Shuttle tank fix not expected to delay next launch
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 24, 2007
Shuttle program managers have ordered repairs to downstream external fuel tanks to remove underlying insulation around propellant feedline support brackets because of cracks found in the wake of a foam-shedding incident that damaged the shuttle Endeavour's heat shield earlier this month. The work is not expected to delay the next shuttle flight, targeted for launch around Oct. 23, but the schedule is extremely tight for the flight after that in December.
But Hale stressed that properly fixing the external tanks - not the pre-existing launch schedule - is NASA's top priority.
"The point is, we will take the amount of time that we need to to get this repair done properly," he said. "We will not rush and if we happen to fall a day or two after the 23rd, that is not a huge impact to our schedule. The schedules for the following flights obviously are more fluid and we'll be reviewing those as the work goes forward."
The shuttle Discovery is scheduled for blastoff Oct. 23 on a flight to deliver a new multi-hatch connecting module to the international space station. That module, after the station crew attaches it to the front of the Destiny laboratory module, will serve as the mounting point for the European Space Agency's Columbus research lab, scheduled for launch in early December, and the Japanese Kibo laboratory complex, which will be carried aloft early next year.
Getting the international partner modules launched is NASA's major near-term priority after years of delays caused in large part by the 2003 Columbia disaster.
Columbia was brought down by a large hole in its left wing leading edge that was caused by the impact of foam debris during launch. Redesigning the tank's insulation in the wake of the accident to minimize foam shedding has been a major challenge for the engineering community.
That point was brought home during the flight of shuttle Endeavour earlier this month when a half-ounce piece of debris popped off a liquid oxygen feedline support bracket, hit a downstream attach strut and ricocheted into the belly of the orbiter. The impact gouged a hole almost all the way through two tiles.
After around-the-clock tests and analyses, NASA managers concluded Endeavour could safely re-enter as is. After the ship's uneventful touchdown Tuesday, engineers found little additional damage and Endeavour should be repaired in short order.
But the incident raised new questions about the insulation on the feedline brackets. NASA already had decided to make a change, ordering titanium brackets for future tanks that will minimize the need for insulation in that area. But the new brackets won't be ready until four flights from now, prompting hurried analysis to determine what, if anything, to do about the next three tanks in the sequence.
"Part of the review of the video (from Endeavour's flight) indicates we potentially lost an underlying thermal protection agent called super-lightweight ablative, SLA as it's commonly called, which was added to this fitting on the tank when we thought we were going to fly different trajectories which have a higher heating during ascent," Hale said.
"We now know, and have known for some time, that the super-lightweight ablative is not really required. The damage that we saw on STS-118, given the trajectory, the size and speed of the debris, could not have been caused, we don't believe, by the lightweight foam alone, but must have had a heavier weight component, either this SLA underlying that foam or potentially ice on the foam."
Hale said engineers this week X-rayed the five oxygen feedline brackets on external tank No. 120, scheduled for use by Discovery in October, and found several small cracks in the underlying SLA material. Other downstream tanks also had cracks in the bracket insulation.
"The exact origin of those cracks is still under investigation," Hale said. "We think it's associated with the manufacturing process, but clearly this could lead to a shedding of foam debris along with this heavier weight SLA, which we now know could have a debris transport path to the underside of the orbiter. Therefore we've decided this is an unacceptable situation."
At the Kennedy Space Center this weekend, engineers from Lockheed Martin's Michoud Assembly Facility in New Orleans, where the huge tanks are built, will begin removing the outer BX foam and underlying SLA from at least the top four feedline brackets on ET-120.
"We will replace that with only the lightweight foam, which will provide us the ice growth inhibiting function and certainly is acceptable for all ascent phases," Hale said. "We still have about five contingency days remaining in our schedule for Oct. 23."
Hale said the super-lightweight ablator is intended to protect the brackets from heating as the shuttle accelerates out of the atmosphere. The foam on the outside is primarily intended to prevent ice from forming on the brackets before launch when the tank is loaded with supercold propellants.
Wind tunnel testing and computer analysis show the trajectories the shuttle currently uses to reach orbit for space station assembly flights do not subject the brackets to the level of heating originally expected and Hale said the SLA can be safely replaced with low-density BX foam instead.
While engineers are hopeful they can keep Discovery on track for launch on or close to its Oct. 23 target date, the picture is less clear for the flight after that. The launch window for Atlantis and the Columbus module, defined in part by temperature constraints imposed by the station's orbit, opens Dec. 6 and closes just one week later.
The problem for NASA is that work to refurbish aging systems in the Vehicle Assembly Building where shuttles, tanks and boosters are assembled has created a bottleneck of sorts, preventing the sort of parallel processing that might otherwise be possible.
As it now stands, Hale said Atlantis can still make the end of its December window. But just barely. The launch window will reopen at the end of December but NASA wants to avoid flying over the New Year break if at all possible.
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