The exhaust velocity scale is the rate at which the cost of the fuel changes relative to the rate at which the exhaust velocity changes. A value of zero means that the fuel cost is independant of the exhaust velocity, higher values mean that the fuel becomes more expensive as the exhaust velocity increases. Higher exhaust velocities generally cost more, partly because the fuels require more energy to produce and partly because the handling cost is higher for colder propellants, which have higher exhaust velocities. Also, more energetic propellants have to be handled very carefully for fear of fire or even explosion, which also adds cost to the craft.

The baseline exhaust velocity is three thousand three hundred and thirty meters per second, which is the value for an engine burning oxygen and kerosene. Exhaust velocity scale is used with the empty mass ratio, mass and payload ratio to calculate fuel cost.

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

This is used in spacecraft cost.
 
 

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