Fuel mass is the combined mass of the fuel, oxidizer and propellant in the craft. Typically about four fifths of the liftoff mass of a spacecraft is fuel. Rockets with dense fuels like quadrcyclene will tend to have a greater ratio of their gross mass as liquid. A Saturn V class craft requires about two million kilograms of fuel.

In pumped rocket and rocket cost given effective tensile, engine mass, fuel volume ratio, interface mass, mass, oxidizer volume ratio, payload ratio, propellant volume ratio and tank pressure, fuel mass is calculated which is in turn used to calculate tank mass.

fixed mass = engine mass + interface mass
fixed payload = payload ratio * fixed mass
relative tank mass = fuel volume ratio + oxidizer volume ratio + propellant volume ratio * tank pressure * 3.0 / effective tensile
tank payload = relative tank mass * ( 1.0 + payload ratio )
remaining mass = mass - fixed mass - fixed payload
fuel mass = remaining mass / ( 1.0 + tank payload )

tank mass = relative tank mass * fuel mass
 

In atmospheric spacecraft, multi stage spacecraft and spacecraft cost given empty mass ratio, mass and payload ratio, fuel mass is calculated which is in turn used to calculate fuel cost.

fuel mass = mass * ( 1.0 - ( 1.0 + payload ratio ) / empty mass ratio )

fuel cost = fuel mass * lox price * pow( ( exhaust velocity / 3330 ), exhaust velocity scale )
 
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