Discovery makes return trip to assembly building
BY JUSTIN RAY
Posted: December 22, 2010
Leaving the launch pad and rolling back to the Vehicle Assembly Building overnight, the space shuttle Discovery will spend the holidays having the external tank digitally X-rayed as engineers look to prove the hypothesis that its structural cracks resulted from pent up stresses accumulated during construction and were unleashed during the strains of cryogenic fueling.
Ever worried about foam breaking free and damaging the shuttle's delicate heat shield during launch, the spaceflight was put on hold while engineers worked to understand why the structural beams, known as stringers, had cracked.
Thus far, they have a good lead on the possible culprit but still don't know for certain.
"It looks like what we probably had happen was during assembly we introduced stress such that this part was kind of pre-loaded with some stress and then the extra stress of cryo loading was enough to then break it and exceed its capacity," said Mike Moses, the shuttle launch integration manager at Kennedy Space Center.
If the team can prove that theory, shuttle engineers could put together sound rationale of why it's safe to launch Discovery with the repaired tank during available windows to the International Space Station in February.
"We're not committing to flying anytime soon. We got to wait until we have a good answer to go fly. We want to make sure we know the risk we have in front of us and not have any unknown risks out there," Moses said.
Detailed examination of the cracked stringers showed no signs of a material flaw or defect that would easily explain why they failed during fueling. Reviews of the stringers' manufacturing history also didn't turn up any indicators of problems.
"We're not probably going to come out of here with a smoking gun, but we're going to come out of here with a family of failures and we're going to have a lot of testing to then make sure we're not fooling ourselves," Moses says.
"So we kind of lost our smoking gun theory and had to go to the more insidious 'what else could it have been?'"
Weeks of work have narrowed the possible explanations into two categories: the stringers had pre-loaded stress that broke them or the more-radical thought that the overall tank itself has something amiss, Moses says.
"Either the stringer was the cause of the problem, that there was assembly tolerance, some build up of stress in that part or a crack that we just didn't go through the system and we couldn't see that caused this stringer to break. So it's an isolated event, it's this stringer and that's the problem.
"Or the other family is that the stringer was a victim here and the design has a problem, the tank itself has a bigger issue to it, the loading had a problem and there's something wrong generically that's causing stress to be concentrated in this area that's never been there before and that this crack is kind of a symptom.
"Now all of our analysis and modeling shows that should not happen with a properly assembled intertank and a properly mated structure should be fine, has plenty of margin actually. And that's one of things that gave us pause is those numbers showed we need a pretty big flaw to have caused this part to fail. So we were thinking we'd see something obvious."
Needing more data on the strains that the external tank experiences during the loading of a half-million gallons of supercold liquid oxygen and liquid hydrogen, the shuttle team conducted a special fueling test last Friday.
Engineers instrumented the tank with 89 strain gauges and temperature sensors, routing 162 wires about 200 feet from the two test zones around the front face of the tank and over to the pad tower to record nearly six terabytes of data. The instruments were placed on the stringers that had been repaired with doublers and another spot where no cracks had occurred.
"So we have stringers instrumented in the repair area so we can look and see how that repair performs under a cryo load. We have stringers instrumented next to that repair that aren't damaged so we can see how they perform. That'll give us some information about the general area to see if there are any non-linearities. And then on the opposite side of the tank, we've instrumented stringers to kind of go to a control type theory to say over here in a completely different area, here's how the tank performs.
"Really what we want to look for, in additional to the details, it's really that big picture. Does the left side and the right side of the tank compare to each other, do both sides perform the way the models show they would?
"We're looking for non linearities, is something obviously not tracking what we think it's going to, are the temperature profiles what we think, is there a spike in stress or strain that we do not expect to see? That would be an indication that there might be a different problem other than the stringer itself having a flaw or a defect that would put us in that second family where the stringer was the victim here and not the cause.
"None of our experts believe that to be the problem, none of our analysis or data mining to date shows that that's going to be the case. This tanking test hopefully will give us the final set of data that lets us kind of declare that to be true."
Quick looks at the data showed the strains and temperatures were within ranges that NASA expected. But it will take weeks to fully sort through the details.
Besides the special fueling exercise, engineers are doing testing on mockups and inducing flaws into the hardware to experiment with the conditions required to crack a stringer.
"When you stack up that there might have been a gap there, and you stack up that you might have clamped it down a little too hard, which made the feet splay out a little, and then when you look at the stringer itself, this particular stringer ... is like a tenth of an inch farther up the tank than the other stringers. These things are pre-formed with a bend in them, that bend is now in a different place than it was supposed to be. So you have these groupings of things that if they line up on you could cause a problem," Moses says.
"That's what the three-stringer and single-stringer tests at Marshall are going to be for, is to go put those flaws, manufacturing defects, so to speak, into a panel and then load it and see if that's big enough and can cause the stresses we need recreate the failure we had. So kind of the proof of the pudding of our theory of what happened really can happen."
The reasoning for today's rollback of Discovery from the launch pad to the Vehicle Assembly Building, only the 20th time that's happened in the three-decade history of the shuttle program, is the pursuit of more data.
The entire backside of the tank's midsection hasn't been inspected yet because it's not reachable at the pad. All of those stringers will be surveyed via the access afforded in the VAB.
"One of the things we're going to go back to the VAB for is to look at the backside of the tank. We call it the -Z, it's the part that doesn't have the orbiter on it. We've never been able to look at that since this first crack appeared, so this will give us the chance to look at the backside. We always thought we might need to do that, we were kinda waiting for some of the analysis to show structurally did we need to do that," Moses said.
Also, the front side where the crack repairs were performed will be re-checked to see how they held up during the fueling test.
"Between Christmas and New Year's, our NDE, our non-destructive evaluation, team will be busy shooting X-rays, and then we'll move into the January timeframe to look at what we need to do, any future work on the tank," Moses said.
Other planned activities in the VAB include removing all of the special instrumentation from the fueling test and applying fresh foam over those areas.
There's also the potential that the program could elect to modify the stringers to reduce the chances of any additional cracks. A meeting is scheduled for Dec. 30 to decide if the data analysis shows modifications are necessary.
Discovery's return to the pad could come in mid-January, setting up the earliest possibly launch date of Feb. 3.