Thursday, October 15, 2009

The Space Shuttle's Gas Tank

Today there are four external tanks in the assembly line at Michoud, ET-135 thru ET-138. All of these tanks will board Pegasus in late 2009 or early 2010 and make the 900-mile trip from Michoud to Kennedy Space Center to play their vital role in supplying the Space Shuttle Main Engines with 145,000 gallons of liquid oxygen and 390,000 gallons of liquid hydrogen during the first eight-and-a-half-minutes of launch.

One additional tank resides at Michoud, but it may never fly. ET-122 was present at Michoud when Hurricane Katrina hit Louisiana in 2005 and was damaged. In fact ET-122 is now being repaired to serve as the very last tank of the Space Shuttle Program, the Launch on Need tank for the last scheduled space shuttle mission, STS-133, in the fall of 2010. If all goes well with that mission ET-122 should never fly.

fter the loss of space shuttle Columbia in February 2003, NASA went to work to redesign and improve many components of the structures of the external tanks and the application processes of the all important foam, also known as the Thermal Protection System or TPS. Major improvements have been made to the tank's forward bipod fitting area, the liquid hydrogen tank Ice Frost Ramps, the intertank flange area, and the liquid oxygen feedline brackets and bellows. The tank's protuberance air load ramps -- known as PAL ramps -- were also removed.

By the summer of 2008 external tank foam application and new designs had reduced the amount of foam being released during launch to very small, if not tiny amounts of foam. The successful reduction in foam debris came as a result of a non-stop process of continuous improvement to make shuttle launches as safe as possible, recognizing that external tanks will still release very small amounts of foam. Even with a few hiccups, the external tanks flying today are the safest and best tanks ever flown in the history of the shuttle program.

The newest tanks, including ET-134, have been welded using a new welding technology called Friction Stir Welding, a technique better than conventional fusion welding. Friction stir welding is different in that the materials are not melted. A rotating tool pin uses friction and pressure to plasticize the metal and join the two parts together.

As a result, weld joints are more efficient, yielding 80 percent of the base strength. Fusion welding averages 40 to 50 percent of the base material's strength. In fact ET-134 is the first external tank to have most of its liquid hydrogen tank welding performed by friction stir welding.

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