Shuttle managers, needing more time to fully understand a valve problem that forced a launch scrub Aug. 25, skipped over a launch window to set their sights on a liftoff just before midnight Aug. 28.

The skipped launch attempt would have taken place at 12:22 a.m. Aug. 28. Instead, the shuttle's Mission Management Team (MMT) will reconvene Friday afternoon to decide whether to begin fueling the orbiter for a launch attempt at 11:59 p.m. this evening.

On Aug. 26, technicians at Launch Pad 39A at Kennedy Space Center performed cycling tests on the PV12 inboard liquid hydrogen fill-and-drain valve in the aft of shuttle Discovery in an attempt to determine whether the valve or its position indicator is faulty. These tests took place at ambient temperatures with the tank empty, as opposed to the cryogenic temperatures of fueling.

The Aug. 25 launch attempt was abandoned late in the tanking process when an indicator for the pneumatically operated valve showed that it did not close when commanded to. The 8-inch valve - through which cryogenic propellant flows both during tanking and de-tanking - must be closed for liftoff.

During the tests, the valve was cycled five times, and each time behaved properly, with the indicator showing the valve as closed. Other tests also were conducted to verify that the valve was actually closed, and that the valve position indicator was reading correctly. Previous problems with this type of valve have been linked to metallic debris created by the grinding of the metal within the mechanism.

Ironically, the seemingly perfect performance "gave some of the team pause," MMT Chairman Mike Moses said during a press conference Aug. 27. Some were concerned over the fact that the performance at ambient temperatures was not corresponding to the performance at cryogenic temperatures.

Even going on the assumption that the indicator is the problem rather than the valve itself, managers decided they needed more time to refine their understanding of the situation, so that if the valve indicator once again shows an open reading after it is commanded to close at the end of tanking, there will be enough data on secondary indicators of the valve's position - such as actuator cycle times - that could give the team enough confidence that the valve is indeed closed to proceed with launch.

Meanwhile, NASA says evaluation of the low-level hydrogen leak detected in the lefthand tail service mast on the shuttle's launch platform following Tuesday's scrub is complete, and no leaks were detected (Aerospace DAILY, Aug. 27).

The 13-day STS-128 mission will deliver supplies and scientific equipment to the International Space Station (ISS). If Discovery can't launch by Sunday, Aug. 30, in all likelihood it will have to stand down until October to avoid conflicts with other missions going to and from the ISS, including Japan's H-II Transfer Vehicle.

Discovery photo: NASA