External Tank Draws Facebook Questions

Space shuttle Discovery's external fuel tank has taken on a bit of a starring role for the STS-133 mission because of the extra work needed to handle the unexpected challenges presented by the tank's stringers.
Technicians have been working on the stringers as the shuttle stands inside the Vehicle Assembly Building. Some of the 108 stringers, which are support beams that make up the corrugated intertank portion of the external tank, developed small cracks during fueling for a Nov. 5, 2010, launch attempt. Examinations performed following a fueling test revealed cracks on other stringers.

In considering the challenges posed by the stringers after a few cracks were discovered, NASA fans and the general public found an appetite for more information about the largest single component of a space shuttle "stack."

As part of NASA's continuing social media interaction, Facebook users were given the chance to ask Kennedy's external tank expert about the modifications and the one-of-a-kind role the tank plays. Kennedy's Facebook page has drawn about 60,000 friends, with some 130,000 following the center on Twitter.

Alicia Mendoza is NASA's External Tank and Solid Rocket Booster vehicle manager at Kennedy and in between preparing the tank for Discovery's targeted launch on Feb. 24, she discussed some of the things that went into designing the tank, why it looks the way it does and why it is not recovered for re-use the way the rest of the stack is.

To save time, the two dozen questions posted to Kennedy's Facebook page were trimmed to six that covered the broadest areas. First, a few basics about the tank:

At 15 stories tall and sporting an outer shell of dark orange insulation but with no engines of its own, the tank serves the shuttle's three main engines. It is actually two tanks on top of each other, with a support ring -- including the stringers -- holding them together. The tank holds about 535,000 gallons of liquid hydrogen and liquid oxygen. Both propellants are cryogenic, which means they are super-cold. The oxygen is chilled and pressurized to minus 297 degrees F in its liquid form and the liquid hydrogen buries the temperature needle to minus 423 degrees F.
A gallon of liquid oxygen weighs about 8.5 pounds while the hydrogen is much lighter, even in liquid form, at about half a pound per gallon. But even though the chemicals may not sound heavy by the gallon, the three main engines use so much propellant that carrying it all into orbit would make an impractically large and heavy orbiter.

Therefore, NASA designed the tank to be jettisoned just as the shuttle makes it into orbit. The tank descends through the atmosphere where some of it burns up and the rest crumples and falls harmlessly into the Indian Ocean.
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