Morphing Structures - Advanced Materials Q&A with Austin Phoenix
Austin Phoenix has been with the U.S. Naval Research Laboratory since 2011. A North Carolina native, Phoenix was hired under a Karl's fellowship studying large space structures. From 2014 to 2016, he participated in NRL’s Select Graduate Training Program and went on to earn a PhD at Virginia Tech with his research in high performance morphing systems.
Today he works in NRL’s Space Mechanical Systems Development Branch, where he’s bringing his background in morphing structures to NRL’s hypersonics and space research. The concept of morphing structures isn’t new. It dates back as early as the Wright Brothers invention of flight, when the brothers used cables to twist and warp the wings of their 1903 flyer to control it in the air. Until recently, the idea had been abandoned in the realm of contemporary aviation in favor of fixed wings and rigid materials.
But these days, engineers like Phoenix are applying morphing structures methodologies to systems designed for space, for the hypersonic regime and underwater. Among the projects Phoenix is currently working on is a design for hypersonic vehicle called a morphing waverider.
For this Q&A, we sat down with Phoenix to discuss the potential he sees for morphing structures in space operations and his group’s ongoing research on advanced materials for hypersonic flight.
Can you tell me what morphing structures are and why they're important?
Morphing structures are structures that change their geometry to meet the needs of their environment. In space, that means basically pushing back against changes in their environment that would make them bend and twist out of shape. In the hypersonic regime, morphing structures methodologies can allow us to change the geometry of a vehicle to improve its aerodynamic performance across its flight envelope.
The Wright brothers used a form of morphing structures with the warped wing concept, but that was quite a while ago.
One of the major changes since their day is that we've moved into engineering materials that are more rigid and less suitable for morphing systems. But there's always been this thread of people in the aerospace community who are looking back to consider the advantages that we can apply by enabling geometry modification.
You’re applying your knowledge of morphing structures to space research. Are morphing structures more common to engineering for space operations?
Not generally. It’s fair to say I’m unusual in my pairing of the two disciplines. Traditionally, when it comes to spacecraft structures the idea is to make sure things don't move. Engineers spend a lot of time and effort working to isolate the space structure from these external disturbances and, internally, isolating the structure’s thermal control from its operational system.
Consider a telescope on a satellite. The idea is to isolate your thermal control system from your optical hardware, so the thermal system doesn't affect its structure. My thought was perhaps we might be able improve the overall system’s performance by coupling these traditionally isolated systems.
How would you couple the thermal system with the optical side of things?
The way we do it currently, like with NASA’s James Webb Space Telescope, is we build a structure that doesn't change with temperature or time. It's very heavy, and it's rigid and it doesn't move. It's strong enough and thermally stable enough that when the environment changes it doesn't notice.
The idea here would be to build a much lighter and adaptive structure that would use the thermal system to change the temperature of our structure to realign an optical system. When environment changes threaten to distort your system and misalign your optics, the structure would apply some thermal energy, resulting in material expansion to maintain your optical alignment.
That sounds like it would be complicated to pull off.
Yes, but if we want to build bigger, more ambitious space projects and more effective space telescopes we're going to need to look toward smarter, more adaptive structures. That will take more complex, integrated systems that can improve performance and enable us to see deeper into space.
As an example, Hubble was really our most successful on orbit telescope, and that used a single mirror. NASA’s James Webb telescope uses multiple mirrors. That means it has some adaptive capability to make all those mirrors act like one mirror.
The next step would be a much more adaptive system that will require new structural paradigms to continue the path of that growth and capability. With a morphing structures methodology, we can design a lighter, larger system that adapts to its environment and provides you the same or better capability.
Morphing structures is not a new concept, but is this idea of applying it to space operations relatively new?
It's still in the research stages. There are a lot of spacecraft systems today that change their shape, but they generally do that through conventional actuation methodologies. There are big ones that use umbrella-type mechanisms, systems that deploy and fold out and use levers and pinions, but there aren't any large scale morphing systems on orbit today. No one's including it on their next launch.
Understandably, the space operations community is fairly risk-averse and they need demonstrated capability prior to launch. The systems we see go on orbit are always several years behind what we can do just because of the technology maturation process. But as we look to achieve more ambitious goals in space, we'll have to start taking these new structural methodologies into account.
Are you talking about something like putting up a space-based solar array to beam solar energy back to a receiver on Earth?
Absolutely, something like that. Because you have to have connections and you have to maintain pointing. If you're looking at putting up large structures like that and still want to use current, non-adaptive methodologies, you're going to have to launch systems that would be much larger than systems that can adapt to their environment.
In addition to your morphing structures research, your group has been conducting advanced materials research for hypersonic flight.
As we look back through aviation history, a lot of the key breakthroughs in capability have come with materials development. Whether it’s the Wright flyer—from cloth and wood, to lightweight aluminum, to higher performance systems like titanium in the SR-71—materials developments have been key to improving the performance of these systems.
The key challenge in hypersonics research is finding materials that can survive not only the structural environment but also the intense thermal environment and maintain oxidation resistance. That’s really a new challenge. So as we look at managing this extremely intense environment, we’re looking at new materials, but we’re also considering new ways to use old materials. That’s primarily what we have focused on in our group. As we look at new ways to use old materials, we're developing systems to actively integrate cooling systems into those materials.
What are the materials generally used in hypersonic flight and what are you developing?
In hypersonics, carbon fiber-reinforced carbon, or carbon-carbon for short, is the material that people are often using. We've been able to make it for a while, but it's still the leading edge. It has enormous thermal capability. It can operate at really high temperatures. But it takes really long to produce. Certain parts can take on the order of months to process.
These material systems are amazing and can provide secondary benefits such as ablation, which has its own benefits and drawbacks. Ablation means the material literally burns. The benefit is that takes a lot of energy to make that phase transition, to burn that material, and so it absorbs a lot of energy when it does that. But the ablation process roughens the surface and alters your vehicle’s aerodynamics.
There's a trade-off there. It makes it practical only on a one-use vehicle. Apollo shuttles used a fully ablative system.
Is your group working with carbon-carbon?
We're working in one avenue to improve the performance of carbon-carbon-like systems through enabling active cooling of those through vascularized networks. The idea is that you generate a vein-like network through the material and you put in a working fluid to control its temperature and improve the material’s maximum operating thermal environment and reduce ablative or secondary concerns.
We’ve also developed a system that uses liquid sodium heat pipes to move thermal energy around and pull energy out of those really hot spots to extend the operational performance of old materials such as titanium, tungsten and stainless steel. The aim is to add active cooling in those systems to extend a conventional material’s operational regime.
Can you talk about advanced materials research with regard to your group’s morphing waverider design? What kind of materials are you considering?
As we design the morphing waverider we've added an additional requirement onto the material and that is some slight elasticity, ductility. Because we're going to introduce these flexible surfaces into this high temperature environment. As we consider that design space, actively cooled metallics really have huge benefits for meeting those morphing needs.
But we're looking at a range of materials, from stainless steels to titanium to a material technology that we've only just scratched the surface of: functionally graded materials. That’s a material that’s graded to another material, going from say stainless steel to tungsten, where you basically make a material that starts out 100 percent titanium and then slowly grades from 100 percent titanium to zero percent titanium to 100 percent stainless steel.
That’s perhaps one of the most promising material sets. Leveraging functionally graded materials may enable us to tune and manage the material properties so we can use lower temperature materials with the higher ductility levels we need to achieve large deformations.
What is the advantage of a material that gradually grades into another material?
You get that gradient in material properties. For example, tungsten is really high temperature performance. So it works really well for the leading edge, but it's a little finicky. It's very heavy and expensive and not something you'd want to build your whole system out of. With a functionally graded material, I can make the leading edge out of tungsten and then slowly grade that material into stainless steel. That allows me to gradually grade from this hard, very high temperature material to a much more conventional engineering material that I can easily manufacture.
The conventional way of doing it would be to build an inch leading edge of tungsten, and then attach that to some stainless steel structure behind it. But that attachment can introduce significant performance issues because it’s a different material; it may grow differently, it may be a thermal barrier—any of those factors cause design and implementation issues. But if instead I'm able to directly grade the material, then I'm able to benefit from both that 100 percent tungsten at the very tip of the leading edge and a clean structural connection all the way back.
That sounds complicated and hard to pull off.
With the additive manufacturing machines that we have at NRL, we're able to dynamically change and insert two different powders at different rates. Basically—and it's clearly not this simple but—in that you have a laser and you have two particle beams where you're injecting small particles of two separate materials. So by doing some very fancy materials design and other things, you may be able to develop a very clean, graded material that way.
Can you describe any notable milestones in your research in recent days?
One of the things we've been doing is we've been looking at lower cost but high temperature vascularized composites. And we've demonstrated high temperature performance using a pumped two-phased system where we've done some cool testing. Just this month, we demonstrated that under an oxy acetylene torch the material with these vascularized networks maintains its integrity. It’s not damaged under incredibly high heat loads.
Can you talk about future plans?
We don't today have the capability to make a full-scale system, but we’re working toward creating an additively manufactured test article with a morphing surface and a fully integrated leading edge thermal management system—maybe not a rocket-powered vehicle but something like the Hypersonic Technology Vehicle 2 (HTV-2), where it's something like a meter by two meters in size.
That’s pretty far down the line, though. In the near term, we'll continue testing both our heat pipe integrated systems as well as our vascularized systems for augmenting both conventional and high-temperature materials.
Ultimately, that’s technology that will be incorporated not just in the morphing waverider design, but elsewhere, like the next generation tactical boost glide vehicle, our next generation hypersonic vehicle—really any other systems where we would augment the performance of the leading edge.