NASA now plans to ship the next external fuel tank to the Kennedy Space Center ahead of schedule and the shuttle Discovery's commander said today the astronauts remain optimistic about launching in May on the second post-Columbia mission. But a variety of technical issues remain on the table, including wind tunnel tests to show fuel tank changes will work as expected, an ice and debris analysis and, most recently, main engine seal leaks and metallic contamination in the main propulsion system.

File image shows Discovery fitted with its three liquid-fueled main engine. Credit: NASA

The latter issue appears to be a generic problem in that metallic debris, estimated to weigh just 0.08 milligrams, has been found in the liquid oxygen prevalve filter screen used by Discovery's main engine No. 1 and also in an oxygen prevalve screen in the shuttle Endeavour. In the latter case, the debris is estimated to weigh 1.1 milligrams. The concern is that such debris, depending on its composition, could trigger a catastrophic fire during engine operation.

It is not yet known whether the shavings detected by boroscope inspections represent a real ignition threat, whether the shuttle can safely fly as is or whether time-consuming work to disassemble the system and remove the debris will be necessary. If so, NASA could be hard-pressed to launch Discovery before the next launch window closes May 22.

In a separate issue, Discovery's main propulsion system failed a helium leak test after engine installation and then failed it again after engineers detached the engines, installed different seals (two per engine) and reattached the powerplants. Engineers now plan to install the best seals in the shuttle inventory and carry out a third helium signature test to verify the integrity of the system.

If the system passes, NASA will press ahead with Discovery's processing for a launch as early as May 10. But the issue will remain open until engineers figure out what caused the original sealing problem or, if the problem involves hardware that does not meet specifications, whether the shuttle can fly safely with any such out-of-spec components.

At Lockheed Martin's Michoud Assembly Facility near New Orleans, meanwhile, engineers are readying external tank No. 119 for shipment to the Kennedy Space Center as early as next Friday, Feb. 24, a week ahead of schedule.

During Discovery's launch last July on the first post-Columbia mission, a large piece of foam insulation broke away from the liquid hydrogen section of the external tank's protuberance air-load - PAL - ramp. In the wake of that incident, NASA managers decided to remove the PAL ramps on all subsequent tanks, starting with ET-119.

The PAL ramps were added to the external tanks before the first shuttle mission to act as aerodynamic dams, shielding two external pressurization lines and a critical cable tray from buffeting as the shuttle goes supersonic in the dense lower atmosphere. Computer modeling now indicates the ramps aren't needed and that the pressurization lines and cable try are tough enough to endure whatever buffeting they might experience.

But wind tunnel tests are needed to confirm the computer results and that work will not be finished until next month.

"We resisted taking the PAL ramps off," shuttle program manager Wayne Hale told Kennedy Space Center workers today during an "all hands" briefing. "We were looking at taking the PAL ramps off eventually and we had this nice program, we were going to do a lot of wind tunnel tests, do all this analysis, instrument a couple of tanks and fly a couple of tanks and get the data that would prove without a shadow of a doubt we didn't need these PAL ramps. ... Clearly, supersonic aerodynamics, the forces that are imposed on the pressurization lines and the cable tray that run up the side of the tank on the outside, you don't want to fool around with that because you can't stand for those things to come loose. So we've got to do it right. That was in the plan."

But in the wake of Discovery's launch last July and inspections of a tank originally scheduled for the next mission, "we knew the PAL ramp had to go," Hale said. "So the big discussion was how can we prove this is safe to do?"

"We've done some very conservative kinds of analyses ... and that indicates we are probably OK to take it off," Hale said. "But it also has big uncertainty factors. And if you put the big uncertainty factors on it, it's not what you'd like. So we've got an expedited set of wind tunnel tests that we're running, first at the Glenn Research Center coming up in March and secondly, at the Arnold Engineering Development Center in Tullahoma, Tenn., in June. You might detect that June is kind of after when we'd like to fly."

But Hale said by around April 1, engineers should have "some numbers that we can at least hang our hat on and if we pass go at that point, which is to say the structure will hold together as we go supersonic on the tank, then life is good. We're not really sure that's going to happen, that's why we've got the Glenn Research Center wind tunnel tests, which has a full-scale part of the tank with these lines and cable trays. Putting that model together and getting it into a wind tunnel is not something they do overnight."

Wind tunnel testing will run through March. Data will be compared to the engineering analysis and "hopefully say, you know, we were too conservative and the loads are actually less than this conservative analysis, we can reduce the uncertainty."

But if the March testing generates less definitive results, additional higher-fidelity tests will be conducted at Arnold in June and Discovery's flight will slip into the July launch window and possibly even later.

"This is a success-oriented schedule," Hale said. "We are betting, as we do frequently, that we will be smart enough to say we're good to go. But if the answer comes out bad, we'll put the red flag out and we'll say we've got to wait until the July launch window to get the AEDC wind tunnel test results back.

"We're all assuming the analysis will show the structure as it stands is good to fly, that it's got the strength necessary to withstand the aerodynamic loads. But I won't kid you, if the answer comes back and says, you know, the cable tray will come off, that's not something we're going to fly with. And then we'll have to sit down and scratch our heads.

"So here's the story," Hale concluded. "We are working towards, as we always seem to in this business, an optimistic (schedule for a May launch). The data has got to prove it is safe to go fly. If the data doesn't come in in time, or the data says it's not safe to fly, then we'll stop what we're doing and go to plan B. I have a great deal of confidence the data will show we are safe to go fly, but it's going to be what it's going to be. ... Welcome to our world."

Other long poles in the processing tent include completion of a detailed ice and debris threat analysis; possible work to replace additional "gap fillers" between heat shield tiles on the orbiter's belly; and work to confirm a long boom that will be used to carry out post-launch heat shield inspections in orbit can withstand launch forces.

Discovery commander Steven Lindsey and five of his six crewmates - pilot Mark Kelly, Michael Fossum, Lisa Nowak, Stephanie Wilson and Piers Sellers - spoke with reporters today at the Kennedy Space Center and expressed confidence about getting off in May.

"The foam problem, obviously we're working on it," Lindsey said. "Everybody knows what happened on 114 (Discovery's last flight). The PAL ramp foam problem, I think we have solved since we've taken the PAL ramp foam off the tank and it won't be there. The data that I've seen ... I'm pretty confident that's the right decision and we're going to be OK on that."

Lindsey said "the program has never advertised that we will never lose any foam. And we'll lose foam on this flight."

"The key is, to make sure the foam we do lose is small enough that it can't hurt us if it hits the vehicle," he said. "And that's what we're working towards. So I feel pretty confident that the decisions we're making based on the information we have and the engineering analysis and wind tunnel testing, we're going in the right direction."

Even so, he cautioned, "this is a test flight and part of the purpose of this test flight is to test the changes we've made to the tank. And so obviously, we won't know for sure until we fly it."

During Discovery's flight last summer, spacewalker Stephen Robinson, riding on the end of the space station's robot arm, ventured under the nose of the shuttle to remove two protruding gap fillers that had shaken loose during launch. The gap fillers provide a sort of protective cushion between tiles during launch and re-entry. Gap fillers that protrude into the airstream during re-entry can cause excessive localized heating.

In the wake of Discovery's last flight, engineers have replaced gap fillers in critical areas.

"The gap fillers, we're in the process of replacing a bunch of them and making sure they're secure," Lindsey said. "As everyone knows, we had one come loose and had to pull it out on the last flight. We've got the primary zones done on the gap fillers, there are some more zones that we just signed up to do that still supports May. There is still some debate going on over another section and whether we need to pull those.

"As far as I know, I don't think that in particular is threatening the May launch date. There is a lot of work going on to get the tank ready, to get the tank shipped on time, to do a good tank flow here to get us all ready to go. There are a whole bunch of things out there. Right now, we're still holding to May, we're training to May and the program is marching toward May. We'll see.

"The only kind of risk we want to take here is a schedule risk, not a technical risk. Nothing technically is being eliminated for the May launch date. We're doing everything technically that we would normally. If we get to a point where, from a schedule standpoint it doesn't make May, then we're going to slip. But we're not going to skip any technical steps."

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