What supersonic aerodynamics phenomenon is seen here? 3

[luchezar]

I'm curious what supersonic aerodynamics phenomenon is seen on this interesting rocket photo? The linked page describes the event, but I'd like to add that this photo is a high-quality colour version of the frame at 0:09:02 (min:sec:frame) of that 1.5 MB greyscale video clip. The exhaust gases seen are from the recent firing of the 8 relatively small solid fuel rockets located at the "tail" of the rocket's first stage whose thrust is directed opposite of the thrust of the propulsion engines in order to quickly jettison the stage.

 

[luchezar]

I don't think that this is the case, for several reasons. First, the shock has already become invisible on this frame from the same footage taken a few moments later. Second, the retro-motor exhaust velocity of 2 km/s calculated above means that the velocity of the counter-rocket air stream must be lower than that, in order for the exhaust to be able to move from the "tail" (where the retro-motors are located) to "nose" of the rocket (as it can be seen it does on the footage). Third, the exhaust lag velocity on the footage is also of this order (easy to estimate if one views it frame-by-frame). And fourth, as Sylvain noted, the shock cone angle indicates a Mach number of about 3 or lower. So it's clearly not a hypersonic flight. But the Apollo 11 flight evaluation report gives a Mach number of 7.7 (p. 4-6) and Earth-fixed velocity of 2.4 km/s (p. 4-7) at this point! Do you see now why all this looks so strange to me?