Lakeroadster's American Rocket Society No. 2

[waltr]

Ever done a bench test? No visible plume expansion / reduction from ignition to ejection. Flame pattern seems pretty tight throughout.

Best viewed at 1/4 speed.

That is what I expected (didn't post until someone showed proof).
Reason is that the combustion chamber is at pressure right up to the nozzle. It is this pressure at the nozzle that determines the exhaust plume size/shape.
The plume may change on some motors due to changing internal pressure, motors that do not have a constant thrust during burn.
This also happen with reduced air pressure on rockets going VERY high, like near space.