Failure of fragile tiles may be source of shuttle failure

By Paul Recer
Associated Press Science Writer

CAPE CANAVERAL, Fla. — The thermal tiles that have come under suspicion in the Columbia disaster seem almost absurdly brittle for something that is supposed to protect the spacecraft from the extreme heat and other rigors of flying into and out of orbit.

The tiles can shatter under finger pressure. In fact, the shuttle cannot be launched in a rainstorm because water droplets smacking into the ship as it hurtles toward orbit can damage the tiles.

Two decades ago, however, space engineers persuaded skeptical experts that the fragile silicate foam tiles could protect the shuttle from thousands of degrees of heat during re-entry and stay firmly glued to the wings, underbelly and nose of the space shuttle.

The tiles were lightweight and they resisted heat better than any other known material.

NASA engineers impressed skeptics with a simple demonstration: The corner of a one-foot square of the tile could be heated red hot with a blowtorch while being held barehanded at the opposite corner. Take the blowtorch away, and the heated section cools within seconds.

Yet, the tiles were so fragile that skin oil from an uncovered hand is enough to damage the surface. A dropped tool could leave a deep gash. Drop a tile while installing it, and it had to be discarded.

Never before had someone tried to coat an entire aircraft with such materials, so NASA and its contractors had to invent new ways of installing the tiles. First they decided how many were needed — more than 24,000. Then each tile had to be designed individually to fit a specific spot on the uneven, curving surface of an aircraft. Eventually, a computer drew up a complex pattern, like a jigsaw puzzle, and gave each tile an individual number, shape and location.

Each tile had to installed by hand. Workers wore gloves and hair nets and took care to make sure their pockets were empty. Dropping a coin from a few feet would have been enough to destroy a tile worth hundreds of dollars.

Once a tile was installed, how could the engineers be sure it was glued on strongly enough? They invented suction machines that tugged against each tile, testing the adhesive. This often had to be done more than once for each tile.

Eventually, the tiles were installed, and Columbia, the original space shuttle, first flew in April 1981.

The tiles proved their worth. They protected the hull of the shuttle and, contrary to forecasts, did not shatter as the craft came through the atmosphere.

But some experts now speculate that Columbia may have been destroyed because some fragile tiles were damaged, exposing the craft's left wing to superheated gases of re-entry.

Norm Carlson, a 38-year-veteran of NASA and a former operations chief at launch control at the Kennedy Space Center, said he believes that damaged tiles are "the No. 1 candidate" for the cause of the tragedy.

Carlson said Columbia would not have been destroyed from the failure of a single tile — it would have taken a whole group. But he said it is possible that if one tile pops off, others may follow, ripping off like a zipper coming open.

He recalled that after an earlier shuttle flight, engineers found that a single tile had slipped off, exposing a body flap to the torrid gas of re-entry.

"It melted the aluminum underneath just like a welding torch," Carlson said. But the damage was limited to a small spot and the mission was not endangered.