Interface mass is the mass of the components controlling, protecting and supporting the rest of the rocket. Interface mass is usually large, typically about half of the empty mass. A Saturn V class rocket has an interface mass of about one hundred and fifty thousand kilograms.

Given effective tensile and mass the interface mass can be calculated which is in turn used to calculate emptymass, fuel mass and tank mass. Interface mass is calculated using the tensile energy of the materials used for the tanks even though different materials will often be used for the computers, insulation, sensors, servos, struts and other interface components. The calculation assumes that if lightweight and expensive materials are used for the tanks, lightweight and expensive materials will also be used for the interface.

interface mass = 13,000 * mass / effective tensile

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
empty mass = fixed mass + tank mass
 
 

This is used in pumped rocket and rocket cost.
 
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