Heat shield parts shed before orbiter disintegrated show "very unusual" deep scoring and pitting of surface

Evidence continues to indicate that a breach in the thermal-protection system led to break-up of the Space Shuttle Columbia during re-entry, but efforts of isolate the source of the breach are still under way. Investigators are being helped by recovery of thermal-protection tiles shed by the orbiter before it disintegrated on 1 February.

A tile found 50km (30 miles) west of Forth Worth, Texas, is heavily eroded. "The ends are worn away, probably by hot gases," says Columbia accident investigation board chairman Hal Gehman, and deep scoring and pitting of the surface facing the heat of re-entry is "very unusual", he says. Which area of Columbia the tile came from is not clear, as the serial number is obliterated. Also unclear is whether the damage was caused while the tile was still attached to the orbiter.

A fragment of tile found 500km west of Fort Worth, the westernmost piece of debris yet to be confirmed, is believed to have come from the upper surface of the left wing glove, near the orbiter's fuselage. The fragment was found near the border with New Mexico. At that point in the re-entry timeline, about a minute before loss of communications with Columbia, tyre pressure and wheel temperature sensors in the left main landing-gear well began to fail.

Board member Scott Hubbard says an initial analysis by NASA suggests that a 100 x 125mm (4 x 5in) breach in the thermal protection under the wing near the main-gear door seal "seems to account for the temperature rise" in the wheel well revealed by telemetry from Columbia. The "hypothetical study" is a first cut, he cautions, but "a good first step".

While the temperature rises and sensor failures seen in telemetry are consistent with the severing of wires passing through the wheel well, Hubbard says a sensor near the front of the orbiter also showed an anomaly. While initial aerodynamic analysis "seems to indicate there was some disturbance going on in the vicinity of the left main landing gear wheel well", eight different calculations all gave different answers, he says.

US Air Force Space Command radar data shows an object separating from Columbia during its second day in orbit. Analysis of nearly 3,200 radar observations from four sites show a piece about 300 x 400mm, "of undetermined composition", accompanying the orbiter then starting to separate, initially semi-stable in a slow rotation, then falling behind and eventually re-entering on 20 January.

The characteristics suggest a lightweight object, and the US Air Force Research Laboratory and NASA plan tests based on radar reflectivity to determine the material type. Board member Brig Gen Duane Deal cautions that Columbia's payload bay was open at the time and could be the source of the object observed.

Investigators continue to focus on the potential for tile damage caused by foam insulation shed by the external tank striking the underside of Columbia 82s after launch on 16 January. Analysis following the incident assumed one piece of polyisocyanurate foam breaking loose, but further analysis of launch video shows three pieces. Controversy surrounds the impact assessments conducted by NASA and Boeing after the incident, concluding the potential tile damage was not a safety of flight issue.

Although engineers at Mission Control in Houston had exchanged e-mails and had "what-if" discussions about the potential ramifications for the left wing after the collision with debris from the external tank during launch, there were no serious concerns about the re-entry of Columbia on 1 February, says flight controller Jeff King.

One theory is that increased roughness resulting from tile damage could have triggered early transition of the boundary layer over Columbia, with turbulent flow bringing peak re-entry temperatures in contact with the tiles.

"Eventually, we would like to see if you can couple the external tank shedding event with TPS [thermal protection system] sensitivities and the calculations being done by the aerodynamics folks," says Hubbard.

Source: Flight International