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If you lift a swing at a children's playground and release it will swing back and forth for a while, losing some altitude in each swing, mostly (?) due to drag. Eventually it has lost all energy and stops.

Couldn't that principle be used when a spacecraft is re-entering the atmosphere? That is, go downwards into the atmosphere and then steer upwards - converting kinetic energy to potential energy but also lose some energy to friction - when the speed/friction-heat is too high, and cool down a bit (when you lose speed by going upwards the friction is reduced). Rinse and repeat until you lost enough energy to avoid the heat from atmospheric friction being an issue.

This would of course require some kind of wings with control surfaces but the space shuttle had exactly that.

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  • $\begingroup$ No, as soon as your velocity drops a bit below orbital velocity you start falling right away. You enter thicker atmosphere which slows you down much faster, and unless you are extremely careful you are first turned to jelly by 10 to 20 gees of deceleration then incinerated by the heat like this. $\endgroup$
    – uhoh
    Apr 6, 2021 at 4:04
  • $\begingroup$ Think of it this way: $E={{1}\over{2}} m v^2$, kinetic energy of a 2.5-ton Soyuz capsule, reentering at orbital 8km/s has energy equivalent of 20 tons of TNT. That energy must go somewhere, no way around it. If you avoid dissipating it early, you'll have to dissipate it later, one way or another, and better your way not involve cooking the crew. $\endgroup$
    – SF.
    Apr 6, 2021 at 10:19
  • $\begingroup$ @uhoh That is why you need wings. Friction requires atmosphere, but atmosphere also gives lift, if you have wings. $\endgroup$
    – d-b
    Apr 6, 2021 at 10:29
  • $\begingroup$ @sf A candle light has more energy than a hand grenade (I have heard). It is all about how long time it takes to release the energy. $\endgroup$
    – d-b
    Apr 6, 2021 at 10:30
  • $\begingroup$ @d-b ...and where. A candle lit right under your head with you unable to move away will kill you just as well as a grenade. Current methodology with reentry is to get great most of the energy into the air surrounding the spacecraft (and you need enough of that air - dense enough to contain it), a small part into ablator of the heatshield which promptly evaporates, and nearly none into hull of the capsule. Change timings and the capsule turns into a slow-cooking oven. And if you want it to heat too slowly to hurt people, your reentry will need to take weeks. $\endgroup$
    – SF.
    Apr 6, 2021 at 10:43

2 Answers 2

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It isn't possible to avoid heat from friction in re-entry, you have to deal with it in some way. What you are describing is called a skip-reentry, and it doesn't require wings. This technique was used by the soviet Zond spacecraft and Apollo spacecraft. The Zonds used the technique to alter trajectory, Apollo used it to avoid heat loads by extending re-entry. It is possible that the technique will be used again for returning missions from the Moon or Mars due to the high re-entry speeds.

Note that this technique helps to reduce friction loads, it doesn't eliminate them, you will still have an atmospheric reentry with significant thermal loads. This isn't an issue as we know how to deal with it.

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  • $\begingroup$ related: Did the Apollo Command module really “skip” within, or off of the atmosphere as a part of it's reentry program? See also Why didn't the Space Shuttle bounce back into space as many times as possible so as to lose a lot of kinetic energy up there? this one is pretty similar to the OP's question. $\endgroup$
    – uhoh
    Apr 6, 2021 at 8:34
  • $\begingroup$ That was kind of the point with my question. Let the vessel heat up a bit and then steer upwards, which would slow it down and reduce friction, which in turn let's it cool down. When cool enough steer downwards again. $\endgroup$
    – d-b
    Apr 6, 2021 at 10:27
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    $\begingroup$ Well, that's not exactly how it works @d-b. Most heat shields ablate, that is they sacrifice material to carry heat away - they don't heat up so they don't need to cool. $\endgroup$
    – GdD
    Apr 6, 2021 at 11:33
  • $\begingroup$ @d-b even for things like the shuttle with re-usable tiles the heat will be soaking inwards as well as out, leaving you with a cooling problem inside the craft while outside is too thin to use conductive cooling but too thick to deploy radiators. A quick descent allows you to cool the heat shield (and interior) with thicker sea level air. A mars return might be very different of course, with potentially hours or days to cool off in a true vacuum where a LEO return gets minutes at most. $\endgroup$
    – GremlinWranger
    Apr 6, 2021 at 12:15
  • $\begingroup$ @GdD SR-71 comes to mind ;-) Yes, but that is motivated by how re-entry takes place today. If, say, the craft only heated up to 200 °C and then made a "U-turn" and cooled off, steel or aluminium could handle the temperature. $\endgroup$
    – d-b
    Apr 6, 2021 at 12:28
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What you're suggesting is an aerodynamic reentry, where aerodynamic surfaces are used to slow the rate of descent into the lower atmosphere. In the real world, heat shields are often shaped in such a way as to generate lift. But wings are almost never used. That's because it's very hard to make an aero-spacecraft that's able to maintain level flight at hyper sonic velocities. It's a common misconception that friction is the primary source of heat on reentry. In actuality, the vehicle creates a zone of extremely compressed air ahead of itself. This increase in pressure super heats the atmosphere. The problem with efficient (level-flight capable) aerodynamics at near-orbital velocities is that sharp edges are needed at the front of the wings. The flatter a surface is, the further away it can hold the reentry plasma. That means significantly less conduction, and less overall heating. But near sharp edges, that plasma can inch much closer to your craft. This is why the space shuttle's leading edges had to use a much tougher, heavier carbon-carbon structure to maintain rigidity. All and all, conventional reentry methods are usually more cost effective. Check out Scott Manley's video here for more information:

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  • $\begingroup$ Interesting, thank you. $\endgroup$
    – d-b
    Apr 6, 2021 at 22:03
  • $\begingroup$ "This is why the space shuttle's wingtips had to use a much tougher, heavier carbon-carbon structure to maintain rigidity." Incorrect, I assume you mean the leading edges, not the wingtips. And, as we found out, it wasn't that tough. $\endgroup$
    – Organic Marble
    Apr 7, 2021 at 1:04
  • $\begingroup$ @OrganicMarble Sorry! I meant leading edges, but the word just didn't come to my head in the moment. I'll edit the post now. $\endgroup$
    – Wesley Adams
    Apr 7, 2021 at 1:07
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    $\begingroup$ Small nitpick: If you explain a misconception, then explain it correctly. Compression comes first, this causes the high pressure and temperature in the post-shock region. To write "In actuality, the vehicle creates a zone of extremely high pressure ahead of itself. This compression super heats the atmosphere." is just confusing and swaps cause and effect. $\endgroup$
    – AtmosphericPrisonEscape
    Apr 7, 2021 at 11:56
  • $\begingroup$ I was using "compression" and "high pressure" interchangeably. I see how that could be confusing! I'll edit it. $\endgroup$
    – Wesley Adams
    Apr 7, 2021 at 16:37

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