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Why do spaceships have a frustum (portion of a cone) shape like e.g. the pressure capsule of the SpaceX Dragon on the image below?

   enter image description here

I think there is some engineering stuff behind this to do with aerodynamics during landing. Can someone explain why this is so?

TildalWave
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Forin
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  • Actually, there was a question about the shape of the Soyuz a year ago http://space.stackexchange.com/questions/3221/what-influenced-the-shape-of-the-soyuz-descent-module/3224#3224 – oefe Dec 08 '14 at 21:13
  • @oefe - true. I've deleted that comment now, but at any rate that question was a lot more specific and didn't cover much about how reentry works in general. – kim holder Dec 08 '14 at 23:26

1 Answers1

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The capsules designed to reenter the atmosphere have to slow down from about 8 km/s to zero by the time they get to the ground. They actually don't use the part that looks like a cone to do that. They all have flat bottoms that they face into the wind to do that. If you compare the Dragon capsule from your link to a Soyuz capsule, the Orion capsule, or the Space Shuttle, their shapes are very different, but they all have a part that is more pointy (though not very much, in the case of the Soyuz), and a part that is flat. The pointier bit is used when they are launched, to lower the friction of passing through the atmosphere (the Soyuz is launched in the middle of the rocket, and so it doesn't need a point). The flat part is used to come back.

blunt shapes of reentry vehicles

The black lines in the picture show where the front of the blast wave occurs when these shapes pass through the air at very high speed. The blast wave is what is slowing the capsule down and you want it to be as big as possible. A sphere is actually the shape that makes the biggest blast wave - in fact, it works too well to be used on a capsule with people inside. It slows down so fast they feel crushed. All the shapes used are partly spherical, on the part that is facing into the wind, behind that they taper in, but how much is optional. The Space Shuttle was an exception - it had only a very slight curve on the bottom because that was best when the overall design was considered. It worked because the surface area it had for slowing down was so large compared to the size of the whole spacecraft.

Notice how the blast wave doesn't touch the craft, it moves along in front of it. The craft moves so fast there is a bubble of air trapped in front of it, compressed between the outside face of the blast wave and the spacecraft. That bubble is also very important. When air is compressed, it heats up. The faster it is compressed, the more it heats. The fastest compression is on the face of the blast wave, and so that part is hottest. The air bubble also gets very hot, but not as hot as that, so it protects the spacecraft from the worst heat.

And see the fuzzy section behind the craft? That is an area where the air is thinner, because the spacecraft is punching a hole through the air, and it takes some time for that hole to fill back in after it has passed. That creates suction pulling on the back of the craft, and that slows it down too.

kim holder
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  • Re: "That creates suction pulling on the back of the craft, and that slows it down too."

    I would have thought that a pointy end would be best for the backside of the capsule to reduce drag or angled fins (like a vortex generator on planes) but this shows otherwise and in fact you want drag via suction during descent. Perhaps a concave end might create even more suction drag.

     – Scott Dec 08 '14 at 20:54
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    Suction drag and "suction pulling" don't actually exist; what you mean is that the air pressure on the front of the craft is not opposed by a force from the near vacuum behind it--that is, the blast wave has a stronger effect. At atmospheric reentry altitudes, the air pressure isn't high enough to make this a major factor. – geometrian Dec 08 '14 at 23:09
  • @Scott - i don't think it would do much over such a small area. The volume the air is rushing back into would be very similar, and you would have lost the space from the inside of the capsule, where things are already cramped. – kim holder Dec 08 '14 at 23:29
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    @imallett - Read 'creates suction, pulling...' Suction is just creation of a partial vacuum, the term applies. It continues to be a factor until the parachutes are deployed deep in the atmosphere. – kim holder Dec 08 '14 at 23:34
  • @briligg suction isn't a scientific term to begin with. What I'm saying is that the fictitious backwards force created by a lack of a forward pressure force is already accounted for in your otherwise correct description of the mechanism of blast wave deceleration. There is no extra force "pulling" backward in addition to the pressure from the blast wave. My second part was just a note that this effect is minor above 30km, where deceleration is highest. – geometrian Dec 09 '14 at 00:32
  • @imallett - I don't know if people are so confused that suction is a force, rather than a large pressure differential created by the overall movement in a system. It is just so convenient to talk about it as though it is - systems in this situation genuinely respond exactly as though there was something pulling them in the direction of the partial vacuum. This is an introductory answer and my main concern is that it be clear. The way it is stated explains the trails in the illustration in brief terms from everyday experience. – kim holder Dec 09 '14 at 15:26
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    Spheres are also less-than-ideal because they have a L/D ratio of zero, meaning all entries are ballistic, with no meaningful maneuverability or steering. Conical capsules are typically weighted so that they have a slight angle of attack, contrary to what is shown in the picture. This angle of attack provides a lift force which allows the reentry to be controlled to a degree by rolling the capsule. Apollo used this to create a skip-profile reentry to lessen peak heating and g-loads. It also provides a small amount of crossrange control. – Tristan Dec 09 '14 at 16:32
  • Can you analyze or provide reference for the analysis of the stability of these shapes? – Hans Jan 28 '19 at 09:23
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    @kimholder you might be surprised how far from "completely flat" the belly of an Orbiter was. It was the biggest surprise I had the first time I saw a real Orbiter. It was like walking under the gently curving belly of a whale. https://imgur.com/a/T50kOiw – Organic Marble Jan 29 '19 at 22:13
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    @OrganicMarble that's worth a correction, i think. I'll edit. – kim holder Jan 30 '19 at 21:41
  • @Hans No, i'm afraid not. If you don't find something here related to that after a search, i'd recommend asking a question about it. – kim holder Jan 30 '19 at 21:42
  • My understanding was that the blunt end keeps the hotter bow shock away from the structure (the "initial concept" picture would be a disaster for that reason), but the conical shape is aerodynamically stable, so the capsule doesn't tumble. A sphere would have no reason not to rotate unless it is actively controlled. – Greg Feb 02 '19 at 00:31