Hot stuff: Tackling extreme temperatures of hypersonic flight
Hypersonic flight is hot — in more ways than one. It’s a big area for research in defense applications and potentially in commercial aircraft to compress travel times in a globalized world. But it’s also just flat-out hot at the leading edges like the nose cone and wings, reaching temperatures in excess of 1,800˚C while traveling at five times the speed of sound.
Materials engineering holds the key to solving the engineering puzzle: how to keep the hypersonic vehicle’s structural materials from degrading and failing in that red-hot environment.
We’re developing ceramic materials and specialized coatings to address this dilemma. Ceramics are characterized by more open crystal structures and therefore lower densities. Any time you want to fly a material, you want it to have a low density because it costs a lot of money to put heavy things in the air. This means the leading edges of hypersonic flight materials are going to be ceramic.
The elevated temperatures the materials must resist are caused by the friction of the molecules in the air flowing over the leading edges of the hypersonic aircraft. Friction is omnipresent in the world whenever you pass through air, even if you’re just walking or running. But at hypersonic speeds, that friction generates a tremendous amount of heat.
Ceramics exhibit a stronger bond between the atoms, providing higher melting temperatures. Our challenge as materials engineers is to limit the effects of the heat on the leading edges because materials almost always show a degradation in their properties as you increase the temperature. If we can effectively cool these leading edges, we can improve the properties of the materials and their performance under hypersonic flight conditions.
The solution we’re developing in our labs is to make coatings with high emissivity, or emittance — and thus the ability to emit, or throw off, thermal energy. Creating a surface with high emissivity re-radiates the heat generated at the leading edge away from that surface. For hypersonic flight, we are working to design coatings that absorb the incoming heat. Then we want that leading edge to re-radiate the incoming heat back into the environment before it gets incorporated into the leading edge material.
We are using a rare-earth material called samarium oxide, alloyed with ceramic-based materials to modify and increase the emissivity of this combination. In addition, we’re investigating infrared (IR) window materials, so hypersonic aircraft can send and receive electromagnetic communication signals. And we’re looking at employing additive manufacturing (AM) techniques, developed by R. Byron Pipes, Purdue’s John L. Bray Distinguished Professor of Engineering, for traditional polymer composites to essentially make 3D prints of hypersonic ceramic structures.
Hypersonics is sometimes described as the new Stealth, referring to the technology that lets aircraft fly without being detected. The United States is very good at stealth technologies; our goal is to help our nation take the leading edge in hypersonic flight as well.
Rodney Trice, PhD
Professor, School of Materials Engineering, College of Engineering, Purdue University
Fellow of The American Ceramic Society (ACerS)