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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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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 will still produce 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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Has SpaceX released any explanation/data on the first stage return? – Rickest Rick Feb 5 at 18:40
    
What do you mean particularly? They've given out a lot of information. – Russell Borogove Feb 5 at 18:40
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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/… – Rickest Rick Feb 5 at 18:57
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Aerodynamic heating is related to the speed cubed! – Brian Lynch Feb 6 at 0:42
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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. – uhoh Feb 6 at 1:52

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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WOW! These photos add a lot of perspective of just what the bottom of the rock goes through. Thanks for the addition! – Rickest Rick Feb 5 at 19:03
    
Hobbes, you may be overstating your case a bit about the panels not needing to be there for ascent. As you can see in this image nasa.gov/sites/default/files/images/… STS had a lot of heat shielding there as well. There is a tremendous amount of exhaust recirculation on the blunt aft ends of these launch vehicles when they get into the thin atmosphere. Also see how burnt the aft end of the ET got. upload.wikimedia.org/wikipedia/commons/2/22/External_Tank.jpg None of this insulation was for entry. – Organic Marble Feb 5 at 19:51
    
Interesting video. I don't wish to challenge the logical chain regarding the heatshield necessity. I think more informed interpretation of video is necessary anyway. How do we know that the "it glows red-hot" is entirely due to re-entry and is not simply a consequence of the engine still being hot after the ascent firing? The colour scale isn't clear. I suspect it hasn't been calibrated for the expected surface emissivity - not least as we'd expect this to change in flight - it measures IR radiation, not temperature. We can't see if the colour white means 200deg or 1000deg. – Puffin Feb 5 at 20:18
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@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. – Puffin Feb 5 at 21:01
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@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. – uhoh Feb 6 at 1:22

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