Loss of One Wing Tile Could Doom Shuttle, Expert Says

The loss of a single ceramic tile from the underside of the wing of the space shuttle could set off a cascading series of events that would bring down the spacecraft, a former NASA engineer who studied the thermal protection system said Monday.

Robert D. Quinn, a heat transfer engineer retired from NASA’s Dryden Flight Research Center at Edwards Air Force Base, told the Los Angeles Times that the Columbia disaster could have been triggered by the loss of just one 6-inch by 6-inch tile early in reentry.

In 1984, Quinn and two other NASA scientists were assigned to analyze the thermal protection system of ceramic tiles and other protections designed by the shuttle’s builder, Rockwell International.

Although the trio essentially came up with the same answer as Rockwell -- that the ceramic tiles would protect the shuttle -- Quinn said Monday, “In some ways, I think it’s quite amazing that we haven’t had a tile episode before.”

Quinn laid out a scenario in which the loss of a single tile under the wing quickly leads to the loss of adjacent tiles through adhesive failure.

Simultaneously, the missing tiles create a turbulent airflow across the affected wing, which increases its drag, or wind resistance, through the atmosphere. That increased drag raises the temperature even higher and causes the wing to lose some of its lift, leading the shuttle to start to roll toward that side.

The scenario depends on a tile loss under the wing early in reentry. Many successful shuttle missions, including Columbia’s maiden flight in 1981, ended with tiles missing by the time the craft had landed. In Houston on Monday, Ron Dittemore, the space shuttle program manager, detailed a series of clues that matched the scenario Quinn described: Sensors on the left wing and in the left landing gear wheel well recorded increasing temperatures before eventually failing.

And the flight control system was trying to keep the shuttle from rolling to the left by firing jet thrusters on the right side and raising the right elevons to force the craft back toward the right.

Dittemore also said a single tile had been found in Fort Worth -- farther west than the other debris. And he said that NASA was interested in the observations of a California astronomer who described seeing tiles coming off the shuttle as it flew above the Owens Valley.

Quinn said the shuttle’s thermal protection system of tiles, blankets and leading edge cuffs made with carbon is designed to keep the aluminum sheet metal beneath from getting hotter than 350 degrees Fahrenheit during atmospheric reentry. Normally, the temperature never exceeds 250 degrees, he said, which is well below the 1,200-degree melting point of the metal.

By being coated black, the tiles on the underside of the shuttle are able to bounce some of the heat back into space through radiation. The rest of the protection for the aluminum skin comes because tiles are made of a silica fiber substance that conducts heat poorly.

However, if even a single tile from the underside of the wing is lost early in reentry, several factors come into play, Quinn says. The aluminum skin is exposed to the full heat, which is generated from the friction of the vehicle as it enters the atmosphere at 17,000 mph.

The aluminum has only half as much ability to radiate heat back into space as the black tile surface, and it is an excellent conductor of heat.

The result is that the bare patch of aluminum melts and the heat radiates through the skin, quickly reaching the point that the adhesive bonding adjacent tiles to the wing gives way.

Meanwhile, the tile loss alters the aerodynamics of the shuttle. With the tiles in place, the wing is designed to have a smooth airflow across it. But when tiles detach, the resulting depressions cause a roughness to the wing surface that creates a turbulent airflow, Quinn said.

Reacting to Dittemore’s description of rising hydraulic fluid temperatures in the left brake lines, Quinn said, “They had lost a tile or it couldn’t have gotten that high.”

He was optimistic that NASA would eventually be able to pinpoint the cause, noting that “all of the tiles will survive.”

At NASA’s news conference Monday, Dittemore noted that each tile is coded and that engineers can tell where each one belonged once they are recovered.