Shuttle meeting to review valve work and launch date
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 3, 2009
NASA managers will meet Wednesday to assess ongoing tests and inspections of suspect hydrogen flow control valves and to discuss whether to press ahead with a flight readiness review Friday that would set the stage for launch of the shuttle Discovery on March 11 or 12. Valve testing has not turned up any major show stoppers, sources say, and a different inspection technique adds confidence three valves being installed aboard Discovery this week are, in fact, free of any cracks that could worsen in flight and release debris inside a critical external tank pressurization line.
Engineers are still debating whether braces should be installed on three 90-degree bends in the pressurization lines just five inches away from the valves in question. Debris released from a cracked valve could hit an elbow joint at high speeds, possibly causing a rupture in a worst-case scenario.
Some engineers believe the braces are not needed because the new inspection technique clears the valves being installed aboard Discovery and there is no evidence a crack could develop and propagate to failure in a single flight. They also argue any change to the pressurization line could alter the acoustics that contribute to valve stress. Others believe the shuttle should not be launched until brand new valves can be built and installed. The valves currently slated for use aboard Discovery have logged four, five and 12 flights each.
If shuttle managers decide Wednesday that test data supports pressing ahead, an executive-level flight readiness review will be held at the Kennedy Space Center on Friday to set an official launch date. As of this writing, the unofficial target is March 12, but sources say managers are optimistic about moving the target up one day, to March 11, if engineers don't encounter any additional problems.
If so, commander Lee Archambault and his six crewmates would return to the Kennedy Space Center on Sunday for the start of the countdown to launch. In that case, liftoff would be targeted for 9:20:10 p.m. next Wednesday. If March 12 eventually is selected, the countdown would begin Monday evening for a liftoff at 8:54:27 p.m. next Thursday.
Either way, it will be a busy week. Space station commander Mike Fincke and flight engineer Yury Lonchakov plan to stage a six-hour spacewalk Tuesday, starting around 12:20 p.m., to install a European experiment on the station's hull and to carry out a variety of other tasks.
On the shuttle front, getting an additional day would give NASA a better chance of getting Discovery off the ground before running into a conflict with the upcoming March 26 launch of a Russian Soyuz spacecraft carrying the international space station's next commander and flight engineer.
The shuttle must be off the ground by March 13 for the crew to conduct a full-duration two-week-long mission. A launch on March 14 or later is still possible, but the crew would have to give up one or more of the mission's four planned spacewalks. If Discovery misses the March window, launch will slip to around April 7.
The valve issue cropped up after the most recent shuttle launch last November when engineers discovered one of three gaseous hydrogen flow control valves used to pressurize the hydrogen section of the external tank suffered a crack and lost a small piece of its poppet assembly.
Subsequent analysis showed the valves were subjected to more stress than engineers originally believed and managers ordered a battery of tests to determine how cracks develop and propagate and to get a better understanding of the consequences of an in-flight failure.
Discovery originally was scheduled for launch Feb. 12. But the flight was delayed because of the valve issue, first to no earlier than Feb. 19, then to Feb. 22, Feb. 27 and eventually to March 12.
Three presumably crack-free flow control valves were installed aboard Discovery that had logged about 12 flights each. Engineers then discovered it was possible for cracks to elude detection using electron microscopy and dye penetration tests because of surface roughness.
Because valve cracks may be related to repeated exposure to stress, engineers were asked to replace Discovery's valves with three valves that had flown four, four and five times respectively.
Over the past week or so, engineers subjected the valves to eddy current inspections, a non-destructive testing technique that uses subtle changes in currents set up by electromagnetic induction to find signs of cracks.
Eddy current testing can detect cracks that surface roughness might mask. Engineers earlier contemplated having to polish valves to remove any such roughness, a procedure that would physically alter a valve and possibly change the way it responds to stress.
Using eddy current testing, one of the two four-flight valves slated for use aboard Discovery showed two "indicators" of cracks. The other two valves were pristine and the results for all three were confirmed by electron microscopy. The valves that had been removed earlier also were tested. Again, one showed a single indicator while the other two were clean.
As a result, engineers were cleared to install crack-free valves with four, five and 12 flights respectively.
"So we have three going in that we know are clean and there's a group that believes the worst you could come out with after a flight is a crack," said one NASA official. "But there are some people who still might feel the best thing to do is not to fly until you have a brand new poppet every flight."
This week's discussions are focused on developing rationale for launching Discovery with valves of the current generation. For downstream flights, NASA managers may order a redesign or simply fly with brand new valves each time. Only eight missions are currently planned beyond Discovery's flight.