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The BBC article Soyuz space capsule brings ISS crew back after five month mission describes the return of ISS Expedition 55's "Russian Anton Shkaplerov, American Scott Tingle and Japan's Norishige Kanai" today. Looking at the capsule in this photo from the article, there's a pronounced angled line that separates blackened areas from lighter-colored areas on the Soyuz reentry capsule.

I'm guessing this has something to do with soot produced by the ablation heat shield and a tilted attitude during reentry.

Which direction does the capsule tilt? Does the heat shield point slightly downward with respect to the reentry trajectory to make more lift? Does that result in the blackened areas on the nadir-facing side of the capsule?

enter image description here

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    $\begingroup$ Soyuz absolutely uses a high AoA lifting reentry, but possibly it varies according to atmospheric conditions etc. A ballistic reentry for Soyuz might peak at 20g - very unsafe for everyone involved. See Soyuz TMA-11 for an example of this. $\endgroup$ – Jack Jun 4 '18 at 10:39
  • $\begingroup$ I believe your suggestion about ablative soot is likely correct too $\endgroup$ – Jack Jun 4 '18 at 10:40
  • $\begingroup$ @Jack thanks! Consider posting an answer? I have no idea about this so anything will be helpful. $\endgroup$ – uhoh Jun 4 '18 at 10:50
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Angle of Attack

As mentioned, the Soyuz flies an AoA of -22.3 degrees. This allows it to use its aft (forward-facing because the capsule reenters retrograde) heat shield as an aerodynamic surface which provides lift.

This is know as a lifting reentry and is done to ensure the capsule descends more slowly; lengthening the deceleration time. This lowers the peak acceleration and spreads the thermal load over a longer time period. Without it, the Soyuz may experience peaks of ~20g acceleration - dangerous for both the crew and the capsule.

The Soyuz achieves its angle of attack by having a nearly symmetrical aerodynamic shape, but with an offset centre of mass. Due to the unpredictable nature of reentry, the amount of lift needs to be varied in order to target the landing site precisely. This is done by rolling the capsule which alters the vertical component of the lift vector.

Ablative Shielding

The Soyuz descent module has an ablative heat shield that is used to dissipate the thermal energy of reentry by 'burning off'. This can be seen on the capsule in the form of soot.

Because of the capsule’s angle of attack, it is in fact the zenith (upward facing) side of the capsule which receives the most ablated material as it is raised slightly above the shelter of the heat shield and more into the airflow.

This effect can be seen in this image (Note: this is NOT the Soyuz descent module)

enter image description here

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    $\begingroup$ @uhoh Yeh it always amazes me how much of a difference to the peak g that a small amount of lift can make. I believe almost all reentry vehicles have used a lifting or skip reentry since the Mercury days $\endgroup$ – Jack Jun 5 '18 at 7:26
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    $\begingroup$ You’re correct about the sideways lift- that’s why the Space Shuttle used alternating roll manoeuvres to average out the horizontal lift component. $\endgroup$ – Jack Jun 5 '18 at 7:35
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    $\begingroup$ @uhoh The shuttle couldn't modulate pitch. "As stated earlier, alpha must be kept within 3° of nominal. This small alpha envelope forces us to use bank to control altitude. The next time you hear someone talk about the shuttle doing roll reversals to bleed off energy, do not listen. The shuttle does roll reversals because it has a very small alpha envelope." From the training manual. $\endgroup$ – Organic Marble Jun 6 '18 at 0:41
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    $\begingroup$ @OrganicMarble starting from that, I've just stumbled across aviation.stackexchange.com/a/23889/14213 There's a nice little diagram there... $\endgroup$ – uhoh Jun 6 '18 at 1:04
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    $\begingroup$ Yeah, that answer is heavily based on that training manual. Which is ok, it's given as a reference. $\endgroup$ – Organic Marble Jun 6 '18 at 1:06
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This article states that the trim angle of attack for Soyuz during entry is -22.3 degrees.

The angle of the char in the picture looks greater than that, though.

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  • $\begingroup$ Oh that's interesting, and open access, yay! I'll give it a read, thanks $\endgroup$ – uhoh Jun 4 '18 at 12:45

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