NASA's next space shuttle will launch no earlier than July 1 and without extra modifications to its already redesigned external fuel tank, the space agency said Friday.
The space shuttle Discovery's STS-121 mission, NASA's second test flight since the 2003 Columbia accident, will not fly with modified, foam-covered ice frost ramps that cover the brackets between vital external tank plumbing lines and its orange exterior.
"It is not without risk to fly these ice frost ramps as they exist," said NASA space shuttle program manager Wayne Hale during a Friday press briefing. "[But] it is more appropriate to make one change at a time, to take care of the biggest problem that we have."
Discovery's external tank has already been retrofitted to fly without a 38-foot (11-meter) protuberance air load (PAL) ramp that once covered the pressurization lines and cable tray that run through the ice frost ramps.
"That change constitutes the largest aerodynamic change that we have made to the space shuttle launch system since it first flew," Hale said, adding that analysis of the PAL ramp fix on the entire launch system will likely continue well towards the liftoff date.
While NASA is targeting July 1 to launch Discovery, the STS-121 mission has a flight window that stretches through July 19.
Ice frost ramp debate
After a series of wind tunnel tests, NASA concluded - with much debate among shuttle engineers - to press ahead with Discovery's July launch, shuttle managers said.
"It was a mixed conclusion," Hale said. "There were opinions on both sides."
Shuttle officials know that ice frost ramps can shed between two and 3.5 ounces (56 to 99 grams) of foam debris during launch, and admit that there is an element of risk associated with a July launch.
In a worst case scenario, in which a 3.5-ounce piece of foam pulls free from a key ice frost ramp at the worst time and inflicts the most damage, it could prove a dangerous hazard to the shuttle and its crew, NASA said.
"It would cause what we call critical damage," Hale said.
Ken Welzyn, NASA's external tank chief engineer at Marshall Space Flight Center in Huntsville, Alabama, said there are 34 ice frost ramps on each shuttle fuel tank. Of those, only the top four along the tank's liquid hydrogen-bearing section are a debris concern during launch, he added.
"Thermally, they warm up at time in the flight that debris poses a risk to the shuttle," Welzyn said.
But NASA does not currently have a viable redesign in hand for the ice frost ramps, shuttle officials said.
"I expect in the next month to six weeks, we will have a really good design that we will implement on subsequent tanks," Hale said, stressing that schedule pressure and the need to complete the International Space Station (ISS) by 2010 did not push shuttle managers to proceed without an ice frost ramp redesign.
"That didn't drive this particular discussion," Hale said. "We're trying to make appropriate decisions in light of the schedule, and not let it drive us to overly risky or foolish decisions just to make a schedule that we know has some time in it to allow for engineering problems to be solved."
Reducing tank foam debris
NASA has been working to reduce the shedding of dangerously large pieces of fuel tank foam insulation since the 2003 Columbia disaster, in which seven astronauts and one orbiter were lost during reentry.
A 1.67-pound (0.8-kilogram) piece of foam breached Columbia's heat shield at launch, leaving it vulnerable to hot atmospheric gases during reentry. A similar foam shedding event occurred during NASA's STS-114 return to flight mission, when a one-pound (0.4-kilogram) piece of foam popped free from the PAL ramp aboard Discovery's tank and flew past the orbiter's heat shield without striking it.
"We do have a serious concern with debris, particularly debris coming of the external tank," Hale said.
Space shuttle external tanks are covered with about 4,000 pounds (1,814 kilograms) of foam insulation covering their aluminum hulls to prevent ice-buildup - also a potential launch debris source - due to the super-cold liquid oxygen and liquid hydrogen they contain. Most of that foam is applied robotically, though about 25 percent is still applied by hand, NASA said.
The PAL and ice frost ramp foam is among the hand-applied material.
From STS-121 to retirement
Commanded by shuttle veteran Steven Lindsey, NASA's STS-121 mission will complete a series of shuttle repair and safety tests required before the space agency resumes ISS construction.
The STS-121 crew will perform at least two spacewalks - and possibly a third if shuttle resources permit - as well as ferry ISS crewmember Thomas Reiter and a fresh load of supplies to the orbital laboratory.
Hale said he was confident NASA could complete the ISS in the planned 16 ISS-bound flights before the space agency retires its shuttle fleet in 2010. A 17th shuttle flight could service the Hubble Space Telescope by 2008, NASA has said.
NASA chief Michael stressed that 2010 is a fixed deadline for the shuttle program.
"We have to pick a year which will be the last year we will fly shuttle flights and stick with that," Griffin said in the briefing. "And that's what we're doing."
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