NASA will
refill the space shuttle Atlantis' large external tank next week in hopes of
isolating a sensor glitch that foiled two attempts to launch an orbital construction
mission this month, the agency's shuttle chief said Tuesday.
Wayne Hale,
NASA's space shuttle program manager, said a team of engineers will outfit
Atlantis' 15-story fuel tank with new monitoring equipment and then fill it
with super-cold propellant on Dec. 18 to better understand why some fuel
gauge-like sensors failed during the shuttle's launch attempts.
"We
have hopes that it will yield the definitive location of our problem and
thereby allow us to solve it," Hale said of the test in an afternoon
teleconference.
Three of
the four fuel gauges, known on engine
cut-off sensors, in the liquid hydrogen portion of Atlantis' external tank failed
standard checks during the shuttle's first launch attempt last Thursday. When
the problem resurfaced on Sunday, NASA postponed Atlantis' mission to deliver a new
European-built lab to the International Space Station (ISS) to no earlier
than Jan. 2.
"We're
going to go find out where this problem is, whether it's the sensors, the
wires, the connectors, what have you," said Hale, adding that other
engineering teams will perform bench tests of individual circuits in parallel
with the fueling test.
During text
week's test, engineers will splice in monitoring equipment into circuits for
the four erratic hydrogen sensors and a fifth sensor that detects when the tank
is about 5 percent full, Hale said.
The
approach uses the same tools as those used on Earth to find breaks in telephone
and cable television lines, and will hopefully trace where the glitch
is hiding among the 100 feet (35 meters) of wiring between the fuel tank
sensors and an electronics box inside Atlantis, he added.
The method
has been used with empty shuttle external tanks in the past, but Tuesday's test
will mark the first on a tank that is fully fueled with more than 500,000
gallons (1.9 million liters) of super-cold liquid hydrogen and liquid oxygen
propellant. Unlike past fueling tests, engineers will be on Atlantis' launch
pad to help monitor the new equipment, Hale said.
Based on
1960s technology developed for NASA's Apollo moon missions, the engine cut-off
sensors are designed as a backup system to shut down a space shuttle's three
main engines before their external tank runs dry of propellant. If the engines continue
to fire without fuel, they could rip apart and cause catastrophic damage, NASA
has said.
Intermittent
glitches with the sensors have afflicted NASA shuttle launch attempts since
2005, when the agency resumed orbiter flights following the Columbia accident, most
recently in September 2006.
Engineers
traced the malfunctions to what appeared to be faulty connectors on some
sensors, replaced them with a new batch and installed a set of voltage monitors
that ensure the new devices work properly. But the source of the glitch's
reappearance aboard Atlantis remains an enigma.
NASA hopes
to root out the sensor problem once and for all before forging ahead with the
up to 12 planned shuttle missions to complete the ISS, as well as one more to
upgrade the Hubble Space Telescope. The agency is facing a looming September
2010 deadline to complete the ISS and retire its three-shuttle fleet.
"Until
we come to the bottom of this mystery, that we are in no better shape for
launching any other orbiter and any other tank than we are with this orbiter
and tank," Hale said. "So we need to find the solution."
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