonic aircraft generate because of the shock waves they create. From the ground, people heard one or two explosive bursts as the supersonic jets of that era passed overhead, but the booms trailed the aircraft continuously like a wake along the flight path, often causing objects on the ground to vibrate when the planes were nearby. 

British Airways and Air France pushed ahead with plans to fly supersonically only over water; both were then government-run operations of the two nations that spent billions of dollars developing the Concorde. They had to overcome lawsuits, local protests and the objections of the U.S. Environmental Protection Agency (then concerned about both noise pollution and ozone layer damage), before beginning service at John F. Kennedy Airport in October 1977.

The Concorde cut travel time between New York and Paris or London in half, to roughly three hours, for a select set of passengers that could afford it, including Princess Diana, Henry Kissinger and Paul McCartney. Flights featured caviar as a starter and entrees like lobster and duck à l’orange served on fine china. Engineering buffs marveled at the Concorde’s then-groundbreaking “fly-by-wire” computer controls, its moveable nose that gave pilots better visibility and its fuselage that expanded with the intense heat of high-speed flight.

But the Concorde never recovered after a New York-bound Air France flight crashed in July 2000 after take-off from Paris, killing 114 people. The fleet was grounded for safety upgrades, and by the time it took flight again—in November 2001—airlines were scaling back due to the drop in air travel and new security costs after the 9/11 terrorist attacks. The airlines slashed the price of a roundtrip transatlantic Concorde ticket in half, to the equivalent of $10,800 in today’s dollars, before retiring the aircraft for good in fall 2003.

“I don’t think there will be too many people around here shedding a tear,” one west London resident told Agence-France Presse, recounting how light bulbs burst, crockery shuddered and window frames shook violently as the aircraft roared overhead. A Green Party official offered an epitaph in a letter to the editor of the local newspaper in Gloucestershire, England, near the Concorde factory: “It might be seen to epitomise the fossil-fuel era—human genius applied through motives of privilege and profit, unintentionally causing all sorts of harmful side-effects.”

Those working on supersonic flight today, including NASA, say they are not aiming to revive the excesses of the Concorde era, but to make ultra-fast commercial air service available to more of the public without adverse environmental impacts.

While the new supersonic designs appear to be more fuel efficient than the Concorde (by one 1979 estimate, it burned 16 times more fuel per passenger-mile than a Boeing 747), they still would be fuel-guzzlers. NASA-funded research at the Massachusetts Institute of Technology concluded new supersonics would burn 3.2 to 10 times more fuel per passenger-mile than conventional aircraft. Researchers at ICCT, a well-regarded research institution that helped uncover the Volkswagen diesel emissions scandal, came up with an estimate in the same ballpark: New supersonics would use five to seven times more fuel.

The technology’s proponents argue either that the additional greenhouse gas emissions would be negligible because the supersonic market would be so small or that the problem could be overcome with the use of “sustainable aviation fuel.”

Recent research on the new supersonic designs, including the NASA-sponsored studies out of MIT, undercut both of those arguments. But a handful of U.S. companies are moving forward with supersonic aircraft, most of which would still produce sonic booms.

The most prominent developer is the 10-year-old Denver-based startup Boom Supersonic. Founded by tech entrepreneur Blake Scholl, it has pre-orders from United, American Airlines and Japan Airlines for its still-in-development Overture aircraft. Boom has attracted more than $700 million in investment, including from founders and backers of Google, Stripe and Airbnb, and—as of last October—an investment fund backed by the kingdom of Saudi Arabia. In a presentation obtained last year by the U.K.-based Centre for Climate Reporting, the Saudi government’s “Oil Sustainability Project” said the nation hoped to fast-track supersonic technology specifically because it “consumes more energy” than conventional aviation. 

In June, Boom held a ribbon-cutting ceremony for its first factory, in Greensboro, North Carolina. Scholl has said Overture aims to debut by 2029 with roundtrip transatlantic flights priced at $4,000 to $5,000—hardly a bargain, but about a quarter of what a full-price Concorde ticket would cost in today’s dollars.

There have been stumbles. Aerion Supersonic, a company chaired by Texas oil billionaire Robert Bass and backed by Boeing, ceased operations in 2021, only a year after Gov. Ron DeSantis announced the company would build a huge headquarters and factory in Florida. The 18-year-old company blamed difficulties in raising capital.

Research shows the business case for supersonic aviation is questionable as long as regulations are in place to restrict sonic booms over land. In one NASA-funded study, a team at MIT’s Laboratory for Aviation and the Environment concluded that more than 78 percent of potential supersonic demand would be for flights over land, with North America accounting for the largest share—36 percent—of the prospective market. 

NASA says this is why its low-boom research is essential. “You can’t really have a transportation system component that’s transformative if it can only be used over water,” Coen said.

‘More Like a Thump Than a Bang’

NASA’s breakthrough was discovering a way to better control the pressure changes, or shock waves, supersonic flight creates.

“Sonic booms happen because the air doesn’t know the plane is coming,” Coen said. NASA researchers found that reshaping the aircraft so shock waves are similar in strength and evenly spaced along the plane, ensures pressure increases gradually instead of instantaneously. 

“By smearing those shockwaves we get a more gradual pressure rise in the sound that’s not so jarring to the ear,” Coen said. “It’s more like a thump than a bang.”

NASA has estimated the Quesst aircraft, the X-59, would generate a 75-decibel thump, which it likened to the sound of a car door slam 20 feet away. At industry workshops, the agency shows this on a scale as louder than distant thunder, but not as loud as a basketball bounce or a hand clap. The Concorde’s sonic boom, in contrast, was about 105 decibels.

Still, a 75-decibel boom would be on the borderline between safe and hearing-damaging noise, according to U.S. health and safety agencies. And it would be louder than the FAA’s 65-decibel threshold for significant aircraft noise, although length of exposure would matter, since the agency uses a 24-hour average to gauge if an aircraft is too loud.

NASA plans to test community tolerance levels for the Quesst aircraft’s sound. Beginning as early as 2026 at locations still to be decided, the agency will carry out month-long “socio-acoustic” studies, flying the X-59 over up to five residential areas across the U.S. and surveying thousands of people on the ground about whether they were startled, bothered, disturbed or annoyed by the sound.

There is a policy purpose to the data-gathering: to make it easier for the new generation of supersonic aircraft to fly over land, opening up the largest potential market for ultra-fast travel. NASA wants its findings to help shape new supersonic flight standards by the FAA and the International Civil Aviation Organization (ICAO), the UN agency that helps set standards worldwide. The speed limits now in place could be replaced with sound limits instead, which would allow supersonics to be louder than today’s conventional aircraft.

President Donald Trump’s administration supercharged this effort, creating a momentum that continues to propel the project in the administration of President Joe Biden. Supersonic research was elevated as a priority for NASA in one of the first bills Trump signed after taking office: the NASA Transition Authorization Act of 2017. The bipartisan legislation, meant to reaffirm the U.S. commitment to human space exploration, sailed through Congress on voice votes, cosponsored by senators from two big space program states, Republican Ted Cruz of Texas and Democrat Bill Nelson of Florida. Nelson, one of a handful of Congress members to have flown in space, was appointed NASA administrator by Biden in 2021.

NASA, which had long been seeking authority to demonstrate its “low-boom” concept, awarded Lockheed Martin the contract to design and build its supersonic X-plane the following year.

Trump also signed the FAA reauthorization bill of 2018, another sprawling bipartisan measure that included a directive for the FAA to “exercise leadership” in developing new policy ​​”relating to the certification and the safe and efficient operation of civil supersonic aircraft.” Among those leading calls for new FAA supersonic policy were three Republicans, Sen. Mike Lee of Utah, who would like to see a U.S. Army site in his state used for supersonic testing, former Sen. Cory Gardner of Colorado, home of Boom Supersonic and former Rep. Mark Sanford of South Carolina, where Lockheed Martin has a major presence. All three received campaign contributions that election cycle from Boom CEO Scholl, who had not previously been a political donor, but doled out $17,500 in 2017 and 2018, according to data compiled by the watchdog group OpenSecrets. Scholl’s recipient list also included then-leaders of the Senate Committee on Commerce, Science and Transportation, which had responsibility for the NASA and FAA legislation: Sen. John Thune (R-S.D.) and Nelson, then the panel’s highest ranking Democrat.

Taking up the Congressional directive, Trump’s FAA in 2020 proposed a dramatic rule change that would have allowed supersonics to be noisier on take-off and landing than conventional aircraft—a plan that was never finalized. But three days before Trump left office, the FAA streamlined its process for authorizing supersonic test flights. Under Biden, the FAA used that process for the first time this past April, granting Boom permission to conduct up to 20 supersonic test flights over the next year in restricted military airspace near Edwards Air Force Base in California.

The jobs and economic activity that a new class of aircraft would generate—especially a class as complex as supersonic—means the program has plenty of supporters on both sides of the aisle. The current chair of the Senate’s Committee on Commerce, Science and Transportation, Democrat Maria Cantwell of Washington, expressed excitement about NASA’s planned low-boom test flight at a hearing earlier this year. Washington is home to Boeing and a $70 billion aerospace industry that exports more aerospace products annually than the next seven aerospace states combined, including California, Texas and Florida.

In May, Biden signed a new bipartisan FAA reauthorization bill co-sponsored by Cantwell and Sen. Jerry Moran of Kansas, the highest ranking Republican on the aviation subcommittee, who in 2020 had helped broker a deal with the FAA establishing a supersonic test flight corridor in federal airspace in his state. The new FAA legislation directs the agency to consult with both NASA and the industry and report back to Congress within a year on what further research is needed to advance new U.S. and international policy on supersonic aviation, including for overland flight.

NASA has made clear that it hopes its Quesst research will help inform decision-making at ICAO, which is now studying sonic boom impacts. The Trump administration had urged ICAO to adopt new international standards, but that effort met pushback from the European Union and the international organization representing airports.

“The environmental impact of this type of aviation remains a major concern in Europe and a challenge that must be overcome before considering their introduction into the global air navigation system,” said an EU coalition in a presentation to ICAO. The Airports Council International (ACI) opposed different standards for supersonic and conventional aircraft, saying it could undermine years of work by airports to reduce noise and carbon emissions. 

Both the EU and the ACI expressed concern that ICAO was limiting its study on supersonics only to the potential impacts of the sonic boom. “Emissions and other effects on the climate have to be addressed as well,” the EU coalition wrote.

Climate and Ozone Trade-Offs, to ‘Shrink the Globe’

Scientific studies indicate the environmental impact of a new fleet of supersonics would be significant. MIT researchers calculated that without overland flight restrictions, supersonics could grab 2.5 percent of the world market for air travel by 2035, increasing global aviation fuel burn 7 percent, and nitrogen oxide (NOx) emissions 10 percent.

Rutherford, of ICCT, estimated the potential impact another way. If Boom met its sales goals of 2,000 supersonic aircraft serving 500 routes by 2035, ICCT calculated that would translate to 5,000 supersonic flights a day (about 5 percent of the number of daily flights worldwide today.) That would generate 96 million metric tons of carbon dioxide annually—roughly the combined emissions of the entire fleets of American, Delta and Southwest airlines in 2017.

“Economics is still the problem with supersonic.”

The prominent atmospheric scientist Donald Wuebbles, who served as a White House climate adviser in President Barack Obama’s administration, has been hired by Boom Supersonic to assess the climate impacts of its airliner model, Overture. He said fleet size is important to consider when talking about the overall impact of supersonic on climate.

“Economics is still the problem with supersonic,” he said. “If we had enough supersonic aircraft flying that they become the most dominant part of the market, then you’d be really concerned about those emissions and their effects on ozone, even using [sustainable aviation fuel].” But whether it is possible for even one company to make supersonic flight profitable remains to be seen, let alone on a much larger scale like NASA envisions, he said. 

Rutherford, however, maintains that if NASA succeeds in solving the sonic boom, it will pave the way for a new supersonic industry that is necessarily large in order for companies to raise capital. “To get any kind of plane built today, you have to have a sizable market,” he said.

“So we’re in this Catch-22,” Rutherford said. “If the environmental impacts are small, it implies that the market is small, which means the plane will never get built. If Boom is correct and there’s a huge market, they will go with the plane and the emissions will be very substantial.”

And there are other environmental concerns. Supersonic aircraft fly in the stratosphere, the thin upper portion of the atmosphere, to reduce drag. In the lower atmosphere, aircraft NOx pollution acts as an indirect greenhouse gas, generating ground-level ozone. But in the stratosphere, NOx deplete the ozone layer that protects Earth and human beings from harmful radiation. It’s a problem that has been understood since the 1970s. Wuebbles was one of the pioneering researchers who demonstrated the effect.

“A single molecule of nitrogen oxide emissions could destroy hundreds of thousands of ozone molecules,” Wuebbles said in a recent interview.

Working again on supersonic flight has given Wuebbles a feeling of “deja vu,” he said. Even though the models used today to study supersonic flight are far more complex than when he began his career, and far more is known about climate effects, he said the results are “very similar.”

 “We captured the fundamental aspects of the issue back then,” Wuebbles said. 

Supersonic aircraft that operate on regular petroleum jet fuel would release another ozone depleter—sulfur pollution—into the stratosphere. They also release two greenhouse pollutants, water vapor and black carbon, which disappear quickly in the lower atmosphere, but would linger for years in the stratosphere.

Boom, which says that it is committed to net zero carbon operations, has pledged to build Overture as an aircraft that is “optimized for 100 percent sustainable aviation fuel,” biofuel made from corn grain or stalks, agricultural waste, used cooking oil or a wide variety of other biomass.

Biofuel still generates carbon emissions, but proponents argue it is a net-zero fuel, releasing to the atmosphere only the amount of carbon that was absorbed by its feedstock. A switch to SAF also would reduce other pollutants, like black carbon and sulfur.

There are complex side effects that undercut claims of the biofuel’s climate benefits, and pressing questions about whether SAF production at the levels needed to make an impact on greenhouse gas emissions is even possible without causing other negative environmental consequences.

Moreover, “sustainable” aviation fuel still would release ozone-depleting NOx and warming water vapor into the stratosphere. The warming effects of the pollutants would be amplified, MIT researchers calculated, because the offsetting cooling effect of sulfur would be eliminated.

“Decision makers considering low-sulfur fuel must therefore consider a trade-off between decreasing global ozone depletion and increasing climate impacts,” the authors said. 

NASA officials say they recognize that there are numerous environmental issues to be resolved. Coen said the agency is focusing first on solving what it sees as the threshold noise issue.

Being able to fly supersonic planes over land is the “key barrier,” Coen said. “It’s not the only barrier. But without that you’ll never get something that’s really acceptable.

“We also recognize that the Quesst mission does not do everything that we’re going to need to have sustainable supersonic air transportation in the future and everybody else should realize that as well,” he said. “If we can manage the sustainability aspect, which is not a given, we see that this makes a transportation option available that can reduce travel time without any real downside.

“It all comes down to reduction in travel time, whether you’re a business traveler, and you want to get home to your family faster. Or you’re a vacation traveler with limited time and would like to get to your vacation destination faster,” Coen said. “This is a way to shrink the globe.”

Rising Costs, Missed Opportunities

Wuebbles compared NASA’s supersonic research investment to its long-term commitment to space exploration, calling the project “certainly worthwhile.” “I think it helps us in other ways, just as the space program has led to so many different innovations for our planet,” he said. “It’s a process of learning.”

The Quesst mission is just a small fraction—about 5 percent—of NASA’s aeronautics budget, which in turn is just 4 percent of the agency’s overall $25.4 billion budget, the bulk of which is for science, including climate science and space exploration.

But Quesst’s costs have risen over time. NASA is asking Congress for a 66 percent increase for the X-59, to $70.9 million, in fiscal year 2025. In its budget justification, the agency told lawmakers that the project, which originally planned its first test flight for 2021, had confronted “significant cost overrun and schedule delay.” The total cost is now expected to be $838.6 million, 80 percent more than the baseline estimate in 2018 when Lockheed Martin won the contract.

Coen blamed both pandemic-related supply chain issues as well as the aircraft’s unique nature. “The complexity of the design, and overcoming these challenges of integrating the systems, COVID … all of these things have stretched out the length of the Quesst mission and the X-59 development,” he said. “We haven’t found anything technically unsound about the design. We haven’t changed the basic requirements. But it’s just taking longer to put it all together and test it to the point where we feel that it’s safe enough to fly.”

Test planes typically are flown in remote restricted airspace, but he said the X-59 must be safe to fly over populated areas so NASA can research community response to the low boom.

As a result, NASA is spending more on Quesst than on any other X-plane. The cost is almost double that of a sustainable aviation X-plane contract NASA awarded to Boeing last year, a $425 million investment in a truss-based wing design aims to cut fuel consumption and carbon emissions 30 percent. 

Rutherford argues that the supersonic project is diverting NASA’s resources from such projects that address the urgent and challenging problem of reducing aviation’s fast-growing climate emissions. He believes NASA is missing at least one specific opportunity being pursued by Japan Airlines, Airbus, startup companies, and European researchers: hydrogen-powered flight. This year’s FAA authorization bill directs the agency to consult with NASA and other agencies and report to Congress within a year on a research and development strategy for the safe use of hydrogen in commercial aviation.

“The really missing X-plane is hydrogen,” Rutherford said. “You can produce it using renewable electricity, so it will have a very, very low carbon footprint, and it’s a very light fuel. So if you can figure out how to store it in a safe way, then it would be applicable to a wide variety of flights, as opposed to supersonic aircraft, which are just going to be for basically wealthy people.”

Coen is not so sure. “Hydrogen is a controversial subject as to whether or not that will actually be a really good technology for air transport operations,” he said.

NASA tries to balance its research portfolio to achieve a number of national objectives, Coen said. He said the agency feels it is focused on the right projects not only to cut emissions, but to achieve other goals including autonomous aviation, improved air safety and—with the Quesst mission—supersonic aviation.

“We are investing a substantial portion of our budget in sustainable activities,” Coen said. “But that doesn’t have to be all we invest in.”