United Airlines has placed firm orders with Boom for 15 supersonic Overture airliners, marking a key milestone towards the revival of faster-than-sound commercial transport and a major boost for the Denver-based start-up aircraft manufacturer.

The United deal, which also covers options for 35 additional aircraft, envisages introduction of the first civil supersonic services on trans-Atlantic and Pacific routes by 2029. The Overture will be designed to cruise at Mach 1.7 and carry up to 88 passengers on routes up to 4,250 nm. using 100% sustainable aviation fuel. 

“It is the first real order for any supersonic airplane since Concorde,” says Blake Scholl, Boom founder and CEO, referring to the Anglo-French supersonic transport which was retired in 2003 after 27 years of service. “These orders have purchase agreements and the terms and conditions you’d expect with that. They also have the financial arrangements in an industry standard way that you’d think about with any agreement between an airline and a manufacturer like Boeing or Airbus. It has all the same things and that’s super meaningful,” he adds. 

Japan Airlines—which is also an investor in Boom—has also secured 20 options for the Overture, though has yet to convert its reserved positions to firm orders. Virgin Atlantic also previously announced purchase options on 10 Overture aircraft, though Boom says it does not have anything “new” to share about the Virgin order. Since announcing its interest in Boom, Virgin has also announced long term plans to study a high Mach long-range transport project of its own. 

United says that executing the firm order deal is still provisional upon the Overture meeting the airline’s “demanding safety, operating and sustainability requirements.” The carrier adds that the Overture “is expected to be the first large commercial aircraft to be net-zero carbon from day one, optimized to run on 100% sustainable aviation fuel (SAF).” 

Commenting on the Overture announcement, United CEO Scott Kirby says the airline “continues on its trajectory to build a more innovative, sustainable airline and today’s advancements in technology are making it more viable for that to include supersonic planes. Boom’s vision for the future of commercial aviation, combined with the industry’s most robust route network in the world, will give business and leisure travelers access to a stellar flight experience.” 

The airline adds that it is working together with Boom on a “corporate sustainability initiative to accelerate production of greater supplies of SAF over the next decade.” Boom is working with California-based Prometheus Fuels on a carbon-neutral fuel for its Overture airliner, which is also produced using a carbon-capture process. Prometheus received a boost in June 2020 when the U.S. startup received an investment from the venture-capital arm of carmaker BMW.

Scholl confirms the Overture will be designed to meet the same Stage 5 noise limits for take-off and landing as current conventional airliners. However, in cruise the delta winged aircraft will generate a sonic boom at speeds over Mach 1 and will not therefore be permitted to operate supersonically overland. Despite this limitation Boom says expected operating costs 75% lower than those of Concorde will enable profitable operations solely on trans-oceanic routes at business class ticket prices or less.

United and Boom are discussing “very specifically three routes to start with, as well as another 100-plus options” Scholl says. The primary introductory routes are expected to be Newark/New York to London Heathrow and Frankfurt, and an initial trans-Pacific route from San Francisco to Narita, Japan. Typical flight times will be around half those of current subsonic equipment with Newark to London in 3.5 hours and San Francisco to Tokyo in 6 hours, despite the likely need for a fuel stop on the longer Pacific route, says Boom.

The United order also revealed Boom’s decision to reduce the Overture’s cruise speed from its original Mach 2.2 design point to Mach 1.7. The move appears to have followed more detailed analysis of propulsion options with Rolls-Royce, which partnered with Boom on engine studies in July 2020, and reflects a closer focus on fuel efficiency and airport noise, says Scholl. “We wanted Overture to be as quiet as the latest subsonic noise standards with no compromises. So that, plus concerns for efficiency and sustainability, drives you to a slightly lower Mach number.”

“We also looked at this from the passenger flight time perspective and Mach 1.7 is the right answer. That’s for long range cruise—not maximum Mach number,” Scholl adds, referencing the effective speed limit of the design at higher altitudes. 

Boom is expected to announce the location for Overture’s assembly and development in 2022, with ground-breaking on the site scheduled for 2023. Production will commence shortly after with roll out of the prototype in 2025. Flight tests are due to be underway in 2026 with entry-into-service slated by 2029. 

Flight tests of the company’s one-third scale supersonic demonstrator, XB-1, are meanwhile expected to begin in Mojave, California, by late 2021 or early 2022. The 71-ft.-long trijet design is undergoing final ground tests at Boom’s Centennial, Colorado headquarters and will shortly begin engine tests.