Exhaust velocity is the effective exhaust velocity of the rocket at ambient pressure. The exhaust velocity is low when the ambient pressure ratio is high, which occurs when the rocket is operating at low altitude. The exhaust velocity is highest when the ambient pressure ratio is zero, which occurs when the rocket is operating in vacuum.

In bipropellant rocket, tripropellant rocket, pumped rocket and rocket cost given the ambient pressure ratio, exit pressure, expanded velocity and relative exit the exhaust velocity can be calculated which is in turn used to calculate final velocity, fuel flow and structure cost.

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

final velocity = exhaust velocity * log( empty mass ratio / ( 1.0 + payload ratio ) )

fuel flow = mass * liftoff acceleration / exhaust velocity

structure cost = pow( ( empty mass / 10,000 ), structure mass scale ) * pow( ( tensile energy / 205,000 ), structure tensile scale ) * pow( ( exhaust velocity / 3,330 ), structure exhaust scale ) * ten ton structure price
 

In spacecraft in vacuum, atmospheric spacecraft, multi stage spacecraft and spacecraft cost the exhaust velocity is chosen. The menu options assume that rocket engine is operating in vacuum. The values for combustion rockets are calculated using the tripropellant rocket script using the space shuttle main engines in vacuum as the reference. This is used to calculate the duration, final velocity, fuel cost and initial structure.

duration = exhaust velocity / liftoff acceleration * ( 1.0 - ( 1.0 + payload ratio ) / empty mass ratio )

final velocity = exhaust velocity * log( empty mass ratio / ( 1.0 + payload ratio ) )

fuel cost = fuel mass * lox price * pow( ( exhaust velocity / 3,330 ), exhaust velocity scale )

initial structure = structure price multiplier * ten ton structure price * pow( ( empty mass / 10,000 ), structure mass scale ) * pow( ( exhaust velocity / 3330 ), structure exhaust scale )
 
 

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