Boom Supersonic’s jet testing worries atmospheric scientists

Boom Supersonic’s Overture would fly at 60,000 feet at Mach 1.7, twice the speed of conventional jetliners.

It may be the most innovative, exciting airplane in the world — and it isn’t being designed at Boeing in Seattle, or Lockheed’s Skunk Works, or at Airbus in Toulouse, but rather at Centennial Airport, 15 miles south of Denver.

Overture, the sleek aircraft under design by Colorado’s Boom Supersonic, is being hailed by company founder and CEO Blake Scholl as “revolutionary” and set to “fundamentally change how we think about distance.”

Last month, Overture was the talk of the Farnborough International Airshow near London, where Boom representatives showed crowds the latest mockup of the faster-than-sound jetliner — a four-engine version that the company says emerged from 26 million hours of software tests and repeated wind tunnel trials.

Here in Colorado a few blocks from Centennial’s control tower, a “Baby Boom” supersonic, one-third the scale of a full-sized 65-passenger airliner, is complete and is undergoing ground engine tests. Boom has told The Denver Gazette that the prototype will take to the air before the end of this year, from California’s Mojave Desert.

Boom says it’s also ready to begin outfitting a 70,000-square-foot testbed for a full-scale Overture, a mile from its Centennial facility. There it will create what designers call an “iron bird” test airplane to allow engineers and pilots to put the jet through all its paces — the moving parts, the electronics and avionics, everything just short of what the craft actually would need to take off.

Boom said it was premature to estimate the testbed’s cost, but that it will hire up to 30 additional full-time employees to staff it.

The craft to emerge from that would be a 21st century version of the Concorde, the Anglo/French supersonic transport that whisked passengers across the Atlantic in three-and-a-half hours between 1976 and 2003. Overture would be a tad slower (still twice as fast as conventional jetliners), cruising at Mach 1.7, 1,300 miles per hour, compared to Concorde’s Mach 2. It’s a tad smaller (65 to 80 passengers, while Concorde carried 92 to 128); and at 201 feet long, just a foot shorter.

But it would benefit from the advances from 50 years of intervening aviation progress — lighter composites, newer electronics and controls. At a moment when aircraft makers are at work designing low-carbon versions of their regular airliners, Boom promises Overture will fly carbon-neutral — no new CO2 added to Earth’s atmosphere, thanks to “sustainable aviation fuel” made with hydrogen and using solar energy and carbon dioxide harvested from the atmosphere.

Boom’s vision isn’t the only supersonic on the drawing board, but it appears more ambitious that any similar concepts. Several competitors have been at work on smaller, corporate jet supersonics, including Las Vegas-based Aerian Supersonic, which had received an investment from Boeing for a 12-passenger jet. The company shut down operations earlier this year.

But Boom’s own visions have gathered financial momentum. With a reported price tag of $200 million per airliner, United Airlines has ordered 15 aircraft with options for more. And Japan Airlines, an early investor, has options on another 20. Scholl has reportedly claimed $600 million in investments, including a $60 million grant from the military.

During the airshow, Boom joined Northrup Grumman to announce a partnership on special-mission variants of the plane. “Using equity fundraising, airline prepayments, supplier commitments, and other sources, we expect to deploy $6-8 billion to bring Overture to market,” a Boom spokesperson told The Denver Gazette.

“We’re seeing strong interest from investors who recognize that Boom has demonstrated measurable progress and a clear path forward to making supersonic travel safe, sustainable, and economically viable.”

For now, Boom is pushing the vision forward with plans for a “super factory” in Greensboro, North Carolina, where it would begin production building. Flight tests of a full-sized Overture would start in 2026 to put passengers in the air by 2029. (Iron-bird tests and other engineering would remain here in Colorado.)

With that kind of momentum, what could possibly go wrong?
But critics are questioning whether Overture will really get off the ground — worrying about its potential environmental impacts, the availability of the clean-burning jet fuel and the aircraft’s ultimate marketability.

And worry No. 1 is that Overture doesn’t yet appear to have the engines to push the plane to supersonic speed.

“There is no airplane without an engine,” Jon Ostrower, editor-and-chief of the aviation journal The Air Current, said in a phone interview with The Denver Gazette.

“Go back to the Wright Brothers, and they also figured out how to create an engine. This has been the ticket true of Boeing and Airbus, and a challenge Boom has to solve.”

On its website and BoomSupersonic.com, Boom lists Rolls-Royce, one of the world’s three big engine manufacturers, as a collaborator. But Ostrower, who interviewed both Scholl and Rolls-Royce CEO Warren East during the airshow, found no concrete relationship expressed that’s likely to deliver a new engine anytime soon.

“What I heard from Rolls is that they’re not willing to commit their own dollars,” Ostrower told The Denver Gazette.

That, Ostrower adds, puts Boom in a position of conjuring up a business model where Rolls or another maker would be paid to develop an engine. Aerian Supersonic, he adds, had that sort of arrangement with GE, the world’s largest jet engine maker, when the would-be airplane manufacturer closed down operations.

“They had to pay for GE to do that,” Ostrower said. “The day Aerian said they were out of money, engineers at GE put their pencils down.”

In a written reply to queries about the engines and other concerns, Boom told The Denver Gazette that they continue to have a relationship with the British engine maker.

“We have matured the Overture configuration significantly, and we’re working with Rolls-Royce and others behind the scenes to develop multiple design options for an engine that is optimized for 100% sustainable aviation fuel,” the Boom spokesperson said.

“Rolls-Royce has completed the contracted work package it had with Boom on the development of the current Overture program,” the spokesperson continued. “Boom is now evaluating the results, as well as assessing market requirements and design alternatives. Our relationship remains active and we continue to discuss the future of supersonic commercial flight and further technical support from Rolls-Royce.”

But regardless of the power plant, scientists also harbor doubts about the consequences of the exhaust load that Overture would put on the thin reaches of the upper atmosphere, where Overture would fly.

“Getting a sustainable source of fuel doesn’t negate the effect that you are creating water vapor and nitrous oxides,” adds David W. Fahey, director of the Chemical Sciences Laboratory at the National Oceanic and Atmospheric Administration in Boulder.

The lab advises government on consequences of atmospheric releases, including impacts on Earth’s protective ozone layer that have been a worldwide focus since the 1987 Montreal Protocol, eventually signed by 197 nations.

In 1995, Fahey and his colleagues carried out a study that sampled Concorde’s exhaust plume in flight in the stratosphere near New Zealand. They concluded based on the measured emissions of small particles that a new larger fleet of supersonics could be a threat to the ozone layer.

Now, 27 years later, Fahey harbors many of the same doubts.

“Basic answer,” Fahey told The Denver Gazette, “the nature of the environmental effects of supersonic aircraft are unchanged from the 1990s when they were last a major focus of the stratospheric science community.

“The issue now is the scale and specifications of a new fleet,” Fahey added, “i.e., the more aircraft, the greater the effects will be. Sustainability (of the fuel) doesn’t obviate the chemical impacts.”

“I’m definitely concerned about the ozone,” Dan Rutherford, program director for aviation and marine programs at the California-based International Council on Clean Transportation, said in a phone interview with The Denver Gazette.

ICCT is a research nonprofit that consults with governmental bodies on reducing carbon emissions from the transportation sector. It gained wide attention after exposing the 2013 Dieselgate scandal at Volkswagen.

In a January paper conducted in partnership with MIT, Rutherford and colleagues studied climate impacts of SSTs — one version assuming that there were constraints on noise and climate, and another with no constraints. “The SSTs investigated are expected to burn seven-to-nine times more fuel per seat-kilometer flown than the subsonic (conventional fleet), creating substantial environmental impacts and poor economics,” they concluded.

Any environmental advantages of burning a carbon-neutral fuel as opposed to regular jet fuel are overcome by that much higher volume of fuel burned, the study notes. The faster flight times of SSTs would do nothing to overcome that gallon-per-seat metric, which doesn’t depend on the time it takes to fly.

“But even a regulated SST will be louder, and emit more air and climate pollution per passenger, than new subsonic planes,” Rutherford reiterated in a follow-up article earlier this year. “Flying faster than the speed of sound is inherently energy-intensive, in part because supersonics use powerful, thirsty engines to produce the high thrust needed to break the sound barrier.”

Supersonic airplanes drag a shock wave behind them that creates a sonic boom on the ground below. And in Concorde’s case, that left airlines limited only to flying transoceanic routes, chiefly London to New York and Paris to New York. The ticket price on those flights, adjusted for inflation, ranged from around $4,000 to $6,000, one way.

Boom is betting that its computer-modeled design will yield a sonic boom much softer and more tolerable, and that the economics of a new fuel would bring ticket prices down.

How many supersonics would be in the air?
Boom’s market model says the company has identified 600 worldwide routes that it claims that it could fly without changes to the existing air regulations.

“Concorde was basically two routes,” said ICCT’s Rutherford. “If you can imagine 600 routes, that’s a crazy high number.”

Critics are also focusing on Boom’s message about its sustainability, prominently mentioned in its promotions of Overture.

“Sustainability is integral to Boom company values, and Overture will be the first airliner optimized to fly on 100% SAF (sustainable aviation fuel),” the spokesperson said in a reply to The Denver Gazette’s queries.

“While Overture can technically fly on (conventional jet fuel), our airline partners have been passionate about partnering with us on sustainability and the future of net-carbon zero flight. Airlines are excited that they can fly faster and support their efforts to decrease their carbon footprint at the same time.”

Boom maintains it has already achieved zero-carbon as a company, but its goal of zero-carbon in the air appears to ride in part on the prospects of a similar startup company, Prometheus Fuels. Boom shows Prometheus’s logo on its Overture website and credits the startup with “enabling net-zero carbon supersonic flight with fuels made from atmospheric carbon dioxide.”

Prometheus and Boom each were funded early on by Y Combinator, based in Mountain View, California, which describes itself as a “startup accelerator.” Y Combinator lists a large number of startups it has helped launch, choosing its recipients in part by way of “Demo Day” presentation events.

On its website, Y Combinator lists the innovative firms it has kickstarted, ranking them by their estimated valuation or market cap. The site ranks Boom Supersonic as No. 70, Prometheus as 47.

Prometheus, which has attracted impressive investors including German carmaker BMW and Danish shipping leviathan Maersk, reportedly has a valuation of some $1.5 billion. But like Boom, Prometheus is gathering its share of skeptics within the environmental community, for its processes for making SAF fuel in volume, and more so for estimates of what SAF will actually cost.

“There is little available evidence it can actually live up to its lofty claims,” senior editor James Temple, who covers energy for MIT Technology Review, wrote in an April 2022 review.

“If these fuels could be produced at the costs and on the scales claimed, Prometheus might well overhaul the global energy marketplace,” Temple went on. “But Prometheus’s assertions have raised eyebrows among researchers, entrepreneurs, and venture capitalists.”

Temple continued that experts who reviewed an investor presentation were dubious the fuel company can achieve the numbers.

Asked whether Boom is focused on possible atmospheric consequences of its operation, the spokesperson responded, “Yes, Boom continues to review the latest research on atmospheric effects and identify mitigations.

“Most relevant,” the spokesperson added, “we are pursuing a next generation of 100% SAF with zero aromatics, which is expected to reduce particulate emissions and contrail impacts.” SAF, the representative said, could produce fewer ice crystal contrails during cruise, reducing the impact.

For now, neither Boom nor its critics are that doubtful about the potential market for a supersonic airliner like Overture, if it meets its operating goals.

“They’re trying to do a pretty difficult job, yet (you) look at market and it is kind of a big need,” Don J. Wuebbles, professor of atmospheric science at the University of Illinois, said in a phone conversation with The Denver Gazette. Wuebbles noted that he had met Boom’s sustainability officer at a recent conference, in the course of conducting studies for FAA on aircraft and their emissions.

“I don’t want to be down on what they’re trying to do,” Wuebbles continued. He added that environmental impacts are difficult to assess with so little known about how many airplanes would fly, and what the particulars of their emissions of water vapor, nitrous oxide and carbon particles will be at Overture’s cruising altitude of 60,000 feet.

He said he doesn’t expect Overture to produce a working airliner in the near term. “I’d be shocked if that occurs,” Wuebbles said. “I have done a lot of work with Boeing, and I know how hard it is.”

“A best guess,” added ICCT’s Dan Rutherford, “is they fly their demonstrator, probably generate additional funds, and three or four years down the road won’t build the aircraft.”

“The higher you fly, the more vulnerable the atmosphere is,” NOAA’s Fahey added. “The bottom line, in a world that wants to become sustainable, society is best served by the best performance per unit of fuel.

“The world is wise to be skeptical, but you want to encourage that out-of-the-box thinking.” denvergazette.com