Shuttle Discovery's launch postponed a few more days
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: February 6, 2009;
Updated with official delay to Feb. 22
Launch of shuttle Discovery on a space station assembly mission, delayed earlier this week from Feb. 12 to no earlier than Feb. 19 because of concern about critical hydrogen flow control valves, now is slipping to at least Feb. 22 to give engineers more time to complete testing, NASA officials said today.
NASA managers initially hoped to assess initial test results next Tuesday and then hold another readiness review Thursday. But officials said today the Tuesday meeting had been called off. A management review now is planned for next Friday and, if test results provide enough data, a "delta" flight readiness review will be held the following week, setting the stage for a possible launch around Feb. 22.
"The Space Shuttle Program will hold a meeting Feb. 13 to review data and determine whether to move forward with a flight readiness review on Feb. 18," NASA said in a statement. "The official launch date will be set at the readiness review, but for planning purposes launch now is no earlier than Feb. 22."
If that schedule holds up, Discovery would take off at 3:31 a.m. on Feb. 22 and dock with the international space station around 11:56 p.m. on Feb. 23. Four spacewalks, beginning between 7 p.m. and 9 p.m., would be on tap Feb. 25, 27, March 1 and March 3. Undocking would be scheduled for around 5:29 p.m. on March 5, following by landing back at the Kennedy Space Center around 9:47 p.m. on March 7.
But is not yet clear whether ongoing tests will support a rationale for launching Discovery with its current set of hydrogen flow control valves.
Each shuttle features three such valves, one associated with each main engine, that operate like lawn sprinklers, popping up as required to route hydrogen gas into pipes leading to the external tank to maintain the internal pressure needed to feed propellant to the main engines.
During the most recent launch last November, one of the three flow control valves aboard the shuttle Endeavour began allowing more hydrogen to pass through than expected. The other two valves reduced flow to maintain the proper pressure and the shuttle's climb to space was uneventful.
After landing, engineers discovered a small crack and missing material on the lip of the valve in question. NASA managers ordered all the valves in Endeavour, Discovery and Atlantis removed for detailed inspections. One other valve was found to have a crack. Based on electron microscope inspections, engineers concluded the defect was the result of high-cycle fatigue. A fresh set of valves that flew previously on Discovery and were known to be in pristine condition was installed for launch.
But engineers were surprised that fatigue played a role. As it turns out, engineers using computational fluid dynamics found that the environment inside the system can allow a harmonic resonance to set up.
"That vibration, if it lined up just with the right structural mode, could end up stressing that piece and causing high cycle fatigue," shuttle Program Manager John Shannon said Tuesday, when the initial launch delay was announced.
While Discovery's valves were thought to be pristine, additional testing was ordered to assess the potential threat posed by metallic debris breaking off during flight and striking the walls of the downstream pressurization line.
"They're just not going to have enough data in from that testing to go to a formal agency FRR next Thursday ... for a launch a week later," a NASA official said, discussing the current Feb. 19 launch target. "And that's just not going to happen. They can't get there."
Complicating the picture, engineers over the past two days have discovered that small grooves in the valves left over from their original machining apparently can mask small cracks. After a valve was subjected to "destructive testing," a source said, engineers discovered evidence that tiny, undetected cracks may have played a role in the failure. That would imply that the supposedly pristine valves aboard Discovery may not, in fact, be crack free.
"If the impact testing comes back and says we can tolerate some conservative bounding cases ... we'll go roll it into a (review) on the 13th, discuss the technical puts and takes and launch the next week," said a shuttle manager. "But it could go the opposite way."
The renewed concern about undetected cracks "kind of puts us back to square one," he said.
Depending on impact test results, NASA managers could opt to launch Discovery as is or stand down long enough to possibly replace the valves with versions that have been polished to remove the grooves and retested to make sure no cracks are present.
Over the next several days, engineers hope to refine the computational fluid dynamics analysis of the acoustic environment inside the pressurization lines to more tightly characterize the sort of stress the valves experience and what sort of velocities debris might achieve. Impact testing will determine what threat any such debris actually poses.
Until test data are assessed, it's not possible to predict how the issue might play out. But sources said Shannon was determined to understand the technical ramifications before making any launch decisions, reflecting a post-Columbia focus on making sure potentially significant problems get the attention they deserve.