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Shuttle team set to debate removing tank foam ramps
Posted: December 8, 2005

NASA managers plan to meet next week to discuss whether to ship a shuttle external fuel tank to Florida in early February without so-called PAL ramp wind delectors in hopes wind tunnel tests and computer modeling will prove the ramps aren't needed to shield external pressurization lines and a cable tray from aerodynamic buffeting.

A piece of the PAL ramp breaks away from Discovery's fuel tank during launch. Credit: NASA
If the tank is shipped without the ramp, and if the upcoming tests confirm it's not needed to prevent a catastrophic failure, NASA will be clear to launch the shuttle Discovery next spring as planned on the second post-Columbia mission. In so doing, the agency will eliminate a potential source of impact debris and put to rest recent concern about cracks in the foam ramps.

But PAL ramps cannot be added at the Florida spaceport and if the tests show the pressurization lines and cable tray do, in fact, need shielding, the tank would have to be shipped back to Lockheed Martin's Michoud Assembly Facility near New Orleans for additional work. And that would put the next shuttle flight on indefinite hold.

In that case, engineers would have to reapply the long ramp, either manually or robotically spraying on foam insulation to build a sort of dam to smooth the flow of supersonic air over the externally mounted components.

More important, and potentially time consuming, they also would have to develop new application techniques or change the composition of the foam - or both - to prevent hard-to-see cracks from forming after the tank is loaded with supercold rocket fuel.

And those issues will be present in the near term if shuttle managers decided to ship a tank with PAL ramps in place.

"I am fairly convinced that we're going to make a decision to build the first tank one way or the other before the end of the year and proceed at some schedule risk while we do the engineering," shuttle program manager Wayne Hale said in an interview with CBS News. "We wouldn't fly until we had a good solid engineering story to back up what we did, but we may ship a tank that we're at some risk we could get surprised and have to turn it around.

"The one thing I keep coming back to, and I keep telling the team is, whatever we do we're going to prove it's safe to go fly or we're not going to fly. So, the rest of it is just kind of how do we maximize the potential to fly as quickly as we can? We're going to do it right, we're going to make sure it's safe to the best of our abilities, and the schedule will wind up being whatever it is."

The protuberance air-load - PAL - ramps have been under the microscope and the center of attention since a 1-pound chunk of foam broke off the hydrogen PAL ramp on Discovery's tank during the first post-Columbia flight last July. The debris did not strike the shuttle, but upcoming flights were put on hold while engineers studied what could be done to prevent such foam shedding in the future.

So far, engineers have been unable to come up with a definitive explanation for the foam loss and, consequently, a fix. Hale ordered all existing PAL ramps removed and engineers were assessing new fabrication techniques as a possible solution when small cracks were discovered prior to the removal of the PAL ramp of a tank slated for the third post-Columbia mission.

While engineers have not come to a formal conclusion on what caused the cracks, the consensus is that it likely is related to the thermal stress the tank undergoes when it's loaded with super cold rocket fuel. The tank in question originally was to be flown by Discovery last July and it was fueled for pre-launch tests. The tank later was replaced for unrelated reasons, but the cracks could have been caused by thermal stress.

"That is clearly their thinking, but that's a preliminary (conclusion)," Hale said. "Where I have to play the bureaucrat in all this is we haven't completed the analysis. Until you get the engineering done, you can't put the stamp on it and say certainly. But I can say that is what everybody is inclined to believe is the cause of the problem, where the preliminary work is leading us to believe, that it's thermally induced cracking, which may have been there for a long time."

If that theory is correct, cracks would be a constant threat for shuttles that were fueled for launch and then delayed for other reasons. Whether similar cracks played a role in the foam loss experienced by Discovery's tank in July is an open question.

"The cracks, most of them, don't show up at the surface and the ones that do you have to be really right up (on them) to (see) them," Hale said. "It's not something you can see from several feet away or with the operational TV cameras the pad. So it's not something you would necessarily detect if you went and tanked and scrubbed."

During a November news conference, Hale said engineers were looking into whether the ramps could simply be removed, eliminating the potential source of debris. At that time, he said it would be difficult to complete the required testing in time for Discovery's May launch window and that it more likely would take until the third flight in the upcoming sequence. That was due in large part to scheduling conflicts delaying the start of wind tunnel testing.

But he said his thinking changed - and the issue became more urgent - when the crack issue surfaced Thanksgiving week, indicating "we've got a more severe problem in that PAL ramp foam than we had initially thought."

Since then, NASA has been able to book wind tunnel time in February to find out, one way or the other, if the ramps can be safely eliminated.

"Getting rid of (the ramp), if you can afford to get rid of it from the structural standpoint, that is clearly a way forward," Hale said. At the same time, he added, "the folks are off working very hard to come up with some options to change the way the foam is sprayed on, to prevent these cracks."

But if the cracks are, in fact, caused by thermal stress and if the upcoming tests show the ramps can't be eliminated, NASA will be stuck with a thorny problem: developing a new PAL ramp and then proving it will not shed debris.

"The way to get around that is if we have some 'eureka' that says we know why we had cracks and we circumvent that with a processing change at MAF (Michoud Assembly Facility)," said LeRoy Cain, manager of launch processing at the Kennedy Space Center. "For flying with the PAL ramps, to get to that point, we're going to have to have confidence that we understand how we got the cracks in these cases and that we have mitigated that."

Said Hale, "that kind of becomes then the operative question, is not only can you come up with a fix and apply it, but how do you prove that the fix to the foam did what you wanted it to do, which is to prevent these cracks from occurring?"

As a result, engineers are hopeful the upcoming tests will allow the ramps to be eliminated.

"I think we all would like to have them gone," Cain said. "On the other hand, the jury's still out on these cracks."

Cracks aside, a "tiger team" of experts and engineers looking into the PAL ramp foam loss during Discovery July flight concluded in October that "in the future, the only way to ensure no foam will be lost from the PAL ramps is to eliminate ramps altogether."

"A wind tunnel test program has been completed for both the LO2 (liquid oxygen) and LH2 (liquid hydrogen) cable trays without the PAL ramps, and initial results indicate the PAL ramps are not required to protect the cable trays from aeroelastic instabilities," the report said.

"STS-114 LO2 tank cable tray data collected to validate test program results is under analysis, but initial results appear to confirm the test program conclusion that the PAL ramp can be eliminated. Wind tunnel limitations did not permit testing to continue beyond the flight envelope required to establish a 32 percent margin for the LH2 tank cable tray as required. ... Additional testing is required before the LH2 PAL ramp can be removed.

"The ET Tiger Team recommends the SSP (space shuttle program) undertake whatever additional testing is required to substantiate wind tunnel results, including instrumentation of flight cable trays if necessary, and that the PAL ramps be eliminated at the earliest opportunity."

The shuttle's external tank is made up of a large hydrogen tank, a so-called intertank section and an upper oxygen tank. Gaseous hydrogen and oxygen, used to pressurize the tanks, are diverted from the propellants feeding the shuttle's main engines and routed up the tops of the respective tanks in externally mounted pipes. The pressurization lines run next to a long cable tray that carries electrical lines routing data and commands between the shuttle and various tank and booster subsystems.

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 PAL ramp "was put on there in the old days as what I would characterize as a suspenders and belt approach," NASA Administrator Mike Griffin told CBS News last month. "The analysis (of the forces involved) is very difficult to do. Most folks, if you asked them, would say I don't really think we need that (but) It was the kind of thing that was technically very difficult to prove because it's a complex aerodynamic flow field, multi-mach number range, yada, yada, yada. So the suspenders and belt approach is, I don't think I really need (the PAL ramp) but I'm going to put it on there anyway because what can it hurt? Well, what it can hurt is if it falls off!

"Now we no longer take the attitude well, I'll put some foam on there, what can it hurt? If we're going to put foam on something it better be that we really need the foam to be there. On the last flight, STS-114 ... we went to the trouble ... to calculate aerodynamically the very complex flow field surrounding the bipod structure and we concluded we don't need a bipod ramp and we don't need bipod ramp foam. That's what we hope to do with the PAL ramp. We're not there yet."

The PAL 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.

"As you go from subsonic to supersonic through the max Q (maximum aerodynamic pressure) region, the shock waves off the top of the SRBs (solid-fuel rocket boosters) cause the flow to cross the cable tray and the press lines at about an 80-degree angle," Hale said. "The important region is just through Mach 1 or maybe 1.2."

To find out whether the pressurization lines and cable tray can withstand the buffeting without suffering a catastrophic failure, a scale section of a tank will be tested in a supersonic wind tunnel in February. At the same time, computer modeling based on computational fluid dynamics (CFD) will be carried out to get a better understanding of the aerodynamics in the region.

Because the testing should be complete before Discovery's launch window opens, shuttle managers can consider the option of shipping the next tank to Florida without a PAL ramp on the assumption the upcoming tests will show it's not needed.

"The thing that I'm keenly interested in is the actual test data," Cain said. "The CFD, it will either confirm or not confirm whatever the test data says is right. But that flow field is non intuitive. It's extremely complex."

Hale agreed, saying "there's a lot of work to be done. We're going to get together before the end of the year to make a decision."

The topic is expected to be discussed - and possibly resolved at the program level - next Thursday at a Program Requirements Control Board meeting at the Johnson Space Center in Houston. Another topic on the agenda is deciding what sorts of foam and wing leading edge repair techniques are worth pursuing.

"If you're going to reapply PAL ramps to the tank, because we're taking them off all the tanks, they're going to go back on at the first of the year," Hale said. "So we would like to decide before the end of the year whether or not to do that.

"So folks are thinking very hard about how we would get the aerodynamic data to make sure we're in good shape, number one, if we take the ramps off. Or, we have a large group that is still off analyzing why these cracks occur and what changes we might make to the foam application to prevent the cracks from happening.

"The one thing that is crystal clear to me is that we cannot launch with these cracks. We just can't."

The PAL ramp foam shedding during the July shuttle mission was the first known case of such debris since the early days of the shuttle program. But NASA only has post-separation photographs of about half the shuttle tanks launched and it's possible more PAL ramps suffered shedding that went unnoticed. Engineers are reviewing photography from past missions to look for any such signs of damage.

"It's interesting, we went back to the history, we dug into the history of how they got on (the tank)," Hale said. "They were actually a late addition before STS-1. And it came in because some of the wind tunnel testing was not as conclusive as they wanted it to be and it was put on, in the words of the report I have in my hand, to ensure that we have margin structurally. Once they put 'em on, and it was cheap and it took care of the problem, we just kind of kept doing it. So there has always been a question from the first flight whether or not they were actually necessary.

"Over the course of time, we've made a lot of changes to the tank," he said. "We've eliminated one of the lines that used to go up there called the anti-geyser line, it's not there any more. The cable tray itself, which is a rectangular metal box, has shrunk to about half its original size and the structure that attaches that to the tank and the underlying tank structure is all different. We've gone from the standard weight tank, to the lightweight tank to the super lightweight tank, all that stuff has changed.

"The question of the day is, is the structure capable of withstanding the aerodynamic loads, some of which are transient - you know, the flag off the flag pole flapping in the breeze kind of effect - and some of which are what the aerodynamicists call steady state. And so we are off to do some computational fluid dynamics work. We thought we were going to have to wait to get into the wind tunnel until May and we have identified a wind tunnel opportunity in early February."