NASA space shuttle managers decided April 20 to adjust the flight plan for the upcoming STS-121 mission aboard the shuttle Discovery, a move that maximizes the agency’s 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 what NASA calls a “Low Q” flight profile that subjects the orbiter and its fuel tank to slightly lower aerodynamic stresses during launch than what it would normally experience, NASA spokesman Kyle Herring said April 21.

NASA also is evaluating a series of recent wind tunnel tests that were conducted to check the effect of modifications to the shuttle fuel tank that involved the removal of a ramp that connects the tank to the orbiter.

“It certainly made sense to do that,” Herring said of the decision to change the flight profile. Herring added that the change gives Discovery a wider margin for the launch because the low flight profile will make it marginally easier to launch in high winds, a key concern in July when the region surrounding Kennedy Space Center (KSC) is prone to stormy weather. Discovery’s launch window is July 1-19. “The biggest benefit is launch probability, and that’s what we want,” Herring said.

During shuttle launches, the orbiter’s three main engines are throttled down for a short time to reduce stress 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, wind tunnel tests at the U.S. Air Force’s Arnold Engineering Development Center in Tullahoma, Tenn., on an external tank — in which a protective protuberance air load (PAL) ramp had been removed — 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 that connect 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 now is undergoing cryogenic tests to determine whether ice — potential launch debris hazard — could form on the now-more-exposed region, Herring said.

Shuttle officials decided in December to remove the PAL ramp, an 11-meter covering that protects pressurization lines from aerodynamic stresses during launch, after a similar ramp shed large pieces — including a 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 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 re-entry.

The upcoming STS-121 launch is NASA’s second post-Columbia mission, 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 all 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 were expected to discuss plans to mate External Tank-119 to Discovery’s twin solid rocket boosters late last week, NASA officials said. If given the go-ahead, engineers could begin uniting the two launch stack components as early as April 24, they added.