Welders inside a large liquid hydrogen tank for NASA's Space Launch System use friction stir welding to plug holes left after the tank was assembled. (NASA/Michoud/Steve Seipel)

Large elements for NASA's Space Launch System  are in production, and will be joined together to create the rocket's 212-foot-tall core stage, the backbone of the SLS rocket. The core stage  is made up of the engine section, liquid hydrogen tank, intertank, liquid oxygen tank, and forward skirt.

Welders inside a large liquid hydrogen tank plug holes using friction stir welding. Using frictional heating and forging pressure, the method produces high-strength bonds virtually free of defects. The process transforms metals from a solid state into a "plastic-like" state, and uses a rotating pin tool to soften, stir, and forge a bond between two metal sections to form a uniform welded joint. At the beginning and end of each weld, holes remain where the rotating pin tool enters and exits the metal.