How to Join Rocket Body Sections

Rocket fuel tanks are made by joining thin, paper-like sheets of special aluminum. But the moment you melt and join this aluminum, the strength in that spot plummets. Tiny, invisible bubbles and micro-cracks start to form inside. Engineers would pre-thicken the seams, X-ray them, dig them out, and re-weld them repeatedly. However, the added weight meant less cargo could be sent to space, and once melted, the joint never regained its original strength. Then, in 1991, British engineers flipped the script. If melting weakens it, why not just avoid melting it altogether? By pressing the seams firmly with a rotating tool, friction heat softens the metal. Then, it's mixed together like kneading clay. Since it was never melted, there are no bubbles, and the strength remains almost fully intact. Today, both Falcon 9 and NASA's super-heavy rocket bodies are joined using this method: 'Friction Stir Welding'.