Digital design, multi-material structures enable a quieter supersonic NASA X-plane
27 May 2022
Folsom adds that unlike commercial passenger aircraft, stiffness was the key mechanical requirement to the design. “Most airplanes — fighter jets, cargo jets — they’re designed to be very strong and to carry lots of payload. Well, with the X-59 you don’t have that. This is actually a stiffness-driven design not a strength-driven design, meaning we’re extremely sensitive to any deflections of the wing or the nose or any of the control surfaces while we’re flying along — if we hit a bump in turbulence or if we go around a corner intentionally or what have you.” Richardson adds that because only one X-59 would ever be built, the team had to overdesign to the mechanical requirements to eliminate any chances for error and ensure the aircraft can meet all load requirements during the life of the airframe.
To optimize the aircraft design, many teams at Lockheed Martin Skunk Works cooperated over a period of seven months: aerodynamics, sonic boom specialists, loads, stress, design. Folsom describes the back-and-forth process as a sort of balancing, requiring compromise and much tweaking in the ply schedule and part geometry to achieve proper global stiffness and tailored flexibility where needed, aerodynamics and strength, all with the goal of producing the most efficient airplane with the lowest sonic boom possible.
During this process, various design software tools were used by each of these teams, working together in “a closed-loop cycle,” Folsom says. This included aerodynamics and model-based design software, such as Dassault Systemes’ (Vélizy-Villacoublay, France) CATIA for design and manufacturing planning. Dassault’s Abaqus, MSC Software’s (Newport Beach, Calif., U.S.) NASTRAN and several in-house tools were all used to analyze the models. On the floor, manufacture and assembly were informed by real-time drawings via Dassault’s Composer on handheld tablets.
The NASA team has been integral to the entire design process as well, providing requirements, helping with design analysis in the beginning and systems testing now. NASA also provided several systems on the aircraft directly, such as the external vision system and — as is common with X-planes — re-used components from defense vehicles, including landing gear and actuators from retired F-16s or F-18s.
X-59 QueSST: Materials, structures
Described on Lockheed Martin’s website as “resembling a futuristic paper airplane,” the X-59 comprises a 99.7-foot-long airframe featuring a 34-foot-long cantilevered nose, a 29.6-foot single-piece delta wing, a cockpit with no forward-facing window and engine and air duct placement above the wing to further muffle sound. The overall appearance is designed to “reshape” or spread out the displaced air — shock waves — that occur during flight of the aircraft, resulting in quieter sonic booms.
Materials were evaluated early in the design process — the Lockheed team determined that because it is a one-off aircraft, only highly mature material systems and technologies would be used. “There’s no new material science technology at all used on the X-59, and all of the materials, whether metallic or composite, have prior heritage within Skunk Works,” Richardson says. “The objective was not to pioneer any manufacturing or material technologies — it was really built to generate a sound from the aircraft [to meet NASA’s requirements].”
According to Buonanno, the resulting design is more metal-centric on the tail and back half of the plane, including the fuselage, with more composites used on the wing, and in the front half of the plane including the nose. This choice of materials was driven by thermal requirements in the engine bay as well as the weight-critical nature of the front part of the aircraft, Buonanno explains. Given the length and narrowness of the nose compared to traditional commercial aircraft, he notes, “The airplane is very unusual because it’s nose-heavy rather than tail-heavy, meaning we needed to reduce the weight of the nose as much as possible.”
Designing the nose. The carbon fiber composite nose comprises a left and right half that are bonded and fastened together. It was designed for overlapping material on the top and bottom seams to add stiffness to prevent bending of the nose during flight. Photo Credit: Lockheed Martin
In the final design, composites were used to manufacture the nose and chines; top and bottom wing skins; flaps, rudder and ailerons; T-tail trailing edges on the top of the aircraft; tips on the stabilator, which is the horizontal tail in the back; the dome over the XVS camera; and the inlet duct. By weight, the composite components make up about 2,050 pounds (22%) of the 9,500-pound empty airframe. The most prevalent composite material chosen for these components is Solvay’s (Brussels, Belgium) MTM-45 carbon fiber/epoxy prepreg, which was originally developed to enable out-of-autoclave (OOA) cure.
Richardson notes, “If you were to go to an operational airliner in the future, say 10 years from now, I believe you’d see a very different range of materials.” For a commercial vehicle, light weight — leading to longer range and/or higher payload capacity — would be even more important, opening the door for more potential composites use, he says.
Wings: In-house, automated fiber placement (AFP)
Putting subsystems above the wing on the X-59 “drove the sound, but that wasn’t what drove us to composites as opposed to metallics on the wing system,” Folsom says. The wing skins, on closer inspection, are far from flat surfaces — “every square inch is different than the square inch around them, the loft is continuously changing” — which would make metallics like stretch-formed aluminum complex and costly to work with. “You need composites in the wing to do the complexity of that curve. Metallics were never even considered,” he says. Composites also allowed the team to tailor the stiffness in different directions. “You can tailor it just by changing the ply laminates — you can’t do that with metals,” Folsom notes.
In-house wing manufacture. The composite wing skins were built by Lockheed Martin using the company’s Ingersoll AFP system and oven cure. Photo Credit: Lockheed Martin
The top and bottom wing skins are manufactured as 0.2-inch-thick solid laminates, from MTM-45 4-inch-wide unidirectional (UD) tapes laid up using Lockheed’s in-house Mongoose automated fiber placement (AFP) machine supplied by Ingersoll Machine Tools (Rockford, Ill., U.S.). “It’s one of the largest AFPs in the world,” Folsom says. “We can do a laminate the size of a room without even batting an eye.” This five-axis, gantry-style AFP system uses a localized infrared heater to temporarily increase the tackiness to improve the layering of each tape and hold them in place during AFP. This is followed by oven cure.
The lower wing skin is one continuous structure, minus a few holes for the landing gear, while the upper skin is manufactured as two separate pieces that sit on either side of a 34-inch metallic skin section in the middle. “That’s so we have places for the access panels, for the fuel pumps and whatnot inside the tanks, as well as it’s just a safe, easy place for maintenance crews to be able to walk on,” Folsom explains.
Building and assembling the wing. Mechanical fasteners were used along with bonding to secure the composite top and bottom wing skins to the metallic ribs and stiffeners in the wing interior. Photo Credit: Lockheed Martin
Long, thin nose: Sandwich construction, autoclave cure
Almost a third of the aircraft is taken up by the 34-foot-long nose. According to NASA, the length and shape of this nose is an essential feature to reconfiguring shock waves during supersonic flight.
The 299-pound nose is fabricated in two halves, right and left, that were adhesively bonded and bolted together along the upper and lower seams. Typically, a nose like this would be built as an upper and lower half, Folsom says, but the length and narrowness of the X-59’s nose required extra stiffness on the top and bottom to prevent bending. “The overlapping of the laminates at the joined edges was strategically placed onto the top and bottom of the nose, meaning there were double the layers of fibers and added stiffness,” he says, noting that the region acts similarly to the caps of an I-beam.
Each half was built as a sandwich panel with seven plies of MTM-45 fabric on the outer skin laminate, three plies on the inner skin and honeycomb core in between. These were laid up on female molds under vacuum bags and then cured in an autoclave. Lockheed Martin initially designed the nose, and then contracted its final laminate design and manufacture to another supplier.
Additional composite components
Like the nose, the rest of the composite components on the X-59 were designed and assembled by Lockheed, but manufactured by fabricating partners.
Inspecting the inlet duct. The composite inlet duct, designed by Lockheed Martin and manufactured by a partner, is designed for placement above the wings of the aircraft along with the engines and several other systems. Photo Credit: Lockheed Martin
The nose and the wing skins are the largest composite components, but Richardson says the most challenging to design were some of the smallest: the flaperons, ailerons and the rudder. “These are parts that get very thin at the trailing edge, and require a lot of handwork and artistry to get them to come together and to meet the loads that are placed on them,” he says. “They’re very highly loaded parts on the aircraft.” The ailerons and flaps are only 3 inches thick.
Semi-automated assembly
Once all of the metallic and composite components were manufactured, Lockheed Martin Skunk Works began assembly of the X-59 in 2018 and completed it in late 2021.
New for the X-59 project, Lockheed Martin Skunk Works recently acquired an Electroimpact (Mukilteo, Wash., U.S.) robotic drilling machine called the Combined Operation: Bolting and Robotic AutoDrill system, or COBRA.
This was acquired specifically for use in wing assembly. According to Folsom, one of the most labor-intensive processes involved in building a composite wing skin — or any large composite laminate via AFP, for that matter — is the assembly. The X-59 wing interior comprises metallic spars and ribs, all of which needed to be mechanically fastened to the wing skins.
“A bunch of guys on their hands and knees, crawling around drilling it by hand is kind of oxymoronic” after so much automation is used to build the skins themselves, he says. He explains that metallic substructures were chosen “since they have linear reactions to forces for ease of analysis. Their initial failure mechanism is to bend rather than snap, and they can easily accommodate the multitude of attach points for subsystems like fuel pumps and probes.”
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