Experts evaluating the space shuttle's readiness to return to flight later this year cautioned NASA yesterday not to rely too heavily on untested computer models to decide whether the orbiter is safe from potentially catastrophic damage to its heat shielding during launch.
The NASA-convened Return to Flight Task Group issued an interim report describing in often blunt language the remaining hurdles the agency must overcome to meet its goal of flying the shuttle by May or June.
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The group expressed reservations about NASA's ability to develop an effective method of repairing the shuttle's thermal protection system in space or to marshal enough funding to manage its flight schedule.
The panel said that NASA'S redesign of the external fuel tank appeared to have lessened the risk of a chunk of foam insulation breaking away and damaging the shuttle's heat insulation but that NASA needs to make sure it is not making "faulty assumptions" about the new design.
Based on computer modeling, NASA redesigned the external tank so that no piece of foam larger than 0.3 ounces will break off during launch, far smaller than the 1.67-pound chunk that punched a hole in the heat shielding protecting the space shuttle Columbia's left wing, causing it burn up on reentry in February 2003.
NASA is unable to test the effects of such a small foam "bullet," but slightly larger pieces -- 0.7 ounces -- have caused potentially catastrophic damage in tests of the "reinforced carbon-carbon" shielding on the leading edge of the shuttle's wings.
"NASA has yet to demonstrate the rigor of the models necessary to certify the space shuttle TPS [thermal protection system] including the ET [external tank]," the report said, without specifically mentioning the tests. "Without validation of models, they should not be used for certification or risk assessment."
Despite these cautions, the group's co-chairman Richard O. Covey, a former astronaut, told reporters in a telephone news conference that NASA had "no show-stoppers" to prevent the shuttle's scheduled launch, essentially the same view the panel expressed Dec. 16, when Covey first discussed the findings of yesterday's report.
The written document, however, displays little optimism, and Covey was noncommittal on what NASA would have to do to enable the panel to sign off on 16 safety recommendations that NASA has said it will fulfill before flying the shuttle again. The task group is scheduled to make its final assessment in April.
Asked how NASA can convince the panel of the adequacy of the tank redesign, Covey said only, "We've seen evidence already that this tank will be the safest ever flown."
NASA has long acknowledged that it will not have a fully developed capability to repair shuttle insulation in space, and it has overcome this deficiency by proposing to use the international space station as a haven for a stranded shuttle crew and preparing a quick-reaction shuttle flight to make a rescue.
Although this plan appeared to be well advanced, the report signaled that the panel may not be willing to accept it, suggesting that NASA "needs to take a hard look" at the repair options before moving forward.
The report was also critical of NASA's view that it has enough money to pay for shuttle operations, citing "recent press reports" of employee concerns about layoffs. It also cast doubt on the viability of NASA's plan to retire the shuttle by 2010 and noted that NASA will have to reallocate funds from other programs to the shuttle, a requirement never so baldly stated by the agency.