No obvious problems seen as Discovery's fuel tank filled
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: December 17, 2010
The shuttle Discovery's external tank has been fully loaded with more than a half-million gallons of liquid oxygen and hydrogen rocket fuel. The three-hour fueling procedure began at 7 a.m. EST and ended at 9:59 a.m. when both the hydrogen and oxygen tanks were both in "stable replenish" mode. So far, no problems have been seen. A hydrogen vent line that leaked during a Nov. 5 launch attempt is operating normally today and engineers have not spotted any problems with the tank's foam insulation.
Launch Director Mike Leinbach said engineers are recording data from strain gauges and temperature sensors, but it will take time to evaluate the information and draw any conclusions about the tank's structural integrity and the effects of ultra-low temperatures on vertical ribs, or stringers, where cracks developed during fueling for the November launch attempt.
"Everything's going extremely well," said Leinbach. "We got into tanking on time and we're processing through the standard tanking procedure, not doing anything different to fill up the tank today. And everything's going really, really well. The ground umbilical carrier plate is working fine, no leaks there. That was one of our major objectives and that guy's performing perfectly fine now.
"We haven't seen any issues with the foam. We've talked to the guys looking at the sensors and the data's being processed right now. Too early to draw any conclusions, of course, but we're getting great data from the tank. So we're pressing on, we feel good about it. ... We're looking forward to the real thing. Hopefully in February we'll do that."
Just like with a real launch countdown, a team of technicians was sent to the pad 39A after the tank was full to begin a close-up inspection, looking for any signs of trouble with the foam insulation. During the Nov. 5 launch attempt, cracks in two stringers making up the ribbed intertank section caused a large crack to appear in the tank's foam insulation.
The stringer cracks were repaired by splicing in undamaged segments and then bolting on doublers to provide additional strength. But engineers looking into what caused the cracks have not found an obvious "smoking gun." Based on an exhaustive investigation, it appears the cracks were the result of a "stack up" of tolerance problems during the tank's construction that resulted in pre-loaded stress in the hardware. Today's fueling test may help engineers confirm that hypothesis.
"So we have stringers instrumented in the repair area so we can look and see how that repair performs under a cryo load," said Mike Moses, the shuttle integration manager at the Kennedy Space Center. "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," he said. "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."
Engineers are hopeful the instrumented fueling test, along with exhaustive analysis and additional testing will clear the way to launch in early February even in the absence of a single obvious cause of the cracks.
"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," Moses said.
When the stringers are attached to the skin of the intertank, he said, "you kind of clamp it together. There could be gaps between the parts such that when you then rivet it up with ... something like a 5,000-pound load, you could trap a lot of that force into the part if there's a gap.
"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.
"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."