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There are a number of questions addressing the desirability and/or feasibility of a slower reentry from Earth orbit in order to reduce thermal load. At least most of them run into the problem that once you are significantly below orbital velocity, the atmosphere is either too thick to decelerate slowly, or too thin to provide lift, so you are committed to a fairly rapid descent.

However, suppose you are going significantly faster than orbital velocity to start with, most likely because you are returning from the Moon or Mars. Is there merit in planning an initial trajectory for aerocapture rather than reentry? You would end up, hopefully, in an elliptical orbit with maybe a 12-24 hour period which would allow for radiating away accumulated heat, and a systems check, possibly followed by a few aerobraking passes to lower the apogee, before finally committing to reentry.

If the systems check revealed problems after aerocapture, a rescue mission might even be possible.

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  • $\begingroup$ Like a skip reentry? Apollo roughly used this on return from the moon, but it didn't 'skip' very high or for very long - nothing like a 12 hour skip $\endgroup$
    – Jack
    May 1, 2019 at 10:03
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    $\begingroup$ 12-24 hours wouldn't be remotely enough for a rescue mission. $\endgroup$
    – GdD
    May 1, 2019 at 12:56
  • $\begingroup$ @GdD It would only take a very small boost to raise the perigee and enter a relatively stable orbit of roughly that duration. Also if SpaceX achieve what they are aiming for (a big if) their rockets are designed for a very quick turnaround and launch. Superheavy and Starship with zero cargo could reach that orbit, and then the crew could transfer. That said, that bit is not really the main point. $\endgroup$
    – Steve Linton
    May 1, 2019 at 13:05
  • $\begingroup$ I can think of very few cases where that would work out @SteveLinton $\endgroup$
    – GdD
    May 1, 2019 at 13:07
  • $\begingroup$ Note that "thermal/heat load" means the time-integrated heat flux, see slide 7, which would be increased in this scenario relative to a direct entry. The proposed 2-stage reentry would reduce the peak (instantaneous) heating and peak (instantaneous) deceleration compared to a direct entry. $\endgroup$
    – BrendanLuke15
    Dec 3, 2021 at 13:08

2 Answers 2

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Absolutely. There are a couple of issues though:

  • 'Aiming' is a little more tricky. Either you need an extra burn to re-aim on the second entry or any deviation from the planned course on the first pass will be greatly amplified on subsequent ones.

  • You still need to do a lot of slowing down in one go. You still have to get down from at least orbital speed without losing to much altitude that your doing this in the lower atmosphere, so there's only so much you can reduce the heating by.

As the aiming in 'just' a precision problem and (re)starting an engine for a small impulse does not inherently need to add more weight, I suspect as the non-delta-v/mass ratio problem get relatively less hard than weight of heat-shielding, we might see this sort of thing become more common. Just a guess though.

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A long period to radiate heat isn't likely to be required. The difference between 20 degrees warmer and 50 degrees warmer then the baseline isn't huge.

As to if this is possible, this is more or less what Apollo did. The Apollo entry took about 9 minutes from entry to parachute deploy. Zond 6-8 used a similar trajectory, but they actually left the atmosphere. These were done to reduce the thermal load to something more reasonable.

Bottom line, there is certainly merit, it was kind of done with Apollo, and was done with Zond 6-8.

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  • $\begingroup$ Note that "thermal/heat load" means the time-integrated heat flux, see slide 7, which would be increased in this scenario relative to a direct entry. The proposed 2-stage reentry would reduce the peak (instantaneous) heating and peak (instantaneous) deceleration compared to a direct entry. $\endgroup$
    – BrendanLuke15
    Dec 3, 2021 at 13:08

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