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External Tank Was Backbone Of Shuttle Launches
by Linda Herridge for NASA's Kennedy Space Center
Kennedy Space Center FL (SPX) Sep 23, 2011

An external tank is lifted inside the Vehicle Assembly Building before it is stacked for launch. Photo credit: NASA/Dimitri Gerondidakis. For a larger version of this image please go here.

It is a riveting sight - the External Fuel Tank attached to a space shuttle, with twin Solid Rocket Boosters on either side, as they emerge from the Vehicle Assembly Building perched atop the mobile launcher and carried by the crawler transporter to the launch pad. Moving slowly along the crawlerway, the tank's distinctive orange color shines like a beacon as if to indicate that something exciting is about to happen.

External Fuel Tank 138, or ET-138, helped launch NASA's last space shuttle flight, Atlantis' STS-135 mission, on July 8, from Launch Pad 39A at Kennedy Space Center. Though not the last tank built for the Space Shuttle Program, it was the last flight tank.

Arriving at Kennedy Space Center on July 13 last year, it had, like all of the tanks before it, made the 900-mile journey to Kennedy from NASA's Michoud Assembly Facility in Louisiana. After offloading, it was transported to the Vehicle Assembly Building (VAB) and stored in a test cell for NASA's last space shuttle mission.

Unfueled, each tank weighs in at 58,000 pounds and 1.6 million pounds when fuel. At more than 15-stories tall, it is the largest single part of a shuttle stack, sometimes referred to as the "backbone" of the space shuttle.

"The external tank actually provides the structural background of the space shuttle system by absorbing the thrust loads produced at launch by the orbiter and the boosters," said Alicia Mendoza, the ET/SRB vehicle manager.

The tank's familiar orange color comes from the foam insulation sprayed on its aluminum structure. The insulation helps the tank act as a thermos bottle to keep the super cold propellants from evaporating too quickly. It also helps prevent ice from forming on the tank's exterior and promotes the right aerodynamic shape for launching into space.

The external tank's main job is to hold about 535,000 gallons of super cold liquid hydrogen and liquid oxygen. The lower portion of the tank holds the liquid hydrogen, which is the fuel for the engines. The second-coldest known chemical, it is stored in the tank at minus 423 degrees Fahrenheit. The upper part of the tank holds liquid oxygen, chilled to minus 297 degrees.

A section call the intertank holds the two tanks together. It is made up of a ring of 108 aluminum alloy support beams known as "stringers" that give the intertank its familiar ribbed appearance. It also contains the tank's instrumentation and a vent to safely release pressure in the hydrogen tank during the countdown.

During final processing in the VAB, technicians fill plug pull test cylindrical holes with a special type of foam that is hand-poured for small or irregularly shaped repairs and install and connect the Ground Umbilical Carrier Plate, or GUP to the Intertank.

"The purpose of the GUP is to provide the interface to the launch pad for purging and venting hydrogen gas as well as provide for electrical pneumatic connections," Mendoza said. The chemicals are funneled from the tank into the shuttle's three main engines at liftoff and throughout the shuttle's eight-and-a-half minute climb into space.

The tank, like the shuttle itself, has undergone numerous upgrades and weight-saving improvements.

The ET was painted white for the first two shuttle flights. But in order to reduce launch weight by 600 pounds, the tank was left in its natural orange state beginning with mission STS-3.

After a few revisions to designs and materials, the latest version of the tank, known as the Super Lightweight tank, is 17,000 pounds lighter than the first one Columbia used in 1981.

"The quality of tanks has definitely improved over the years," Mendoza said. "For instance, the tanks are lighter and several process improvements were implemented at the Michoud plant."

After the Columbia accident in 2003, engineers implemented changes to the foam and the way it is applied to the ET. Some foam was removed altogether to eliminate further risk. Cameras were attached to the tank so engineers could see any possible debris coming off the tank during ascent.

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