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Most spacecraft (or parts thereof) intended to survive atmospheric (re-)entry use an ablative heatshield, which, when exposed to the strong aerodynamic heating of (re-)entry, gradually ablates away, expelling a layer of hot gas which shields the vehicle from the far hotter gas and plasma that would otherwise destroy it; this allows the vehicle to use the atmosphere to slow down from near-orbital speed without succumbing to the massive heating produced by hitting the atmosphere at near-orbital speed.

When a heatshield ablates away, all of the ex-heatshield gasses produced thereby are expelled forwards, in the vehicle's direction of motion; they can't escape backwards, since the capsule is in the way. This should produce a small additional braking force on top of that provided by aerodynamic drag; how much additional deceleration does the average reentry vehicle experience as a result of the recoil from the gasses expelled from the ablating heatshield?

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    $\begingroup$ The ablation gas would be expelled perpendicular to the surface, therefore it depends on the shape of the vehicle (Apollo CM vs Vostok/Voskhod). Also the mass flowrate of the expelled gas (and therefore the corresponding velocity of the gas) would depend on ablative heat shield surface temperature, and the latter would depend on vehicle velocity and atmosphere density and would be changing as the vehicle descends. This would be highly nonlinear effect across the descending altitude. And perhaps would add a miniscule amount of decceleration. $\endgroup$ – Sergiy Lenzion Dec 19 '19 at 4:41
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    $\begingroup$ Your assumptions, as LeoS points out, are incorrect. There is a stagnant, high-pressure, layer of air at the heatshield. I doubt much of the vaporized material escapes that region in the first place. $\endgroup$ – Carl Witthoft Dec 19 '19 at 14:47
  • $\begingroup$ There is no nozzle giving ex-heatshield gasses a certain direction like the nozzle of a rocket engine. $\endgroup$ – Uwe Dec 19 '19 at 23:54
  • $\begingroup$ Related question space.stackexchange.com/q/33413/33950 $\endgroup$ – Sergiy Lenzion Dec 22 '19 at 9:30
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I don’t think you can separately consider just the ablative products.

There’s a region of high pressure hot gas that’s decelerating the total mass of the craft. That pressure is providing the force.

The pressure in turn is made up of the motion of molecules from both the ablative material and (mostly) the incident gas.

That pressure profile is almost entirely dominated by the incident flow of tons/sec of high-speed air; the addition of at most a few kg/sec of ablative gas isn’t going to change it detectably.

Looking microscopically, the pressure profile comes from gas molecules bouncing off the surface at high-T thermal speeds. At least in the subsonic region nearest the surface, they’re bouncing back and forth in near-equilibrium many times. Adding a few parts-per-thousand that are just emitted for a first time instead of bouncing isn’t going to change that equilibrium, hence the pressure, hence the force.

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  • $\begingroup$ I highly doubt that in the subsonic region there would be enough temperature to ablate the gas out of the shield. $\endgroup$ – Sergiy Lenzion Dec 20 '19 at 21:36
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    $\begingroup$ @LeoS I must not have been clear. There’s a subsonic region of gas entrained right next to the skin, inside the separation shocks. Except where shocks attach, and there’s a lot of effort to keep shocks from attaching. $\endgroup$ – Bob Jacobsen Dec 20 '19 at 22:14

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