It probably won't you folks long to come back with the explanation,
but my google searches have come up with nothing . . .

What is the explanation for the dark band in the exhaust of the five
F-I engines of the Saturn-V? It almost seems to look like a band of
dark static smoke which makes no sense to me. It appears to be opaque
and dark where you might expect the exhaust to be the brightest. I
don't remember ever seeing this on the exhaust of any other engine.

Thanks in advance for any (reasonable) explanation.

John Pelchat