The external tank in its final stages of fabrication at Lockheed Martin Michoud Space Systems. (Photo courtesy of Lockheed Martin.)
BY PHILIP CHIEN
Improving the Space Shuttle's performance is always a concern of NASA. Every pound of deadweight translates into less cargo that the shuttle can lift into orbit, and that is especially ciritical for flights to the International Space Station.
A rocket performs best when it launches due east, benefiting from the Earth's rotation. But the Space Station requires launches to the northeast in order to achieve an orbit that can be reached by Russian launch vehicles. This results in a drastic decrease in the amount of cargo that the shuttle can lift into orbit. Instead of reducing the number of assembly flights, collaborating with Russia in the Space Station program has actually increased the number of shuttle flights and the time required to complete the Space Station. For a while, NASA funded the Advanced Solid Rocket Motor (ASRM) as a means of increasing lift capability, but that project got mired in regional politics and was eventually canceled. The decision was made to fund a weight reduction program for the external tank, primarily replacing the present aluminum alloy with an aluminum-lithium alloy.
Strength at Cryogenic Temperatures
The 2195 aluminum-lithium alloy in the super lightweight external tank (SLWT) is only 5% lighter than the 2219 aluminum alloy used in the current lightweight external tank. But its 30% greater strength at cryogenic temperatures results in a significant weight savings. Overall, the SLWTs are 7500 lb lighter than the older external tanks. NASA external tank project manager Parker Counts said, "Each pound we can take from the external tank is one more pound we can take to orbit. This becomes especially important when launching the International Space Station into its proper orbit in 1998."
Very little lithium is used in the 2195 aluminum-lithium alloy, The specifications for 2195 call for 1% lithium, 4% copper, 0.4% silver, 0.4% magnesium, 94% aluminum and the remainder minor amounts of other elements. Reynolds Metals, McCook, Ill., produces the alloy and Alcoa in Pittsburgh, Pa., has been qualified as a second supplier.
Oxygen Is a Concern
The same tooling (Fig. 1) used to manufacture the current external tanks is used with the new alloy, making the new tanks more affordable than other techniques to improve the shuttle's performance. The shuttle's external tanks are manufactured by Lockheed Martin Michoud Space Systems (LMMSS), New Orleans, La.
Terry Hibbard, Lockheed Martin's vice president for the external tank program, said, "The basic difference is with the lithium in the aluminum; it's easily attacked by oxygen. So we have to purge both sides of the weld to protect oxygen from getting to the lithium content in the metal. With the 2219 Aluminum we only had to purge from the torch side." A mixture of argon and helium is used for the purge. For 2219 welding, the filler metal was 2319, but it has been replaced by 4043 for the 2195 alloy.
Welding the Tanks
The early shuttle tanks were welded using gas tungsten arc welding. Hibbard said, "In the early 1980s, we developed with NASA Marshall the variable polarity plasma arc (VPPA) welding process." That process creates a keyhole in the metal and the molten metal then solidifies at the back of the hole as the torch progresses. "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," he said. At that point General Digital Inc. supplied the welding equipment. Hibbard said, "In the early 1980s we worked with Hobart to provide us with an improved computer control system for that equipment and worked with them up until this last year when they sold their rights to Advanced Manufacturing Engineering Technology (AMET). AMET now supplies us with the welding equipment, using the same technology that Hobart developed. We have on order six AMET welding units, which should show up this year. It's specialized for this VPPA welding and there are very few companies who are using it." The new equipment is worth approximately $250,000. The only other manufacturers with a requirement for this specialized welding equipment are aerospace firms, primarily ones that build lightweight pressurized vessels for launch vehicles.
As the largest rocket component ever, the external tank has had challenges since its original design. One early proposal called for it to be carried from its manufacturing plant to the launch site piggyback on top of a modified Boeing 747. But a more conventional sea shipment via barge through the Gulf of Mexico and around the Florida peninsula was selected instead.
Hibbard indicated three challenges in the tank's welding process. "The major issue is having something that is 28 ft in diameter. You have to have the constant purge where the torch is, that's one of the bigger tooling challenges we had, controlling that backside shield.
"We also found out that when welding aluminum-lithium you have to speed up the travel. Aluminum-lithium material likes to be welded at a higher travel speed." The travel speed is 10 in./min, which is faster than the 2219 welding speed.
"The real difficulty is 2195 doesn't like to be repaired, When you add heat for repair in a localized area you get residual stress buildup. We actually go in and relieve the residual stresses by planishing. Weld repair has been the area of development where we've had to work real hard." Stresses are perpendicular to the weld direction. Hibbard said, "We have a slightly reduced strength in repair areas, therefore, early on in the development of the tanks, we designed the weld thicknesses around the repair strength. That was accommodated in our design development activities."
However, one of the key advantages of the alloy is it becomes stronger when it's reduced to the cryogenic temperature when it's filled with its liquid hydrogen and liquid oxygen propellants.
For a 0.32-in. thickness, the typical initial weld strength at room temperature is 38,000 lb/in.2. At cryogenic temperatures, the weld is 1.2 times as strong. The allowable repair strength is 30,000 lb/in.2, which gives a point of comparison for the strength of a weld repair and an initial weld.
Nondestructive examination is performed via X-ray images and a level three fluorescent dye penetration method. All welds are X-rayed, a total of approximately 32,000 in. per tank.
The first super lightweight tank had approximately 600 repairs. Typically, the repaired area is a 0.1-in. crack, but with site work the total amount of repairs amounted to about 1000 in. The figure is similar to the amount of repairs required for the first external tank. However, experience is the best teacher, and Hibbard said, "We're seeing a decreasing number of weld repairs as we get more familiar with our process. The latest tank has about half the number of repairs required previously."
Other changes to the SLWTs include fine-tuning the orange spray-on insulation and machining the walls of the hydrogen tank. The metal is machined in an orthogrid pattern and removing a percentage of the metal actually increases the tank's strength. Since the hydrogen is the larger quantity of the two propellants, it's a greater weight savings.
A quarter-length full-diameter test article was tested at the Marshall Space Flight Center in Huntsville, Ala., from February to July 1996 to certify the new tank.
The first SLWT (Fig. 2) was scheduled to be launched with the shuttle in late May for the STS-91 mission. That SLWT weighed 7668 lb (3478 kg) lighter than a standard light-weight external tank -- 165 lb (76 kg) better than the contract specifications. *
PHILIP CHIEN is a freelance writer, Earth News, Merritt Island, Fla.
Fig. 1 -- The dome of the external tank is rotated during a stage of fabrication. (Photo courtesy of Lockheed Martin.)
Fig. 2 -- The external tank is backed into the Vehicle Assembly Building at Cape Canaveral, Fla., in preparation for its attachment to the shuttle. (Photo courtesy of NASA.)