But it should be noted that after the rocket enters the atmosphere, space and vacuum, the pressure of the gas exiting the nozzle is much higher than the pressure of the outside environment (vacuum - pressure is almost 0), so the exiting gas expands to such an extent that from the edges of the nozzle to Piron and the sides are thrown! And the efficiency goes down again!

This is one of the most basic rules that caused the engines to be built in two types of sea level and vacuum!