NASA Space Shuttle main tank aluminum 2219 / aluminum-lithium

2219 was the original weldable shuttle main tank alloy; later replaced with aluminum-lithium (8000 series) for weight savings.

The heat-treatable alloys — the oldest is 2024, an aluminum-copper alloy. Like I said, the Wright Brothers' aircraft engine is made out of aluminum-copper, precipitation hardened for strength. Doesn't have particularly good corrosion resistance, most of it doesn't have very good weldability. 2219 is a modern version which NASA was using for the early space shuttle main tanks because it was weldable — they had to weld the great big tank. They've now gone to aluminum-lithium alloys, which are even lighter, part of the 8000 series.