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More fuel tank tests will delay next shuttle launch
Posted: July 23, 2009

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NASA managers decided Thursday to order another 125 foam "plug-pull" tests on the external tank scheduled for use by the shuttle Discovery next month to make sure there are no adhesion problems like those that resulted in unusual foam shedding during Endeavour's takeoff last week.

Technicians examine Discovery's tank in the Vehicle Assembly Building. Credit: NASA-KSC
The additional work will delay Discovery's rollover from the processing hangar to the Vehicle Assembly Building at the Kennedy Space Center, where external tank No. 132 and a set of solid-fuel boosters are waiting, and will delay launch by several days at least.

"We're going to have to re-evaluate the launch date," LeRoy Cain, deputy manager of the space shuttle program, told CBS News in an interview Thursday. "We had been kind of targeting (Aug.) 21st and ... with this work we're adding in, it's unlikely we can make the 21st. I don't have a new target date for you yet but it'll be probably the 25th or 26th, somewhere in that ball park."

During Endeavour's launching July 15, an unusual amount of foam insulation pulled away from the external tank's central "intertank" region, a structurally rigid part of the tank that is not exposed to the sort of ultra-low cryogenic temperatures experienced by the hydrogen and oxygen sections.

Most of the debris fell off after the shuttle was out of the dense lower atmosphere when impacts pose the greatest threat to the shuttle's heat shield. But several tiles were dinged and while Endeavour has been cleared for entry as is, NASA managers want to understand the mechanism and make sure whatever caused the shedding can't happen earlier.

Earlier this week, engineers carried out 26 so-called "plug-pull" tests, drilling small cores in the foam of the intertank and pulling them to determine how firmly the insulation was bonded with the underlying structure. In all 26 cases, the foam ultimately failed, but it did so in the expected manner and did not pull away from the metal below.

But some engineers argued more tests were needed to provide statistically significant results and managers decided Thursday to do another 125 pull tests, mostly on the back side of the tank. About 5 percent of such pull tests fail, so the final number likely will be a bit higher than 151.

"We were trying to understand this problem we have, what would constitute a statistically significant sample," Cain said. "We're still working on that. What we've determined so far, though, is there are some more areas that we want to sample that would give us higher confidence, given we have positive results, that ET-132 is a good tank. And so we're going to go do that, we're going to go sample some more areas and as a result of that, we're not going to be ready to roll out of the OPF (Orbiter Processing Facility) tomorrow.

"We think the number of plug pulls we're planning on doing right now is going to delay the rollout from the OPF by at least a couple of days and subsequent to that, we're going to be adding a couple of days, probably of work in the VAB, even after we get the orbiter rolled over there and mated up. There are a certain number of things we have to do before we can remove the scaffolding and do the mating."

NASA managers would like to get Discovery off the ground as soon as possible to avoid any conflicts with the planned launch of Japan's new HTV unmanned cargo ship.

The HTV's initial launch window opens Sept. 10 and runs through Sept. 20. Two more days are available Sept. 29 and 30, but the HTV team would have to stand down between Oct. 1 and Nov. 1 because of Japanese fishing industry constraints. The next HTV launch window runs between Nov. 1 and Dec. 31.

"We're off talking to the station and international partner folks about that right now," Cain said. "It's very complicated because of their window, the station has certain constraints that they're trying to honor and so we need to let station go talk some more with their international partner friends."

Cain said it's simply too early to say when Discovery might fly or what impact it might have on downstream missions. But he seemed optimistic NASA could avoid any major disruptions.

"The bottom line is we're not going to make the 21st, we're probably only a few days down from that," he said. "It's too early for me to be able to put a fine point on that. But certainly, we'll be able to do something before the end of August, assuming, again, the results of the work we currently have planned are positive."

The plug-pull tests do not address the underlying cause of the foam shedding seen during Endeavour's launch. Cain said engineers are reviewing the tank's manufacturing history and other instances of foam loss to help pin that down. But root cause is not required to clear Discovery for flight.

Even so, Cain said, "we have to understand with some confidence, we have to have reason to believe we don't have the same circumstances with this tank with some degree of certainty. We'll continue to work toward root cause like we always do ... but we don't absolutely have to have root cause. We do need to have some understanding of the risk exposure with some degree of confidence."

While the intertank is the focus of attention, Endeavour's tank also lost foam from two other areas - an ice-frost ramp on the oxygen section of the tank and around one of the attachment points of a bi-pod strut that helps hold the shuttle's nose to the tank.

Cain said engineers are looking into a wide variety of issues to help determine what happened, including the number of times Endeavour's tank was loaded with rocket fuel.

"If you look at the number of cycles we put on this tank, if you look at the amount of weather that this tank saw sitting out at the launch pad, the severity of some of that weather and the potential for water intrusion ... we're looking at all those special circumstances, or the potential that special circumstances contributed to those foam losses," he said.

"And we're not at the bottom of that yet. We're in the process of trying to ascertain whether or not there was something unique. We've got to go back and look at our foam loss distribution assessments that we've done previously and make sure we still have the right overall risk assessment."

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