The Auto-Adjustable Pin Tool designed at NASA’s Marshall Space Flight Center (MSFC) is an improvement to the friction stir welding (FSW) process—a process already widely recognized for providing greatly im-proved weld properties over conventional fusion welds. Introduced and patented by The Welding Institute (TWI) in the United Kingdom, FSW has been widely recognized for improved weld properties over conven-tional fusion welds.The FSW process uses a metallic pin tool that is comprised of a threaded pin extend-ing from a shoulder. The diameter of the threaded pin is much smaller than the shoul-der’s diameter, and the length of the pin is roughly the same as the weld workpiece thickness. To affect a weld, the rotating pin is forced into the abutting edges of a weld joint until the surface of the shoulder con-tacts the upper surface of the workpiece. The contact (rubbing) between the shoul-der and the upper surface of the workpiece induces sufficient frictional energy into the workpiece to elevate the temperature of the workpiece into a plastic state. The threaded rotating pin then “stirs” together the plas-tic, soft material of the two abutting edges as the pin tool traverses the weld joint. No melting occurs during the weld process. In spite of its advantages, FSW has had two major drawbacks: the reliance on a pin tool that left a “keyhole” at the end of welds and the requirement for different length pin tools when welding materials of varying thickness.The NASA technology eliminates the “key-hole,” and it also enables the welding of material that tapers from one thickness to another, as found in the Space Shuttle’s external tank. The auto-adjustable pin tool uses a computer-controlled motor to auto-matically retract the pin into the shoulder of the tool at the end of a weld, resulting in a smooth hole closure. The pin tool will be an essential, integral component of the baseline weld process used to fabricate/weld the upper stage cryo-genic hardware for the ARES I rocket manu-facturing program, and it is also expected to be used for ARES V. FSW systems using the pin tool technology concept, with numerous applications in a wide range of industries, are enabling FSW applications that are ver-satile, efficient, and cost-competitive.