New Clues Emerge About Boom’s Supersonic Engine Game Plan

As the dust settles following American Airlines’ surprise decision to acquire up to 60 Boom Supersonic Overture airliners, the all-important question of what will power the Mach 1.7 aircraft remains unanswered.

• Boom focuses on engine operating model
• External-compression inlets likely are key

Engine technology traditionally drives aircraft design and, unless relying on an existing or derivative propulsion system, airframe development is usually paced by the powerplant. Engine testing, therefore, typically begins years ahead of first flight for all-new designs, particularly for those with challenging performance requirements such as the Overture—an aircraft designed to cruise at Mach 1.7 and carry up to 80 passengers on routes up to 4,250 nm using 100% sustainable aviation fuel.

In the case of the Overture, Boom is also trying to achieve routine supersonic flight while simultaneously meeting stringent International Civil Aviation Organization Chapter 14/FAA Stage 5 takeoff-and-landing noise limits—a prerequisite for any new passenger aircraft in the 2020s. To some extent, however, Boom has eased the pressures on its engine requirement by reducing cruise Mach speed to 1.7 from 2.2 and selecting a more pragmatic four-engine, high-lift configuration over its earlier trijet Concorde-like configuration.

“We’ve got technology options that meet all of our needs, including the design work that [Rolls-Royce] has finished,” Boom founder and CEO Blake Scholl tells Aviation Week, referring to long-running study work with the U.K. engine-maker. “Supersonic needs large cores and smaller fans. Overture is at the current limit of core size,” he noted on social media.

“Our focus is very much on developing the right business model for the engine,” Scholl says. “Historically, those models have not been very customer-friendly, and we want to do something that’s not just a breakthrough in the engine technology but . . . a breakthrough in the business model.

“Getting that right is a big focus,” he adds. “When we make the engine announcement, you’re not just going to hear about the engine technology, you’re going to hear about a very different approach to the business.”

Whether Rolls-Royce will be an active partner in the Overture beyond the initial study work remains to be seen. Since the manufacturer developed Concorde’s Olympus 593 engine with Safran from an earlier military version designed for sustained Mach 2.2 operation, it does not appear to be in any hurry to commit to the new venture. “[We] just completed some studies of engine technology with Boom,” says Chris Cholerton, president of civil aerospace at Rolls-Royce, “so we are talking about what we might do next—but nothing specific has been agreed. It’s been a valuable exercise, and we continue talks.”

Scholl’s focus on the engine operating model rather than the baseline technology points to several potential clues about Boom’s development approach and why the powerplant will almost certainly be based on an existing core. Illustrations of the redesigned Overture revealed at the recent Farnborough Airshow also support this view. The engines appear to incorporate fixed-geometry external-compression inlets and an extended nozzle shroud and plug nozzle, all of which would enable a suitably adapted version of an existing turbofan to operate supersonically.

The low drag inlet design includes an annular bypass duct that routes flow subsonically to the front face of the fan—a critically important operational feature. In this case, the Boom inlet includes an isentropic compression spike and a subsonic diffuser area that would combine to slow inlet air to around Mach 0.6 from Mach 1.7 while providing good recovery pressure for efficient long-range cruise.

While the arrangement of the swept inlet guide vanes may simply be for illustration purposes, the image appears to show a bypass duct splitter in the throat region. This would channel flow subsonically around the engine before expanding it back to supersonic in converging-diverging passages and exhausting it around the engine flow in the exhaust. At slower speeds, this bypass flow could provide noise shielding; in supersonic flight, the bypass duct would divert the low total pressure flow from the shock forming at the lip of the inlet cowl and, simultaneously, provide clean flow to the tips of the fan blades.

The configuration appears to share some basic features with a propulsion concept studied by Gulfstream, Rolls-Royce and NASA under a quiet supersonic jet project in 2009-10. Aimed at a small, low-boom aircraft with a design cruise of Mach 1.6 at 45,000 ft., the study showed significant promise and was based on a 15,000-lb.-takeoff-thrust-class Tay engine. The larger Overture, which will be built predominantly of lightweight composite materials, could conceivably utilize derivatives of the higher-thrust BR700/Pearl series in a similar arrangement.

Part of Boom’s confidence in still meeting an end-of-decade in-service target date is also likely based on the reduced development costs and time involved in a derivative, compared to an all-new engine. The derivative approach also potentially opens the program beyond a larger Rolls-Royce core engine to include similarly sized modern powerplants such as the Pratt & Whitney PW800 or General Electric Passport. The latter recently flew just past Mach 1 during flight tests of Bombardier’s Global 8000 business jet.   

Propulsion system selection, along with other major systems, is expected over the next few months, says Boom. As with other novel aspects of an airliner developed by a startup, the funding model for the engine program could be as different from the conventional approach as the sustainment and support concept outlined by Scholl. “The most important thing is the operation of the engine itself,” he says about regular in-service operational support and maintenance. “How do we service them, and how do we ensure that airlines have reliable low-cost thrust? I can’t premake any announcement, but we think there’s a far better way. There’s a significant opportunity for innovation.”

Boom sees the American Airlines move, which covers nonrefundable deposits on 20 firm orders and 40 options, as a key endorsement of its concept as it pushes to raise additional support for Overture’s development. Overall program costs are expected to be in the $6-8 billion range, which, according to a report in the Denver Gazette, will be supported through equity fundraising, airline prepayments, supplier commitments and other sources.

American’s supersonic plan also appears to guard against losing premium traffic next decade to its largest rival, United Airlines, which became the first U.S. carrier to announce an order for the Overture last year. Together with United's 15 firm orders and 35 options for the Overture and 20 “preorder” production positions reserved in 2017 by Japan Airlines, the American commitment brings the total Boom orderbook tally to 130 aircraft, of which 35 are considered “firm.”

Scholl acknowledges that the definition of “firm” varies from manufacturer to manufacturer but hints that American and United’s due diligence indicates satisfaction with the maturity of the design. “Going from zero to one [customer] last year with United was obviously huge for us, but I think going from one to two in some ways is even more significant,” Scholl says. “That’s because it shows that lots of airlines are reaching the same conclusion, and for that reason, it’s a big build of momentum.”

Despite Boom’s sales uptick, the uncertainty over the engine continues to drive industry skepticism. “We talk about nonrefundable deposits. We don’t know how big they are, putting aside the fact that it’s really hard to place firm orders when there’s no engine,” says Richard Aboulafia, a managing director at AeroDynamic Advisory. “[Without an engine,] you can’t guarantee performance because you don’t have any engine to provide performance specifications,” he adds.

American, meanwhile, says the Overture will “introduce an important new speed advantage” to its fleet. In a cautiously worded joint statement issued with Boom, the airline adds: “Under the terms of the agreement, Boom must meet industry-standard operating, performance and safety requirements as well as American’s other customary conditions before delivery of any Overtures.”

“Looking to the future, supersonic travel will be an important part of our ability to deliver for our customers,” American Chief Financial Officer Derek Kerr says about the airline’s selection of the Overture. The carrier previously dallied with supersonic flight in the 1960s, reserving options on six Concordes in 1963-65. However, these were canceled amid mounting environmental concerns and the global oil crisis in 1973.