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This question already has an answer here:

Why do SpaceX rockets' re-entries seem to burn (as in heat the hull) less than the space shuttle ? Is it because of less friction surface ? Thanks !

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marked as duplicate by Russell Borogove, uhoh, Polygnome, Organic Marble, Nick T Sep 7 '17 at 16:40

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    $\begingroup$ tl;dr: Falcon first stages re-enter at speeds < 2000 m/s; orbital spacecraft like the shuttle re-enter from > 7000 m/s. $\endgroup$ – Russell Borogove Sep 7 '17 at 14:17
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The Space Shuttle re-enters from full orbital velocity.

The Falcon 9 first stage cuts off after only 2 minutes of flight. Much more comparable to the SRB flight times. Velocity and height are much lower.

They do a reentry burn to slow down as they enter the atmosphere.

All this suffices to keep the heating down to the level below the melting point of the Aluminum hull (The material it is mostly made of).

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The answer of geoffc is only partially true, as some parts of the Falcon 9 are forged out of titanium to sustain more heat than aluminium.
And considering material science the heating has to be kept even lower than to the level of the melting point to avoid that the aluminium alloys undergo a phase change of the crystal structure.

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  • $\begingroup$ We've never brought anything back from Titan, let alone used it to build a rocket out of! :) $\endgroup$ – Loren Pechtel Sep 9 '17 at 1:59
  • $\begingroup$ Titanium has been introduced this year for the grid fins, AFAIK no other parts of the F9 use titanium. $\endgroup$ – Hobbes Sep 10 '17 at 6:27
  • $\begingroup$ @Hobbes yeah, that's why I wrote "some parts". And besides that my second point considering alloys still applies. :) $\endgroup$ – Scotty1- Sep 11 '17 at 11:12

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