Two promising methods of friction stir lap welding were investigated and demonstrated on large-scale thin-gage hardware in order to address the growing need to save weight and cost on spaceflight pressure vessels.  The first method investigated the productivity that could be achieved using conventional tooling to lap weld extruded stiffeners onto skin panels to form a 7-m (23-ft) diameter 3.2-mm (0.125-in) thick Al7075 payload fairing barrel section.  The second method followed a more unconventional approach to circumferentially lap weld three 5.5-m (18-ft) diameter 2-mm (0.080-in) thick Al2090 Al-Li barrels together using a retractable pin tool with a SIN-pattern transverse oscillation on the first weld and a straight self-reacting process on the other.  The SIN-pattern lap weld in particular was proven to substantially increase strength compared to a straight lap weld, and with a weld length of 32-m (1260-in) it became the longest friction stir weld made to date at the NASA Michoud Assembly Facility.  These efforts were undertaken primarily to optimize spaceflight structures, however the aeronautic and automotive industries could benefit as well by using similar techniques.