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ET: A New Generation

Redesigned External Tanks for the Space Shuttle are prepared for flight

by Michelle Evans

Near the northern limit of New Orleans, situated in the lowlands of the Intracoastal Canal between Lake Pontchartrain and Lake Borgne, is the NASA Michoud Assembly Facility. Here is where the largest single component of the Space Transportation System, more commonly known as the Space Shuttle, takes shape: the External Tank.

The black-and-white winged beauty of the space shuttle orbiter is recognizable to nearly everyone. This is where the crew of astronauts rides the fury hundreds of miles into space to perform their duties: constructing a space station, servicing a telescope that peers at the beginnings of the universe, or performs experiments that will hopefully further our goals of permanent human settlements off our home planet.

To get the orbiter into space to accomplish these tasks requires the help of two Solid Rocket Boosters (SRBs) that look like tall, white candlesticks, and the 153.8 foot tall, giant orange External Tank (ET). Without this fuel tank, nothing gets off the pad and nothing achieves orbit, since this is the reservoir for the Space Shuttle Main Engines (SSMEs), three of which reside at the rear of the orbiter. When discussions of the ET come up, many often refer to it as a huge soda can. After all, the basic structure is thin aluminum covered by a layer of its signature orange insulating foam. This keeps the fuels in its liquid oxygen (LOX) and liquid hydrogen (LH2) tanks chilled. There are no fancy engines attached to the tank and no crew ride aboard it.

The ET-fed SSMEs fire in concert with the twin SRBs to lift the stack off the launch pad. Two minutes later, the solids are finished and fall away, but the ET keeps fuel flowing to the shuttle engines for another six-and-a-half minutes to complete the ride to orbit. Once the fuels in the ET are depleted, it is jettisoned to fall back into the ocean, the only major component of the shuttle that is not recycled. During this eight-and-a-half minutes, over half a million gallons of fuel carried by the ET, or about 62,000 gallons a minute, is transferred to the SSMEs to burn—over a thousand gallons each second are converted to thrust to propel the Space Shuttle and its payload to orbit.

I had the pleasure to visit Michoud (pronounced me-shu) recently under the guidance of Harry Wadsworth of Lockheed Martin, the primary contractor for the ET. My purpose was to understand and record the assembly of the External Tank; however, it became much more than that as I learned of the devastation and hardship that these people have overcome from the onslaught of Hurricane Katrina last August.

Originally built to make wooden cargo planes in World War 2, the Michoud factory came on line too late in the war. Only two planes ever left the assembly line of this 832-acre facility, 43 acres of which are under a single roof—the production factory. NASA took over the area in 1961 to create the facility to build the first stage of the Saturn V booster. As part of the Space Shuttle program, Michoud was converted to External Tank production in the mid-1970s.

Following Katrina, the facility was forced to shut down all work. Although the levees held around the facility, they suffered extensive flooding as a 20-foot wall of water topped the levees in two places and 13 inches of rain pounded Michoud, extensively damaging buildings and tearing off roofs. As Harry explained, “We kept our pumps running the entire storm via a huge generator, so water was constantly being pumped out. That saved our factory. Had the factory flooded we could have lost the Space Shuttle program. Credit goes to our ride-out crew, the 38 men and women who stayed behind to care for the facility.”

The entire area was near the eye of the storm, and 47 percent of the 2,000 person Michoud workforce found themselves unable to return home. Nonetheless, even with their own personal losses, the Lockheed Martin and NASA employees returned quickly to clean up the damage and re-open the plant within only three weeks, all this while also serving to aid the community as a whole in their recovery effort. Even nearly a year later, there is much damage still evident. Levees are being rebuilt and buildings get patched in anticipation of the start of a new summer hurricane season. The dedication of the workers to bring our space program back to full throttle is amazing to witness.

The External Tank came to prominent focus after the loss of Columbia on February 1, 2003. Pieces of the foam insulation dislodged during launch ascent and struck the leading edge of Columbia’s left wing, leaving damage that caused the wing to fail 16 days later, during reentry. The insulation that caused the fatal damage was at the base of the bipod area of the ET, which is the forward attach point of the shuttle orbiter.

In response to the Columbia Accident Investigation Board, the ET foam installation procedures have been greatly modified. In addition, after the first post-Columbia test flight of Discovery on mission STS-114 last summer proved, the fix was not complete. Engineers went back to their computers and wind tunnels. In the end, it has been decided that one of the safest ways to continue flying is to actually remove some of the areas of foam causing the most difficulty.

One such area is called the Protuberance Airloads (PAL) ramps. Original testing showed the PAL ramp near the top of the liquid hydrogen tank to be acceptable for flight, but a large piece of this ramp came off late in the STS-114 ascent, nearly missing Discovery’s underbelly heat tiles. This PAL ramp has now been removed, and wind tunnel testing confirms the aerodynamics of the shuttle should not be significantly affected. At this point, the only thing left to do is fly the mission and see where we stand on the ET foam issue, and if the remainder of the Space Shuttle missions will fly as planned before retirement in 2010.

You might believe that since the ET is similar to a huge soda can, it would be simple and quick to manufacture. In actuality, each External Tank takes 20 months to construct. The 97-foot long LH2 tank and 55-foot long LOX tank are mated to an intertank assembly, fuel feed lines are installed, and a thermal protection coating made of a spray-on foam insulation is added. Ranging from one to two inches in thickness, this foam adds 4,100 pounds to the gross weight of the ET, and it also serves to hold the tank together as reentry breakup starts to occur to create a smaller footprint where its pieces fall into the ocean.

I found it extremely interesting to watch people at work on the tank. In some cases their jobs resembled more a sculptor rather than an aerospace technician. I saw the foam areas being meticulously worked with carving tools to create exactly the correct shape. At one point I saw a couple men, carefully walking on black pads to protect the tank foam, vacuuming the insulation to remove particulate matter that could shed. The head of the vacuum wand was similar to what you would use in your home to clean upholstery.

Spaceflight is an art form, and the people at Michoud are definitely artists. Their work reflects their dedication to returning spaceflight to a safe and reliable proposition, and is vital to our continued presence in space. Major elements of a new generation of human launch vehicles will be derived from their work on the External Tank. Their personal dedication to our future in the face of adversity from the extreme forces of nature, under which the Gulf Coast must live, cannot be equalled.