Aluminum alloy and carbon fiber, because both have high strength to weight ratios. Steel is denser, so its specific strength isn't as high. But it retains strength at both lower & higher temperatures, and it's much cheaper. It's also cheaper/quicker to modify a steel design, as welding is easier than aluminum (at least for low carbon steels used in Starship). And if the same structure were made of carbon fiber, it would likely have to be fabricated from scratch.

I think choosing steel has helped Starship, as it allowed the ship to survive reentry and land in the ocean despite heat shield failures. Had the ship been made of aluminum, I'm certain earlier missions would have broken up when the aluminum heated up and lost its strength.

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y0ssar1an 11 hours ago | parent [-]

you should come back to this comment in 10 years. stainless steel starship will not have left LEO.

	▲	ggreer 10 hours ago \| parent [-]
		Does this mean you think Starship HLS will fail? It will use a steel structure and is planned to be part of future Artemis missions. Also are you willing to stake money on that claim? How about this: If before July 25th, 2036, any version of Starship made primarily of steel (HLS, future Mars versions, etc) has an apogee above 2,000km, you pay me $1,000. If not, I pay you $1,000. If you like, we can bet some other amount up to $10k. And if you prefer, the loser could donate the amount to a non-profit/charity of the winner's choosing.