NASA once
again fueled the space shuttle Discovery's external tank for Friday in a test
to iron out sensor and valve glitches.
Over the
course of 11 hours, shuttle workers pumped and drained more than 500,000
gallons (1.9 million liters) of supercold propellant into Discovery's external
tank, which stands upright with the rest of the launch stack at Kennedy Space
Center's Pad 39B in Cape Canaveral, Florida.
Unlike an
April 14 test,
engineers detected no fuel gauge sensor malfunctions during the fueling
process, but did monitor an identical pressure valve anomaly from the earlier
run.
"We had
just a perfect test as far as I'm concerned," said William Parsons, NASA's shuttle
program manager, during a post-test teleconference. "[But] we still have a lot
of data to look at."
Ripe weather
for launch
If
Discovery's STS-114 crew - NASA's first shuttle astronauts to fly since the
2003 Columbia disaster - had been aboard the orbiter, conditions would have
been ripe to launch them into space.
"It was a
good day," said Sandy Coleman, NASA's external tank project manager, during the
teleconference. "We could have launched today."
The
humidity today allowed more ice to form on Discovery's tank today than in the
previous test, but amounted to little more than frost, shuttle officials said.
Concerns
over the potential fatal danger posed by ice
debris shaking off the tank prompted shuttle engineers to devise a new
heater to limit the hazard.
Discovery
and the STS-114 crew are set to launch no earlier than July 13 on mission to
test new hardware and procedures expected enhance space shuttle safety. Their
spaceflight will also deliver vital consumables, science equipment and other
cargo to the International Space Station, where the Discovery crew plans to
perform no less than three space walks.
NASA's
space shuttle fleet has been grounded since Feb. 1, 2003, when the Columbia
broke apart during reentry, killing its crew. Investigators later found that
launch debris, which struck Columbia's left wing during liftoff, caused the
accident.
Potential
delay
Despite
today's test, Discovery will eventually fly with a completely different
external tank, one which already stands ready to accept the orbiter at the
massive 52-story Vehicle Assembly Building (VAB) just over four miles away from
the launch pad. The tank was original slated for NASA's follow-up mission to
STS-114, STS-121 aboard Atlantis.
But there
are no current plans to check that replacement tank as well by pumping it full
of the liquid oxygen and liquid hydrogen fuel Discovery consumes during
liftoff. NASA officials, however, could call for that extra test after
reviewing data from Friday's activity.
"Right now,
I don't have a third tanking test in my plan," Parsons said, conceding that
some shuttle officials believe such a test could be warranted, but have agreed
to look over the data from today's test first. "But I wouldn't rule it out
completely."
NASA launch
director Michael Lienbach said the addition of a third tanking test could push
Discovery up to a week into its current launch window, which runs from July 13
to July 31. If shuttle engineers push hard, however, they could accomplish the
feat by the window's opening, he added.
Shuttle
engineers will soon install the new heater to the expandable bellows portion of
a liquid oxygen feed line on Discovery's replacement fuel tank to prevent ice
buildup.
Another
concern revolving adequate clearance between Discovery's new orbital
boom - which astronauts will use to scan the orbiter for damage - and a
vital communication antenna has been addressed, NASA officials said, adding
that the separation between the two devices is slim during deployment.
"We've been
aware of this and we have a way to ensure we deploy the [boom]," Parsons said.
"I don't believe this is going to be a real big problem, this is just an
operational workaround."
Getting
the kinks out
Shuttle
managers ordered Friday's test to reexamine a set of four sensors that monitor
liquid hydrogen propellant levels inside Discovery's external tank to ensure
main engine cutoff during launch. The sensors make sure Discovery's engine shut
down before its fuel tank runs completely dry, NASA officials said.
Launch
restrictions require that all four sensors function properly before liftoff,
though two of the devices cut out in the April 14 test. Since then, pad
engineers have swapped out wiring and rechecked the connections running between
Discovery and its tank. The result, apparently, was a fully function sensor
suite during the second test fueling.
"The main
engine cutoff sensors worked perfectly," Parsons said. "It probably points to
the fact that...some kind of connection wasn't exactly right."
More to
unravel
One concern
not solved in the May 20 test revolved around a pressurization valve at the
very top of Discovery's external tank. The valve apparently cycled the exact
number of times - 13- as it did during the April 14 tanking test, which is just
shy of breaching launch restrictions for an actual space shot. In a normal
launch, the valve cycles between eight and nine times.
"We've got
to understand it," Coleman said of the glitch. "We don't like flying right on
the edge."
Coleman
said one potential culprit is a defuser, a sort of mesh screen that that sits
at the very top of the hydrogen tank and disperses fuel in a manner that
prevents hot spots from developing on the tanks aluminum skin. The defuser on
Discovery's tank is different from those on all past external tanks and will
eventually be swapped out with an older version, she added.
Lienbach
said that Discovery will begin to make the 4.2-mile (6.7-kilometer) journey
back to the VAB no earlier than May 24, after any remaining hydrogen has boiled
out of its external tank.
There, the
171,000-pound (77,564-kilogram) orbiter will be demated from its launch stack
and attached to its new one. The enter assembly should once again reach the
launch pad by mid-June, shuttle officials said.
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