NASA will delay the launch of its
next space shuttle mission until July in order to replace potentially faulty
fuel sensors inside the orbiter's massive external tank, the agency's shuttle
program chief said Tuesday.
Wayne Hale, NASA's shuttle program
manager, told reporters that the shuttle Discovery and its STS-121
return to flight mission will now launch no earlier than July 1,
weeks later than its earlier May
10-22 flight window.
The extra time will allow for an
invasive, three-week swap of four engine cut-off (ECO) sensors inside the
liquid hydrogen portion of the orbiter's 15-story fuel tank, he said.
"This is not a decision about
schedule," Hale said of the delay during a press conference at NASA's Johnson
Space Center in Houston, Texas. "This is a question of safety."
Discovery's STS-121 mission,
commanded by veteran astronaut Steven
Lindsey, is the second shuttle flight set to fly since the 2003 Columbia disaster that
killed seven astronauts. It is also the second test flight before NASA returns
to its International Space Station (ISS) construction launch schedule.
Hale said he remained optimistic
that NASA could launch three shuttle flights - STS-115 aboard Atlantis in late
August and STS-116 in the fall - by the end of 2006.
Critical sensors
ECO sensors
are designed to monitor shuttle fuel tank levels and shut down an orbiter's
three main engines before its fuel tank runs dry.
But recent studies found that wiring
defects in the manufacturing of some sensors could lead them to falsely report
a dry tank, which could force an early engine shut down before a shuttle
reaches its proper orbit, Hale said, adding that one such sensor on Discovery's
external tank has also been returning some errant readings.
"If a number of these sensors fail
to the 'dry' state they would shut the engines down early, prematurely, which
is not a good thing in spaceflight," Hale said, adding that the chances of that
occurring is admittedly remote. "This is what we call a criticality one, life
or death, kind of situation in that you want those sensors to work properly
either way...we need to have a good set."
Errant ECO sensor readings scrubbed
the attempted July 13 launch of NASA's first post-Columbia mission - STS-114 also aboard Discovery -
though the orbiter launched
13 days later without incident. Problems with the sensors also cropped up on a
separate tank during an April
2005 fueling test.
Hale said that unrelated problems
have led shuttle engines to shut down early in a previous orbiter launch during
NASA's STS-51F mission in 1985. A hydrogen leak impaired engine performance during
1999's STS-93 mission launch, he added.
Sensor swap
Tank engineers will replace all four
liquid hydrogen fuel ECO sensors on Discovery's External Tank-119 (ET-119)
which now sits inside NASA's 52-story Vehicle Assembly Building at the Kennedy
Space Center in Cape Canaveral, Florida.
The tank will be hoisted into the
vertical position, where engineers will remove its foam covered surface and pry
open a manhole at its bottom to gain access to the liquid hydrogen tank, Hale
said. After swapping out the old ECO sensors - which were built in 1996 - with
new ones, engineers will then close out the tank and set it back in a
horizontal cradle to reapply the foam insulation, he added.
"There are certain risks that you
might damage the tank," Hale said of the sensor swap operation.
Extra time
Discovery's launch window, which
runs from July 1 to July 19, also allows shuttle workers about six additional
weeks to close out several issues that have cropped up in the last few weeks.
Engineers will now be able to repair
Discovery's damaged
robotic arm - which was dinged inside the shuttle's hangar-like Orbiter
Processing Facility on March 4 while workers tried to clean up broken glass
from a light bulb that fell into the orbiter's payload bay. The arm is
instrumented to evaluate its performance in the upcoming flight, and would have
had to be replaced with an un-instrumented arm with out the extra work time, Hale
said.
Hale also said shuttle workers will
be able to replace outer windows on Discovery's cabin and remove additional
foam from ice frost ramps on the orbiter's fuel tank.
NASA has been working to reduce the
amount of foam insulation aboard shuttle fuel tanks since 2003, when a chunk of
foam fell from Columbia's tank and breached the heat shield along its left wing
leading edge, leading to the orbiter's Feb. 1, 2003 destruction. The problem
cropped up again during Discovery's launch when cameras caught unacceptably
large pieces of foam falling from the tank, primarily from a protective
ramp that has since removed from current and future tanks.
All of the work to be completed on
Discovery and its external tank should be completed well before the July launch
window, especially since the agency was pushing hard to make the May launch
window, Hale said.
"We were racing very hard to get to
the mid-May launch period," Hale said, adding that the extra six weeks could
allow some shuttle workers to take weekend rests in what has been a daily
effort to reach the May launch window. "I would expect it will allow us to
slacken the pace is other areas."
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