Protective Tiles Efficient, Fragile

The fragile thermal tiles that protect space shuttles on their fiery reentries to Earth’s atmosphere are extremely effective in dissipating heat, but they also have a history of problems that make them a constant concern.

The focus on the tiles’ role in the Columbia disaster intensified Sunday as shuttle program manager Ron Dittemore discussed sensor readings that suggested the shuttle may have been losing tiles before it disintegrated in the skies over Texas.

One problem that could have affected Columbia’s tiles had not been resolved before the shuttle launch: the possibility of tile damage from insulation coming off the shuttle’s main fuel tank during liftoff. The decision to keep launching shuttle missions even though the insulation problem had not been eliminated is certain to be a subject of attention for the commission appointed Sunday to investigate the loss of Columbia.

Dittemore said NASA experts had analyzed the situation and concluded there was no hazard to the orbiter and its crew.

The agency had fixed a previous condition in which little pieces of “popcorning” insulation would strike and nick the tiles, he said. But a different type of problem emerged on recent launches, with bigger chunks of the spray-on material peeling off.

Shuttles are shielded by about 24,300 tiles and 2,300 high-tech insulation blankets, according to NASA. The spacecraft experiences extremes of temperature, from minus 150 degrees in space to a peak of about 3,000 degrees during reentry.

Black tiles known as high-temperature, reusable surface insulation are bonded to the bottom of the orbiter. They are made of a very pure silica, which is derived from sand, and are about 6 inches square with thicknesses varying from 1 inch to 5 inches.

The signature property of the tiles is their ability to deflect heat. “Surface heat dissipates so quickly that a tile can be held by its corners with a bare hand only seconds after removal from a 2,300-degree oven, while the center of the tile still glows red with heat,” a NASA briefing paper said.

But the tiles can be damaged in many ways, ranging from the heat and stress of reentry to handling by ground personnel and being struck by ice or debris on launch.

“They do a great job of insulating the shuttle, but they are somewhat fragile,” said Donald Kutyna, a retired Air Force four-star general who served on the presidential commission that investigated the 1986 loss of the shuttle Challenger.

Dittemore said the loss of one or even several tiles would not automatically doom a shuttle.

But missing or damaged tiles can increase risks in more than one way. A crack in a tile can allow heat to penetrate to the aluminum structure of the shuttle. Damaged tiles can also change the aerodynamic profile of the wing, increasing drag and heat and possibly precipitating further damage to the shuttle’s thermal protection system.

Dittemore said sensors aboard Columbia had noted an unusual increase in drag on the left side of the shuttle shortly before catastrophe struck. The computer flying the shuttle tried to compensate by rolling the orbiter toward the right.

“It could be an indication of rough tile,” Dittemore said. “It could be an indication of missing tile.”

The insulation that had fallen off the main fuel tank struck the left side of Columbia. Dittemore cautioned, however, that no conclusions have been reached about the causes of the disaster.

“We are not looking at tile exclusively,” he said.

Kutyna, a former test pilot, said damaged or lost tiles are one possible explanation for the increased drag on the shuttle’s wing. Other sensor readings recorded that the temperature on the left side of the shuttle was rising more quickly than on the right side.

“Generally, when you change the form of the wing, you can increase drag,” Kutyna said. “So it would logically follow that if there were a deformation or loss of tile on one side of the wing, that would increase drag and the control system would try to correct it.”

A computer controls the shuttle’s reentry, guiding the orbiter through a series of turning maneuvers called “roll reversals” that help reduce speed and the buildup of heat.

The investigation appears to be a long way from establishing a connection between the insulation that came loose 80 seconds after launch and the mission’s disintegration 16 days later.

Dittemore said the situation was first detected the day after Columbia was launched, during a routine analysis of liftoff films. NASA experts scrutinize the films frame by frame, looking for anything that could damage a shuttle.

Concerns about falling insulation had previously prompted NASA to try to capture the phenomenon with special cameras, but film proved too fuzzy.

After the Columbia launch, Dittemore said, engineering experts analyzed what had happened. They tried to ascertain where the falling insulation had struck the shuttle, what the potential for damage to the tiles was, and what the consequences might be.

There was some discussion of whether the reentry sequence could be changed to reduce heating, he said. But it was determined that Columbia was already scheduled to follow a path that would minimize heat as much as possible.

“We concluded that it did not represent a safety concern,” Dittemore said. The NASA experts scheduled the issue for action before the next launch. The Columbia crew was informed, Dittemore said.

In the past, NASA had considered whether crews could be trained to perform spacewalk repairs of damaged tiles.

“Early in the program, we recognized that if we lost a tile, we didn’t have any repair technique,” Dittemore said. “We abandoned pursuing that idea [because] we were also very concerned that a crew member underneath a vehicle could cause more damage. The risk was greater sending the crew over the side.”

Dittemore noted shuttles had flown more than 100 missions with no serious failures of the thermal protection system.

NASA expects to fly space shuttles for perhaps another 20 years or more, and it has been searching for a tougher tile, thus far without success.

“We have had difficulty in formulating a new tile,” Dittemore said. “We are continuing to investigate whether we can develop a stronger, tougher tile. We haven’t been successful yet, but we are making progress.”