For some reason Elon Musk is extremely enamored of welding technology. I remember seeing a page on an old version of the SpaceX site containing a high-level overview of the Falcon 9, where they went out of the way to mention how great circumferential friction stir welding is compared to whatever NASA used to do. And here he is bragging about it being the large stir welded thing ever created: http://www.nasaspaceflight.com/2009/01/musk-ambition-spacex-.... It's a revolutionary vehicle in many regards, but Elon seems to have a particular soft spot for welds.