Lockheed Martin technicians in the Vehicle Assembly Building at NASA's Kennedy Space Center begin to apply new foam over the manhole cover on the lower end of external tank No. 119. The manhole was removed to access the area where the tank's four liquid hydrogen engine cutoff sensors were replaced.
Credit: NASA/Jim Grossman.
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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