Structures Technology Research at AFRL

As many of you know, I changed jobs in March right as the world shut down due to the coronavirus. After 31 years in the AFRL Materials and Manufacturing Directorate, I moved to our sister organization, the Aerospace Systems Directorate, to lead the Structures Technology Branch. Some of you have asked what I’ve been up to in the new job, so I thought I’d update you.

Our mission is to drive step function improvements in cost, performance (speed, range, loiter, and payload capacity), life, and time to market for military aircraft structures. We do this through developing transformative technologies to improve weapon system capability and by informing Air Force policy on airframe certification and structural integrity. Our technology investment portfolio is a direct response to guidance from many sources, including but not limited to, the 2018 National Defense Strategy, the 2019 USAF Science and Technology Strategy, the AFRL Commander’s Intent, the Air Force Warfighting Integration Capability, and USAF Lifecycle Management Center Engineering and Program Offices.

So why after all these years should AFRL continue to invest in aircraft structures research when mission effectiveness and cost are driven primarily by specialized payloads or operational requirements? First, military aircraft are required to operate at the edge of a typical design envelope, potentially in severe conditions and while taking fire. Next, for new aircraft concepts, structures drive the weight and durability of a design, which then impacts the range/loiter, speed, payload, cost, and longevity of the system. Finally for the existing fleet, our research helps us understand and address issues due to flying our aircraft longer than planned. As our structures wear out due to fatigue and corrosion, we seek to understand how structures degrade to then develop solutions that mitigate the issues. In other words, we can’t afford to not invest in aircraft structures research.

Our new strategy over the Future Years Defense Program (FYDP) starting in 2021 focuses on three areas: 

1.) Understanding the impact of airframe life on structural design, airworthiness certification, and structural integrity policy for both conventional and new attritable aircraft concepts

2.) Reducing airframe weight to improve the performance of both conventional and attritable aircraft concepts

3.) Study the role of structures in aeroelasticity and develop a plan to address the key issues.

For airframe life, we have three ongoing efforts for conventional aircraft. The first is the Airframe Digital Twin, where we are developing uncertainty quantified structural lifing methods and tracking processes for making maintenance decisions or anticipating structural modification needs to meet a required aircraft service life for individual aircraft. Next is the Laser Peening demonstration to develop the rules and tools necessary to increase the service life of metallic structural components using laser peening to impart beneficial residual stresses. Finally, we have the Composite Airframe Life Extension program which looks at approaches to extend the service life of composite airframes.

For the future, we are planning a new effort to look specifically at the new attritable aircraft concept. Whether it’s based on aircraft similar to the XQ-58A or the DARPA Gremlins, these concepts are envisioned to have a fraction of the service life of conventional aircraft, perhaps just hundreds of flight hours. We aim to clearly define “attritable” by examining the USAF structural requirement documents line-by-line and striking requirements that should not apply to limited life airframes. From this, we will develop a “Recommended Attritable Airframe Requirements Document” supported by research that quantifies through demonstration the potential cost, performance, and safety impact that reducing these requirements may have.

Parallel to the attritable requirements effort, we are investigating the impact that new materials and manufacturing methods can have on the structural design of attritable aircraft: with limited flight hour expectations and of the possibility of no depot maintenance, the aerospace industry can consider new and novel processes to help meet weight and cost targets. For our current efforts, we asked our industrial partners to build a structure similar to the XA-58A but forego the design and manufacturing implications of published airworthiness certification criteria and joint service specification guidance required for manned military aircraft. Additionally, they were to rethink aerospace structural manufacturing norms and instead consider relying on low capital investment tooling and equipment, low-cost composite materials and processes, simple geometry parts with self-locating details, selective polymer additive manufacturing for tooling, edges and joint features, and an all-bonded assembly. All of this was to explore the art of the possible in reducing cost without the constraint of aircraft performance goals. 

Moving beyond current efforts, we plan to balance cost and performance. Using the Recommended Attritable Airframe Requirements Document concept mentioned above, we intend to design and build an attritable aircraft structure to verify that the new materials and manufacturing concepts meet attritable design criteria through ground test. We are also going to explore the impact of using additive manufacturing to build primary structures for attritable aircraft. With much lower fight hour expectations, concerns about durability in metal additive manufactured parts are lessened considerably.

To reduce airframe weight for conventional aircraft, we are looking at the next generation of bonded structures. Building off of the Composites Affordability Initiative and the X-55A Advanced Composite Cargo Aircraft, we are considering methods to enable fail-safe certification approaches for bonded structures by exploring several damage arrest features. In concert, we are also looking at new impact damage analysis tools to assess the durability and damage tolerance of these structures.

To reduce airframe weight for attritable aircraft, we are exploring topology optimization. This has been successful in additive manufacturing to place material only where needed to carry structural loads. We have been exploring how to do this at the airframe level. In our laboratory, we have designed and built a wing structure using topology optimization and are seeing great promise in reducing structural weight. We plan to continue this work by exploring additional load cases and structural component concepts. Additionally, we are looking to improve topology optimization tools and methods for translating their predicted load paths into structural layouts.

Finally, we are initiating a focused study investigating the interplay between aircraft structures and aeroelasticity. Several potential applications, such as a high-altitude, long-endurance vehicle, require multi-disciplinary technologies that we have not considered to date but are well positioned to pursue. Over the next year, we will be hosting workshops focused on exploring this multi-disciplinary concept and reaching out to our partners in academia, industry, and other government agencies to help us identify our role.

In summary, aircraft structures research is still important for the Air Force. Through our investments, we are developing transformative structures technologies and policy recommendations for certification and structural integrity that enable step function improvements in cost, performance, and time to market of military aircraft. As we move forward with our new investments, we look forward to working with our partners to improve capability for the warfighter.

DISTRIBUTION STATEMENT A. Approved for public release: distribution unlimited. Case Number:  88ABW-2020-3558