Exit area is the cross sectional area of the nozzle exit usually expressed in square meters. Exit areas will be roughly proportional to the mass and will increase as the operating altitude increases. Exit areas range from one five thousanth of a square meter for a model rocket operating at sea level to two hundred square meters for a Saturn V class rocket optimized for high altitude operation.

The exit area adds to the overall aerodynamic drag of the craft. Given exhaust velocity, area ratio, chamber pressure, combustion temperature, exhaust molecular, k, liftoff acceleration and mass the exit area can be calculated which is in turn used to calculate area and exit radius.

fuel flow = mass * liftoff acceleration / exhaust velocity

k = 1.21

throat temperature = combustion temperature / ( 0.5 + 0.5 * k )

throat pressure = chamber pressure * pow( 0.5 + 0.5 * k, - k / ( k - 1.0 ) )

throat area = sqrt( 8,314 J * K / kmol * throat temperature / exhaust molecular / k ) / throat pressure

exit area = fuel flow * throat area * area ratio
 

area = pow( ( mass / 13000 ), 0.6666 ) + exit area

exit radius = sqrt( exit area / pi )
 
 

This is used in pumped rocket and rocket cost.
 
 

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