Shuttle valve tests continue but yet not complete
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: February 11, 2009
Testing to determine the threat posed by suspect hydrogen flow control valves - work that has delayed the shuttle Discovery's planned launch from this week to no earlier than Feb. 22 - is proceeding at multiple NASA field centers and so far, engineers say, no show stoppers have been identified. But testing is far from complete and it's not yet known whether NASA can develop an acceptable near-term flight rationale.
NASA is running high-fidelity tests with mockups using air and hydrogen gas in an attempt to accurately model the flow environment inside the 79-foot-long, 0.6-inch-wide line used to pressurize the hydrogen section of the shuttle's external tank during the climb to space. Engineers are trying to make sure a valve fragment, should a piece break off in flight, could not puncture the line with potentially catastrophic results.
So far, no major signs of damage have been seen, although "witness marks" on the inside of the line show where test fragments have impacted as they were blown along. But the testing is not yet complete.
Even so, NASA managers plan to meet Friday to assess the progress of the testing and to decide whether to proceed with another flight readiness review next Wednesday. For a launch on Feb. 22, Discovery's crew would have to go into medical quarantine Sunday and fly to the Kennedy Space Center on Tuesday, before the FRR could set an official launch date.
"They're going to press on for the (meeting) Friday and present what data they have, recognizing that testing is still going on, obviously," said NASA spokesman Kyle Herring. "They haven't seen anything so far that I'd characterize as a show stopper, but there definitely is going to be some open testing, analysis, whatever, that will not be ready by Friday. But they felt comfortable enough that they would have enough information to meet Friday."
The issue came to light after the most recent shuttle flight last November. During launch, 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 part of the valve in question had broken off. 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 vales 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 last week, when the initial launch delay was announced.
The valves currently installed aboard Discovery have flown about a dozen times in the same positions they're currently in. They were subjected to dye penetration tests to look for cracks, as well as inspections by a scanning electron microscope. All three valves were thought to be pristine and crack free.
But in subsequent tests, a valve was forced to fail and engineers were surprised to find signs of small cracks even though pre-test inspections indicated the article was crack free. Engineers suspect grooves in the valve material from its original machining may have masked the cracks. If so, Discovery's valves could be suspect as well.
The hydrogen pressurization line features three major bends: a 60-degree bend near the valves, a 90-degree bend down stream and a 102-degree turn inside the external tank. The ongoing impact testing is designed to make sure any fragment of a valve that does break off doesn't puncture the line at a bend or anywhere else.
If the impact testing definitively shows no significant additional risk due to possible valve fragment "liberation," NASA managers may be able to develop a rationale for proceeding with Discovery's launch. Only one such incident has been recorded over the history of the shuttle program - Endeavour's flight last November - and the valves currently installed have flown multiple times without any signs of damage.
One possible long-range fix is to polish off the grooves on other flow control valves that might mask small underlying cracks. Assuming those valves passed subsequent inspections, engineers would have confidence the valves were crack free. But such work would require additional testing to make sure the removal of the grooves does not change the flow pattern in the pressurization line and and along with it, the acoustic conditions that can lead to fatigue.
Polishing is not currently an option for Discovery. The current emphasis is on finding out if Discovery can be launched as is.
The issue is complicated and engineers do not want to repeat the errors in judgement that led to a decision in late 2002 to keep flying space shuttles while a fix was implemented to resolve ongoing concerns about foam insulation falling off the external tank. The shuttle Columbia was destroyed during re-entry Feb. 1, 2003, because of damage from the impact of foam debris during launch.