Discovery is targeted for launch on the first post-Columbia mission around May 15. Three spacewalks are planned, two of them devoted to space station repairs and outfitting and one to testing new tile and wing leading edge repair materials and procedures.

NASA calls that spacewalk a "detailed test objective," or DTO, and until recently, it was not certain such tests would even be attempted. But Hale said it now appears repair techniques of some sort will, in fact, be tested by spacewalkers Stephen Robinson and Japanese astronaut Soichi Noguchi.

"I think it is a done deal we are going to have a DTO and test whatever we get to carry," Hale said.

But with just three-and-a-half months to go before launch, engineers, astronauts and managers are still assessing which techniques to test. At least part of the reason it's taken so long to reach a consensus is NASA's post-Columbia effort to improve communications between managers and engineers.

"We have spent the last two years in this culture change, where we say that everybody needs to be involved, everybody's opinion needs to be heard and I think we've been successful, because we're hearing from everybody," Hale said. "And I've got to tell you, some of the gray beards, the retired NASA alumni league, former managers, have told me face to face that we are absolutely nuts to try to manage the program this way, that we will never reach a conclusion, fly the vehicle, you name it, because you get so much (feedback).

"I don't know, maybe they're right, but I've got to tell you our goal is to try to hear folks out and apply some logic and where possible, test and analysis, to come to a conclusion. But if you don't hear people out, you may miss something. That's the lesson I think we've heard."

In the meantime, he said, "what you're seeing is the sausage making."

In the RCC arena, the engineering community is focused on developing repairs to fix everything from coating damage, which in some circumstances could be "entry critical," to holes up to six inches across.

To fix cracks, engineers are testing a technique that calls for the astronauts to first heat a damaged RCC panel and then to apply an adhesive ceramic material that would be squirted from a gun-like applicator and smoothed out with trowels. A heated frame then would be used to cure it in place for half an hour at 300 degrees.

A team of Boeing engineers is working on a patch repair concept that potentially could be used to fix everything from coating damage to holes up to four inches across.

For this technique, the astronauts would first sand and smooth the damaged RCC panel. A two- to three-layer patch made up of silicon carbide and zirconia fabric impregnated with a "preceramic polymer matrix" then would be applied with a hand roller or paddle and cured in place for 30 minutes at 450 degrees.

Yet another technique, one that is perhaps the most technically mature, calls for using a flexible carbon silicon-carbide patch seven inches across to plug holes up to six inches in diameter. After fit checks and application of a sealant, the plugs would be held in place from behind by expansion bolts inserted into the hole.

Between 20 and 30 different plugs, each with slightly different geometries, would be needed in a repair kit to ensure a good fit virtually anywhere in the curving leading edge.

"The plug is very mature, the crack repair, it's very mature if we can develop a heater because this stuff has got to be heated after it's applied," Hale said. "We're going through some difficulty in building that heater. They're off doing a whole bunch of engineering tests to see if this crack repair would work in an unheated state so that it actually cures during the early part of re-entry. Sounds kind of dicey, but they're off looking at that. So we may fly that and bring it back."

The heater requires a battery pack and must be mounted to the leading edge to do its work. "It looks like a Rube Goldberg kind of device," Hale said. "It's not as simple as you would like EVA equipment to be. But there is this group of experts who say (the repair material) could work in a non-cured state, the early re-entry actually allows it to cure before you get to the maximum heating. Of course, there are a lot of folks who have a lot of concerns that that may not work. So here we are."

Engineers currently are studying ways to combine components of different RCC repair techniques to come up with procedures that might be easier to implement address a wider range of damage scenarios.

As for tile damage, engineers have been focusing on the caulk-gun/STA-54 technique to fill major cracks or voids in a shuttle's protective tiles. Minor damage may be addressed with yet another technique, known as "emittance wash," in which a coating is applied to damaged tile to help reject heat.

The STA-54 backpack applicator is known by the acronym CIPAA, which stands for "cure in-place ablator applicator." The constituents of STA-54 are carried inside two tanks and, like epoxy, are mixed together as they are pumped out. The astronauts then smooth and shape the viscous mixture before letting it harden and cure in the vacuum of space.

In ground tests last summer, however, engineers discovered air bubbles can form in the material as it is mixed together in the applicator. In the weightlessness of space, such bubbles could form large voids, weakening the material's strength and insulating ability.

Engineers do not yet understand the mechanism responsible for the air bubbles and while they've made changes that have reduced the amount of bubbling, they want to test the technique in orbit to find out exactly how the space environment affects the material.

"The problem the office has is not actually the CIPAA itself, by which I mean the idea of squirting the material out," Thomas said. "It's the material itself. It's not performing the way you would like ... because of the bubbling, it's outgassing and so on. So I think a lot of work needs to be done on the material itself."

Even so, he agreed with Hale that a CIPAA demonstration during a Discovery spacewalk "has some merit in so far as you could address some of the questions of how you deploy a goo like this in zero gravity, that's a tough problem. But the office has taken a position that the systems are really just not developed along enough to do an effective demonstration."

For Rominger, the backpack and equipment required for the CIPAA/STA-54 repair technique pose problems in their own right.

"For the repair with the STA-54, you have this backpack, for the EVA guys it's very technique sensitive," he said. "If you've ever caulked around your bathtub or other areas, you (know you) need to have a little bit of the artist in you. So to do that kind of work, we need a pretty secure work platform."

STA-54, he said, "is still in the research-and-development stage and it really is still a science project."

By not flying STA-54 aboard Discovery, Robinson and Noguchi would have more time during the repair DTO to test other procedures more thoroughly. Like the potentially more reliable overlay technique.

Instead of filling a void in tiles with a material that must cure in space, the idea with the overlay is to fill a void with packets of insulation and then to cover it with a thin, flexible sheet of high-temperature carbon silicon-carbide material. The sheet would be held in place by screws driven directly into the surrounding tiles like drywall bolts. A thin temperature-resistant fabric would serve as a gasket between surrounding tile and the cover sheet.

In recent tests, a section of tile repaired with this "overlay" sheet endured back-to-back sessions in NASA's arcjet facility getting blasted by temperatures similar to those that would be experienced during re-entry. The test sample survived both sessions in good shape.

Supporters argue the overlay technique is inherently superior to STA-54 because it is a mechanical fix and as such, immune to the effects of the space environment. And unlike STA-54, which has a limited shelf life, the overlay sheets could be stored indefinitely for use as required.

"It's actually pretty interesting what they've come up with," Thomas said. "It's a material that you would basically screw on and they've come up with methods of making fasteners that can take all the heat. ... The material is impregnated with a glass so it kind of liquefies a bit and fills the voids and potentially has applications for RCC (the shuttle's leading edge).

"On tile, you could use it in conjunction with something underneath it. If you had a big gouge in a tile, you'd put something in there to fill it in and then put this wrap over it. ... You don't have the problem of the material flowing, the adhesion problem, which is really a tricky problem. It kind of circumvents that."

Despite its initial promise, the overlay technique, like STA-54, cannot be certified before Discovery's launch. But testing the procedure in orbit "should tell us whether we should have more or less confidence in this as a repair," said one manager.

It may even be possible to combine the two repair techniques by using STA-54 to fill in voids and then covering them with the carbon silicon-carbide sheets. But much more testing is needed.

All in all, however, NASA has made solid progress toward meeting the intent of the Columbia Accident Investigation Board's recommendations, Covey told reporters in a recent teleconference.

"If we look at it in the total context of all the things that are being done, and if you look at the intent of the CAIB recommendation, which is one that I will admit we continue to have considerable debate over, then you could say well, OK, the real control for the hazards that come from the external tank is to reduce the debris that comes off the external tank to acceptable levels," he said. "That then reduces the requirement to be able to repair and if you look at the total context, even if we can't repair something, then for the first two flights while we gain confidence in the changes made to the external tank, the agency is prepared to have a safe haven capability on the space station and be able to launch a rescue mission.

"If you look at all those things together, then whether you have a repair capability that can be tested on STS-114 or not really probably isn't as significant as the overall control (of debris and the ability to inspect the thermal protection system during launch and in orbit).

"So, whereas that's probably still in my mind the remaining most difficult technical challenge, I still believe that the agency has gone a long ways toward meeting the intent of the CAIB recommendation, which was to develop and implement a program to develop those capabilities to the greatest range possible."

END