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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

3 Answers 3

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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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  • $\begingroup$ 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. $\endgroup$ 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. $\endgroup$ 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. $\endgroup$ 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. $\endgroup$ 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. $\endgroup$ Apr 7, 2021 at 16:37
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What you're talking about is a skip-reentry, where you cut a series of passes through the high atmosphere to bleed off speed before making the final descent. But note that there's no need for wings, and not really a U-turn happening here -- from the perspective of an outside observer floating in space above the north pole, your orbit is just a curved path through the atmosphere that curves less than the surface of the earth does. If you watch your altimeter as you do this, you will appear to descend and then rise back into the sky, but that's just because your altitude is being measured relative to a ball instead of a flat surface. The vessel doesn't have to do anything to make this happen, it's just their orbital path, which happens to pass through some air along the way.

To explain that point more, consider an eccentric orbit that doesn't even touch the atmosphere -- at one end of the orbit it's 30,000 miles above the surface of Earth, and at the other end it's 80,000 miles away. Is the spacecraft "making a U-turn" on each orbit? Not at all, it's just going around in an ellipse. But if you watch the radio altimeter, it says you're going up and down all the time.

Anyway, there are several reasons we don't use skip-reentry paths. Skip-reentry is a trade-off. You can reduce the immediate heating but you have to stretch it out into a low-and-slow bake, which is actually harder to deal with for a spacecraft. Contrary to popular belief, space is not cold (at least, not the way we would think of the word), and things in space (or the extremely high atmosphere) cool very slowly, so if you give the hot shield time to cool, the easiest place for heat to go is into the cabin, and you're going to have to plan for how to manage that. You really want your reentry to be as fast as you can make it without causing damage to the ship or crew. It's usually better to just build a more robust heat shield that can power through the worst of it and get down into the thick air that will convect the heat away, instead of a lighter shield that requires you to tippy-toe into the atmosphere.

There's also a safety issue with a long reentry path. It requires your vessel to stay stable and operational for longer in between between "safely in orbit" and "safely on the ground". You need more air, more battery capacity (since you had to ditch or stow any solar cells before reentry), and potentially more fuel for the control jets. That makes it more risky in general -- there's more time in the critical zone for something to fail -- and it makes the vessel less useful in emergency situations. If something has gone wrong on orbit and you need to get down as soon as possible, a lazy multi-hour reentry path that swoops through the atmosphere multiple times is probably a lot less desirable than just pushing through and getting to a place where you can get assistance.

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  • $\begingroup$ Another thing that determines reentry path is g loads on the astronauts. The Soyuz capsule is quite capable of a fast reentry using no lift, known as a ballistic entry. However this creates about 8-9 g which is uncomfortable at best and can in some cases cause injuries. So they use lift to extend the reentry and reduce the load to around 4-5 g. There is a failsafe mode in Soyuz that will change to ballistic entry when the control system is in question. This happened in 2008 on the TMA-11 landing with Peggy Whitson on board, causing it to land 475 km short of the normal landing site. $\endgroup$ Jul 17, 2023 at 18:46
  • $\begingroup$ Yes, that's true, I should have said "as fast as you can make it without causing damage to the ship or crew". I was thinking of human limitations as part of the ship's designed max loading, but you're correct that the comfort and safety of the crew is often a bigger limitation than the strict structural capability of the capsule. $\endgroup$ Jul 17, 2023 at 19:49

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