Welding Wonders

Welding Wonder: Space Shuttle External Tank

For 30 years, the Space Shuttle stood as the beacon of American spaceflight. Between 1981 and 2011, a total of 135 missions were launched from Kennedy Space Center in Florida. Thanks to the Space Shuttle, numerous satellites and interplanetary probes were launched (including the Hubble Space Telescope), important science experiments were conducted in orbit, and the International Space Station was constructed.

And all of this was made possible thanks to the welding wonders that brought the Shuttle’s external tank to life.

The external tank was designed by the Martin Marietta Corporation, and manufactured and assembled by the Lockheed Martin Space Systems Company at NASA’s Michoud Assembly Facility in New Orleans, Louisiana. It was the largest and heaviest component of the Space Shuttle. It contained the liquid hydrogen fuel and liquid oxygen oxidizer which were supplied to the three Space Shuttle Main Engines during lift-off. It was also the only Space Shuttle component that was not recovered after launch. The tanks simply broke apart before impacting the ocean.

Since its first mission in 1981, the tank went through two important upgrades. Starting with the Standard Weight Tank and ending with the third-generation Super Lightweight Tank, the changes made were done to reduce the weight of the tank and increase the Shuttle’s payload capacity.

This photograph shows the liquid hydrogen tank and liquid oxygen tank for the Space Shuttle external tank being assembled in the weld assembly area of the Michoud Assembly Facility.

The original tank, which was built until 1983, weighted about 76,000 pounds when loaded with fuel and oxidizer. The basic structure was made of aluminum alloy 2219. Tank sections, which were comprised of many thicknesses of aluminum sheeting, were assembled using gas tungsten arc welding.

This tank lasted for just six Space Shuttle missions, with the last of the Standard Weight tanks being flown on Challenger’s STS-7 mission in 1982.

The year before that, Michoud had begun production on the Lightweight Tank, which trimmed about 10,000 pounds from the tank that preceded it.

The weight reduction was accomplished through several different methods. First, the thickness of several aluminum skin panels was reduced, and several stringers in the liquid hydrogen tank were eliminated. Dome caps which were chemically milled on one side only, were now milled on both sides to reduce weight without reducing strength. A new titanium alloy, which was stronger, lighter and less expensive, was also used by the Solid Rocket Booster attachments.

New welding processes also made the Lightweight Tank production more labor and cost efficient. In 1984, the Marshall Spaceflight Center adopted Variable Polarity Plasma Arc welding as the method used in the tank construction. Terry Hibbard, Lockheed Martin’s vice president for the external tank program said: “In the early 1980s, we developed with NASA Marshall the variable polarity plasma arc welding process. We used those processes for the 2195, plus a hybrid process where the plasma arc alternates current and does some cathodic cleaning at the torch,”.

The Lightweight Tank flew on 85 Space Shuttle missions, with its last flight being aboard STS-99 in 2000.

In 1993, NASA had asked Lockheed Martin to develop a new high-strength, light-weight replacement for the aluminum alloy Al 2219 being used on the External Tanks. After years of research, Lockheed Martin, Reynolds Aluminum, and the labs at Marshall Space Flight Center were able to successfully develop a new alloy known as Aluminum Lithium Al-Li 2195 – which reduced the weight of the External Tank by another 7,500 pounds.

The External Tank rolls out of the Michoud Assembly Facility.

This Super Lightweight Tank gave the Shuttle the chance to carry heavy components for the assembly of the International Space Station. However, the new alloy used in the tank’s construction did not come without problems.

NASA and Lockheed Martin Engineers faced several difficulties as they learned to form, weld, and repair the new material. Myron Pessin, former Chief Engineer for the External Tank Program, noted that weld repairs were a significant challenge.

Many weld lands on the Super Lightweight Tanks were increased in thickness by up to 0.35″ to allow more margin for potential weld repairs. A “second generation” of the Super Lightweight used a different alloy in the intertank thrust panels. The change allowed for even more weight savings – though they were offset by the conversion back to Al 2219 for the dome gores – which were easier to weld, and which drastically reduced repairs.

With repair welds becoming more difficult to make, and production costs increasing on the tank, NASA began researching alternative welding techniques. Project Managers eventually chose the friction stir process, which produced a stronger joint than the fusion arc welding used in the earlier Lightweight Tank. Another significant benefit of friction stir welding was that it had far less elements to control. For example, in fusion welding you must control purge gas, voltage, amperage, wire feed, travel speed, shield gas, and arc gap. However, in friction stir there are only three process to control: rotation speed, travel speed, and pressure.

Friction stir welding works by rotating a dowel between 180 to 300 revolutions per minute depending on the thickness of the material.  The tip of the dowel is forced into the material, and as it continues rotating, friction heat softens the area around the pin and forces it to forge and create a bond with the other material.

The barge carries the external tank to Cape Canaveral for the launch of STS-121.

STS-132 in 2010 was the first mission to fly using an External Tank that was constructed using friction stir welding. It featured friction stir welds on two of the liquid hydrogen tank barrels. STS-134 was the first mission which featured friction stir welds on all four liquid hydrogen tank barrels and the liquid oxygen barrel.

In the end, welding played a huge part in the decision-making for certain alloys and construction methods used on the Space Shuttle’s external tank.

After it’s retirement in 2011, NASA was left without a working spacecraft to take it’s astronauts into space. The Russian Soyuz spacecraft is now the main vehicle used by NASA astronauts to get to and from the International Space Station. But the lessons learned during its construction – from alloys to welding to construction methods – will live on in future NASA efforts.

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