Part 1 - “Aluminum plays a pivotal role in both the structural integrity and aerodynamic efficiency of NASA’s X-59 QueSST aircraft,” Kalea Texeira, National Training Program Manager at Federal Aviation Administration

In conversation with Kalea Texeira, who explained about NASA’s X-59 Quiet Supersonic Technology (QueSST) aircraft. In this interview she highlighted how it used materials and shaped its airframe to minimise sonic booms something earlier aircraft like the Concorde and F-18 never addressed. She also pointed out the importance of aluminium for aircraft construction.

To know more of her insights, read the full interview below:

Al Circle: What distinguishes the X-59's airframe and materials from earlier supersonic aircraft like Concorde or the F-18?

Kalea Texeira:  The X-59 Quiet Supersonic Technology (QueSST) aircraft represents a significant leap in supersonic aircraft design, particularly in its approach to airframe and material selection, distinguishing it markedly from predecessors like the Concorde and the F-18. While all three aircraft leverage robust materials to withstand the rigors of supersonic flight, the X-59's design is fundamentally driven by its primary mission: to mitigate the sonic boom.

Concorde, a marvel of 20th-century engineering, was primarily constructed from aluminium alloys, specifically Hiduminium ‒ RR58. This material choice was critical for its structural integrity and ability to endure the thermal stresses of sustained Mach 2 flight. The Concorde's aluminium nose, for instance, could reach temperatures of approximately 127 °C (261 °F) during cruise, causing the aircraft to lengthen by nearly a foot due to thermal expansion. While advanced for its time, the Concorde's design did not actively seek to suppress the sonic boom, which ultimately restricted its overland flight paths.

The F-18 Hornet, a versatile multirole combat jet, incorporates a blend of materials. Its airframe is predominantly aluminium, but it was a pioneer in extensively utilising composite materials, particularly carbon/epoxy composites, for structural elements like wings, doors, and control surfaces. Composites comprise a significant portion of its structural weight and external surface area, offering a superior strength-to-weight ratio an enhanced performance characteristics for combat manoeuvres. However, like the Concorde, the F-18's design is not optimised for sonic boom reduction.

In contrast, the X-59 is a purpose-built experimental aircraft, and its airframe and material selection are intrinsically linked to its low-boom design philosophy. While the X-59 is also constructed from conventional aviation-grade aluminium, its distinguishing features lie in its unique aerodynamic shaping and the strategic integration of components to achieve a quiet sonic boom. Its exceptionally long and slender fuselage, particularly the elongated nose cone, is designed to prevent shockwaves from coalescing into a loud boom. Instead, these shockwaves are spread out, resulting in a much quieter “thump”. This design priority means that while it uses familiar materials, their application and the overall structural geometry are revolutionary. The X-59 also incorporates components from other aircraft, such as parts from the F-18 and F-35, demonstrating a pragmatic approach to leveraging existing, proven technologies within its novel design.

AL Circle: How does aluminium factor into the structural and aerodynamic design of the X-59? What percentage of the 9's structure is made of aluminium or aluminium alloys?

Kalea Texeira:  Aluminum plays a pivotal role in both the structural integrity and aerodynamic efficiency of NASA’s X-59 Quiet Supersonic Technology (QueSST) aircraft. While specific percentage figures for aluminium content are not widely publicised in exact numerical terms, sources consistently indicate that the X-59’s construction features “mostly aluminium” or is primarily made of “conventional aviation-grade aluminium”. This reliance on aluminium is a deliberate choice that underpins several critical aspects of the aircraft’s design and mission.

Structural Design

Aluminium alloys are favoured in aerospace for their exceptional strength-to-weight ratio, corrosion resistance, and ease of fabrication. For the X-59, these properties are crucial for creating a robust yet lightweight airframe capable of enduring the stresses of supersonic flight. The aircraft’s unique, elongated shape, which is fundamental to its low-boom design, necessitates a material that can be precisely formed and reliably maintain its structural integrity under varying flight conditions. Aluminum allows for the intricate shaping required for the X-59’s distinctive nose, slender fuselage, and specialised wing structures, all of which are engineered to manipulate shockwaves.

Aerodynamic Design and Sonic Boom Mitigation:

The primary objective of the X-59 is to reduce the perceived loudness of a sonic boom to a mere “thump.” Aluminium’s role in achieving this is indirect but essential. The material enables the construction of the aircraft’s highly unconventional aerodynamic profile, which is the key to its quiet supersonic capability. This includes:

• Long, Slender Fuselage: The X-59 is nearly 100 feet long but has a relatively small wingspan. This extreme length, facilitated by the structural properties of aluminium, is critical for spreading out the shockwaves that typically coalesce into a loud sonic boom.
• Elongated Nose Cone: A significant portion of the X-59’s length is its extended nose, which is designed to break up and diffuse shockwaves before they reach the ground. Aluminum allows for the precise manufacturing of this critical aerodynamic feature.
• Strategic Shaping: The overall shaping of the X-59’s airframe, from its nose to its tail, is meticulously designed to prevent shockwaves from merging. The malleability and strength of aluminium alloys enable the complex curves and surfaces necessary for this shockwave management.

While the exact percentage of aluminium in the X-59’s structure is not publicly detailed, the consistent emphasis on it’s “mostly aluminium” construction and the frequent reference to it as an “aluminium bird” during testing phases underscore its pervasive presence. This indicates that aluminium, likely in various high-strength alloys, forms the vast majority of the aircraft’s primary structure, with other materials (such as composites or specialised alloys for specific components) used strategically where their unique properties offer an advantage. The choice of aluminium also aligns with the project’s goal of demonstrating a viable path for future commercial supersonic aircraft, as aluminium is a well-understood and cost-effective material for large-scale aircraft production.

AL Circle: Could you explain the significance of the "aluminium bird" test in comparison to traditional "iron bird" systems testing?

Kalea Texeira The "aluminium bird" test conducted for NASA's X-59 Quiet Supersonic Technology (QueSST) aircraft represents a significant evolution in aerospace systems testing, particularly when compared to the traditional "iron bird" methodology. Both testing approaches are crucial for verifying an aircraft's integrated systems before its maiden flight, but they differ fundamentally in their scope and the fidelity of the test platform.

Traditional "Iron Bird" Testing:

Historically, an "iron bird" test involves a full-scale, non-flying mock-up of an aircraft, often constructed from steel or other robust, readily available materials (hence "iron"). This mock- up is equipped with the actual flight control systems, hydraulics, electrical systems, and other critical components that would be present in the real aircraft. The primary purpose of an iron bird is to:

• System Integration Verification: Test how all the aircraft's subsystems interact and function together in a controlled ground environment.
• Software Validation: Validate the flight control software and avionics in a realistic hardware-in-the-loop setup.
• Failure Mode Analysis: Simulate various failure scenarios to understand system responses and develop appropriate emergency procedures.
• Pilot Familiarisation: Allow test pilots and engineers to interact with the aircraft's controls and systems before actual flight. While highly effective for system integration and software validation, the traditional iron bird is typically a static, non-aerodynamic structure. It does not accurately replicate the dynamic structural behavior or thermal responses of the actual aircraft's airframe during flight.

The "Aluminum Bird" Test for X-59:

The X-59's "aluminium bird" test takes the concept of ground-based systems verification a significant step further. Instead of a generic mock-up, the X-59 itself the actual flight-ready aircraft, constructed predominantly from aluminium was used as the test platform. This approach is highly significant for several reasons:

High-Fidelity Structural and System Interaction: By using the actual aircraft, the test accurately reflects how the flight systems interact with the real airframe, including its structural flexibility and thermal characteristics. This is particularly crucial for the X-59, where the precise aerodynamic shape and structural response are integral to its low- boom design. The test verified how the aircraft's hardware and software work together, responding to pilot inputs and handling injected system failures.

Realistic Simulation of Flight Conditions: The X-59 was put through its paces in simulated flight environments without ever leaving the ground. This involved commanding the aircraft's onboard subsystems and tricking the plane into believing it was flying, allowing engineers to verify integrated performance under conditions that closely mimic actual flight.

Early Identification of Issues: Testing the actual aircraft at this stage allows for the early identification of any unforeseen interactions or discrepancies between the systems and the physical airframe that might not be apparent in a less representative iron bird setup. This includes electromagnetic interference testing, ensuring that all systems (radios, navigation, sensors) can work together safely without interference.

Efficiency and Risk Reduction: By performing these comprehensive tests on the real aircraft, NASA and Lockheed Martin can gain a higher degree of confidence in the X-59's readiness for flight, potentially reducing the number of costly and time-consuming flight tests required. It's a pivotal step in ensuring safe flight and successful achievement of mission goals.

In essence, the "aluminium bird" test for the X-59 signifies a move towards more integrated and realistic ground testing, leveraging the flight article itself to provide a higher fidelity assessment of system performance and interaction within its actual structural and material context. This approach is particularly valuable for an experimental aircraft like the X-59, where novel design principles demand the most accurate pre-flight validation possible.

Stay tuned for the next part...