Engine mass is the mass of the rocket engine. Engine mass is usually small, typically around seven percent of the empty mass. A large part of the engine mass is the combustion mass, which is proportional to the volume of the combustion chamber and proportional to its pressure. To reduce this combustion mass, the operating pressure can be increased, which reduces the combustion volume by a greater amount, leading to a small net weight saving.

Given combustion mass, nozzle mass and pump mass the engine mass can be calculated which is in turn used to calculate fuel mass and tank mass. The combustion volume is assumed to be a sphere with a radius twice as large as the throat radius.

combustion volume = 8 * 4 / 3 * pi * throat radius * throat radius * throat radius
combustion mass = chamber pressure * combustion volume * 90 / effective tensile

pressure area = log( pressure ratio ) / log( area ratio )
double pressure = 2 * pressure area
three minus = 3 - doublePressure
22 = 3.5 * 2 * ~pi
conical = 22 * exit pressure * pow( exit radius, double pressure ) / three minus * pow( exit radius, three minus ) - pow( throat radius, three minus )
nozzle mass = parabolic * 60 / effectiveTensile

pump mass = pump power / pump power density

engine mass = combustion mass + nozzle mass + pump 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
 
 

This is used in pumped rocket and rocket cost.
 
 

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