2 Inches Between Life, Death

At 18 times the speed of sound, the line between life and death aboard the space shuttle is about 2 inches thick.

It is the measure of most of its ceramic heat-shield tiles.

NASA investigators are still struggling to determine what caused the destruction of the space shuttle Columbia as it hurtled home Saturday. But its protective sheath of about 26,000 tiles is under intense scrutiny in the investigation of the accident that killed seven astronauts and scattered wreckage across the Southwest.

At that speed and that moment in flight -- when the temperature is almost twice the melting point of the shuttle’s aluminum alloy airframe -- almost any calamity ultimately would involve the breach of the heat shield and the breakup of the spacecraft.

The exotic tiles of silica -- fused from the purest river sand -- are in many ways the essence of NASA itself, embodying the agency’s engineering resourcefulness and its daring, as well as its false starts and misjudgments.

They are also, perhaps more than any other part of the most complicated flying machine ever built, pieces that crystallize a basic dilemma of modern spaceflight: how to combine strength and durability in featherweight materials that can survive launch and reentry.

Despite 30 years of refinements, researchers have yet to devise a fundamentally simpler, lighter and tougher system.

Columbia was the first manned experiment in the trial-and-error engineering of a reusable heat shield. Loose tiles, cracked tiles, broken tiles, abraded tiles or missing tiles were part of every shuttle flight.

The shuttle’s tiles were conceived in the twilight of the Apollo program, when NASA was desperate to keep manned space flight alive, former agency engineers, space analysts, and retired aerospace executives said.

As a technical matter, the tiles were an ingenious effort to balance heat, strength, weight, flexibility and the desperate need to protect the shuttle’s aluminum alloy frame from the furnace torch of reentry into Earth’s atmosphere, said Paul Dimotakis, a professor of aeronautics and applied physics at Caltech.

Tiles begin as glassy 6-inch squares of silica molded from a slurry of sand, seasoned with traces of exotic chemicals and baked in the world’s largest microwave oven. The finished slabs are the size of Texas toast. They have the heft of balsa wood. They are as porous as a sponge and as brittle as a coffee mug dropped on the floor, said Charles McMahon, a metallurgist at the University of Pennsylvania.

At liftoff, even a bouncing piece of cork could damage them, NASA mission records show.

Yet these featherweight wafers dissipate heat so rapidly that they can be held under a blowtorch and still remain cool to the touch.

Political compromises burdened the heat shield, space historians said.

To secure crucial political support from the U.S. military decades ago, NASA designers agreed to build a larger, more maneuverable space shuttle that could also carry bigger payloads.

That made the spacecraft heavier and put more demands on the heat shield. Other hardware changes added bulk, making a light heat shield even more of a necessity. Engineering could barely keep up with the promises of low-cost, low-maintenance, reusable space systems. From the time the first construction contract for Columbia was awarded in 1972, misgivings and misjudgments about the tiles dogged the project.

Costs Monitored

The political case for a reusable spaceship was as much a matter of economics as engineering design.

A square foot of shuttle tile cost $10,000 to manufacture and install, by NASA’s original cost accounting, and could last -- at least in theory -- for 100 missions.

By comparison, a more conventional heat shield absorbed the heat of reentry by ablation -- handling heat by charring and burning up a little at a time during descent. It cost about $30,000 a square foot and could be used only once.

NASA engineers quickly discovered that layers of insulation and padding were needed. But the tiles pulled every which way as materials expanded and contracted depending on the temperature. They seriously misjudged how much force the tiles must withstand. A heavy rainstorm, on the launch pad or in flight, was cause for consternation.

The shuttle’s aluminum alloy frame and skin also flexed in flight, adding to the strains. Every square inch of the spacecraft behaved differently, said Subra Suresh, a materials engineering expert at the Massachusetts Institute of Technology.

Consequently, each tile is custom-fitted to its exact spot on the shuttle more precisely than a Savile Row suit. There was to be no such thing as a standard, off-the-shelf tile.

NASA engineers also underestimated how much even a relatively small change in the surface of the shuttle tiles could alter the way the craft glided through the air at velocities so many times the speed of sound, said Michael Cima, an authority on advanced ceramics at MIT.

Worse still, they learned that the loss of one tile could undermine the stability of those around it, causing rows of tiles to break away. All told, engineering problems with the tiles helped delay the first shuttle launch for two years.

At its first launch, almost half of Columbia’s tiles had already cracked, fallen off, or been rebuilt at a cost in 1980 of about $600 per tile. Each one took two weeks to fix. It was like gluing eggs to an anvil.

For 112 flights, the repair records for every space shuttle were a litany of often inexplicable tile dings, gouges and cracks. A 1994 analysis of debris strikes during the first 50 shuttle launches concluded that about 25 thermal tiles per flight sustained damage of at least 1 inch.

The undersides of the wings, close to the fuselage and right under the crew compartment, are the most vulnerable parts of the shuttle because they face the external fuel tank and solid rocket boosters during launch.

When Columbia returned from a 16-day mission in December 1997, technicians reportedly found 308 scratches, streaks and cracks in tiles. The damaged tiles were only 2 inches thick and the deepest hits penetrated three-quarters of the way through.

“It has always been one of the most difficult challenges that the shuttle program has had to deal with,” said David Spencer, an aerospace engineer at Penn State.

Even so, almost nothing else among the shuttle’s 1 million parts comes quite so close to realizing the ambitious dream of a truly reusable spaceship, said Howard Goldstein, former chief scientist of the space technology division at NASA’s Ames Research Center in the San Francisco Bay Area.

Eli Pearce, a materials scientist at Polytechnic University in Brooklyn, termed the tiles revolutionary.

Some Original Tiles Left

After 22 years of wear and two major overhauls, three-quarters of Columbia’s original tiles were still in place when it launched its 28th flight on Jan. 16.

Columbia’s final voyage began, in so many ways, like its first. On its maiden flight in 1981, the raw fury of its rockets shook the 2,200-ton spacecraft like an empty garbage can. Debris and chunks of ice bounced off the spacecraft windows.

The violence of its liftoff knocked the wing controls out of kilter, bent fuel tank supports and shook loose so many of its heat-shield tiles that NASA officials feared for the lives of the two astronauts aboard.

The damage to tiles that mission managers could see on the engine cowlings was bad enough; what they could not see unsettled them even more. Astronauts had no way to inspect the underbelly of the shuttle, which would bear the brunt of heat encountered in the course of returning to Earth.

So they pleaded for astronomers to scan for tile damage. The Pentagon trained its secret sensors on the spacecraft, then surreptitiously shared the images with NASA.

The critical tiles appeared undamaged. The shuttle and its first crew landed safely.

In all, Columbia had lost 15 tiles, but NASA replaced hundreds more before allowing Columbia to fly again.

Twenty-two years later -- on Jan. 16 -- Columbia again shuddered off the launchpad.

The vibration of its twin boosters shook loose a chunk of foam insulation from its external fuel tank 80 seconds after liftoff. As the shuttle accelerated to 1,900 mph, the foam struck the left wing so hard that the fragment disintegrated.

Once again, the astronauts in orbit had no way to inspect for tile damage underneath the spacecraft and, despite so many problems with the tiles, still no way to repair them in space.

This time, however, no one thought an inspection necessary. NASA shuttle engineers analyzed film of the impact, ran a series of computer simulations of the potential damage, and, on the mission’s 12th day, decided they could live with the risk.

Tile damage had become an absolutely routine part of every shuttle mission. After every flight, about 40 tiles needed some attention.

Four days later, Columbia reentered Earth’s atmosphere for the last time.

Only Subtle Alterations

NASA scientists and engineers have never stopped looking for a better way to protect human beings returning to Earth from orbit.

For the next generation of spacecraft, they have tested titanium plates, exotic alloy shields and more advanced ceramics.In its X-34 reusable rocket project, NASA researchers sought to develop tiles that could survive 25 flights at up to 2,300 degrees Fahrenheit, a more modest goal than the 100-flight lifetime goal set by the shuttle’s original designers.

For its X-33 space plane project, NASA designers developed more rugged metallic thermal panels and flexible tiles. But flight tests in 1998 showed that even these more advanced materials still cracked and eroded in the rain.

With experience gained over the decades, NASA altered the shuttle tiles in subtle ways, the way tailors change lapels in response to fashion

When Columbia made its first flight, it was armored in 34,000 tiles. Today, there are 24,000 to 26,000 tiles on every space shuttle, as NASA has refined and improved the materials used in the heat shield.

Each tile is a ceramic souffle that today costs between $2,000 and $3,000 to make, NASA officials said.

Every tile is still unique. Some are white; some black; some have been consolidated into small thermal blankets. Engineers use four kinds of insulation that vary in the temperatures they can tolerate.

Some of the newer tiles are as much as 100 times more impact-resistant than Columbia’s original tiles. But they also are heavier and are used only in areas where damage in the past was most common.

The space shuttle fleet still requires many people working with hand-held laser scanners to inspect, tag, document and replace damaged tiles after each mission. To keep track, each of the thousands of tiles is cataloged according to size, type of tile and location on the vehicle. Each one is marked with an indelible digital barcode.

Fewer Makers

At its height, the manufacture of shuttle tiles was a booming cottage industry, involving 13 contractors from Los Angeles to Dedham, Mass.

But after 22 years, tile operations have been consolidated under one contractor at the Kennedy Space Center in Florida, and the job itself has become more of a museum-restoration task, like replacing a broken plate in a pattern of china.

Even before the Columbia accident, scientists were worrying about the effects of age on the reusable tiles.

There were concerns that repeated exposure to the stresses and extreme temperatures of spaceflight could alter the ceramics in ways normal inspections would not reveal.

“By stressing the tiles over and over, you could make them more susceptible to damage,” Suresh said.

Goldstein at NASA’s Ames center said that concerns about long-term corrosion had prompted tests and analyses, which showed that the tiles still meet performance standards.

The tiles, in a sense, will always be an experiment. No one has ever flown such materials back and forth from space so many times. No one can really tell how long they might endure.

“The question of what may be the real lifetime of these tiles is still up in the air,” Goldstein said.

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Times staff writer J. Michael Kennedy contributed to this report.