Exit pressure is the pressure of the expanded gases at the exit of the rocket nozzle. In an optimized rocket, the exit pressure will usually be slightly higher than the outside pressure, because the nozzle will be slightly underextended to reduce its mass. Operating in vaccum, the exit pressure will always be higher than the outside pressure, because the nozzle would have to be infinitely large to expand the exhaust gases to zero pressure. The exit pressure is used to calculate the boiling temperature of water, pressure ratio and pressure velocity.

boiling temperature = 1.0 / ( 0.002681 - 0.00020361 * Math.log( exit pressure / 101300.0 ) )

pressure ratio = chamber pressure / exit pressure

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

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

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