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What are the small triangular fins on the lowest part of the boosters of the Soyuz 2.1 rocket? What function do they have?

I circled one of them in the upper image here. They are not visible in the lower image, which is a Soyuz launcher in transport almost a year later. Are these fins somehow attached or extended only after the rocket reaches the launch pad?

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A frame I stole from this youtube clip A very beautiful filming of a Soyuz launch 28 April 2016!

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A January 2017 photo I found on spaceflightnow.com. The "fins" are not visible there.

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The Soyuz (booster) User's Manual from Arianespace calls them "aerofins" and says they are part of the attitude control system.

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An additional image I ran across showing the aerofins and stating that they are for "auxiliary course correction in the atmosphere".

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    $\begingroup$ "Soyuz aerofins" seems to be a good search term. But I wonder how that works, they are so tiny compared to the rocket and its many powerful gimballing engines, especially since it doesn't move very fast through the thick lower atmosphere. Do they reduce the vibrations or something rather than actually steering the rocket? And why aren't they visible when the Soyuz is transported horizontally? $\endgroup$
    – LocalFluff
    Mar 23, 2017 at 14:37
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    $\begingroup$ @LocalFluff I would assume that they aren't visible during transport because they are added on the ramp. They seem easy to remove and transport separately, but would make the transport more difficult due to increasing the diameter of the whole rocket. Regarding them not looking like they could do much: Even small fins can drastically improve stability and make gimballing unnecessary. Source: Playing lots of Kerbal Space Program :) $\endgroup$
    – Philipp
    Mar 23, 2017 at 16:00
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    $\begingroup$ @LocalFluff: Powerful or precise, pick one. Since the gimbals and verniers are quite strong, it's hard to perform very minuscule adjustments with them. Such tiny fins could be exactly what's needed to get rid of these the few arc seconds of error on ascent. $\endgroup$
    – SF.
    Mar 23, 2017 at 16:04
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    $\begingroup$ According to Spaceflight101 the main thrust chambers of the Soyuz engines aren't gimbaled; the verniers associated with them are. $\endgroup$
    – Russell Borogove
    Mar 23, 2017 at 17:53
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    $\begingroup$ There are a few reasons fins might be preferable to verniers for attitude control. For example, they have a different frequency response which could be used to fill gaps in the main control system. They are also exceptionally well understood and reliable. Another potential reason for there use is: fins do more than just control, they add stability. They can move the aero-dynamic-centre backwards/downwards. The further behind the centre of mass, the more stable the vehicle is. $\endgroup$
    – ANone
    Apr 16, 2019 at 12:24

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