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Following this question, could a spacecraft approaching upper atmosphere from very low circular Earth orbit, produce lift to soar propulsively, bleeding speed at constant apoapsis, by sublimating material on its lower surface until entry speed is low enough to avoid very high temperatures when entering more dense regions of atmosphere?

Could it also raise its orbit, or explore very low orbits for extended period of time, as long as there is material to sublimate, converting its mass into lift or downwash?

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There would be thrust but it is going to be very slight. This is basically a rocket where there is no nozzle, so looking at specific impulse the exhaust velocity would be defined by velocity of the evaporating molecules (100s of meters per second)*sin45 due to the random departure direction time the mass per second of ablation.

If you wanted to improve this, you would have an internal pipe network in the vehicle skin that heated the working medium to vapor and exhausted it through proper nozzles, at least getting rid of the sine losses by constraining the thrust to the needed direction but the exhaust velocity of this system is still pretty low, quite possibly above Mach one but nowhere near the kilometers a second a chemical rocket has.

In general doing a powered re-entry will need similar performance to reaching orbit since you are doing the reverse so you need either similar efficiency per unit weight of fuel or a massive amount of 'fuel'. If your objective is to land bulk material, say a water from a comet there are some options here, but all will involve turning >90% of your payload to vapor. For things like manned vehicles a conventional heat shield does a better job for far less mass.

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  • $\begingroup$ I agree with this answer, but what does this mean in the first sentence "...which is not noticeably heavier."? $\endgroup$
    – Organic Marble
    Commented Jan 8, 2019 at 14:33
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    $\begingroup$ A pot of boiling water will thrust downwards due to escaping steam. However there is not enough thrust to make it noticeably heavier than a pot of non-boiling water $\endgroup$
    – Steve Linton
    Commented Jan 8, 2019 at 14:34
  • $\begingroup$ I think you are not making something quite clear yet. At 373K the speed of a water molecule at $k_B T$ is almost 600 m/s. At heat shield temperatures the speeds of small molecules are way over 1,000 m/s. While the steam drifts slowly in air, it's because collisions that happen within microns cause the momentum to mix into the dense atmosphere, and that whole situation gets a little complicated to describe in one sentence. At very low pressures in space, that km/s is substantial thrust, and dismissing it with a kettle is misleading. It's not a good analogy. $\endgroup$
    – uhoh
    Commented Jan 9, 2019 at 3:14
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    $\begingroup$ @ Uhoh, thanks for the numbers on the exhaust velocity of boiling water and open space. I had thought it was down at 50m/s but only spent a couple of minutes looking. Guess that means I need to rethink that opening paragraph. $\endgroup$
    – GremlinWranger
    Commented Jan 9, 2019 at 8:26
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    $\begingroup$ @uhoh That's interesting. This basically resembles Project Orion, or the ways in which high intensity lasers are used to propel small objects by ablating their surfaces. How this works is going to depend a great deal on the details of how the molecules actually leave the surface. One could maybe get an idea with a flamethrower and a block of dry ice -- anyone happen to have those lying around? $\endgroup$
    – Steve Linton
    Commented Jan 10, 2019 at 16:18

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