NASA managers today delayed the shuttle Endeavour's launch yet another day, from Tuesday to Wednesday, because of time need to bring in equipment to test a replacement nitrogen valve in the ship's left-side orbital maneuvering system rocket pod. The valve is in a regulator used to supply the pressurized nitrogen gas needed to operate other valves that feed propellant to the left-side OMS engine.

The regulator valve replacement work began this morning and if all goes well, engineers will be ready Monday to top off the shuttle's on-board supplies of liquid hydrogen and oxygen to power Endeavour's electrical generators. With a full load of fuel cell reactants, Endeavour's crew will be able to make three launch attempts in four days, starting Wednesday.

But the weather forecast is uncertain, with meteorologists calling for a roughly 50-50 chance of good weather. Conditions are expected to worsen Thursday and Friday.

NASA's mission management team will meet again Monday to assess the valve replacement work and an updated forecast for Wednesday and later in the week. While Wednesday is the current launch target, nothing is set in stone at this point and the date could change again depending on a variety of factors.

Endeavour originally was scheduled for launch last Thursday, but the countdown was called off because of stormy weather. Late in the countdown, engineers noticed an apparent leak in the OMS-pod nitrogen valve, but additional tests indicated the problem had cleared itself. In any case, the countdown was recycled for Friday, but the next day NASA managers ruled out that option because of a dismal weekend forecast. Launch was re-targeted for Monday.

Overnight Friday, however, engineers noticed additional leakage in the OMS pod regulator valve and early Saturday, NASA managers ordered the valve replaced and delayed launch to Tuesday. The unexpected time needed to bring in test equipment forced today's delay to Wednesday.