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Now that a Falcon 9 first stage has successfully landed after a launch mission, I want to know how the first stage can avoid burning up when coming back down to earth.

There doesn't appear to be any heat shield on the bottom of the rocket, and I assume the first stage would be well above the atmosphere, meaning that it would have to re-enter.

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    $\begingroup$ I also noticed during the latest decent video, that the first stage engines were firing in pairs at a very low thrust for about 5 seconds per cycle, then switching to another pair. This is most likely to assist in shielding the vehicle using the exhaust gases during the most critical part of re-entry. $\endgroup$
    – svrima
    May 1, 2017 at 19:43
  • $\begingroup$ @svrima I am quite sure they do not switch the engine pairs.. only 3 engines are restartable. $\endgroup$
    – jkavalik
    Feb 15, 2018 at 13:39
  • $\begingroup$ @svrima: There are two kinds of entry burns. There's the single-engine entry burn using engine #9, and there's the 1-3-1 engine burn using engine #9, then starting up #3 and #7, shutting down #3 and #7, and finally shut down #5. At no point do they use pairs or switch pairs or switch engines. $\endgroup$ Nov 13, 2023 at 14:09

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The heat of re-entry is highly dependent on speed. The second stage of the rocket is responsible for providing most of the speed needed for orbit, after the first stage lifts it out of dense atmosphere.

Falcon 9 separates its first and second stages at relatively low speed, so its reentry starts off drastically slower than a reentry from orbit -- about 1650 m/s for the return-to-launch-site flight in December 2015, compared to orbital speed of 7700 m/s. Stage separation can be quite a bit faster in their downrange, barge landing flights, but the first stage is still moving much slower than orbital velocity.

That's still up around Mach 5 or 6, though, which produces a lot of heat. So the rocket fires three of its engines to slow down further before entering the thicker part of the atmosphere. The exhaust plume from that burn, as well, forces the atmospheric compression that creates reentry heat to occur well away from the rocket.

The end result is that the heat load is light enough that the body of the rocket can survive it.

There's a protective shell on the underside of the rocket, rather than a heavyweight ablative heat shield. The engine bells themselves are bearing the brunt of what reentry heat there is, and they are obviously able to cope with very high temperatures.

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  • $\begingroup$ Has SpaceX released any explanation/data on the first stage return? $\endgroup$ Feb 5, 2016 at 18:40
  • $\begingroup$ What do you mean particularly? They've given out a lot of information. $\endgroup$ Feb 5, 2016 at 18:40
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    $\begingroup$ Confimred! Physicist Mark Adler stated that the original first stage(s) DID burn up, at which point SpaceX added the refiring of the main engines to slow the rocket down enough to avaoid burning up. quora.com/… $\endgroup$ Feb 5, 2016 at 18:57
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    $\begingroup$ Aerodynamic heating is related to the speed cubed! $\endgroup$ Feb 6, 2016 at 0:42
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    $\begingroup$ That's right @BrianLynch! I always forget - the drag force is speed squared, but work is force times distance, so power is force times speed, or speed cubed. $\endgroup$
    – uhoh
    Feb 6, 2016 at 1:52
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Here's an image of the bottom of the stage before launch.

F9 first stage

As you can see, the entire bottom is covered in white panels. I suspect those panels are a heat shield.

This SpaceX press release on the introduction of the Falcon 9 v1.1 refers to a heat shield. The reference is a bit oblique, but I think this refers to the first stage.

Here's the same area after a successful landing:

after landing

The panels are now charred black, but are still intact.

Here's an infrared video that shows the rocket gets pretty hot - at one point it glows red-hot, and that's before the engines are started for the reentry burn.

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    $\begingroup$ WOW! These photos add a lot of perspective of just what the bottom of the rock goes through. Thanks for the addition! $\endgroup$ Feb 5, 2016 at 19:03
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    $\begingroup$ @Hobbes I've now looked at the scale with a better pair of glasses, the regular numbers are constantly changing, still a puzzle to me. I agree its of value, its just the creators haven't passed on as much as they could to us. To me, an IR camera can only measure thermal power. One would have to calibrate for emissivity (maybe = 1.0, though its not obvious that bare metal = soot = plume in that regard) and also absorption to the distant camera. All this is a distraction though: its "glowing" after MECO, during its manoeuvre to get out of the 2nd stage plume and afterwards. $\endgroup$
    – Puffin
    Feb 5, 2016 at 21:01
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    $\begingroup$ @Puffin in this screen shot of the beginning of this video it's stated "color scale: MWIR sensor counts". Counts refers to ADC counts (search here for the word "count") or read the answers to this question. Converting a MWIR signal to absolute temperature reliably would be more than a little tricky. $\endgroup$
    – uhoh
    Feb 6, 2016 at 1:22
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    $\begingroup$ "Charred black" - couldn't this just be the same soot buildup we saw on the sides of the landed stage? $\endgroup$ Feb 6, 2016 at 7:32
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    $\begingroup$ I'm pretty sure this is not actually charred. Charred would imply an ablative heat shield, and that is not reusable without replacement as the F9 is supposed to be. $\endgroup$
    – JanKanis
    Sep 15, 2017 at 20:01

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