Shuttle engineers prepare to test liquid hydrogen valve

Shuttle engineers prepare to test liquid hydrogen valve
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 26, 2009

Engineers finished draining the shuttle Discovery's external tank Wednesday and geared up to remotely test a suspect valve in the orbiter's engine compartment to find out whether the shuttle can be launched "as is" early Friday or whether the flight might face a more significant delay, possibly to mid October.

Credit: NASA TV

During fueling Tuesday for a planned 1:10 a.m. EDT Wednesday launch, readings from Discovery's inboard liquid hydrogen fill-and-drain valve indicated it failed to fully close on command when the shuttle's external tank was nearing a full load of propellant.

Engineers suspect a position sensor, part of a valve actuator system, might be the culprit and not the valve itself. To find out, they planned to cycle the 8-inch valve in a series of tests at ambient temperature Wednesday evening to determine if the valve is, in fact, operating normally.

But first, engineers had to finish draining the huge tank after Tuesday's interrupted countdown, a procedure that was completed around 1:40 p.m. A final procedure to "inert" the tank, flushing out residual hydrogen, was expected to be complete by 5:30 p.m. Valve testing was expected to begin after that.

If test data shows the valve functions normally, and if NASA managers can get comfortable launching Discovery without full instrumentation in a critical system, Discovery could be ready for a third launch attempt Friday at 12:22:11 a.m. A decision on whether that course of action is a viable option is expected late Wednesday.

If so, NASA's Mission Management Team will meet Thursday at noon to discuss a waiver that would allow the agency to proceed using alternative data to make sure the valve is properly positioned. A scheduling assessment shows the fly-as-is option is the only approach currently under discussion that gives NASA a chance of getting Discovery off the ground before its current launch window closes Sunday.

Window considerations aside, if engineers determine the position sensor and actuator must be replaced, the scheduling timeline shows the repair work would delay launch to Sept. 3. The schedule shows actuator replacement work Friday and Saturday, extensive retest and work to close out the aft compartment before the start of a new countdown Monday.

If the valve itself must be replaced, launch would slip to around Sept. 5 to accommodate the more invasive work in the engine compartment. In that case, engineers would not be ready to start a new countdown until next Wednesday.

But Discovery's current launch window only runs through Sunday, Aug. 30. The Air Force Eastern Range, which provides tracking and telemetry support, is unavailable from Aug. 31 through the next several days because of an unidentified, presumably military operation.

Assuming nothing changes on that front, Discovery's crew then would have to wait on upcoming Japanese and Russian space station missions. As it now stands, the next shuttle launch opportunity after Aug. 30 is around Oct. 17.

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The shuttle is equipped with four propellant fill-and-drain valves: inboard and outboard liquid hydrogen valves and two corresponding liquid oxygen valves. When the shuttle is loaded with fuel before launch, the liquid hydrogen and oxygen propellants are routed through launch pad tail service masts on each side of the shuttle, into the engine compartment, through the main engine plumbing and into the external tank.

The fill-and-drain valves are closed before launch and then opened in orbit, after external tank separation, to rid the engine lines of residual propellant that may be trapped in the system. The valves also must be opened to drain the external tank if a launch is delayed.

The inboard fill-and-drain valve must be operational and confirmed closed before launch.

NASA flight rules forbid cycling the valve for troubleshooting once fueling has started and the hardware is in a supercold cryogenic environment. The concern is that additional cycles could contribute to an existing problem, possibly causing galling and creating debris that could cause the valve to break or get stuck in the closed position.

Testing the valve in ambient conditions puts less stress on the hardware and should provide insights into its actual operation.
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