No, the outlines of any hypersonic aircraft need to be sleek and smooth, allowing the craft to slice through the air as friction-free as possible. And it's not just the design; the materials of your craft need to be slick, too, and tough enough to withstand incredibly low pressure and unbelievably hot, fast-moving plasma.

Vanderbilt Assistant Professor of Chemical Engineering Bridget Rogers is developing thin-film ceramic composite coatings that can perform under the extreme and unique conditions encountered in flight by hypersonic vehicles, such as the NASA X-43A space plane that achieved a record-breaking Mach 7-5,000 miles per hour-on March 27 last spring.

These materials can also be used in other extreme conditions, such as on power plant turbines, military and civilian aircraft, and NASA space craft.

"Our ultimate goal," Professor Rogers says, "Is to develop an optimal thin-film coating that adheres well to the surface of the vehicle or machinery, resists corrosion and protects the equipment from high temperatures, low pressure and high-speed plasma flows during flight."

Her research, which focuses specifically on the processes of oxidation under hypersonic conditions, is funded by a five-year grant from the Presidential Early Career Award for Scientists and Engineers. This award, which she received at the White House last spring, is the highest honor bestowed by the U.S. government on outstanding scientists and engineers early in their research careers.

O No

Professor Rogers and her associates are testing materials to be used on the nose cone and the leading edges of wings of hypersonic aircraft.

The material most often used to protect hypersonic vehicles is a ceramic or mixture of ceramics. Ceramics are desirable for this work because of their extreme hardness and resistance to both heat and corrosion. They are also inexpensive, being made of substances found commonly in the ground.

While ceramics are indisputably tough, they are also brittle, and they vary a lot in how they react to something rather important in atmospheric flight: Oxygen.

Oxygen, which is reactive enough under everyday conditions (think of rust and silver tarnish), becomes highly reactive when it is broken apart from its preferred O2 bonding to the lonely and volatile O atom. Which is exactly what happens when a plane's nose cone and wing edges, traveling Mach 7 or so, smash into air molecules in the way.

"The current understanding of the oxidation processes under hypersonic conditions is very limited," Professor Rogers points out. "Our first job will be to study the ceramics themselves and the materials they're made of to determine the oxidative properties under hypersonic flight conditions. Once we have a better understanding of these processes, we will focus on developing thin-film barrier coatings for the ceramic materials."

Partnering with NASA Ames and SRI International, Professor Rogers' team has already done initial studies of the oxidation properties of silicon carbide and silicon nitride. "We are exploring now whether we can make lightweight thin-film composites of these ceramics, perhaps using them to coat a carbon matrix."

She is mapping a wide range of properties of these and other thin-film candidate materials as they perform under hypersonic-like conditions. "We are evaluating thermal expansion coefficients, thermal conductivity, stress, adhesion and barrier properties of both layered and alloyed coatings," she says.

Coatings are deposited at Vanderbilt by an Ultra High Vacuum - Chemical Vapor Deposit (UHV-CVD) reactor Professor Rogers designed. This reactor can deposit layers of thin films on the order of tens of nanometers thick. A nanometer is roughly four atoms long and about 1/1000th the diameter of a human cell.

The thin film is next analyzed in situ using spectroscopic ellipsometry, which can assess thickness, consistency, and composition and other properties of the film based on its reaction to different wavelengths of light. The reactor is also equipped with a quadrupole mass spectrometer for kinetic and mechanistic studies. A mass spectrometer pours ions through a small opening in the reactor and measures the electrical emissions that result.

Additional thermal and mechanical studies are done at the Oak Ridge National Laboratories High Temperature Materials Laboratory.

Shuttles, Planes and CEVs

Hypersonic research nationwide underwent a course correction last spring. Only months before the air-breathing space plane X-43A's record-breaking flight, NASA announced that the Orbital Space Plane (OSP) program would be cancelled. Instead, work would begin on the CEV, the Crew Exploration Vehicle described in President George W. Bush's 2004 State of the Union Address. NASA's new Office of Exploration Systems announced that, while the X-43C hypersonic demonstrator program would be cancelled, work on hypersonic flight would continue.

The OSP was conceived as a replacement vehicle for the Space Shuttle, capable of transporting crew and supplies to and from the International Space Station. The CEV will be designed for lunar, possibly inter-planetary, flight. President Bush hopes to land an American on the moon by 2015.

The change does not affect Professor Rogers' research at this point.

"Our research is fundamental science, applicable to any hypersonic vehicle, regardless of the propulsion system used, that must fly at some point through the air in Earth's atmosphere and hence will be subjected to the effects of oxidation on the vehicle's surface," Professor Rogers says.