Spaceflight Now: STS-92 Mission Report

Replacement of shock absorbing fuel valve begins
BY WILLIAM HARWOOD
SPACEFLIGHT NOW

Posted: October 6, 2000

  Discovery
Discovery before dawn on Thursday. Photo: NASA
 
NASA managers say work to replace a sluggish valve in the shuttle Discovery's engine compartment should be complete by late today, setting the stage for a delayed launch attempt at 8:05 p.m. Monday.

But that assumes ongoing work to assess a separate problem -- concern about a suspect bolt in the shuttle's external fuel tank -- can be resolved by Monday morning.

It was that issue that stopped Discovery's countdown just before fueling Thursday and prompted a one-day delay. While backing out of the countdown, engineers noticed sluggish behavior in a 2-inch valve in the orbiter's main propulsion system.

The valve was cycled three times and each time, it moved slower than expected. As a result, NASA managers Thursday evening ordered engineers to replace the valve, delaying launch to Monday.

"After much, much technical discussion, it was decided the prudent and responsible thing to do was to stand down ... to change out the valve so we could launch with a pristine system," said James Halsell, director of shuttle operations at the Kennedy Space Center.

Shuttle launch director Michael Leinbach said engineers opened Discovery's aft engine compartment earlier today. If all goes well, the valve will be replaced by the end of the second shift and leak tested.

Normally, such a repair would require a time-consuming helium signature test to make sure the propulsion system remained leak free after the work was complete.

But Leinbach said a waiver will be signed to bypass the system-wide helium test in favor of two leak checks focused on the valve and the eight-bolt flange where it is attached.

"It does require a waiver. We are breaking into the liquid oxygen system," he said. "But it's a very simple mechanism, a very simple flange construction and we feel very comfortable that a bubble soak leak check and a mass spectrometer leak check around that flange is adequate for this situation."

The so-called pogo accumulator recirculation valve is one of two in a system that prevents propellant from surging through the fuel lines during ascent. Without such a shock absorbing system, the shuttle would experience high vibrations during the climb to space.

  Discovery
The first launch attempt was not to be as NASA scrubbed Discovery on Friday. Photo: NASA
 
The valve in question is normally opened a few seconds before main engine ignition. Had NASA not stopped the countdown Thursday because of the external tank bolt issue, the flight would have been automatically scrubbed when the countdown computer discovered the problem.

"It would normally be opened in the launch countdown at T-minus 12-and-a-half seconds," Halsell said. "If it had improperly worked at that point in time, the computers would have realized it shortly thereafter and would have shut down the count just prior to (engine ignition)."

While the valve replacement work is relatively straight forward, Halsell would not speculate on the ultimate outcome of the external tank issue.

During analysis of film shot by an automatic camera during the shuttle Atlantis's climb to orbit last month, engineers noticed that one of two huge bolts holding the bottom of the tank to the orbiter failed to retract into the tank properly at tank separation.

Engineers are concerned that a hung bolt could remain inside the shuttle attach fitting long enough for the orbiter to impart a force on the tank during separation. If so, the tank could possibly hit and damage the shuttle during the separation sequence.

Halsell said three teams of engineers investigating the issue have been given additional time to review past flights, the design of the bolt retraction system and possible workarounds.

The teams now are scheduled to report the results of the analysis to senior managers just before fuel loading Monday.

"Our current plans," Halsell said, "would be to have a review meeting on Monday morning to understand where they are with their analysis and if they're at a point where they can make the engineering community and also the mission management community feel comfortable that it's safe to go fly."

If all goes well, Discovery's countdown will resume at 5:10 a.m. Monday at the T-minus 11-hour mark. The astronauts will be awakened at 7:30 a.m. and NASA managers will meet around 9:10 a.m. to discuss the shuttle's readiness for flight.

Engineers will begin pumping a half-million gallons of supercold liquid oxygen and liquid hydrogen rocket fuel into Discovery's external tank around 10:45 a.m. The process will take about three hours to complete.

NASA television coverage will begin at 2:30 p.m. The astronauts will suit up and head for pad 39A at 4:15 p.m. and the shuttle's side hatch should be closed and locked for flight by 6 p.m.

A 10-minute hold at the T-minus 20-minute mark will begin at 6:50 p.m. and end at 7 p.m. A final 45-minute hold at the T-minus nine-minute mark will begin at 7:11 p.m.

Assuming the current launch time -- 8:04:58 p.m. -- holds up, the countdown will resume at 7:56 p.m. and the 100th shuttle flight will finally be on its way.