Tank mass is the combined mass of the fuel, oxidizer and propellant tanks. Tank mass is typically one quarter of the empty mass. Tank mass divided by the empty mass will tend to be higher in pressure fed rockets. A Saturn V class rocket has a tank mass of about fifty thousand kilograms.

Given effective tensile, engine mass, fuel volume ratio, interface mass, mass, oxidizer volume ratio, payload ratio, propellant volume ratio and tank pressure, tank mass can be calculated which is in turn used to calculate empty 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

payload mass = fixed payload + tank mass * payload ratio
empty mass = fixed mass + tank mass
 
 

This is used in pumped rocket and rocket cost.
 
 

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