IIRC, Musk explained it in a SSB-BPA joint meeting last year. They considered carbon-fiber, aluminum lithium alloy, and 301 steel. It was a fascinating discussi

IIRC, Musk explained it in a SSB-BPA joint meeting last year. They considered carbon-fiber, aluminum lithium alloy, and 301 steel. It was a fascinating discussion. The materials were great under low temperature, but had other issues as well as degraded performance under high temperature. For instance, carbon-fibers can leak gas (or react with the percolated fuel?). Starship eventually used 30X, a steel alloy.