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Blake Scholl stands on a stairwell overlooking a vast hangar where a 71-foot airplane is being built—combining the cockpit of a jetfighter with the body of a small puddle-jumper. He describes the plane as aviation’s future.

Think: environmentally-friendly flights from San Francisco to Tokyo in just four hours.

The aircraft is a scaled-down version of a planned 65-passenger commercial supersonic plane that, if all goes according to schedule, will fly for the first time in 2026 and then ferry paying passengers three years later. The prototype, one-third of the airliner’s planned size, should take its first test-flight early next year.

“I have to pinch myself, I’m so excited,” said Scholl, the 40-year-old founder of Boom Supersonic, the startup building the airplane. “There are two places in the world where you can see a supersonic civil aircraft: a museum and this building.”

There’s a reason that supersonic planes are typically found in museums. The Concorde supersonic jet, which first flew in 1969, retired in 2003 after governments refused to further subsidize its costly operations. Supersonic flights have since been banned over land because of the thunderous rumble when they break the sound barrier, dooming further development.

In seventeen years, few have sought to replicate the Concorde. Aerion Supersonic, a Reno startup that GE and Boeing backed, tried to build a quieter 10-person supersonic plane, but it shut down earlier this year.

The 232-employee Boom is the outgrowth of Scholl’s personal obsession to build an airliner that can exceed the sound barrier—part of his greater vision of shrinking the world and paving the way for faster, more sustainable air travel.

Walking through the hangar, Scholl speaks at breakneck speeds, rattling off specs for the prototype aircraft and tech jargon: “carbon-fiber composites” and “software-defined aerodynamics.”

Yet Scholl isn’t your typical aerospace executive. A computer scientist, he spent several years at Amazon and at a startup, before building an ecommerce app company, Kima Labs, which Groupon bought in 2012. A private pilot, Scholl knew he wanted to start another company and for years had been fascinated by supersonic flight after an ex-girlfriend’s flight was delayed.

Rather than assuming the idea wasn’t viable—as nearly all industry officials did—Scholl spent most of 2013 studying how he could build such a plane. He read aerospace textbooks, took an airplane design class, and ignored conventional wisdom that said few people would pay to shave time off their trips or that the amount of fuel used would be an environmental disaster.

“You find that none of the conventional wisdom stands up even a little bit to careful analysis,” Scholl said.

Seven years since its start, Boom has raised $270 million from investors including Japan Airlines. It has also partnered with Rolls Royce, which built the Concorde’s engines to design Boom’s. Boom also allied itself with Prometheus Fuels, a startup that takes carbon dioxide from the air and converts it into liquid jet fuel. This, Scholl said, will ultimately let Boom’s planes be carbon neutral because any carbon they emit will be offset by the fuel they use.

In June, Boom landed perhaps its biggest coup. United Airlines agreed to buy up to 15 Boom planes for $3 billion with an option to buy 35 more. That deal required a “significant” non-refundable upfront payment. But the deal is contingent on Boom’s ability to create a plane that meets safety, operating, and sustainability requirements.

That last sentence? It’s big. Those milestones will be challenging to meet. After all, building a 100,000-pound machine that can safely shuttle millions of passengers across oceans at supersonic speeds is a bit different than designing an app.

Take, for instance, the engine. Designing one requires years, and then there’s myriad testing on top of that. The shape and size of the engine could affect the plane’s wing design, to single out just one potential complication.

“It’s very complex and it’s very unique—you can’t come off the streets and design a supersonic plane,” said Henry H. Harteveldt, a longtime aviation analyst and president of Atmosphere Research Group.

In the 1990s, Boeing attempted to build the Sonic Cruiser, a plane that would fly 20% faster than traditional airliners. But buyers weren’t interested. The Concorde’s fares were about 30% more than subsonic first class fares. Today, in the wake of COVID-19, the fear is that there will be fewer air travelers willing to pay extra to shave a few hours off of long-haul flights. “There’s not much of a market,” Harteveldt said, outside of high-powered executives, celebrities, and the super wealthy.

Environmentalists also express skepticism. Traditional subsonic flight is still hugely inefficient and far from environmentally sustainable, said Andrew Murphy, director of aviation for Transport & Environment, an organization pushing for cleaner transportation. And the faster you fly, the more fuel you use. Boom’s project, he said, “seems like a waste of good resources, human talent, and capital.”

Scholl brushes off such criticism. He’s convinced that the Prometheus partnership will make Boom a leader in aviation sustainability.

Scholl dons safety glasses and walks around the rear of the prototype. The metal skin of the aircraft’s tail has been removed, exposing the innards. There, engineers connect wires to a loud humming machine—which looks similar to a large metal kiln—and test the mechanisms controlling the plane’s pitch.

No pictures of the plane’s tail, Scholl asks. “The back end of the airplane,” he said, “is considered the real secret sauce.”

Test pilot Chris Guarente sits in the plane’s cockpit while talking to a member of the technical team standing on scaffolding. Guarente is a former Air Force and Northrop Grumman test pilot hired by Boom to train to fly the plane.

Meanwhile, Inside Boom’s avionics lab, a team of 30 control room engineers perform regular dress rehearsals of flight with pilots, who will have flown hundreds of hours in a flight simulator before the first real flight.

In anticipation of safety regulations, Boom also began working with the FAA early to help design its planes and avoid safety problems and delays. Boom plans to use plenty of standard aircraft materials and components to make it easier for regulators, aircraft maintenance, and manufacturing.

When the COVID virus hit the U.S. last year, Boom shut down for two weeks and board meetings got heated, as directors debated whether to pull back on its aggressive plans or to press the throttle. The company pushed forward, accelerated hiring to ensure the planes are tested and built on time. The move opened up opportunity: Boom was able to step up engine design with Rolls-Royce because the lack of airline travel quashed the engine maintenance revenues at the London company. Boom’s decision to accelerate also led to the deal with United.

Before hurriedly running off to another meeting, Scholl admitted he can nerd out on the technology of building a supersonic aircraft. But he said that human connection is the real mission behind Boom and why supersonic is needed going forward. Eventually YouTube videos, virtual reality, and Zoom meetings of the pandemic will get old. “Life,” he said, “is better lived in person.”