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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