Feb., 10, 2003 /Anyone who has dropped an overheated platter and watched it shatter has seen firsthand the problems scientists face with the thermal tiles that shield space shuttles from blistering heat and their dilemma in developing new materials.

The materials used in the thermal tiles were developed initially decades ago. Scientists had to balance two characteristics: resistance to heat against resistance to damage - all while keeping the material lightweight.


The shuttle faces temperatures as high as 3,000 degrees upon re- entry, so thermal protection must come first, but at a cost. The 24,000 or so ceramic thermal tiles sheathing the shuttle's aluminum skin are brittle, coarse and easily chipped or broken.

Current tiles are made of a silica - sand - manufactured as a network of tiny fibers with many tiny air pockets to make them light. To toughen tiles in areas of the shuttle that are exposed to the greatest heat, the exterior of some tiles are coated with a glassy substance to further protect them from impacts, adding weight.

Engineers would like to develop so-called "hot structures," spacecraft with a basic structure strong enough to operate in the extreme heat of re-entry with minimal protective material. Its thermal-protection system would be the aircraft, not something glued or bolted to it.

The tiles' weaknesses have been highlighted in the ongoing investigation into the Columbia disaster Saturday, just minutes before scheduled landing.

Retrofitting the current fleet is not practical, said Virginia Tech structural engineer Dr. Rakesh K. Kapania, who is focusing research mostly on products for the second- and third-generation of reusable launch vehicles.

Scientists at NASA's Langley Research Center are developing a metal-based system to protect the next generation of shuttles. "ARMOR" is designed to be an "adaptable, robust, metallic, operable and reusable" thermal protection system.

In addition to deflecting the destructive heat the space shuttle generates as it returns through Earth's friction-producing atmosphere, Langley's new system would save on the current tiles' extensive maintenance.

"We're looking at something a little bit more robust," said Dr. Mark J. Shuart, director for Langley's structures and materials group. Langley scientists have been studying metal-based thermal- protection systems for more than 20 years.

With the ability of metal to flex without breaking, called its ductility, "you'll get a local dent, while in a ceramic you might get a spalling off [flaking] of something," Shuart said.

The metallic anti-heat system sandwiches silica-fiber insulation between thin layers of what might be thought of as "super-stainless steel" made from a high-strength, heat-resistant alloy of nickel, chromium and iron.

The sandwich formed by the metal and insulation would be attached to the spacecraft with metal brackets, rather than being glued, as are the ceramic tiles.

In Langley's testing so far, the alloys used for the panels withstand about 2,000 degrees. Researchers are working on new metal alloys that could protect against even higher temperatures.

Other researchers are advancing ceramics.

NASA's Ames Research Center, near Sunnyvale, Calif., is studying several materials called "ultrahigh-temperature ceramics," based on boride and carbon. They withstand high temperatures and breakage but are difficult to make.

At Virginia Commonwealth University, Dr. M. Samy El-Shall is creating ceramics with particles 1 billionth of a meter. These nanomaterials - silicon-nitrite, carbon-nitrite and several forms of titanium - are designed to retain ceramics' ability to withstand heat while introducing flexibility, he said.

The research is preliminary. Scientists are devising new ways to "mill" the particles to the right size, control their behavior and then assemble them into a useful application - tiles, for example - that function exactly as intended.

"It's the structure of the material . . . at the atomic level . . . that will dictate the performance of that material," said Dr. David Clark, head of materials science and engineering at Virginia Tech.

El-Shall uses lasers to synthesize both ceramic and metallic semiconducting particles. But there is a vast gulf from the fine piles of powder he creates to covering a spacecraft flying more than 12,000 mph through space, he said. "Things work very well in the lab, you know, in the form of particles, but this does not mean that it will translate" into a real-world application.

Getting the chance to apply these high-tech materials to traveling in space could be problematic.

In 2001, NASA canceled the $1 billion X-33 program - considered to have been a potential replacement for the space shuttle - after five years of development, technical problems and soaring costs.

With the X-33's demise, Langley researchers lost the opportunity to test their metallic thermal barrier in space, Shuart said.

NASA's Ames ended up licensing to a Blacksburg company a new protective coating for ceramics it had developed for the X-33 and others of its generation.

Blacksburg-based Wessex Inc. now manufactures and commercializes the product under the name Emisshield. Company President John Olver said the coating now is in 36 products, ranging from furnaces to Xenon headlights to firewalls to NASCAR race c