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The paintings of the Apollo command module and space shuttle below portray the heating effects of re-entry. The whitest parts of the spacecraft get the hottest, with cooler parts in red and darker colors. If the spacecraft has to fall through the atmosphere to return to Earth, why don't the bottom parts of the craft (green arrows) get the hottest?

Apollo re-entry heat Shuttle re-entry heat

Related: What are the top temperatures occurring during reentry?

DrSheldon
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  • This is an aerodynamic shadow. http://www.buran.ru/htm/tersaf4.htm – A. Rumlin Jun 06 '19 at 03:22
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    (Intentionally written with a common misconception. A good answer will address that common misconception. For the record, I do know the answer.) It's not a very good-faith question then. So in that spirit, I offer this not-very-good-faith answer: https://en.wikipedia.org/wiki/Atmospheric_entry (see caption on related image).

     – Axel Magnuson Jun 06 '19 at 02:04
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    If you have a good answer to this already, I'd recommend you answer yourself so others can review – Jack Jun 06 '19 at 07:51
  • Even if people only suspect you know the answer but aren't telling, they still get a little persnickety. While people like challenge questions, in my experience they don't like it when they feel like they are being quizzed. It's your choice of course, but in this particular SE site explicit quiz questions are sometimes not well received. It's unpredictable. I have suggested once or twice before we have a "space golf" or a "puzzler" tag. (reminiscent of code golf). – uhoh Jun 06 '19 at 12:20
  • @uhoh: answer posted – DrSheldon Jun 06 '19 at 12:28
  • @Jack: answer posted – DrSheldon Jun 06 '19 at 12:28
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    Aren't the two images provided artist renditions? I don't think artist renditions are proof that it is indeed hotter at the top, it could just be how they drew it. – Magic Octopus Urn Jun 06 '19 at 13:44
  • @MagicOctopusUrn: They are; the question calls them "paintings". It's doubtful that we will ever get real photographs of reentry, especially from the perspective shown. The answer provides evidence that those paintings are reasonably accurate about the hot spots. – DrSheldon Jun 06 '19 at 14:12
  • @DrSheldon there are some decent images of reentering Shuttles in the CAIB report; there is at least one picture of a reentering Shuttle's top side https://space.stackexchange.com/q/18932/6944 – Organic Marble Jun 07 '19 at 00:29
  • @OrganicMarble: The SILTS pod is interesting. I've been looking through the CAIB report and can't find suitable images; can you help me there? It's not enough to spot the re-entering orbiter (e.g. the Albuquerque or Kirtland AFB pix), it needs to show where the heat occurs on the vehicle. – DrSheldon Jun 07 '19 at 00:52
  • I'm away from home and working on memory; I thought there were photos in there good enough to show disturbed flow on the damaged wing. – Organic Marble Jun 07 '19 at 02:18
  • @MagicOctopusUrn, the Space Shuttle used different heat-protective materials in different areas. The areas drawn as the hottest mostly correspond to the parts that used reinforced carbon-carbon, the most heat-resistant of the materials. It may be a drawing, but it seems to be a drawing that reflects reality. – Mark Jun 07 '19 at 02:26

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The answer belies the common misconception that getting to and from space is merely a matter of going up and down through the atmosphere. At this phase of re-entry, the velocity of the spacecraft is mostly in the direction across the atmosphere, not down through the atmosphere. You can see that the hottest points on both spacecraft are actually along the leading edges, not the bottom.

We've visited the topic of orbital speed before (as has XKCD). Although you have to get "high enough" to get into space, you also need to get "fast enough" to stay there. In particular, there must be a significant component of the velocity tangential to the orbit (that is, in the direction around the Earth). The Shuttle did this by pitching over quite early in its ascent, reaching an orbital velocity in the range of 7.5 to 8.0 km/s.

XKCD not fast enough for space

Returning from orbit requires the spacecraft to lose this velocity, which is done by atmospheric drag. The hottest part of the spacecraft will be at the edges facing the direction of the velocity (bow shock). Because the velocity is still mostly in the tangential direction, it is the edges along the side facing the orbital velocity. As you can see in the Shuttle painting, these leading edges are the nose and the forward edges of the wings and vertical stabilizer. The hottest parts of the Shuttle were covered in black tiles, and you can see them in the picture below covering the nose, wing edges, and tail edges (even in places that are on the "top side"!).

Shuttle orbiter in profile

Re-entry for Apollo was somewhat more complicated, as it was a trans-Earth injection rather than a low Earth orbit. The re-entry speed was 11 km/s. However, the direction of this velocity was still mostly across the atmosphere, rather than down through the atmosphere (see "relative wind" in the diagram below). Again, the hottest part of the spacecraft will be the side facing the direction of travel (the right side in the painting above). Indeed, the heat shields of Apollo capsules were not uniformly constructed, but instead were thickest on one edge of the base of the spacecraft. From Apollo Experience Report: Thermal Protection System (Tech Note D-7564):

Apollo ablator thickness

Why did the Apollo and Shuttle descend pitched at an angle, rather than purely horizontal or vertical? Well, that's the subject of another question...

DrSheldon
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    wasn't that obvious from the whizz lines? –  Jun 06 '19 at 12:30
  • @JCRM: Great observation, but perhaps not obvious to those who have the "falling down" misconception. – DrSheldon Jun 06 '19 at 12:36
  • Can we get a source on https://i.stack.imgur.com/91A7l.png? It's the main image proving that they knew re-entry was hotter at the top than the bottom. – Magic Octopus Urn Jun 06 '19 at 14:29
  • Also, I'm going to guess the answer is bow-shock. The front-most piece of the aircraft will split the atmosphere further and further from the rest of the craft, meaning that the first edges, along the velocity vector, will hit the atmosphere first, causing the pressure there to be at maximum. The further from this initial point of impact we go the pressure and heat would be lessened. The gradient on that diagram definitely looks like a pressure gradient of something that's in flight (and pressure can equal heat). This is a total guess by the way, but it sounds plausible. – Magic Octopus Urn Jun 06 '19 at 14:35
  • @MagicOctopusUrn: Citation added. You are absolutely correct about this being a bow shock effect; it's a matter of establishing why a side of the spacecraft is the "bow", and not the bottom (i.e. that the velocity is more horizontal than vertical). – DrSheldon Jun 06 '19 at 15:24
  • @MagicOctopusUrn: I can't open that link. – DrSheldon Jun 06 '19 at 16:11
  • Page 3 of https://elib.dlr.de/53288/1/Manuskript.pdf - I was talking about strong viscous interaction versus weak viscous interaction. I didn't know anything about it before today, but that looks pretty similar to what you were talking about. It's the colorful diagram which is apparently unlinkable :). – Magic Octopus Urn Jun 06 '19 at 16:52
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    What a great picture of Atlantis (sniffle). – Organic Marble Jun 11 '19 at 01:25