Ambient pressure ratio is the outside pressure divided by the pressure of the expanded gases at the exit of the rocket nozzle. An adapted nozzle is one in which the outside pressure is equal to the pressure of the expanded gases at the end of the rocket nozzle. This balanced situation leads to the highest possible exhaust velocity. Usually the nozzle will be slightly underextended to reduce the engine mass. When a rocket lifts off from the ground heading towards low density atmosphere, it will generally be in the overextended condition at ground level and in an underextended condition at high altitude. The ambient pressure ratio is used along with exit pressure and relative exit to calculate the pressure velocity which is in turn used along with expanded velocity to calculate the exhaust velocity.

pressure velocity = exit pressure * ( 1.0 - ambient pressure ratio ) * relative exit

exhaust velocity = expanded velocity + pressure velocity
 
 

This is used in bipropellantrocket, tripropellant rocket, pumped rocket and rocket cost.
 
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