Cause of Discovery fuel tank foam loss still unresolved
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: November 22, 2005
Four months after the shuttle Discovery's long-awaited return to flight last July, NASA engineers still don't know what caused a large piece of potentially dangerous foam to break away from a so-called PAL ramp on the side of the ship's external fuel tank.
Figuring out how to keep the long, hand-sprayed foam wind deflectors intact is driving NASA's launch schedule. Despite the loss of one to three months due to the impact of Hurricane Katrina, agency managers hope to have Discovery ready for another launch next spring.
But shuttle program manager Wayne Hale said today that assumes engineers figure out what to do about the foam problem that marred Discovery's July launching.
"Guys, people talk to us about schedule pressure," he said, bristling somewhat after a question about the presumed May launch target. "I'm going to tell you something. The only people I get schedule pressure from is the media. We have not set a launch date. I've said that, I don't know how many times, in media briefings. We have not set a launch date. We are working through the investigation.
"We have assessed the schedule and if the testing gets completed as is planned, and if it comes out the way we think it might likely come out, then we think there is a probability that we will be able to launch in May. But we have not set that as a schedule, it is not driving us.
"I'll put this in very simple words and plain English. The results of the technical investigation are going to allow us to fly. We are not driving to a date on a calendar that was set arbitrarily. We're not allowing ourselves to be driven to do something stupid. We are not doing that."
That said, the next realistic shuttle launch window opens in May with the shuttle Discovery and external tank 119. Assuming a liftoff on May 3, ET-119 must be shipped from Lockheed Martin's Michoud Assembly Facility near New Orleans to the Kennedy Space Center by around Feb. 2. The trip by barge takes about five days.
Another tank, ET-120 - the one with cracks in its hydrogen PAL ramp - would need to be at KSC by around March 23 to support a possible June rescue flight by the shuttle Atlantis.
Having a second shuttle on standby, ready to carry out an emergency rescue mission in case of major problems, is a post-Columbia safety enhancement. NASA managers will meet in December to discuss whether flights beyond Discovery's will require such standby rescue missions.
"Those two pipes carry the pressurization gas, hydrogen and oxygen, that are bled off the main engines that keep the pressure up inside the tank," Hale said. "You can't fly without pressurizing the tanks."
When the shuttle was designed in the 1970s, engineers believed shock waves during the vehicle's transition to supersonic speeds could cause potentially serious damage to the pressurization lines and/or the cable tray. As a result, the tank was equipped with two protuberance air load, or PAL, ramps, one running along the upper section of the hydrogen tank and the other along the outside of the oxygen tank.
The ramps are built up by workers at Michoud, who manually spray on foam and then sculpt it, making ramps that smooth the flow of supersonic air over the pressurization lines and cable tray.
The shuttle Columbia was destroyed by foam insulation that broke away from a different area of the tank during launch in January 2003. That foam was eliminated during the post-disaster recovery. But during Discovery's flight last July, a large piece of the hydrogen PAL ramp broke off.
"With all the testing we've done so far, and we're only part way into it - we expect to complete our testing, by the way, between mid December and mid January - we haven't found any 'eureka,' or smoking gun so far," said John Chapman, external tank project manager.
Hale said engineers are assessing three options. All three require the PAL ramps on existing tanks to be removed:
"In the long run, we have decided we would like to remove this fairly large piece of foam, just eliminate the hazard that it might cause," Hale said. "We think we have a very strong case to be ready to take that ramp off by the third flight tank. Some folks believe we can accelerate that and potentially even remove it for the STS-121 tank."
"Before you remove the wind deflector, you need to make sure those structures won't come off in the supersonic flight regime that we see," Hale said. "So the aerodynamic (analysis) process, which is greatly improved over where we were in 1979, 1980 and 1981, is looking to see if we have the structural margin in the system to fly without that protection that the foam ramp provides.
"The reason the foam ramps were put on in those days nearly 30 years ago is that the aerodynamics indicated the structure was marginal to hold together during those supersonic loading periods when the shock waves off the tips of the solid rocket boosters interact with the side of the tank. Today, there is a great deal of hope that we can prove that the structure, which has been improved over time, strengthened over time, can hold up to that buffet and not come off.
"But that is a risk that you do not take without having a good engineering analysis," Hale said. "So we're starting a campaign of wind tunnel tests, computational fluid dynamics modeling with supercomputers to try to understand that better and feed that back into the structural analysis to make sure those parts stay on."
He said it is possible engineers could complete the work in time for Discovery's next flight, but "it is more likely that it will take us to the fall of next year" to complete the work.
As for ET-120, Chapman said a close inspection found nine hairline cracks distributed across the length of the hydrogen PAL ramp. The cracks were not present when the tank was shipped to the Kennedy Space Center. They were discovered when the tank was shipped back following Hurricane Katrina.
ET-120 originally was slated to fly with Discovery last July. It was loaded with supercold rocket fuel twice before other issues prompted NASA to replace it with a different tank.
Interestingly, another tank, one that was not subjected to fuel loading, did not experience any PAL ramp cracking.
"We've looked at the tank that wasn't on the launch pad, it didn't get filled and pressurized," Hale said. "It has no indications (of cracks). And the tank out on the pad, did. That's about as close to conclusive as you get in this business.
"It appears the thermal cycles, the stretch and contraction of thermal cycles, or pressurization, is a factor here. All the engineering reports haven't been done, we've got to get to the bottom of it. Now whether or not these particular little indications are significant and could cause the liberation of foam, that's what the testing is going to tell us in the next few weeks."
Steve Poulos, manager of the orbiter projects office at the Johnson Space Center, updated reporters on three other open items left over from Discovery's July flight.
He said a post-flight inspection of the shuttle's reinforced carbon carbon wing leading edge panels showed no major signs of damage, adding that new inspection techniques will allow engineers to spot trouble areas earlier.
In addition, two relatively minor in-flight problems have now been resolved.
Missing or broken stitching on an insulation blanket under the commander's cockpit window was responsible for damage spotted in orbit. Poulos said about 40 blankets on Discovery and another 60 on Atlantis will be replaced to eliminate any such suspect stitching.
Two protruding gap fillers - spacers between heat shield tiles on the belly of the orbiter - had to be manually removed during a spacewalk after they were seen extending up above the surrounding tiles. Left in place, the protruding gap fillers could have led to increased heating, and subsequent damage, during the return to Earth.
Poulos said engineers understood what caused the gap fillers to pull away and were in the process of conducting tests to weed out any more loose spacers. The work is being prioritized and Poulos said the most critical areas near the nose of the shuttle would be tested first. Any problems will be corrected before Discovery flies again.