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In the following images, it can be seen that the interstage (separating the first stage PS1 and the second stage PS2) of the Indian Space Research Organisation's (ISRO) Polar Satellite Launch Vehicle (PSLV) contains pairs of cylinders as indicated by the yellow and red rectangles respectively:

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Original Image Source: PSLV-C47 Gallery

What is the purpose of these cylinders? Are they separation motors? Or, are they present for passive stability? Why are they flat at the top, will they not increase drag during the atmospheric regime?

If possible, kindly support your answer with references.

Thank you in advance

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This highly annotated photo says they are separation motors aka "retrorockets".

enter image description here

Also

enter image description here

Ullage & Retro Rockets, attached to the outer surface of the PSLV, are another interesting mechanism utilised in the Jettison process. The PSLV uses 12 Retro Rockets - 8 for Stage I & 4 for Stage II; while 4 Ullage Rockets mounted on the body of Stage II activate during Stage I separation. This photograph of the PSLV-C32, above, shows a clear view of the Ullage & Retro rockets mounted on the body. Retro Rockets, attached to the jettisoning body, imparts the jettison a velocity, away from the PSLV, decelerating it, increasing the distance between the two.

As far as being flat on the top, this last picture shows that they are at least angled.

Found here

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    $\begingroup$ +1: Thanks a lot! Now, I could be familiar while watching its launch on 27th :) $\endgroup$
    – Vishnu
    Nov 25, 2019 at 16:12
  • $\begingroup$ @Intellex that linked website seems pretty informative about PSLV - thanks for the question that led me to find it. $\endgroup$
    – Organic Marble
    Nov 25, 2019 at 16:12
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    $\begingroup$ Yes, it's more informative than the official website! $\endgroup$
    – Vishnu
    Nov 25, 2019 at 16:14

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