Elon Musk changed the name of BFR to Starship. While people were curious to know the reason of the sudden change of the name.He made another change in the plan and started to build the Starship of stainless steel. So, what led him to the sudden change?

“To the best of my knowledge, this has never been proposed before.” Musk said when he was asked about why he changed the plan to make the Starship of stainless steel.

Elon Musk was quite busy these days in redesigning the Starship and the Super Heavy rocket booster. He changed to a special alloy of stainless steel which he was thinking to do for quite a long time. Musk said, “And this is somewhat counterintuitive. It took me quite a bit of effort to convince the team to go in this direction.”

At last, Musk convinced everyone. They were chasing an advanced carbon-fiber structure, which costs $135per kilogram. Cutting the fiber would make the scrap rate about 35 percent. Some of which you can’t even use. While if you look at the properties of high-quality stainless steel, the thing that isn’t obvious is that at cryogenic temperatures, the strength is boosted by 50 percent, as well it’s the lightest.

Musk said, “Most steels, as you get to cryogenic temperatures, they become very brittle. You’ve seen the trick with liquid nitrogen on typical carbon steel: You spray liquid nitrogen, you can hit it with a hammer, it shatters like glass. That’s true of most steels, but not of stainless steel that has a high chrome-nickel content. That actually increases in strength, and ductility is still very high. So you have, like, 12 to 18 percent ductility at, say, minus 330 degrees Fahrenheit. Very ductile, very tough. No fracture issues.”

As well, Musk was thinking to abandon the heat shield thing. So, he needed something that’s strong at cryogenic temperatures. The thickness of the heat shield tiles is actually driven by the heat soak from the heat shield getting to the bond line of the tile onto the shell. 1500 F interface temperature looks quite comfortable when you’re in a steel rocket instead of, say, a 300 F, so you have five times the temperature capability at interface point. What that means is that for a steel structure, the leeward side of the back shell does not need any heat shielding.

As well, it is very easy to work with steel, if you look then the carbon fiber is $135 a kilogram, 35 percent scrap, so you’re starting to approach almost $200 a kilogram. The steel is $3 a kilogram.

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