NASA space shuttle managers decided
Thursday to adjust the flight plan for the upcoming STS-121
mission aboard Discovery, a move that maximizes launch options while reducing
stress on the orbiter's external
fuel tank, space agency officials said.
Discovery's next launch - currently
set for no
earlier than July 1 - will follow a "Low Q" flight profile that subjects
the orbiter and its fuel tank to slightly lower aerodynamic stresses during
launch, NASA spokesperson Kyle Herring told SPACE.com Thursday.
NASA is also evaluating a series of
wind tunnel tests to check shuttle fuel tank modifications that removed
a foam insulation ramp from the structure.
"It certainly made sense to do
that," Herring said of the flight plan decision, adding that the change also
gives Discovery a wider margin to launch around thunderstorms and high winds
from NASA's Florida-based Kennedy Space Center (KSC) spaceport earlier in its
July 1-19 window. "The biggest benefit is launch probability, and that's
what we want."
During shuttle launches, the
orbiter's three main engines are throttled down for a short time to reduce
stress on the spacecraft as it passes through the denser portion of the Earth's
lower atmosphere. In Low Q flight profiles, the orbiter throttles down its
engines earlier, and for longer, during ascent.
"We've flown Low Q for almost every
mission until we started flying
to the Mir Space Station and we wanted to maximize our ascent performance
and payload weight to orbit capability," Herring said, adding that the Low Q
plan does reduce the weight Discovery can haul into orbit.
Meanwhile, early external tank wind
tunnel tests - in which a protective protuberance air load (PAL) ramp has been removed -
at the U.S. Air Force's Arnold Engineering Development Center in Tullahoma,
Tennessee kicked off two sizeable chunks of foam insulation from ice-frost
ramps, NASA officials said. Following that test, engineers shaved additional
foam insulation from the ice-frost ramps, which appeared to improve
performance.
"They got good results from that,"
Herring said of the second modification.
Ice-frost ramps cover brackets
connecting a tray of cables and pressurization lines to the external tank exterior.
The latest ice-frost ramp modification left a thin layer of protection over the
brackets, which is now undergoing cryogenic tests to determine whether ice - a
potential launch debris hazard - could form on the now more exposed region,
Herring said.
Shuttle officials decided in
December to remove the PAL ramp, a 38-foot (11-meter)
covering that protects pressurization lines from aerodynamic stresses during
launch, after a similar ramp shed
large pieces - including a one-pound (0.4-kilogram) chunk - during
Discovery's STS-114 spaceflight
in July 2005.
NASA has focused on reducing the
shedding of large foam debris from shuttle tanks since the 2003 Columbia accident, in
which a 1.67-pound (0.8-kilogram) piece of foam breached the orbiter's heat
shield at launch and led to the loss of the vehicle and its astronaut
crew during reentry.
The upcoming STS-121 launch is
NASA's second post-Columbia accident spaceflight, as well as the final test
flight before the agency resumes construction
missions to the International
Space Station. NASA's three remaining orbiters - Discovery, Atlantis and
Endeavour - are due for retirement
in 2010.
Herring said that shuttle fuel tank
wind tunnel tests should continue for another two weeks, with the resulting
analysis expected to continue through early June. Any changes can then be applied to
Discovery's External
Tank-119 at KSC, he added.
KSC shuttle managers are expected to
discuss plans to mate ET-119 to Discovery's twin solid rocket boosters later
today, NASA officials said. If given the go-ahead, engineers could begin
uniting the two launch stack components on Monday, they added.
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