Investigators are trying to understand how NASA came to accept debris strikes on Space Shuttle thermal-protection tiles as a maintenance issue rather than a safety concern, despite its guiding principle that nothing should hit the orbiter.
Insulation foam shed by the external tank, which hit the left wing leading-edge during launch, is still the prime suspected cause of the thermal breach that led to the break-up of Columbia during re-entry on 1 February.
Columbia accident investigation board member Sally Ride says she sees "echoes" of the 1986 Challenger disaster, which was caused by a problem with the solid-rocket boosters that NASA was aware of, but had begun to accept as normal. Tile damage was a "huge concern" on early Shuttle flights, says Ride. "As time went on...people got used to tile damage from debris off the external tank...got used to treating it as a turnaround issue."
The Crater computer program used to assess potential tile damage after the foam strike was observed during ascent, which suggested there was no safety-of-flight issue, is "rudimentary", says board chairman Hal Gehman. "Neither NASA nor the board is satisfied with the model," which is essentially a Microsoft Excel spreadsheet using data from previous flights and small-scale impact tests. "Obviously it was wrong, but that is hindsight," he says. "We are not ready to conclude the analysis and decision-making was incorrectly carried out."
Ride says the Crater program had not been used as an in-flight decision-making tool before, but those involved seemed "fairly comfortable" with the analysis. The model's ability to predict damage was tested against a foam strike on a previous Shuttle mission - albeit after the flight - and "appeared to be in the ballpark", she says.
But a team of engineers meeting on 21 January decided to request imagery of Columbia in orbit, to check for tile damage. "They decided they needed more data," Ride says. "Now the question is: why didn't that request make it to the programme managers?" The US National Imaging and Mapping Agency has agreed to take satellite images of all future Shuttle flights.
Investigators are examining the bipod area where struts supporting the nose of the orbiter attach to the external tank. A dissection of a tank revealed more than 120 voids and defects, including a piece of duct tape, in the hand-sprayed foam ramps that form aerodynamic fairings over the attachments. These probably explain the shedding of ramp pieces during Columbia's ascent and previous flights. NASA proposes installing a lightweight Inconel metal cover over the ramps to prevent shedding.
Radar cross-section testing to identify the object that separated from Columbia on its second day in orbit has been extended to include partial leading-edge reinforced carbon-carbon (RCC) panels, as well as T-seals and carrier panels with backing structure and three or four tiles. A broken panel would have edges that would increase radar reflectivity. Most RCC panels recovered have been split in two, for reasons yet to be determined.
Source: Flight International
