CAPE
CANAVERAL, Fla. - NASA's plans to launch the space shuttle Discovery are
stretching further into July as engineers struggle to understand a fuel sensor
anomaly that scrubbed an attempted space shot Wednesday.
If the
problem is something as simple as a case of loose wiring, NASA could launch
Discovery - its first orbiter to fly since the 2003 Columbia tragedy - at about
2:14:21 p.m. EDT (1814:21 GMT) on Sunday, July 17, but the chance of that is
remote at best, NASA officials said here Thursday.
"If we were
to get extremely lucky, it is theoretically possible we could still launch on
Sunday," said Wayne Hale, deputy manager of NASA's shuttle program, during a
press briefing here at Kennedy Space Center (KSC). "This represents a really
optimistic, good luck scenario which I think is not very credible."
Shuttle
engineers are more likely to spend several days troubleshooting Discovery's
external tank fuel sensor system in hopes of isolating the problem, Hale added.
In that
case the launch could push into early next week, shuttle officials said. While
Discovery's STS-114 astronaut crew - which was strapping into the orbiter when
their launch was scrubbed - will stay on at KSC for the time being, they will
likely return to NASA's Johnson Space Center if the liftoff date stretches
further out, they added. Discovery must launch by July 31 to make its current
launch window. The next opportunity to launch after that runs between Sept.
9-24.
Discovery's
mission management team expects to meet at 3:30 p.m. EDT (1030 GMT) Friday for
an update on roubleshooting plans for Discovery. A press briefing is expected
pending that meeting, NASA officials said.
During July
13 countdown for Discovery's STS-114 mission, launch controllers detected a
failure in one of four engine cut-off (ECO) sensors in the liquid hydrogen
section of the orbiter's external tank. In a countdown test designed to force
the sensors to report an empty fuel tank, one sensor reported it full - or 'wet'.
Since all four must perform perfectly in order to launch, flight controllers
called off the launch attempt.
The sensors
track the amount of liquid hydrogen propellant so the orbiter can shut down its
three main engines before propellant runs out. A similar sensor suite performs
the same function for the external tank's liquid oxygen supply.
As pad
engineers emptied Discovery's fuel tank late Wednesday, the faulty fuel sensor continued
to report a full tank despite the absence of liquid hydrogen. In later tests,
the sensor reported the proper, dry tank status.
In even
more subsequent tests, engineers were able to get the sensor to again report a full
external tank before it settled into normal operations, Hale said.
"We now
have an intermittent, transient kind of failure which is the worst kind of
thing to troubleshoot," Hale said.
Engineers
are still unclear whether the fuel sensor glitch resides with the actual sensor
itself, the wiring between it and Discovery or the sensor box inside the
orbiter that processes the fuel readings. The box does contain transistors that
engineers have found faulty in the past, but the malfunction seen Wednesday is
not indicative of a transistor failure, shuttle managers said.
"The
indications are not consistent with what a transistor failure would indicate,"
Hale said. "That's part of the trouble here...if we really thought that was the
smoking gun, I think we'd have something to go after."
For now,
all troubleshooting plans assume that fixes to Discovery's ECO sensor system
can be made at the pad, with mission managers hoping they will not have to roll
the shuttle launch stack back to the 52-story Vehicle Assembly Building for work or send engineers inside the fume-filled liquid hydrogen tank.
"We would
like to not go into the liquid hydrogen tank," Hale said, adding that special
precautions and care must be taken to ensure engineers safe access. "It becomes
an operation you'd just as soon not do."
Shuttle
officials and engineers hope to determine a specific troubleshooting plan for Discovery
tonight and decide whether to begin unloading the orbiter's cryogenic fuel cell
reactants to allow pad workers safe access to its aft section where the sensor
box resides.
"At this
point we are following the normal 72-hour scrub turnaround process," Michael
Wetmore, NASA's director of space shuttle processing at KSC, told reporters in
the briefing.
But NASA
cannot keep its launch team on alert indefinitely, and if the mission is not
expected to launch by early next week, the team will shift away from a
countdown hold status and into full troubleshooting mode.
"Early next
week is the break point at which we need to get out of the countdown procedure and
get everybody some crew rest," Wetmore said.
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