Five sprayers were trained to apply foam to the intertank, and Richard hopes to certify five more. One crew is trained for "closing out," or finishing, the bipod area, another for protuberance-air-load (PAL) ramps, and another for the longeron, a structural support for the tank's aft orbiter attachment struts. Before Columbia, a close-out sprayer could have done the applications on any of those areas; today the workforce is divided into specialties.
Ron McQueen, a production supervisor of foam applications who has been with Lockheed Martin for 23 years, watches a video screen while two sprayers apply foam. A third sprayer is on hand only to watch what the other two are doing. Immediately after the application is complete, the video is run again, and the sprayers, the quality control people, the production supervisor, and several engineers watch the replay, looking, says McQueen, to see if anything has contaminated the foam, or if a void has developed, FOD has been introduced, or more than 45 seconds has elapsed between the first spray and the second. If they spot a defect over a large enough area, the foam will have to be removed and replaced.
"These new [procedures] were developed over a long period of time," says McQueen. "On the bipod, we probably worked for a year and a half coming up with a new [process], which we were all involved in-the supervisors, the hourly employees. [NASA and Lockheed Martin managers] took our suggestions right along with theirs."
Application is now a six-part process, not counting the video review and other quality control steps. Before the foam is applied to a real tank, it is applied to a high-fidelity mockup of the tank area to be sprayed. The sprayer applies foam to a "lead-in" test panel, then to the mockup, then to a "lead-out" test panel. All three are later dissected to see if any voids or weak areas developed in the foam. The steps are repeated for the tank that will eventually be sent into space, but only the lead-in and lead-out panels are dissected, unless the technicians see something unusual.
And yet, despite all the safeguards, when Discovery is launched this spring, some foam will almost certainly shed from the tank.
Wayne Hale, the deputy space shuttle program manager, explained last December that NASA managers interpreted the CAIB's recommendation as "eliminate all debris that could cause damage." After hundreds of tests, the engineers determined that a piece of foam weighing as little as .023 pound, if it came off the top of the tank, could damage the wing leading edge so severely that safe reentry would be questionable. (The piece that struck Columbia's wing weighed an estimated 1.67 pound.) NASA believes that no debris larger than .008 pound will come off; that leaves a safety margin of only .015 pound. "We were very clear from day one...that if in fact the requirement becomes 'No debris,' we are not going to be able to make it-not with this foam system," Neil Otte said.
When the CAIB required "an aggressive program to eliminate all External Tank Thermal Protection System debris-shedding," was it envisioning zero debris? That's the type of question debated by a group of 26 experts appointed by NASA Administrator Sean O'Keefe in June 2003, as NASA began responding to the CAIB's preliminary recommendations. The Return to Flight Task Group, co-chaired by Apollo astronaut Thomas Stafford and shuttle astronaut Richard Covey, who piloted Discovery in 1988 on the shuttle's first flight after the 1986 Challenger accident, shadowed NASA's employees and contractors at every step of their return to flight, questioning their analyses and decisions and compiling its own report, to be delivered to the administrator about six weeks before the next shuttle is launched. (The task group reports to a different administrator from the one who chartered it; O'Keefe left the agency in February to return to academia.)
The task group's job is to make an independent assessment of NASA's response to the CAIB's 15 return-to-flight recommendations. It is a self-described "umpire calling balls and strikes in a zone defined by the CAIB." Last December, the umpire seemed disposed to approve NASA's solution to external tank debris-shedding. Dan Crippen heads the group's panel evaluating actions taken to improve NASA management. A former director of the Congressional Budget Office, Crippen holds a doctorate in public finance and is the only one of the group's leaders who had never worked in the space program.
"The CAIB clearly understood some limitations," he said at a December press conference. "Their intention was to eliminate all debris. Well, if they thought that was possible, they wouldn't have gone on to say, 'Oh, by the way, you ought to be able to inspect for damage, you ought to be able to repair damage.' Because if you had eliminated all debris, then you wouldn't need that. So they clearly understood in their own discourse that their recommendations were subject to imperfection."
Reading the first four CAIB recommendations, the ones that directly address the physical cause of the Columbia tragedy, you can hear What if ? whispered after each one, as if the committee were trying to lock the shuttle's survivability inside a strong box, then inside a combination safe, then inside a bank vault. In the CAIB plan, foam will not shed from the tank. But what if it does? Then the RCC panels on the orbiter's leading edge will be more impact-resistant than they were. But what if they're not? Then the astronauts will be able to inspect and repair them. But what if they can't?