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External Tank Draws Facebook Questions
The external tank with boostersThe dark orange external tank is the largest component of the space shuttle stack. It holds 535,000 gallons of liquid hydrogen and liquid oxygen for the shuttle's three main engines. Photo credit: NASA/Troy Cryder
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The junction between the two tanks. A look inside the external tank shows the junction of the liquid oxygen tank on top and the liquid hydrogen tank on the bottoom. The intertank joins the two tanks together to make a complete external tank. Photo credit: NASA/Frank Michaux
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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.

And now, the questions and Mendoza's answers:

Question: You're making unprecedented changes to the intertank. How do you model or simulate the results to prevent unintended consequences?

Answer: On ET-137 [the one for Discovery], we are performing a stringer reinforcement modification. This mod consists of installing aluminum reinforcement strips called 'radius blocks' on selected stringers. Radius block enhancement is a routine approved production repair method used during intertank assembly at the manufacturing plant, but not normally implemented at KSC. We performed an instrumented tanking test prior to the mod. Data from this test was used to validate existing engineering analysis models. Engineering tests have also determined that the mod increases the capabilities of the stringers and the design change does not affect the structural stiffness or integrated loads, while increasing margins of safety.

Q: The ET is being worked on while the SRBs are attached to the stack. Could you describe some of the safety procedures used while working in such close proximity to these fueled motors?

A: Working on the ET while attached to the SRBs is standard operations at KSC. The tank spends most of its processing time at KSC attached to the SRBs. It is considered a hazardous environment which requires specially certified technicians. Typical safety precautions include grounding, electromagnetic interference restrictions, and specially designed equipment that meet National Electric Code standards for hazardous and explosive environments.

Q: How can we make these external tanks reuseable?

A: At 500 seconds after liftoff, the ET separates from the orbiter and plunges through the atmosphere and breaks apart as it falls into the ocean. A re-design to increase its structural ability to survive the 52-mile plunge would increase the weight beyond the shuttle's capability to launch. In addition, the tank would need to be transported from the Indian Ocean to New Orleans, La.

Q: Why not just replace the tank instead of fixing it?

A: There are no additional tanks in production. The three tanks at KSC are suspect until the investigation is complete.

Q: What's the material used for it and why?

A: The tank is manufactured using mainly lightweight aluminum alloys for weight reduction so that we maximize payload capability of the Shuttle.

Q: Are the liquid oxygen and liquid hydrogen filled at the same time during tanking? And why do we need slow and fast fill? If liquid hydrogen is continuously coming out (I think that's why the pre-burn before SSME ignition) how do you maintain the pressure inside the liquid hydrogen tank?

A: Yes, the liquid oxygen and liquid hydrogen tanking process begin simultaneously. The process begins by chilling the transfer lines and main engines. Both commodities begin with slow fill to avoid thermal shocking of the aft domes. We then transition to fast fill up to about 98 percent capacity. We slow back down at this point to avoid overfilling. Once filled, we move into a stable replenish flow rate to maintain proper propellant levels and conditioning.

Regarding the liquid hydrogen tank, we use helium to pressurize the tank for flight. The preburn you mention is actually for the liquid oxygen exhaust at the main engines.

Discovery continues to go through its launch processing inside the VAB. Analysis and testing showed Space Shuttle Program managers the root cause for the stringer problem and thin support structures called radius blocks were prescribed to cure the issue. When the tank's repairs and modifications are completed, the entire circumference of the intertank will have been strengthened. That work is slated to finish in time for the shuttle to rollout to Launch Pad 39A on Jan. 31.

The crew for the mission also was changed recently after Mission Specialist Tim Kopra was injured in a bicycle accident. Veteran astronaut and spacewalker Steve Bowen will take Kopra's place on the mission to the International Space Station. Bowen last flew on STS-132 in May 2010, which means he will become the first astronaut ever to fly consecutive missions into space.

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Steven Siceloff
NASA's John F. Kennedy Space Center