The disintegration of Space Shuttle Columbia has highlighted one stark fact - even if NASA had determined that thermal-protection tile damage during launch was a safety risk, there is little the space agency could have done. This could have long-term implications for the operation of the three surviving orbiters.
There is no way to repair or replace tiles in orbit, and Columbia's crew was not equipped to spacewalk outside the payload bay. Columbia did not have enough fuel to reach the International Space Station, and it was not equipped for docking. A more benign re-entry was impossible, says Space Shuttle programme manager Ron Dittemore.
Returning to Earth on 1 February after the 16-day STS 107 Freestar science mission, en route to a 09.16 landing at Kennedy Space Center, Florida, Columbia broke up over Texas just before 09.00, during the period of maximum re-entry heating. Tile damage has not been confirmed, but NASA believes a "thermal event, rather than a structural event" caused the accident.
Telemetry data has revealed a sequence of thermal events, affecting the left side of the vehicle, that began as Columbia crossed the California coast. This led NASA to re-examine the incident during launch, on 16 January, when foam insulation shed from the external tank 82s after lift-off struck the underside of the left wing, potentially damaging tiles.
NASA's analysis concluded the probable tile damage was "within the family" of previous debris impacts, and was "not a safety of flight issue". The "worst case" analysis assumed a 500 x 400mm (20 x 16in) section of foam weighing 1.2kg (2.67lb) and travelling at 460m/s (1,500ft/s) and concluded there was potential for damage or removal of tiles over an area 180 x 800mm - "not sufficient to cause a catastrophic event", says Dittemore.
Shuttle orbiters average over 100 tile impact points per flight - "25 to 35 impacts greater than 1in are well within our experience base," says Dittemore. There have been 11 flights with impacts greater than 25mm, but in the previous 112 Shuttle flights tile damage "was never a safety of flight concern," he says.
Tests are under way to determine the damage potential of the foam and tiles, but "it does not make sense that a piece [of foam] could be the root cause of the loss of Columbia and its crew," says Dittemore. The spray-on polyurethane insulation is fragile and lightweight, virtually waterproof, and the bipod area was checked for ice before launch, he says. "There has got to be another reason."
Investigators are analysing the timeline of temperature rises and sensor losses across the left wing, to "back out to where the heat source had to be to generate the indications," says Dittemore. Similar "reverse analysis" is under way on the flight-control data. Reaction-control thrusters fired to help the elevons counteract increasing drag on the left side, but the rates before loss of communications indicate the flight control system "was beginning to lose the battle", he says.
Damaged or missing tiles would increase drag, while triggering turbulent boundary-layer flow over the wing could increase drag and heating. But "damage to the underside of the wing would not affect handling. It would not change the boundary layer or trip the flow over the wing," Dittemore says.
NASA did not attempt to image the underside of Columbia in orbit because pictures taken in 1998 of Discovery's drag-chute compartment were "not very useful". Columbia did not have a robot arm, which could have been used to look under the orbiter. If damage had been confirmed, "there was nothing we could have done," Dittemore says.
Source: Flight International
