KENNEDY SPACE CENTER, Fla.--NASA managers reviewing the progress of repairs to the shuttle Discovery's external tank and the rationale for making another launch attempt decided today to pass up an early December launch window, delaying the flight to at least December 17 and possibly all the way to February.
Shuttle Program Manager John Shannon said engineers need more time to understand what caused cracks to develop in structural ribs, or stringers, during fueling on November 5 so they can better assess the likelihood of additional cracks forming during flight that could lead to potentially damaging foam insulation losses or even structural failure. While the latter is believed to be a remote possibility, engineers need to make sure, and that will take time.
"Clearly we're not ready for the December 3-7 window that's coming up," Shannon said. "We're going to leave the option open for the next launch window that starts December 17. But a lot of data has to come together for us to support that. I don't think the team is worried about any specific launch date at all right now. They're just worried about getting the proper data so we can determine what our risk exposure is to this problem. Once we determine that and determine what the appropriate path forward is, then we'll go off and pick the right launch date."
Shannon cautioned that the December 17 "no-earlier-than" date is little more than a planning target at this point, and is the first day a launch would be possible. As it now stands, the shuttle could launch through December 20 without any major impacts.
Launches after that also are possible, but in those cases the shuttle would be in orbit during the New Year transition, requiring the ship's flight computers to be reconfigured. That would have to be done during a period of light activity, requiring close coordination with space station planners. If Discovery remains grounded through December, the flight would slip to February because of conflicts with other unmanned cargo launches and temperature constraints related to the station's orbit.
A key element of the "flight rationale" discussion is an understanding of what caused cracks in the external tank stringers spotted after a November 5 launch scrub and whether they represent a generic problem that poses a broader threat. While there are several theories, engineers have not yet identified the root cause.
Cracks became more common after the 1998 debut of new super lightweight tanks built with a more brittle aluminum-lithium alloy. Shannon said a review of manufacturing records showed that some 5,000 stringers were built for 43 aluminum-lithium tanks. Out of that total, 31 cracks were found that required repairs, including four in Discovery's tank and three in a tank scheduled for use by the shuttle Atlantis next summer.
But Discovery's cracks were the first to be discovered at the launch pad, the presumed result of exposure to ultra-low temperatures during fueling. Engineers spotted a large crack in the foam insulation of the intertank flange area near the base of the upper liquid oxygen tank. When the insulation was cut away for repairs, four cracks in two underlying stringers were found.
Using a certified repair technique, the damaged stringer sections were cut out and replacements were spliced in, along with "doublers" to provide additional strength. Fresh BX foam then was sprayed on and shaped to blend in with the surrounding insulation.
But before Discovery can be cleared for launch, engineers must show that any additional cracks that might form in the tank stringers after the shuttle is refueled won't cause foam to pop off while the shuttle is climbing out of the dense lower atmosphere. During the first two minutes or so of flight, the density of the atmosphere is still high enough to quickly slow lightweight foam debris, allowing the rapidly accelerating shuttle to fly into it at a high relative velocity.
At a more fundamental level, the crack issue raises questions about the overall structural integrity of the tank and the ability of weakened stringers to handle the forces experienced during launch. The stringers are not considered load-bearing components, but engineers need to verify the tank's safety margins. Whether cracks in the aluminum-lithium ribs represent a generic problem remains to be seen.
"As we started looking at it, we have had a history of these stringers during assembly occasionally getting cracks in them that are caused, we believe, by the assembly process," Shannon said. "They have plenty of design margin. So a properly assembled stringer in the expected flight environment will not crack."
But problems during assembly, including fastener misalignment or mishandling, could have caused a problem that escaped detection and later showed up under the thermal stresses of fueling.
"The hope last week, and why we were kind of sequentially slipping launch dates, was that we would show that there was a flaw in this material, that you could go find some material defect that got through our process," he said. "We also hoped the initial cryo loading stress on that part would be the maximum stress that that part saw during the entire flight phase. As we're going through the investigation, neither one of those points is as clear as we need it to be to commit to go fly."
However the discussion plays out, NASA's options are limited. The agency only plans three more missions: Discovery's, a flight by Endeavour at the end of February, and a final flight by Atlantis next summer. There are no spare tanks and little time to make major changes without significant downstream impacts.
Even so, Shannon said his team will not be driven by schedule pressure and that Discovery will only be launched if the community agrees it is safe to do so.