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During Earth's atmospheric entry of an Apollo-like capsule, what would happen if oxygen hitting heatshield was mixed with fuel injected through heatshield, and ignited?

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Edit & addendum : During hypersonic entry, is there a boundary layer, with velocity gradient, from 0 to hypersonic, close to heatshield's skin, where combustion may occur, and what about such a heatshield's shape behaviour? Would it act like a combustion chamber? (illustration below)

Ultimately, would this slow down entry by creating a bigger hypersonic draggy bubble around the capsule, some sort of entry burn, using no oxydizer.

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    $\begingroup$ I guess you're wondering whether that would slow down the capsule or something? $\endgroup$
    – DarkDust
    Dec 13, 2018 at 16:54
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    $\begingroup$ Ignition would not be necessary, the very high temperature will do it. Just remember how fuel is ignited within a diesel engine. But it would be a waste of precious mass transported together with the reentry capsule. $\endgroup$
    – Uwe
    Dec 13, 2018 at 16:55
  • $\begingroup$ @DarkDust exactly, Uwe nice point! $\endgroup$
    – qq jkztd
    Dec 13, 2018 at 17:27
  • $\begingroup$ effectively you'd have an ablative heatshield. Except the ablation generates heat, instead of carrying it away. $\endgroup$
    – user20636
    Dec 13, 2018 at 17:31
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    $\begingroup$ I think the mostly hypersonic airflow would be too fast for combustion to keep up? $\endgroup$
    – geoffc
    Dec 13, 2018 at 17:39

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"Doctor, it hurts when I burn fuel in front of my heat shield!" "Don't do it then."

The procedure you're proposing wouldn't do anything useful, it'd just put a bit more load on to your heatshield (apart from being an engineering challenge to implement).

Your "combustion chamber" wouldn't generate any thrust, because that's not what combustion chambers do. They're just good at converting the chemical energy of the propellant into the internal energy of the resulting gas. And that's useless without a nozzle. You need a nozzle to take that internal energy (temperature and pressure; chaotic motion of particles bumping into each other) and convert it into a directed macroscopic flow that has a large total momentum, which in turn pushes the nozzle in the opposite direction.

So the burning propellant would just generate some extra heat before escaping away. Although it'd also push some of the hot gas away from the capsule, you can do that in a much more efficient way without combustion. This is exactly what ordinary ablative heat shields do: absorbing heat and converting it into a lot of gas that keeps the hot gases away from the capsule. The only difference is that ablative heatshields don't generate any extra heat, while your combustive heat shield would.

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  • $\begingroup$ saying combustion chamber i thought nozzle. bad wording of mine. $\endgroup$
    – qq jkztd
    Dec 14, 2018 at 20:58
  • $\begingroup$ by the way nice answer thank you even if comment section is not a place to say thank you $\endgroup$
    – qq jkztd
    Dec 14, 2018 at 21:04

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