Recently, there was news that SpaceX changed the material used for the BFS/Starship (upper stage of the BFR) from initially planned carbon fiber to metal which seems quite counter-intuitive as carbon fiber has a few desirable physical properties and, more importantly, low weight.

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What were the main reasons for choosing stainless steel over carbon fiber composite?

  • $\begingroup$ I'm sure they'll tell us at some point, until then this question will only invite speculation. $\endgroup$ – JCRM Dec 29 '18 at 10:43
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    $\begingroup$ Musk has hinted at a presentation in March or April 2019. $\endgroup$ – Hobbes Dec 29 '18 at 11:18
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    $\begingroup$ will it be like the last one where he described the Dunning-Kruger effect as if it were something he'd come up with himself @Hobbes $\endgroup$ – JCRM Dec 29 '18 at 11:51

According to tweets from Elon, at least part of the decision is due to their design for dealing with the reentry heat: instead of adding ablators to cool the craft on reentry, Starship is going to actively cool the hot side with liquid methane. Steel is better at coping with this than carbon fiber.

Tweet 1:

Usable strength/weight of full hard stainless at cryo is slightly better than carbon fiber, room temp is worse, high temp is vastly better

Tweet 2:

Leeward side needs nothing, windward side will be activity cooled with residual (cryo) liquid methane, so will appear liquid silver even on hot side

(Also: Skin will get too hot for paint. Stainless mirror finish. Maximum relfectivity. [sic])

  • $\begingroup$ "so will appear liquid silver even on hot side" If this should be the melting point of silver at about 960 °C, that is light cherry red color of hot steel, see wikipedia. Steel looses a lot of its strength at this temperature and may be bend or cut by a blacksmith easily. $\endgroup$ – Uwe Dec 30 '18 at 21:19
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    $\begingroup$ @Uwe: I interpret it that he means the steel will still look silver/grey during reentry and not glow red. But we'll know for sure once SpaceX is releasing all the details (Elon mentioned that's scheduled to happen in March or April). $\endgroup$ – DarkDust Dec 30 '18 at 21:28
  • $\begingroup$ If Elon Musk means the steel will still look silver/grey during reentry, why he used the word liquid? Or is "liquid silver" meant as the metal mercury or quicksilver? $\endgroup$ – Uwe Dec 30 '18 at 21:36
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    $\begingroup$ @Uwe: Your guess is as good as mine here. We'd have to ask him. :-) $\endgroup$ – DarkDust Dec 30 '18 at 21:46
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    $\begingroup$ May be I am wrong, but some sentences by Elon Musk seems to be inprecise. Remember the space suit "tested to double vacuum pressure"? $\endgroup$ – Uwe Dec 30 '18 at 21:54

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