Casey is a retired three-star general with decades of experience in missiles and rocket systems.
"High speed impacts of material on the shuttle wings are beyond the qualification envelope of the orbiter. The known debris from the tank hitting the left wing is incontrovertible," Casey said in reference to the pieces of insulating foam that cameras captured falling from Columbia's external tank about 81 seconds after the Jan. 16 launch.
Although NASA managers concluded Columbia and its crew were in no danger from the debris striking the wing, during re-entry the left wing's heat shield was breached, most likely along the leading edge where the debris is believed to have hit. This allowed hot gases inside the wing, which eventually caused a structural failure and the loss of vehicle and crew.
"Regardless of the specific sequence and the details of the failure events, it seems to me that the remedy is to preclude debris from impacting critical systems during ascent or anytime they have to operate. I believe this was doubtless an original design requirement. However, I think this is a design requirement that was not achieved, demonstrably not achieved," Casey said.
Casey explained that to understand the margins of a particular system it must be tested under conditions more extreme than it would see during a flight. Only then would that system be considered qualified to fly.
Moreover, the space agency needs to do a better job in quantifying the safety margins for all of the shuttle's various systems and be more sensitive to clues the hardware may be offering when things don't work as designed, he said.
NASA must especially avoid the potential trap in believing a particular system that hasn't been working exactly as designed is safe just because it has done so for several flights in a row, Casey said -- a point that board chairman Harold Gehman was quick to agree with during the hearing.
"Your comments are very helpful because in some of the readings we've done as part of our review of some of these programs, that subject of 'successful flights don't re-establish margins' has come back again and again," Gehman said.
Specifically, foam shedding from external tanks has been seen on numerous shuttle flights before Columbias final mission.
On four occasions officials identified large chunks falling from a ramp of insulating material that is hand-sculpted on the external tank near the two forward struts --known as the bipod -- that attach the orange tank to the spaceplane's black belly.
Considered individually, none of those previous missions presented a safety of flight concern and all were dispositioned during NASA's standard safety review process that ultimately leads to the Flight Readiness Review (FRR), a meeting of senior managers in which the shuttle is cleared for launch.
"There's no question that NASA looks at the shuttle program through a microscope. Several people have indicated they also need to stand back and look at it through a telescope," Gehman said after the hearing.
During the hearing, KSC director Roy Bridges appeared as the opening speaker and noted he never considered the shedding foam and their effect on the shuttle's thousands of heat protection tiles a safety issue.
"To be honest I did not think that the bipod foam coming off had caused any significant damage in the program to date. I personally looked at every shuttle that's come back during my tenure here and I've seen no significant damage from any of the foam coming off," Bridges said. "It's certainly been a maintenance concern. It's a lot of work to go out and repair all of those things."
Bridges said if there was any evidence to suspect there was a serious issue with the foam he would have said something.
"Had I been aware of that, I certainly would have put my hand up at the FRR that we would stop flying. I think this is certainly a surprise to all of us," Bridges said.
Gehman later commented on that way of thinking as he discussed with Casey NASA's decision-making philosophy, a philosophy the Rogers Commission investigating the 1986 Challenger disaster labeled "flawed" and cited as a contributing factor.
"We should change the operative question on the table here. The present question is that you've got to prove to me that something is unsafe before I'll change it," Gehman said.
Instead, "we should require the system to prove it is safe, particularly if we have something that appears to be exhibiting anomalies. The impetus should be to prove it's safe. The burden shouldn't be on me to prove it's not safe. The burden should be on the system to prove it is safe," Gehman said.
After suggesting that NASA consider using less insulation foam on the tank, Casey also recommended:
- Shuttles be launched only on missions in which unmanned rockets couldn't be used.
- Future crew size be kept to a minimum.
- Waivers and deviations in processing hardware should be avoided as much as possible.
Acknowledging those ideas, after the hearing Gehman told reporters he didn't expect to make such specific operational recommendations in the final report, which is still weeks -- if not months -- away.
"What we'll do is attempt to put in our own words a description of what we have found to be the real risks, the real costs, the real benefits and the fixes you need to increase the margin," Gehman said. "I do wish we had the answer by now and that we were writing the final report."
A fourth public hearing is scheduled to begin at 9 a.m. EST (1400 GMT) Wednesday at the Radisson Resort at Port Canaveral here on Floridas Space Coast. NASA TV is scheduled to broadcast the event.
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