In this video at 7:12 you can see a thermal shot of a landed space shuttle. There is a periodic white cloud coming out of the top. What is this cloud?


Supplementary answer because I like pictures.

Here's a schematic of the APU system. You can see the 3 exhaust ducts at the top, two on one side of where the vertical tail goes, one on the other. This is from the 1982 Shuttle Press Reference - so realize that it's not what the system looked like at the end of the program because the APU system underwent some extensive redesign. But this is still an accurate depiction of the exhaust ducts and vents.

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Here's a top view of Discovery approaching the ISS on STS-121.

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I cropped the picture and indicated the 3 vents.

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You can see an ignited APU exhaust plume and the characteristic pulsing of the APU exhaust (it used a bang/bang speed control scheme) at about 4:20 into this video of the STS-134 landing.

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    $\begingroup$ A picture is worth a thousand words, which makes this a very long answer :) $\endgroup$
    – Criggie
    May 19 '18 at 0:25

That is the exhaust of one of the Shuttle's three APUs. These hydrazine powered turbines provide power to Shuttle hydraulic systems.

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    $\begingroup$ That link is wonderfully detailed, is there any chance you could find the relevant part and include it in the answer please. $\endgroup$ May 17 '18 at 14:36
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    $\begingroup$ Sometimes an APU exhaust plume would ignite; spectacular at night landings. $\endgroup$ May 17 '18 at 15:12
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    $\begingroup$ @OrganicMarble: Is this the exhaust plume ignite shot you were speaking of? I couldn't find anything clearer, but this looks more like heating than ignition. collectspace.com/review/sts123_landing06.jpg $\endgroup$ May 17 '18 at 17:09
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    $\begingroup$ Note that APU stands for auxiliary power unit $\endgroup$ May 17 '18 at 18:10
  • $\begingroup$ @SethRobertson see supplemental answer I posted. $\endgroup$ May 17 '18 at 21:00

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