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After a successful reentry burn of the stage, around 10 seconds after engine shutdown, one of the fins glows red, then begins to burn. My question is why would this burn after the stage already completed to reentry burn to slow it down. And if this grid fin began to burn, why didn't the others?

Here's a picture of the fin just as it began to start to burn.

enter image description here

I know there was already a question about whether the grid fins are flammable but I'm just wondering why they burned when they did and what stopped the others from doing so. I read that SpaceX uses ablative paint on their gridfins which would explain why it is burning in the first place, but why would it do so after slowing down for the reentry burn?

The reentry burn typically happens at around 70-40 km in altitude. I was unable to find data on the rate of descent after the burn.

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  • $\begingroup$ Possible duplicate of When I see the space x reentry vidos I see no flaming ionizaton $\endgroup$ – Russell Borogove Mar 31 '17 at 15:48
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    $\begingroup$ @RussellBorogove That question only explains why there is no flaming ionization of the first stage and how fast it is moving, I'm just confused on why the fin would start ablating after slowing down with the reentry burn already. $\endgroup$ – Jake Blocker Mar 31 '17 at 16:09
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    $\begingroup$ The reentry burn doesn't do most of the deceleration; it does just enough so that the stage can survive the reentry. Atmospheric resistance during the reentry provides most of the deceleration. $\endgroup$ – Russell Borogove Mar 31 '17 at 19:08
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According to Elon Musk (during the post-flight press conference):

New design coming for Grid Fin. Will be largest titanium forging in the world. Current Grid Fin is aluminum and gets so hot it lights on fire... which isn't good for reuse.

After the reentry burn, the stage still flies at high speed (I don't have an exact figure, but it's at least supersonic). That means there's a lot of aerodynamic heating, especially on items like the grid fins that don't have an aerodynamic shape.

The grid fins are indeed covered in ablative paint.

The 4 grid fins are controlled independently to steer the trajectory of the stage. A fin rotated to a large angle interacts less with the air (presents a smaller cross-section to the airstream), so it would experience less heating. Fins may also end up in the lee of the rocket body when the rocket isn't pointed exactly along its direction of flight. That's why you can get 1 grid fin that's more charred than the others.

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    $\begingroup$ The "top" grid fins might also be protected from the supersonic flow by the rocket body. $\endgroup$ – Antzi Mar 31 '17 at 8:36
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    $\begingroup$ The video screenshot says 17539 km/h ~ 4872 m/s. I think that qualifies as supersonic. $\endgroup$ – a CVn Mar 31 '17 at 8:42
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    $\begingroup$ But that's telemetry from the second stage. $\endgroup$ – Hobbes Mar 31 '17 at 9:00
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    $\begingroup$ @MichaelKjörling I think it's safe to say if the first stage came into the atmosphere that fast it wouldn't be a stage anymore. $\endgroup$ – Jake Blocker Mar 31 '17 at 16:47
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    $\begingroup$ I'm not quite sure what the last paragraph of your answer is trying to say; can you clarify how the control of the fins affects their heat load? Is it just that some will be deflecting while others aren't, or is it that they're using body lift at that point in the descent and some of the fins are in the lee of the rocket body? $\endgroup$ – Russell Borogove Apr 1 '17 at 1:21
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After the entry burn, the first stage is still going pretty fast - About 8200km/h, or Mach 6.6 or so. Still plenty fast enough for atmospheric friction and compression heating to take their toll.

The entry burn reduces the booster's speed from 8200km/h down to ~5800km/h - from deadly to mostly-survivable. The landing burn only slows the booster down from maybe 900km/h to zero. The other 4900km/h of deceleration is all done through drag.

(all these numbers are extracted from telemetry from the SES-10 launch from this video )

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It slows down at 70-40 km in altitude before the atmosphere pressure increased. As it descends the atmospheric density will increase and heating will increase if speed is unchanged. See air density vs. altitude

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