CAPE CANAVERAL, Fla. -- A sluggish fuel valve would have triggered a show-stopping halt to shuttle Discoverys launch a mere 9.7 seconds before a planned liftoff Thursday, prompting a lengthier delay in the postponed flight.
Shuttle Discovery waits on the launch pad on Thursday before NASA was forced to scrub a launch attempt later in the day. NASA image.
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But the serendipitous discovery of the problem will enable NASA to swap out the valve and press ahead with plans to launch the agencys milestone 100th shuttle flight Monday night.
If, that is, engineers can convince mission managers that a suspect bolt on the shuttles 15-story external tank wont trigger a collision between Discovery and the bullet-shaped fuel reservoir in orbit.
Thats the story today as NASA began an effort to replace the balky valve, a tedious job that prompted NASA to delay Discoverys flight for four days.
A planned launch attempt Thursday was called off when engineers discovered a potential problem with the fuel-tank bolt. The subsequent discovery of the valve problem then scuttled initial plans to make a launch attempt tonight.As it stands now, Discovery and its crew -- which includes six U.S. astronauts and a Japanese mission specialist -- are tentatively scheduled to blast off on an International Space Station construction mission at 8:05 p.m. Eastern Daylight Time Monday (Tuesday, 00:05 GMT).
~Close quarters work
Working in the shuttles cramped rear engine compartment, technicians today will replace a main propulsion system valve that controls the flow of liquid hydrogen and liquid oxygen into the shuttles three powerful main engines.
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The valve is one of two so-called -- Pogo Accumulator Recirculation Valves -- that are designed to make certain the flow of propellant into the engines remains constant, rather than spurting in surges.
A surging flow would alternatively slow and speed up a soaring shuttle during what would be a rough 8.5-minute ride toward orbit.
"Pogo is not a NASA acronym -- it is rather a reference to a pogo stick that a kid would jump up and down on," said veteran astronaut Jim Halsell, now a senior shuttle program manager based here at NASAs Kennedy Space Center (KSC).
"And thats exactly the feeling that you would get if you were riding this rocket and the accumulator did not work properly," he added. "It would feel all the world like you were jumping up and down on a pogo stick."
Such a bumpy ride potentially could leave Discoverys crew in a lower-than-intended orbit. And in that case, the shuttle might not have enough fuel to make up the shortfall and dock with the International Space Station -- a link-up obviously critical to mission success.
The valve in question began operating sluggishly during routine scrub turnaround work after Thursdays launch attempt was called off.
NASA Launch Director Mike Leinbach said engineers discovered that the valve was opening more slowly than it was supposed to during a trio of tests.
The valve is designed to open 12.5 seconds before a planned liftoff. In Discoverys case, the shuttles on-board computers would have to detect its sluggish operation 9.7 seconds before liftoff, prompting an immediate cutoff of the countdown.
~Suspect bolt
Coming hand in hand with the valve replacement work will be an ongoing analysis of the potential problem with the suspect bolt, which plays a key role in the shuttles external-tank separation system.
During a standard postflight film review, engineers discovered that an identical bolt failed to retract properly after the September 8 launch of shuttle Atlantis on a mission to outfit the International Space Station for the arrival in early November of its first full-time crew.
The 14-inch (35.5-centimeter) bolt is one of several that attach the shuttle orbiter to its external tank, which holds the 500,000 gallons (1.9-million liters) of propellant needed to power the shuttles main engines.
Equipped with an explosive nut, the bolt is designed to retract back into the external tank as the massive fuel reservoir is jettisoned from the shuttle about nine minutes into flight.
The way the system is designed, small explosive charges break the nut apart and the then bolt draws back into place with the tank. The expendable tank then makes a destructive plunge back through the atmosphere.
On Atlantis flight, the nut broke apart as expected, but the bolt failed to fully retract into the tank. Consequently, about 2 and one-quarter inches (5.72 centimeters) of the bolt was left sticking out of the tank when it was jettisoned from the shuttle.
NASA managers are concerned that a protruding bolt could come into contact with a shuttle orbiter and cause the massive tank to pitch up and collide with the ship -- a potentially dangerous situation for an astronaut crew.
Engineers hope to prove that the probability of an orbital collision is so slim that Discoverys crew can be given a green light for flight.
An analysis of the bolt problem will continue through the weekend. Engineers then will brief mission managers Monday morning.
At that point, NASA will decide whether to gas up Discovery for a launch attempt Monday night. The other options: Delaying the launch for further analysis, or hauling Discovery back to KSCs Vehicle Assembly Building for close-up inspections of the bolt in question.
The latter option -- now considered a worst case scenario -- likely would delay Discoverys flight for several weeks.
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