Pressure velocity is the extra exhaust velocity which comes from the pressure difference between the exit of the rocket nozzle and the ambient pressure. In an underextended nozzle this will be positive. In an adapted nozzle this will be zero. In an overexpanded nozzle this will be negative. As the rocket rises through the atmosphere, a fixed nozzle will be in an increasingly underextended state and the pressure velocity will increase.

Given ambient pressure ratio, exit pressure and relative exit, pressure velocity can be calculated 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 bipropellant rocket, tripropellant rocket, pumped rocket and rocket cost.
 
 

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