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NASA Finds Cracks In Insulation On Space Shuttle Discovery Tank

By | August 27, 2007

      Shuttle Endeavour Damage, Despite Safe Landing, Sparks Safety Rework

      NASA technicians discovered cracks in ablative insulation on the external fuel tank of Space Shuttle Discovery, which is less than two months away from its scheduled launch.

      Space agency leaders worry that such cracks could pose a danger of the insulation breaking off and striking the orbiter vehicle.

      As a case in point, insulation-caused damage to Space Shuttle Endeavour during a launch Aug. 8 — while it didn’t prevent the orbiter vehicle from executing a safe reentry and landing almost two weeks later — prompted a new NASA initiative to lessen the risk of foam-insulation-caused damage to spaceships in future launches.

      The working assumption is that most likely, cracks in ablative insulation on the Endeavour tank insulation caused the foam loss and resultant damage to the orbiter vehicle.

      The cracks were found on the Discovery tank during that new safety initiative, and officials responded by devising both short-term and longer-term solutions.

      The safety moves were decided in a meeting Friday at Michoud Assembly Facility in Louisiana led by N. Wayne Hale, the space shuttle program manager. He later spoke with reporters in a teleconference.

      NASA officials are concerned that foam insulation on shuttle external fuel tanks can break loose and strike orbiter vehicles, causing damage to critically important protective heat tiles.

      Space Shuttle Columbia was lost during reentry in 2003, because of undetected damage caused by foam insulation that broke loose during the earlier launch.

      And damage has been seen in later shuttle flights.

      For example, in the shuttle Endeavour STS-118 Mission to the International Space Station this month, a piece of foam, possibly accompanied by ice and/or the denser-heavier ablative foam insulation (there is no total certainty here), broke loose from the external fuel tank, a foam hunk that was ripped away from the liquid oxygen feed line bracket.

      The chunk of foam then bounced off a structural attachment member, and dinged the underside of the spacecraft, gouging a hole through the heat tile. (Please see Space & Missile Defense Report, Monday, Aug. 20, 2007.)

      After carefully inspecting the gouge while Endeavour still was in space, and replicating the damage to the tile in tests on the ground, NASA leaders decided it would be safe to permit the orbiter vehicle to go through the cauldron of reentry without attempting a risky repair to the heat tile while Endeavour still was in space.

      They were right. Endeavour handily survived reentry and landed in good shape.

      To lessen the threat of foam strikes in future, NASA will rework the bracket insulation to lessen chances of foam or ice breaking loose from it, Hale said.

      For a longer-term fix, NASA eventually will install a newly-designed feedline yoke bracket made of titanium, which has better thermal properties so that ice doesn’t readily grow on it, compared to ice formation on the existing aluminum bracket. Ice forms on the external fuel tank and its structures because the tank holds extremely cold oxygen and hydrogen.

      An added plus in this fix is that technicians don’t have to apply as much insulation to prevent ice from forming.

      But that fix won’t be in place until four shuttle flights from now, meaning the fix won’t be ready in time for the liftoff of Space Shuttle Discovery as early as 11:38 a.m. ET Oct. 23 on the STS-120 Mission to the International Space Station, Hale said.

      So, meanwhile, NASA is taking some interim steps to avert potentially worse damage to orbiter vehicles in future missions.

      First, launch and ascent videos from cameras in the tank appear to show that the foam insulation lost included an underlying layer of one type of foam, called super lightweight ablative (SLA), as well as the BX foam more generally used on the tank.

      Hale has viewed SLA with suspicion for about a year at least.

      While SLA earlier was thought to be required because of very high heat generated by the friction with the atmosphere of a space shuttle as it accelerates from the launch pad to thousands of miles an hour, Hale said temperatures actually may not be so high, and thus SLA is not really required.

      The damage to Endeavour couldn’t have been caused by the regular lightweight foam alone, Hale said, so there had to be some heavier component attached to the regular foam: either SLA and/or ice.

      All this prompted a closer look at the SLA and regular foam, so NASA technicians X-rayed foam on five brackets of the liquid oxygen feed line outside the next shuttle tank to fly, the one on the Discovery mission that launches Oct. 23.

      Those X-rays revealed that the SLA was cracked for reasons unknown, perhaps because of some factor in the manufacturing process, Hale said.

      Whatever the cause, this cracking may be dangerous.

      “This could lead to a shedding of foam debris,” Hale cautioned. And that threat is magnified by the heavier weight of the SLA, a worrisome consideration given the fact that SLA could strike the orbiter vehicle.

      “This is an unacceptable situation,” Hale said. “We could not let this go, and really just had to fix it.” Therefore, NASA will make that interim fix: all foam — both SLA and regular — will be removed from the first four brackets on the external fuel tank oxygen feed line, and replaced with only the regular, lighter-weight BX foam insulation to prevent ice formation.

      That repair work over about nine days probably won’t mean any problem in Discovery lifting off from Kennedy Space Center on schedule Oct. 23, Hale said. But the repairs will be performed, even if that should mean that Discovery is delayed in lifting off by a day or two.

      And, hopefully, tank insulation repairs won’t cause NASA to have to delay the launch of Space Shuttle Atlantis that is scheduled as early as Dec. 6 on the STS-122 Mission to the space station.

      Hale certainly didn’t promise that the interim fix, or the permanent fix, will guarantee there never will be any further instances of foam shedding on shuttle missions. Further, these fixes that he outlined won’t be the last that will be initiated on the spaceships and their external fuel tanks, he indicated. The foam insulation will continue to pose a problem.

      “It’s a basic design situation that we are going to continue to battle as we continue to fly the shuttle” fleet, he said.

      NASA will continue “to eliminate areas of risk as we have resources available to do it,” he said. “You’ll see continuing efforts with the external tank, and, in fact, the entire shuttle vehicle to reduce risk throughout the life of the program.”

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