Brittle and weak welds on SLS tanks?

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Government in action! The hydrogen tanks that will be used for the first SLS rocket flight were welded using a technique that NASA has since found to be untrustworthy.

Although the weld strength issue stopped welding the qualification and flight articles of the LOX tank before it could start, the issue wasn’t caught until after both LH2 tanks were welded with the modified pin tool last summer. The implications of the two tanks possibly having below design strength welds disrupted the original, post-weld plans.

The LH2 qualification tank, which will be used for structural testing at Marshall Space Flight Center in Huntsville, Alabama, was welded first and after setup and configuration was taken to Building 451 in December of last year both for proof testing of the welds and to qualify the test facility and procedures for subsequent flight tanks. Hydrogen tanks are proof tested by pressurizing them with nitrogen gas while a hydraulic test rig applies loads to the structure. “We wanted to wring out…the control system – 451 was another building that was made bigger to fit the hydrogen tank,” Doering said. “The control system is all new, the reaction fittings are all new, along with all the actuators. We didn’t want to put the flight asset in there to try to use it for the first time, so [using] the qual[ification] article [first] was also trying to wring out the pressurization and the actuation of the control system in 451.”

Originally, the plan included a test case to pressurize the qualification tank to slightly above flight pressure to help as a part of that “pathfinding” work; however, the discovery that the welds may be below design strength forced plans to be reconsidered.“We couldn’t say with any real degree of certainty that these welds would make it to [flight pressure],” Doering said. “In a pneumatic test, pressurizing it like that, it’s like a balloon…there’s a good portion of the community that thinks it will survive, there’s another portion of the community that says you don’t know enough to be able to say that, [and] there’s another portion of the community that says…’no way.’ [emphasis mine]

This is merely the qualification tank, built to find out if the tank design, which appears to be overly complicated to begin with, will work. The flight tank?

Lower pressure isn’t an option for the LH2 flight tank, which must perform at flight pressures both in testing and in flight. The SLS Program developed and is working on multiple, parallel options for consideration that include repairs and/or replacement of the already-welded flight tank. “We’re looking at use as-is – can I get to the point where I’m comfortable using that flight tank?” Doering said. “The answer to that is probably not, just because the analysis tools don’t exist yet to do this.” [emphasis mine]

They are faced with the likely possibility that they will have to repair the tank, which will likely cause the now 2019 launch date for the first unmanned test to be delayed further.

The rumors that NASA is considering making that first test flight a manned one makes me think that they are considering that decision as a cover for these additional delays. “We need more time to make this work as a manned flight,” NASA management will claim, using that extra time to fix the tanks as well. They will also claim they need more money, as they always do.

Meanwhile, NASA is having trouble building rocket tanks, an item that aerospace engineers figured out how to build half a century ago. Way to go, NASA!



  • LocalFluff

    Falcon 9’s two mishaps were caused by, if not directly bad welding so anyway structural rupture in the upper stage tankage. However they use super cooling and put helium tanks inside of oxidizer tanks, so that of course means new challenges no one ever tried before. SLS fails without even trying such things.

    Maybe it is SLS’ huge size that’s the challenge? SLS Block I will lift amazing 70 tons to LEO while Falcon Heavy will lift only 64 tons to LEO. That’s a huuuge difference!

    “Lower pressure isn’t an option”
    Oh, I think it is! Reclassify SLS’ capacity to that of an Atlas V by leaving the tanks mostly empty from start. But oh, then it cannot lift Orion crewed to orbit with its heavy launch abort tower system. Maybe it can be parked empty in LEO by a Delta IV Heavy and then the crew can be launched with an F9+Dragon to dock with it? That’s interesting because it is like doing the asteroid retrieval mission. This can become an important new “stepping stone to Mars”!

    Or they could pretend doing like SpaceX who puts their historical breakthrough first stages in museum after they have landed, and send this SLS test tank directly to a museum, just without testing it. It was made using the largest welding machine in the galaxy (any aliens out there don’t weld anymore), so that is an achievement in itself. I know a place that would welcome them, they have room just next to the plastic bicycle the frame of which was too flimsy to actually ride upon:

  • Juan

    I do not pretend to understand the technical aspects of all of this, but would someone answer a question for me because to me it is not clear? Did they use a completely new process for welding these tanks? If so, why? Was the way they welded the shuttle tanks not sufficient or was it just another example of government just having to do something “new, untried and expensive” to justify its existence?

  • Rashomon

    The welding technique used is self-reacting friction-stir welding, otherwise known as bobbin-tool friction-stir welding. Friction-stir welding is a relatively new process that has been used to join aluminum on the Eclipse light jet and on Space X boosters, but the practice generally involves pressing a spinning tool over the joint while the joint is supported at the back by an anvil. The process NASA is using for this tank deletes the anvil but uses a tool that spans the joint, applying pressure from both sides. It has never been used to weld sections as thick as on the SLS tank, and the process is still being developed and categorized. NASA has found it be to extremely tool and process critical, with even slight changes in either leading to unacceptable welds.

  • Dick Eagleson

    What Rashomon said.

    I would note only that, while friction stir welding is a recent innovation, compared to other welding techniques, it nonetheless dates from the early 90’s which is a quarter-century ago. That is long enough for SpaceX to have employed the technique extensively going back to the Falcon 1 days.

    I don’t know exactly what variant of friction stir welding SpaceX employs for its structures fabrication but I’m inclined to doubt it uses the bobbin-tool technique described as used for SLS tankage. The technique is well adapted for flat sections, but seems ill-suited for curved and compound-curved joints which, of course, are the only kinds one finds on rocket tankage.

    SpaceX has frequently acknowledged its use of NASA-developed know-how. This appears to be one of those situations in which the “mentor” organization could have profited by seeking advice from its nominal “student.”

  • air_and_space92

    “Lower pressure isn’t an option”
    Oh, I think it is!

    Absolutely not and dropping the payload mass won’t help one bit here — it actually could make flight loads worse. Launch vehicle flight tanks are pressurized to alleviate the running load stresses caused by the combination of axial stresses by acceleration *and* air-induced bending moments. Dropping the pressure actually increases the axial stress within the tank walls and reduces the buckling load the isogrid shell can hold regardless of payload.

    Additionally, lowering the payload reduces the amount of inertia relief applied to canceling out the lateral forces produced by the engine gimbals and body normal lift forces generated during trimmed flight. This also changes the shear and bending moment curves (hence component thicknesses and materials) along the entire length of the vehicle.

  • LocalFluff

    Interesting. Rockets to orbit is a crazy complicated thing. All animals watch in amazement as the apes do all this crazy stuff.

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