NASA returns to roots for tile repair
Engineers coming up with shuttle fix learn from past tests  
A NASA official shows how the tile repair equipment would be mounted onto a spacewalker's backpack and applied using the "gun" in the astronaut's left hand.
By James Oberg
Sept. 18 —  NASA showed off its current ideas for in-flight repair of space shuttle tiles this week, a demonstration that went a long way toward finally explaining why the space agency canceled its first tile repair efforts a quarter century ago.
  Image: James Oberg

       THE DEMONSTRATION was part of a special media workshop that NASA held this week to showcase its “Return to Flight” efforts. Officials in Houston set up a semicircle of tables with equipment and hands-on demonstrations in a high-bay room next to a giant vacuum chamber for testing spaceships. Groups of journalists visited the exhibit as part of a round-robin tour that included other testing and training facilities.
       Engineers directly involved in the tile repair process explained in detail how the tile damage could occur, how it would be detected, and how the several different proposed repair techniques were being developed, evaluated and improved.
       Even before the first space shuttle mission in 1981, NASA had been making serious efforts to develop an in-flight tile repair capability. Engineers developed an applicator gun for injecting a fast-setting paste and pre-cast tile sections into cavities, and space-suited astronauts had tested it on zero-G airplane flights.

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       The original intent of that 1980-era effort was to replace entire missing tiles, explained NASA flight director Paul Hill, since NASA had been having a difficult time with the adhesive to hold them onto the shuttle’s outer skin. But following the success of an intensive effort to find stronger attachment techniques, the perceived need for in-flight replacement faded, said Hill, who is heading up the new tile repair effort.
       At that time, NASA engineers could not imagine any need to repair tiles damaged by impacts from insulation coming off the shuttle’s giant belly-mounted fuel tank, since the spacecraft’s official design specifications explicitly stated that no such debris release would be allowed.
The tests a quarter century ago revealed significant problems with the spray-on materials foaming uncontrollably, creating bulging “repairs” that would have created damaging air turbulence during entry into the atmosphere. Also, NASA’s lack of experience with spacewalks led to fears that clumsy astronauts would induce more damage into the tiles than the original problems they would be sent outside to fix.
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       Testing also showed that the repair material was sticky in the wrong direction — it clung to the applicator tools and pulled off the often-dusty cavities where tiles were missing. “The material stuck better to the tools than to the tiles,” spacewalk tools expert Dana Weigel told reporters.
       “Most of what killed [the effort] in 1980 was the operational aspect,” explained Dr. Michael Fowler, a materials scientist who was describing the chemistry of the compound called MA-25S. “But we’re a lot smarter on EVA [extravehicular activity, or space walks] now.”
       In the years following the cancellation of its first tile repair effort, NASA began to notice tile damage not from the loss of whole tiles (as originally feared) but from impact damage with debris (which had originally not been considered). But the damage tended to be localized and survivable, so a pressing need for repair capability was never recognized.
       Then came the Columbia disaster on Feb. 1, when launch debris impact damage to the shuttle’s thermal protection system led to the catastrophic loss of the shuttle and its seven astronauts. NASA resolved never to be caught without a repair capability again.
At this week’s demonstration, the famous MA-25S “goop” paste, intended for filling cavities gouged in shuttle thermal tiles from impacts with debris or other causes, was made available for seeing, touching — even smelling and tasting. Dried segments of the rubbery compound, looking like rubbery bologna (the salmon color is due to iron oxide in the mix), were handed out as souvenirs.


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       Fowler said his team had settled on the same basic silicone-based foam material as picked in 1980, with the substitution of a few materials no longer manufactured. They changed the foam processing in order to reduce the air and water contamination that had originally caused the excess foaming in the 1980 tests. “We do the mixing in vacuum now,” he explained.
       When two compounds from separate tanks are mixed and extruded through an applicator gun, they deposit a paste that does not foam. And even under arc jet flame tests above 2000 degrees Fahrenheit, the paste expands only slightly and provides full thermal protection.
       Another old problem was solved serendipitously, said Weigel.
        “We discovered recently that foam doesn’t stick to MA-25S,”, she said, referring to commercially bought foam paint brushes and rollers.
       A hands-on demonstration confirmed that the widely used home-improvement tools could easily shape and smooth out a hunk of goop that had been squirted into a tile cavity a foot wide. With enough effort, some of the goop could be made to stick to the foam tool, but only in minor amounts.
However promising these new tile-repair tools are, however, they would not have been able to repair the actual damage that destroyed Columbia. Then, a debris strike on the left wing leading edge had punched a hole in the high-temperature material called ‘reenforced carbon carbon’, or RCC. Techniques being developed to repair such material are much less mature, NASA admits.
       Furthermore, said Hill, the project manager, “Holes the size you see here in tiles, we probably wouldn’t even need to repair.” In the past, such tile damage has proved tolerable, and not worth taking the chance that space repair astronauts would accidentally inflict damage while applying the goop.

       Where the goop application would be critically needed, Hill said, was if there was wide-area tile destruction, and for damage that interrupted the “step” of the tile pattern across the skin. In such cases, significant disturbances in the hypersonic entry air flow across the shuttle’s skin could create heating downwind of the damage that was “three or four times as hot as normal”.
       “I can imagine having to lay a bead of material along a broken edge,” Hill explained further, “to better smooth the surface and reduce turbulence.” With a combination of improved tricks, hard work and some luck, Hill’s team appeared well on its way to adding a highly flexible capability to a space crew’s emergency kit.
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