Testing Clears Shuttle Mission
By Frank Morring, Jr. 
NASA engineers adapted eddy-current testing techniques to find tiny cracks in suspect space shuttle engine valves that scanning electronic microscopes missed, using the data to clear the space shuttle Discovery for launch on March 11.
John Shannon, the space shuttle program manager, said March 6 the space agency was able to develop a rationale for launching the STS-119 mission to the International Space Station (ISS) by using eddy current tests - essentially measuring changes in an induced magnetic field to pinpoint flaws - to find three gaseous hydrogen valves in the NASA inventory that don't have the cracks.
Shuttle engineers and other experts have been conducting an exhaustive analysis of the valves after finding a piece had broken off one of them on a past shuttle mission in November 2008.
That damage wasn't discovered until after the mission had ended safely, but it raised concerns that debris from a similar mishap could cause a dangerous hydrogen leak in the shuttle orbiter's aft compartment.
The valves at the top of each of an orbiter's three main engines maintain pressure in the external liquid hydrogen tank at the proper level. In addition to finding three good valves, engineers determined that the chance a piece of debris could cause a leak was "extremely remote," and that if it did it would be too small to cause a fire hazard or lower the tank pressure below the 32 psi needed for the shuttle engines to operate properly.
"We really attacked this problem from all the different areas," Shannon said at the conclusion of the final flight readiness review for the STS-119 mission.
Weather permitting, Discovery will lift off at 9:20 p.m. EST March 11 on a 14-day mission to deliver the final 240-foot-long solar array wing to the ISS. NASA must get the mission under way by March 16 to have Discovery out of the way in time for the arrival of the next Soyuz mission at the station on March 28.
William Gerstenmaier, associate administrator for space operations, said the station program is prepared to give up two of four planned spacewalks on the mission if the launch goes that late. Those extravehicular activities (EVAs) would be shifted into a period when no vehicles are docked, and carried out by station crew members.
To ease that potential load on the station crew after the Soyuz departs April 7, the station program advanced a Russian-side EVA to March 10, when Expedition 18 Commander Mike Fincke and Flight Engineer Yuri Lonchakov will install a European space-exposure experiment on the station hull.
