More than a decade of joint investigation lies behind NASA’s Jan. 18 selection of Boeing for a public-private partnership to develop a full-scale experimental aircraft to validate the low-drag Transonic Truss-Braced Wing concept for potential use on future single-aisle airliners.
NASA has signed a funded Space Act Agreement with Boeing under which it is to provide $425 million in funding through milestone payments while Boeing and its industry partners contribute $725 million. A yearlong flight-test campaign is planned to begin at NASA Armstrong Flight Research Center, California, in 2028.
The Sustainable Flight Demonstrator (SFD) program gives Boeing a public response to the multiple technology-demonstration projects Airbus has launched with funding from European governments and its industry partners. These range from long-span wings and open-rotor engines to hybrid and hydrogen propulsion.
The award is not a surprise, as NASA and Boeing have been working together on the high-aspect-ratio Transonic Truss-Braced Wing (TTBW) configuration for almost 15 years. Selection of the SFD was a competition, NASA Administrator Bill Nelson says, but Boeing’s proposal “was by far the best.” NASA declines to identify the other bidders.
Boeing’s proposal outlined a possible future family of single-aisle TTBW aircraft, the VS-1 and VS-2 (for Vision System). Based on the MD-90 fuselage, the smaller VS-1 seats 130-160 passengers while the 180-210-seat VS-2 has a bigger wing and engines. Using one wing and engine size for both variants would have unacceptably penalized the smaller aircraft, Boeing says.
The TTBW is a long, slender wing structurally braced by trusses that also generate lift. Increasing span reduces lift-induced drag, and the TTBW’s span is about 50% longer than the cantilever wing on an equivalent conventional single-aisle airliner, says Mike Sinnett, vice president and general manager for product development at Boeing Commercial Airplanes.
The wing design alone is expected to reduce fuel burn by up to 10% compared with the conventional cantilevered wings on today’s most efficient single-aisles. When combined with advanced propulsion, composite structures and other technologies, the TTBW is expected to reduce fuel consumption and emissions by at least 30% for an aircraft entering service in the 2030s, NASA says.
The goal of the SFD program is to validate the benefits of the TTBW concept at the aircraft level. Whether Boeing will then use the configuration for a future single-aisle will depend on the success of flight tests. But the scale of the industry investment, combined with the $110 million it has already spent on TTBW development, indicates that Boeing is “optimistic” about the design, Sinnett says.
Airbus has said it plans to make a decision by 2028 on development of a new aircraft to enter service in 2035. Based on that time frame, a first flight of NASA’s SFD in 2028 would allow the TTBW configuration to be considered for use by Boeing in a next-generation single-aisle entering service in 2035-40. NASA’s objective in funding the SFD is to help aviation achieve its goal of reaching net-zero emissions by 2050, NASA Associate Administrator Robert Pearce says.
The demonstrator will use a shortened MD-90 airframe. The SFD program is focused on demonstrating the TTBW and related technologies required to integrate the wing onto an aircraft. These include digital fly-by-wire flight controls that enable active load alleviation and flutter suppression to minimize the weight penalty of the longer, thinner wing, Sinnett says.
Boeing is finalizing agreements with its industry partners, including the engine supplier, Chief Technology Officer Todd Citron says. Specific technical challenges with the TTBW include construction of the long, thin wing, which will be a combination of composite and metallic materials, and design of actuation systems for the multiple movable surfaces that can fit with the thin airfoil.
While the concept of a long-span truss-braced wing is not new—it was used by France’s Hurel-Dubois for a series of propeller-driven aircraft in the 1950s—work by NASA and Boeing on the jet-speed TTBW began in 2008. A series of seven wind tunnel test campaigns were conducted through to 2022 under the Subsonic Ultra-Green Aircraft Research (SUGAR) program.
Over that time, the concept has evolved from a wing optimized to fly at Mach 0.745 to save fuel to a higher-sweep design optimized for the Mach 0.80 cruise speed of today’s narrowbodies. Early testing validated that bracing the slender wing structurally with struts minimized the weight penalty and made the concept feasible. Later tests focused on low-speed, high-lift performance and transonic buffet.
Evaluation of the TTBW concept under the SUGAR program is planned to continue, to investigate aspects such as icing and ditching. But Boeing is close to finalizing the high-speed lines for the SFD and aims to begin a separate series of wind tunnel tests to validate the specific design of the demonstrator, as this requires changes to the TTBW to adapt it to the structural constraints of the existing MD-90 fuselage.
Under the funded Space Act Agreement, NASA will support the Boeing-led industry team with technical experts and test facilities, says Brent Cobleigh, NASA’s SFD project manager. But the Boeing team will own the demonstrator, set the program schedule and be responsible for covering any cost overruns. It will be Boeing’s decision whether or not to apply to the U.S. Air Force for an X-plane designation, he says.
A more flexible and hands-off approach has been chosen for the SFD program, one of only a handful of funded Space Act Agreements signed by NASA, in an effort to ensure the TTBW crosses the “valley of death” and transitions from research into product development, Cobleigh says.
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