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Mon, June 16, 2008 05:55:02 PMFrom:
CBS Space News
Subject:
545p 6/16 Update: Engineers confident about pad repair in time for Hubble mission
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CBS NEWS STS-124 STATUS REPORT: 70
Posted: 5:35 PM, 6/16/08
By William Harwood
CBS News Space Analyst
Changes and additions:
SR-67 (06/14/08): Shuttle Discovery lands in Florida
SR-68 (06/14/08): Reisman joins crewmates on runway; commander pleased with landing
SR-69 (06/14/08): Reisman looks fit at crew news conference
SR-70 (06/16/08): NASA managers optimistic about pad repair in time for Hubble mission
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5:35 PM, 6/16/08, Update: Pad repair likely will involve brick removal, application of spray-on coating to protect underlying wall; engineers confident repairs can be made in time to support Hubble mission
Engineers assessing extensive damage to launch pad 39A during the shuttle Discovery's May 31 takeoff said today they are confident the "flame trench" that diverts exhaust to either side can be repaired in time for NASA's next mission, the Oct. 8 launch of shuttle Atlantis on a flight to service the Hubble Space Telescope.
Repair options will be presented to shuttle Program Manager John Shannon on June 26, leaving about two months to complete the pad rehab before the planned Aug. 29 rollout of Atlantis.
"We feel we're on the right path for a design solution and we're working to get there," Perry Becker, chief of NASA's structural systems branch at the Kennedy Space Center, told reporters today. "We're working very extensively with a few vendors in industry, we're starting to put that plan together. ... We're confident, regardless of the scope of the work here, that we can repair this pad in time and support the rollout."
During Discovery's takeoff, some 5,300 heat-resistant bricks lining the northeast wall of the flame trench under the shuttle's mobile launch platform were blown away, some blasted more than 1,800 feet, heavily damaging a security fence around the pad perimeter. The interlocking bricks, held in place by epoxy and metal clips anchored in concrete, are used to protect an underlying 3-foot-thick concrete wall that helps form the structural backbone of the pad.
The missing bricks exposed an irregular area of the concrete wall measuring roughly 20 feet by 75 feet. New bricks cannot be manufactured in time to support the Hubble mission, but Becker said engineers believe the trench can be repaired by stripping away additional bricks around the damage area, erecting a steel mesh framework and then spraying on a thick coating of a refractory material like Fondu Fyre.
A five-inch-thick coating of Fondu Fyre currently covers the inverted V-shaped flame deflector that diverts main engine exhaust to one side of the pad and booster exhaust to the other. About 20 feet of the flame trench extending from the deflector on the booster side already is covered by Fondu Fyre, giving way to bricks. The idea would be to extend that coating to cover the areas damaged during Discovery's launching.
"We're certainly looking strongly at Fondu Fyre, we've got a history with it out here, we know its properties," Becker said. "There are a couple of other materials on the market that we're looking at, so we haven't down selected that definitively. But it is a leading candidate."
Despite the intense heat and pressure produced by the shuttle's solid-fuel boosters, Becker said the Fondu Fyre coating the flame deflector holds up well to the extreme heat and pressures produced by the shuttle's huge solid-fuel boosters.
"The erosion rate we would expect on the side walls (of the flame trench) would be very, very minimal based on some Fondu Fyre that we have up near the main flame deflector itself," he said. "That has performed very well."
It is not yet known how many more bricks will need to be stripped away or how large an area might ultimately be covered by Fondu Fyre, assuming program managers approve that approach.
"I like to talk about it as if you're redoing tile in your bathroom at your house," said Ed Mango, launch director for the Hubble servicing mission. "One tile gets loose, then you've got to chase it. This is a similar thing. The tile in your shower is there to protect the wall behind it. We have the brick here to protect the concrete behind it. Of course, the over pressure and the amount of water is much worse!"
Looking at trowel marks on the exposed concrete, engineers believe the epoxy used to help hold the bricks to the wall was not uniformly applied. So-called "tap tests" have revealed possible voids behind other sections of the flame trench where bricks are still in place. Engineers also are taking core samples and performing "pull tests" to further characterize the remaining bricks.
"We have seen indications on the visibly damaged sections of trowel marks in some areas on the wall there that shows less than full engagement of the bricks to the back wall structure when it was originally manufactured," Becker said. "Certainly, that is one component of several possibilities of the root cause of failure."
Becker downplayed the question of whether the original construction was flawed, saying "there's no such thing as a perfectly vertical or smooth wall. So there are going to be voids and surface imperfections, that's common in the construction industry."
The Hubble crew cannot take advantage of "safe haven" aboard the international space station if Atlantis suffers any damage that might prevent a safe re-entry. As a result, NASA plans to have a second shuttle, Endeavour, ready for takeoff from nearby pad 39B if a rescue mission is required.
Both shuttle pads were built in the 1960s for the Apollo moon program and engineers are carrying out tests and inspections to assess the health of pad B.
"We're evaluating that state of pad B as we speak and if we find anything, we'll take the appropriate action," Becker said.
First used in 1967, pad 39A has withstood 12 Saturn 5 launchings, including the first Apollo moon landing mission, and 70 shuttle flights. Pad 39B, first used in 1969, supported one Saturn 5 launch, four Saturn 1B flights and 53 shuttle missions, including Challenger's final flight.
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