Blake Scholl’s is a familiar Silicon Valley story. He graduated from Carnegie Mellon University in computer science, worked at Amazon and Groupon, among others, and in 2014 sold a startup he had developed. An instrument-rated private pilot and flight enthusiast, he used the proceeds to create, with two partners, a new company, which they named Boom Technology. The company does business under a less generic and more dramatic name: Boom Supersonic.
The none-too-modest aim of Boom Supersonic is to design and manufacture a supersonic airliner, which it calls Overture. Early this year, Boom successfully passed Mach 1 with a one-third scale demonstrator called XB-1. Boom boasts that XB-1 is the first privately developed jet to break the sound barrier. (Privately developed rocket planes, like Burt Rutan’s SpaceShipOne, did so previously.) In its brief supersonic flights, XB-1 also provided a practical demonstration of so-called “boomless cruise.”
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Like travel to the moon, supersonic passenger flight was achieved half a century ago and eventually abandoned. Concorde flew transatlantic routes at Mach 2 for three decades.
Whether it ever earned back its development cost—equivalent to around $ 15 billion, supplied by British and French taxpayers—is doubtful, but its three-and-a-half-hour transatlantic flight time supported boutique ticket prices on the order of 9,000 of today’s dollars. Concorde was a beautiful airplane, and it remains the object of much nostalgic admiration, but the world did not hurry to replace it.
- READ MORE: An Inaudible Sonic Boom, Visualized
The reason was not some daunting technical difficulty. Perhaps it was not even economics. It was the shock wave, the so-called sonic boom, that accompanies supersonic objects and arrives at the ground without warning, like an explosion. A series of experimental supersonic flights over Oklahoma City in the 1970s left behind a trail of rattled citizens and broken windows, which, possibly aided by sour grapes over our own failure to develop an airplane to compete with Concorde, led the FAA in 1973 to ban civilian supersonic flight over the United States.
Now that ban, whose obtuse wording would apply even to a perfectly silent supersonic airplane if there were such a thing, is under new scrutiny, and NASA is studying ways of making sonic booms less annoying.
“Lots of things are controversial in D.C., but this is not one of them,” Scholl said. “The regulation says, ‘Thou shalt not exceed Mach 1.’ What it should say is, ‘Thou shalt not make bad noises.’ I haven’t found a single person, left, right, center, that sees it differently.”
Boomless cruise, which was one of the energetically hyped aspects of XB-1’s supersonic flights, is not a new discovery. It has long been known that certain winds and temperature gradients can keep the boom of an airplane flying at high altitude and low supersonic speed from reaching the ground. When XB-1 flew at Mach 1.12 in February, microphones on the ground, Scholl says, recorded no boom.
Boomless cruise holds out the possibility of flight speeds 50 percent higher than those of subsonic airliners. But it is not without difficulties, most of which are shared by supersonic flight in general.
The power required to move an object through air is proportional to nearly the cube of its true speed. Almost eight times as much fuel is needed to propel an airplane at 400 knots as at 200. Approaching and passing through Mach 1, however, an airplane acquires, in addition to the drag it already has, a new kind of drag, called wave drag.
Wave drag is part of what made the speed of sound a barrier in the 1950s. Today, with more powerful engines and more astute aerodynamic design, the sound barrier has been reduced to a mere speed bump. Nevertheless, total drag still temporarily triples as an airplane passes through Mach 1. Drag gradually diminishes as the Mach number increases, but never returns to subsonic levels. Aerodynamic efficiency at supersonic speeds—which is directly related to seat-mile costs and therefore to commercial viability—is only half that of subsonic flight.
The problem for boomless cruise is that it takes place within the transonic drag rise. Scholl claims, however, that the overall economics are not very different from those of subsonic flight.
“There is a fuel burn penalty, but there is a speed dividend,” Scholl said.. “When one flies faster, one can do more flights with the same airplane and crew. Engine maintenance is reduced. Airframe maintenance is reduced. Pilot costs are reduced. All these other costs are proportional to hours. Use fewer hours, the cost comes down. We ran the math comparing subsonic overland to boomless overland, and it’s within 1 percent of the same cost.” Somewhat the same economic reasoning would apply to Overture.
The original conception was of a three-engine airplane with a cruising speed of Mach 2.2. Eventually cooler heads prevailed, and Overture is now a four-engine airplane cruising at Mach 1.7, a sweet spot where economics and aerodynamics come, if not into perfect alignment, at least into proximity. Frictional heating is at a level tolerable for aluminum and composites, wave drag has dropped off considerably, and engine inlet design, which requires complicated variable geometry at higher Mach numbers, is relatively straightforward.
Being limited to subsonic speed over land was a fatal disadvantage for Concorde. Boom aims to alleviate it (assuming the blanket supersonic ban will be rescinded or revised) by flying overland legs boomless at 700 knots and accelerating to 980 knots over oceans. Computers would use information from atmospheric soundings to select speeds and altitudes for boomless cruise, and would plot routings that best balance over-land and over-ocean
segments.
In renderings and three-views—there is no physical airplane—Overture looks strikingly similar to the Boeing 2707-300, the proposed American SST whose development was finally abandoned in 1971 after it had metamorphosed through several different forms. Overture, with a double-delta wing, aft stabilizer, and four engines in separate cylindrical nacelles slung under the wings, differs from the final version of the 2707 mainly in size. The Boeing’s fuselage was nearly 300 feet long, while Overture’s is 200. Passenger capacity for airplanes of this shape apparently scales roughly with the cube of fuselage length—the 2707 would have held 244 passengers to Overture’s 65-80. Part of the reason for the collapse of the 2707 program was the airplane’s gigantic size and proportionate cost. Another part was its forbidding intended speed of Mach 3. In 1971, even mighty Boeing had bitten off more than it could chew.
The excess drag of supersonic flight can be minimized by characteristics visible in all SSTs, real or imagined—long, slender fuselages with pointy noses and highly-swept delta wings of short span. (Fighters, which use supersonic speed briefly, if at all, look different.) Concorde and the Tupolev Tu-144 cruised at Mach 2 at 60,000 feet, where their indicated airspeed was only 380 knots, but the friction of air heated their noses to 260 degrees Fahrenheit. Boom proposes a less hostile environment for Overture, but the slender fuselage, which limits
capacity, and the narrow wing, which complicates landing, remain obligatory.
The purpose of XB-1, construction of which began in 2017, was to validate the design of Overture which, however, changed after the building of XB-1 was underway. XB-1 now resembles Overture mainly in the slenderness of its forward fuselage and planform of its wing. In other important respects, such as the shoulder rather than low placement of its wing and the embedding of the engines in the aft fuselage, it is quite different.
That may have been one of the reasons XB-1 flight tests ended once it had surpassed Mach 1. The test program had been without surprises—there is excellent in-cockpit video of some flights on YouTube, by the way—and the test pilot reported that the airplane’s flying qualities above Mach 1 were actually nicer than at subsonic speed. I asked Scholl why Boom hadn’t continued testing to higher Mach numbers since, if its three 4,300-pounds-thrust afterburning J-85s could have driven XB-1 to Mach 1.7, it would have made an impressive case for Boom’s capabilities.
“We don’t optimize for optics,” Scholl said. “All the learning is right around Mach 1. The transonic regime, when some of the flow is subsonic and some of it is supersonic, is where everything is the hardest to predict, where you’re most likely to be surprised in flight test. You go faster, it actually gets easier.”
The eight-year effort expended on XB-1 paid other dividends, however.
“We did underestimate the challenge by a lot,” he said. “If we went back and started over knowing what we know today, it would have been maybe a third of the time and a quarter of the cost. If we had gone straight to the Overture airliner, we would have made the same mistakes, but they would not have been recoverable. They’d have been too expensive.”
One system planned for Overture and successfully tested on XB-1 was landing with synthetic vision. Airplanes with narrow delta wings touch down in an extremely nose-high attitude. To provide their pilots with a view of the runway, both Concorde and the Russian Tu-144 used “droop snoots”—nose cones hinged ahead of the windscreen to tilt out of the way for landing. XB-1 instead used cameras on the nosewheel leg. Just for good measure, Boom brought in a Navy aircraft carrier landing signal officer to talk the pilot down.
Boom’s attention now turns to Overture.
Boom has said that Overture will enter airline service in 2030. That is the sort of claim companies have to make when more realistic ones would discourage investment.
To bring any four-engine airliner, let alone a supersonic one, from CAD screen to runway in five years would be astounding. It’s not just the airframe. It’s the tooling and systems integration and manufacturing methods and capacity as well, not to mention the certification of an airplane of a type with which the FAA has no institutional experience.
But Boom’s challenge is doubly difficult. It’s not just developing an airplane. It’s also developing an engine.
Boom suffered what was widely assumed to be a fatal blow in fall 2022 when Rolls-Royce, which it had collaborated with since 2020, withdrew from the project. The British firm, which manufactured Concorde’s engines, had brought both credibility and know-how to the Overture program. Without an existing engine, Overture looked sunk. But Scholl “pulled a rabbit out of a hat,” as an industry watcher put it, when he enlisted three smaller firms already involved in engine development and parts manufacturing to create a new engine called Symphony. Symphony will be a medium-bypass turbofan of 35,000 pounds thrust designed with three goals in mind—efficient supercruise (that is, supersonic cruise without afterburner), low takeoff and approach noise levels (Concorde was famously noisy), and the ability to run on sustainable (that is, synthetic or plant-based) fuel.
Overture, if and when it enters service, will face some headwinds. It will compete with more efficient subsonic airliners for limited supplies of sustainable fuel. Its route structures will be constrained by its boom. High fuel consumption will limit the profit margin, now quite large, that airlines enjoy on first- and business-class tickets. Small passenger capacity will nullify the advantage gained from higher flight frequency.
My mother taught me it is impolite to talk about money, and so I did not ask Scholl about Boom’s financial situation. It has been reported that he has so far raised $ 700 million. Three airlines—American, United, and Japan—have placed orders for Overture. The cost of bringing the airplane into production is impossible to foresee, but it’s certainly multiple billions. As you may have noticed, however, if you follow the news these days, trillion is the new billion. Quite a few private individuals could spend $ 10 billion or $ 20 billion out of pocket if they were sufficiently entranced by the idea of supersonic travel. Scholl is a convincing fellow— perhaps he can convince one of those.
After all, there’s something irresistible about that word: supersonic!
This feature first appeared in the May Issue 958 of the FLYING print edition.
