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How to Design a Supersonic Plane for the (Fairly Rich) Masses

One would think that 2020 is, perhaps, not a great year to bring up reviving supersonic passenger travel. Thanks to Covid, nobody is flying, and airlines are parking entire fleets of aircraft. Plus, anything that highlights the economic divide is going to get dragged on Twitter for sure. A new Concorde? Hold my beer.

Unfazed, Denver-based aviation startup Boom Supersonic last week unapologetically unveiled a subscale prototype for the high-speed commercial airliner it has been developing since 2014. Dubbed XB-1, the 71-foot-long, single-seat test vehicle was built to validate the design and technology of the company’s planned eventual final product, a $200 million airliner called Overture that will be three times XB-1’s size and carry 55 passengers to Mach 2.2. Sporting a crisp black, white, and yellow livery, the XB-1 looks like a fighter jet absent the Sidewinder missiles and military paint job.

The three-engine, delta-wing airplane is the first privately funded supersonic aircraft to be developed, and Boom is the first of at least three startups pursuing supersonic flight to get a human-piloted test aircraft out the door. (Aerion and Spike are the other two big ones, though their craft will both be smaller business jets.) The XB-1 will begin ground trials, including engine and taxi tests, at the company’s headquarters at Centennial Airport within a few months, before being trucked to Mojave, California, for its first flight sometime next year. Engineers will integrate findings from the program into the design of Overture, which is expected to begin flight tests by 2025. Ultimately the goal is to fly passengers between New York and London, for example, in three hours instead of six.

As for whether the time is right to debut such a flashy, presumably exclusive, and potentially very divisive aircraft—even a test prototype, and even considering the fact that it won’t actually enter service until well past the current health and economic crises—Boom founder and CEO Blake Scholl is adamant: “This year we’re all suffering from the tyranny of missing human connection,” he says. Zoom calls deliver a fraction of the benefit of an in-person interaction, after all. “People are as hungry as ever to build a world in which more people can go more places more often—only they now want to spend less time in airports and on airplanes around other people while getting to them. That’s the role of supersonic airplanes.”

Granted, it’s an easy argument to make when it's your own day-glo sky rocket streaking across the stratosphere, but reality might hold a few surprises for Boom if delays and cost spirals set in, as they often do during flight test programs. In fact, cash flow could be the biggest hurdle. “Technologically, there’s no reason this airplane cannot be built, but the question is whether there’s really a market for more than 30 or 40 of them,” says Teal Group aviation analyst Richard Aboulafia. “This isn’t Airbus or Boeing or Northrop Grumman or someone with deep R&D pockets, and venture capital investors tend to have different expectations in terms of the returns on their investment. Aerospace is a slow, long-term investment.”

So far, Boom has garnered investments from Japan Airlines and Virgin Group, totaling presales of 30 aircraft, and more than $160 million in investment from Emerson Collective, Y Combinator Continuity, Caffeinated Capital, SV Angel, and others. But that’s all still a long way from the $6 billion to $8 billion needed to reach certification of the new aircraft.

Still, the CEO remains optimistic, even arguing that, in light of our current travel woes, the airplane might be the solution for more than mere time-is-money C-suiters. In response to the pandemic, Boom says it will integrate technologies for touch-free experiences and seating environments designed with physical and airflow barriers to help boost passenger safety, which Scholl notes can’t readily be retrofit into existing airplanes. Scholl goes so far as to say the global pandemic will actually bolster Boom’s prospects, including with the airlines. “I’ve seen estimates of anywhere from two to four years until airlines are going to want to grow again,” he says. “Airlines have a long memory, and they won't forget this period for some time. They’re thinking about how to operate in such a way that no matter what happens next, people will still be happy to travel.”

That optimism doesn’t mean the airlines will necessarily see it that way, and Scholl and his engineers are still up against significant challenges making a new, economically viable, reasonably affordable—at least by business travel standards—version of the simultaneously adored and derided Concorde, which flew from 1976 to 2003. That aircraft was an aerospace triumph but also a financial disaster and an environmental menace. Boom has stated that its flights will cost the equivalent of an international business-class seat—Concorde’s were at least double that equivalent—and the XB-1 demonstrator unveiled last week will be carbon-neutral, thanks to its collaboration with sustainable-fuels maker Prometheus Fuels. (Prometheus uses a process called direct air carbon capture to extract carbon from the air and convert it to fuel with electricity, a process it’s also developing for carmakers BMW and Toyota.) The more important question, however, is whether Overture itself, the actual production aircraft, will be even remotely environmentally friendly. That depends on how successful Boom and engine maker Rolls-Royce are at producing an efficient final package—but critics are again skeptical that will occur: The International Council on Clean Transportation argues that even purpose-built engines will burn up to seven times more fuel per passenger than subsonic aircraft, which themselves are already massive contributors to greenhouse gas emissions.

Then there’s the issue of Concorde’s infamous racket on takeoff and the ear-splitting sonic booms that followed it when it flew above the sound barrier. The XB-1 prototype won’t be particularly quiet—it will use old-school but reliable and easily serviced GE J85-15 military engines originally developed in the 1950s—but it will be flying over open desert in California, so noise won’t be an issue. The final Overture, however, could be as quiet as a comparably sized commercial airplane on takeoff, if the new engines Rolls-Royce is developing come through as anticipated. Aboulafia notes that even though propulsion engineering has come a long way in the 50 years since Concorde debuted, there really isn’t a comparable engine for Overture at the moment. Most aircraft engines are either compact and powerful for fighter jets, or large and efficient for commercial airliners. The engines for Overture will need to fit inside the fuselage and rocket the airplane to supersonic velocities without the fuel-gulping and noisy afterburners that Concorde required, yet still meet noise and emissions standards.

As for the notorious sonic booms that can rattle windows and wake up babies, Boom will skirt the issue by having Overture avoid overland routing while at supersonic speeds—and in any case, its shape should generate a muffled boom that won’t be nearly as pronounced as that of the Concorde or the average fighter jet, Scholl says.

Supersonic flight poses materials challenges as well. The craft are exposed to far greater heat and stress than conventional aircraft, and aluminum, for example, loses strength at high temperatures. Carbon fiber, however, retains both its shape and strength, giving designers more latitude in shaping the wing and fuselage so as to minimize airflow disturbances and reduce drag.

For the aft fuselage, engineers used titanium, to better withstand the high impact forces during landing, estimated to be 112,000 pounds of force at each wheel, and better support the weight of the three engines. The XB-1 also relies on a material called Ultem 9085, a thermoplastic that can be 3D-printed into strong, lightweight, and fire-resistant parts, the company says. The ability to print hundreds of spacers, ducts, brackets, and more at the hangar saved significant time and money.

Advanced as they are, those materials are well known within the industry. Where things are more unknown is the fussy, often spooky aerodynamics of supersonic flight. At high speed, air vortices coming off the nose can interact with vortices coming off the wing and tail, affecting how the airplane behaves, so engineers have to tune the aerodynamics to avoid these collisions. And since the wing won’t mechanically sweep forward and back to optimize for both low-speed and high-speed performance—think the F-14 in Top Gun—configuring the wing’s delta shape primarily for high-speed flight impacts its stability when it flies more slowly during takeoff and landing. So engineers developed a hybrid fly-by-wire system that uses conventional hydraulic mechanical linkages for the controls, supplemented with electrical actuators to help improve stability, and they prescribed a higher nose angle in order to maximize airflow under the wing at low speed. Since that could cause a tail strike on landing, they opted for taller landing gear. Of course, that higher nose angle also limits the pilots’ forward visibility. Concorde addressed this with a drooping nose mechanism, but Boom didn’t like the mechanical complexity of that system, so it’s opting for an augmented-reality-based camera system in the nose gear to help pilots see during takeoffs and landings.

Flight tests of the subscale XB-1 will also show how the plane deals with a phenomenon known as “Mach tuck.” Here, as an aircraft approaches supersonic speed, the nose tends to dip down as shock waves, migrating rearward as speed increases, create pressure differentials that increase lift at the back of the wing, destabilizing it. “We have predictions for this, but it’s really critical that we go collect data on a real aircraft so we can fully counteract that,” says chief engineer Greg Krauland, who worked at rocket maker SpaceX and aerospace innovator Scaled Composites before joining Boom. “There might be implications for how the flight controls are programmed, so these are the kinds of issues we’ll look at as we push the XB-1 from subsonic testing through transonic and then up to supersonic.”

As the XB-1 reveals its secrets, work will move forward on the design and construction of the first Overture prototype, organizing all the production processes and partners, and then entry into service with the commercial airlines. So far, the airlines that have stepped up have been lured by the appeal of halving the flying time between major cities. The airplane’s approximate anticipated range is 4,000 miles, meaning some trips will require “tech breaks”—as Boom calls them—to refuel. This will include flights across the Pacific Ocean from, say, Tokyo to Los Angeles or San Francisco, with a pit stop in Alaska, but flights across the Atlantic will mostly be feasible in single hops. Boom’s published time estimates factor in that additional refueling stop.

Beyond airlines, Scholl says he’s drawn interest from the US Air Force, as well. Not as a combat aircraft, but as a potential “executive airlift” tool. That, of course, is code for a supersonic Air Force One, something any president would likely approve in a heartbeat.

Updated 10-15-2020, 6:15 pm EDT. The story has been updated to reflect new certification data provided by the company.

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