Astronauts have been practicing tile repair concepts and various materials in preparation for return to flight. Credit: NASA

As of this writing, NASA managers continue to debate the sequence of the three planned spacewalks and the relative priorities of the tasks. The current plan calls for Robinson and Noguchi to test tile repair procedures during the first spacewalk on Flight Day 5, even if those procedures are not formally certified and even if questions remain about whether the techniques will work as planned. The plan calls for the spacewalkers to install the new control moment gyroscope during the second spacewalk and to attach a space station tool kit known as ESP-2 during the third excursion.

"Our current requirement puts (the tile repair demonstration) as one of the highest priority objectives for this mission," Hill said. "Because there's so much focus on the ET foam and the health of the TPS (thermal protection system) after we've made it into orbit, one of the things we've talked about is, if we really thought there was a risk we'd have to repair tile, our interest in getting the (tile repair tests) done goes up even more because we know that even when we resolve the technical issues with tile repair, it's still going to be a difficult job.

"Since we haven't repaired tile in orbit and we don't have the ability to do all of it in a full up orbit-like environment, except when we're in orbit, if we thought there was a significant risk of needing a repair, we absolutely would want to be out on EVA-1 with this crew practicing and confirming to us the tools will work and we really can put this material into tile and it really will cure in orbit. In this case, the test would be important as practice for an actual repair.

"But that aside, from a nominal perspective our mission priorities have this DTO as one of the highest objectives of the mission."

But those priorities could change if the shuttle encounters a problem with some other system that would require the crew to make an early landing and NASA managers are still discussing what to do if the crew only has time to carry out a single spacewalk.

Flight Day 5 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/18/05
   Wed  04:11 AM...03...12...00...STS crew wakeup
   Wed  04:41 AM...03...12...30...ISS crew wakeup
   Wed  05:26 AM...03...13...15...EVA-1: EVA preparations begin
   Wed  07:41 AM...03...15...30...Equipment transfer operations resume
   Wed  08:11 AM...03...16...00...SSRMS re-grapples a fixture on the lab module
   Wed  08:56 AM...03...16...45...SSRMS ungrapples the mobile base system
   Wed  09:31 AM...03...17...20...EVA-1: Airlock depressurization
   Wed  10:16 AM...03...18...05...EVA-1: Airlock egress
   Wed  11:21 AM...03...19...10...EVA-1: Payload bay equipment/tools setup
   Wed  12:16 PM...03...20...05...EVA-1: Tile sample repair demo
   Wed  02:46 PM...03...22...35...EVA-1: Payload bay cleanup
   Wed  03:46 PM...03...23...35...EVA-1: Airlock ingress
   Wed  04:46 PM...04...00...35...EVA-1: Airlock repressurization
   Wed  08:11 PM...04...04...00...STS/ISS crew sleep begins

The astronauts may also perform rudimentary RCC repairs, although those procedures are not nearly as mature.

In recent weeks, tile repair tests have raised questions about the curing process and whether the material will, in fact, form the shield a damaged shuttle might need.

"The biggest concern we have right now ... is we have started seeing bubbling in the material in tests in vacuum," Hill said. "This is a problem we thought we had solved in May of last year. There are a couple of things that have changed and we're not sure where the bubbling is creeping in."

One theory is that air is somehow leaking into the supply canisters, which are designed to keep the repair material at vacuum. Another theory involves the applicator. In previous ground tests, the material was not subjected to the sort of pressures it will see in the gun.

"This material has little micro balloons of silica, little glass balls, in it," Hill said. "The little glass balls have air in them. One of the concerns was that as we put this material in the flight gun, the pressures we're operating at as we push it through the static mixer ... causes those little micro balloons to break and the very small quantity of air in those micro balloons, we've shown on paper would be enough to show the bubbles we're seeing."

Based on testing, engineers decided to reduce the pressure in the gun. But that effectively reduces the rate at which the material leaves the nozzle and that, in turn, could affect how the material bonds with the damaged tile. Testing continues.

On the ground, bubbling is not a serious issue because bubbles tend to rise to the surface, pop and disappear. But in the weightlessness of space, the bubbles would remain suspended in place.

"In orbit, there is some concern amongst the materials guys that as these bubbles form inside the material, they're going to coalesce together and they may form a big pocket," Hill explained. "Then during deorbit, instead of getting a nice, glassy external surface, instead we'll burn down a little bit and hit that pocket and then the plasma will be channeled down into that damage cavity and will dig that patch and then the damaged tile right out. That's the concern.

"The two main technical questions about this repair technique are one, if we can't make this bubbling go away like we thought we had, can we tolerate it? Or will it cause large voids that will end up burning through? We can't test that anywhere but in orbit. So that's question number one.

"Then the second one would be, when we extrude this into tile in zero G and in vacuum and in the orbit thermal environment where we could be swinging significantly in temperature, are we confident we're going to get a good enough bond to that powdery tile surface that the patch is not going to break loose? And the only place we can fully test both of those is in orbit."

If engineers are unable to resolve the bubbling issue, or otherwise demonstrate the techniques will work as required, "it means we go into 114 having less confidence than we had hoped we would have in this repair technique being effective," Hill said. "And it means we really need to get the test samples down from 114 ... and cut them open and look at them under the microscope and stick them in the arc jet before we can really have confidence in this tile repair technique."

In the end, NASA may not have a "certified" procedure for repairing damaged tiles before Discovery's launch period opens. Agency managers decided earlier that STS-114 could be safetly launched without a certified RCC repair procedure "because we absolutely are going to have the capability to detect entry critical damage and protect the crew from entry critical damage," Hill said. "Whether we can repair or not, we'll know we have it and if necessary, we'll invoke CSCS (contingency shuttle crew support, or safe haven), launch on need and we'll rescue the crew."

A similar argument could be applied to tile repairs as well. But Parsons, Hale and other senior managers say a tile repair kit of some sort - and possibly a rudimentary RCC kit - almost certainly will be on board Discovery whether the procedures are formally certified or not.

The question is, will those repair kits meet the intent of the Columbia Accident Investigation Board? The recommendation in question calls for development of "a practicable capability to inspect and effect emergency repairs to the widest possible range of damage to the thermal protection system, including both tile and reinforced carbon carbon."

The Stafford-Covey Return to Flight Task Group, an independent panel of experts set up by outgoing NASA Administrator O'Keefe, is monitoring the agency's compliance with the CAIB's recommendations.

"The way I interpret the CAIB, I think a practicable repair technique is a requirement," said James Adamson, a member of the Stafford-Covey task force. "I don't believe it needs to be certified. It's an emergency technique for an emergency situation. I don't think it necessarily has to have completed all its testing. It has to be reasonable, doable and practicable. And I think NASA's going to have that."

A former shuttle astronaut, Adamson said it's possible "we might disagree that they have met the full intent, or goal, of the CAIB recommendation and still be OK with them deciding that it's OK to fly because of this over-arching reduction of risk.

"But it's really not our call to say the shuttle's safe to fly," he said. "We're looking at a very tiny subset of all the things NASA has to consider to fly again so we really can't be in a position of declaring the shuttle safe to fly. That's their call."

PREVIEW REPORT PART 8 --->