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I've never noticed this before, so I'm asking. Shortly after SRB ejection, the RD-180 plume appears to be very asymmetric. Why is this? If I recall correctly, these are full-flow & there isn't any gas generator exhaust, so I was confused.

Is this "bulge" in the plane between the two nozzles & thus representing a higher-pressure region between the two plumes? Is there some other reason?enter image description here

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    $\begingroup$ Wow. Gut feel, something is venting, but I don't know what it would be. $\endgroup$
    – Organic Marble
    Jan 21 at 19:33
  • $\begingroup$ that was my inclination after I mentally ruled out gas generator. thought it odd that it would have been venting directly onto the mounted SRB $\endgroup$
    – Anton Hengst
    Jan 21 at 19:56
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    $\begingroup$ I was able to find that the hydraulic system uses kerosene as its fluid; I wonder if it's an open-loop system? So far haven't found any details. $\endgroup$
    – Organic Marble
    Jan 21 at 20:02

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Looking at the engine configurations for the Atlas V:

Atlas V engine configs Credit: ULA, from Spaceflight Now article

I think it's likely a plume impingement effect from the two nozzles of the RD-180 engine. The plume expands out from each nozzle and between them has nowhere to go but out sideways, I suspect there is a nearly identical one on the other side of the rocket.

This is seen in other Atlas V launches, for example the Lucy launch (401 config), SBIRS GEO May 2021 (421 config), NROL-101 (531 config) to show a few.

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