Beat the heat

Boeing team develops tiles to make shuttle safer, easier to maintain

BY BOB HOWARD

Karrie Hinkle's favorite hobby is cooking good things. Picking the perfect ingredients also makes her an outstanding Boeing chemist, one who spearheads the kind of innovation that makes NASA stand up and take notice—especially when it affects Space Shuttles.

Each NASA Shuttle is covered with more than 21,000 lightweight heat-insulating "tiles" that protect the Shuttle and crew from the searing heat of atmospheric reentry. Flying at 17,500 miles per hour, this 20-minute descent generates friction and, ultimately, a scorching environment hotter than molten lava.

Yet these tiles are so good at quickly throwing off intense heat that you can hold this material with your bare fingers just seconds after it's been taken, still glowing red hot to the core, from a 2,200-degree furnace. The tiles, however, are fragile and therefore prone to damage.

Boeing has improved Shuttle tiles substantially by developing Boeing Rigid
Insulation (BRI), which is five to 10 times stronger and more durable than any tile used before it. For NASA and the Shuttle crew, this means maintenance costs decrease and safety margins radically increase.

"Chemistry is like cooking. What we did is like cooking high-temperature cake," Hinkle said. "You mix the right amount of the right things and cook at the right temperature. The result is a stronger, safer tile."

While ceramics engineer and chemist Hinkle picked out the new ingredients, it was Vann Heng and MaryAnn Santos, both Boeing materials engineers and scientists, who actually developed BRI. They rounded out this three-woman team, which earned a U.S. patent for its work.

Heng and Santos' job was to painstakingly develop how to mix and cook the new tile to get the best strength and thermal efficiencies for when the tile was doing its job. They spent several years changing the formulation and fine-tuning each of the processes. Then, of course, the material was tested extensively.

"With BRI, we took something we had reason to believe would work, and Vann and MaryAnn mixed it into the normal tile mixture," Hinkle explained. "They really did the important development work on this, experimenting to develop the right 'cooking' temperatures and right amounts of ingredients."

Most Space Shuttle tiles are made with pure silica oxide fibers. A Space Shuttle tile is super-light because it is 90 percent air and 10 percent silica fibers. It resembles and feels like Styrofoam. Dive down into this material, magnifying it hundreds of times, and its fibers look like a loosely woven bird's nest. This porosity helps the material throw off heat.

Yet some Shuttle areas experience higher temperatures than others. To address these differences and make BRI, the trio added alumina oxide, because it can take higher reentry temperatures without allowing warping. A pure silica tile might deform and influence an adjacent tile.

Then their breakthrough came. "The secret to BRI, beyond adding alumina fibers, was processing them so the fibers lay flat and conducted heat out horizontally rather than in, vertically, toward the Shuttle's skin," Heng said.

Because the alumina fiber makes the BRI tile free from warping, it also could take advantage of a surface treatment invented by NASA. This toughened the tile's outer surface and gave it five to 10 times higher resistance to damage. The treatment involves infiltration of small particles into the tile's surface, thus filling in between the bird's nest silica strands. The introduction of this material improved resistance to impact damage.

"Technicians are putting new BRI tile on Discovery in the hotter, more critical areas, like the nose landing gear doors. We expect to see a couple of dozen installed on that Shuttle before we fly mission STS-121," Hinkle said.

Then, more than 700 of the new BRI tiles will be installed in selected, critical areas of each Shuttle—the underbelly, wing tops and bottoms and the window areas—to offer more protection than ever. Boeing continues to issue the blueprints for this work, planned over the next four years.

The BRI fabrication work was done at the Huntington Beach Development Center in California. NASA relied heavily on Boeing's Huntington Beach team and this center when preparing Space Shuttles to fly again after Space Shuttle Columbia tragically broke up on reentry in 2003.

All in all, it's been quite a ride getting to this level.

"We developed each other's potential," Santos said. "We were open-minded with each other's ideas all through the development process. This was important as we were limited in both time and budget.

"When there was a question about which idea to use, we analyzed logically, practically, to arrive at a consensus for which was best," Santos added. "We couldn't get this far if we were limited to just a few ideas. We needed everybody's ideas."

robert.howard@boeing.com