Engineers plan to begin tests late this week to verify the health of helium regulator assemblies downstream from a failed isolation valve in the shuttle Discovery's right-side orbital maneuvering system rocket pod. The valve cannot be replaced or repaired at the launch pad, but if the regulators check out - and no other problems crop up - mission managers could opt to launch Discovery as is next month, relying on the system's redundancy to justify proceeding with flight.

Discovery currently is targeted for launch April 5 on a three-spacewalk space station assembly and resupply mission. If shuttle managers cannot get comfortable with a "fly-as-is" scenario, Discovery likely would have to be hauled to its hangar for removal of its right OMS pod, work that would trigger a significant launch delay.

In the meantime, NASA is pressing ahead with launch processing.

During flight, helium is used to push propellants to the OMS pod's rocket thrusters. During propellant loading overnight Friday, one of two electrically driven helium isolation valves downstream of the tank used by the right-side pod's reaction control system steering jets either failed in the open position or failed to seat, resulting in a major leak.

The isolation valves feed two downstream helium regulators before the two lines recombine. If the regulators work properly, the failed isolation valve would not be an issue beyond the loss of one level of redundancy. And even if subsequent failures occur, a downstream pressure relief disk would prevent the OMS pod's steering jets from being over-pressurized.

As such, the failed isolation valve is not viewed as a "safety of flight" issue, officials say. But subsequent failures could prevent Discovery's crew from completing its planned mission to the International Space Station.

Before managers can discuss possible flight rationale - NASA-speak for the justification needed to fly with a known problem - engineers must make sure the helium regulators are working normally. A full-up regulator test has not been carried out in years, officials say, because the devices have operated in near-flawless fashion. To test Discovery's, engineers first must remove test equipment from launch pad 39B and install it at pad 39A where Discovery is being processed for flight.

"The test is expected to occur late this week once a ground support equipment test panel has been calibrated," according to a NASA statement. "The helium system will be brought to flight pressure and engineers monitoring the panel will ascertain whether the regulators function properly.

"Managers also are discussing when to take the STS-131 payload to the launch pad and whether to continue with its installation. As of now, the payload will go to the pad Wednesday."

Discovery's crew plans to install an ammonia coolant tank on the space station's main power truss and carry up a logistics module loaded with science gear, crew equipment and needed supplies. Assuming a launch on April 5, Discovery would land back at the Kennedy Space Center on April 18.

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