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Whilst writing this answer about the PSLV, on this informative page about its Secondary Injection Thrust Vector Control system, this image (cropped) is displayed, showing that for the Core Alone version, "pods" that are roughly equivalent in size to the SITVC/Reaction Control System (RCS) units are installed, here labeled as "unknown function":

enter image description here

In some informative comments on that answer, user Ohsin identified those "pods" as Aerodynamic Stabilizers (ASTs) and supplied some images, from which I've cropped these (and added annotations to the last one):

enter image description here

enter image description here

In my research, I had run across mention of these "pods" being ASTs, but I disregarded it until Ohsin provided this info, because to me, they didn't appear capable of providing a beneficial contribution to aerodynamic stability - I would have expected some kind of fin, if that was truly the intent. But it looks like my intuition was wrong.

How do these two cylindrical "pods", symmetrically placed on either side of a cylindrical vehicle, contribute to its aerodynamic stability?

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I would have expected some kind of fin... How do these two cylindrical "pods", symmetrically placed on either side of a cylindrical vehicle, contribute to its aerodynamic stability?

Any source of drag placed well behind the center of gravity of the launcher will improve aerodynamic stability. The advantage of fins over cylindrical pods are that fins are lightweight and produce little drag when the rocket is flying at a zero angle of attack.

At a guess, the reason for using cylindrical pods instead of fins here is that the aerodynamic effect of the SITVC/RCS pods is matched by the AST pods, so the rocket behaves similarly in pitch as it does in yaw.

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    $\begingroup$ Makes sense, and probably match the aerodynamics of the (missing) SRBs better than fins would. $\endgroup$ – Organic Marble Dec 29 '20 at 21:06

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