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Thought experiment illustrated below:

Suppose a weird spacecraft entering Earth's atmosphere from low Earth orbit, payload first. Behind it there is an inverted nozzle shaped structure, attached with struts.

Let's assume everything is made out of some heat resistant material, except the blue coating inside the nozzle, which is made of ablative material sublimating.

Blue material expands into a gas trying to escape the nozzle, while air molecules compress it inside the nozzle.

Could this device produce thrust, reducing entry velocity?

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    $\begingroup$ To work, there needs to be some air, and that means some drag. To be significant, the thrust needs to be larger than the drag that's already slowing down your parachute-shaped device very effectively. Your idea proposes to use the available kinetic energy to liberate gas and heat it, then direct it forward as a reaction mass. At very low pressure (molecular flow) this might be great, but as pressure increases you'll get a shock wave and pushback from the dominant atmosphere, and it will become just a parachute protected from melting by ablation. Same results, just a bit faster. $\endgroup$
    – uhoh
    Commented Jan 8, 2019 at 1:02
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    $\begingroup$ So look for low pressure applications, like gentle aerobraking at high altitude, rather than reentry. After crunching the numbers, it might have a significant thrust compared to drag, and so you could aerobrake at a higher altitude. That's handy once you release it, because the remaining small payload is now at a higher periapsis and could remain a lot longer. Normally it's strange to think of thrust to drag ratio when they both point in the same direction but here it's what you'd like if you want to aerobrake into an orbit and then stay there a while without a further propulsive boost. $\endgroup$
    – uhoh
    Commented Jan 8, 2019 at 1:11
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    $\begingroup$ I'd recommend you ask that as a separate question, comments here should focus on the current question. BUT I strongly recommend that you don't use the term "K-plane". I used it once and it made a big mess! $\endgroup$
    – uhoh
    Commented Jan 8, 2019 at 1:42
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    $\begingroup$ The nozzle shape is not going to help you at all with thrust here, because your propellant is just evaporating off the walls. For the diverging nozzle shape to increase thrust, you need to have supersonic axial flow (in the direction of the increasing area). The thrust from something like this will be negligible and as @uhoh says, buried in the shock cone coming off your "payload". I don't see you would ever get any significant thrust off the bottom of your wing either. $\endgroup$
    – Organic Marble
    Commented Jan 8, 2019 at 1:52
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    $\begingroup$ My gut feeling: Any ablative material will make your device worse. The gas will fill up the inner part of the cone, hinder air from flowing in and essentially makes it behave more as if it were solid, like a regular capsule for reentry. $\endgroup$
    – asdfex
    Commented Jan 8, 2019 at 14:47

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