Mass is defined as the gross liftoff mass of the spacecraft. Mass is used to determine the frontal area of the spacecraft. It is assumed that the rocket has the density of water, one thousand kg / m^3 and that it has a length to width ratio of thirteen. The greater the mass of the spacecraft, the lower its drag to mass ratio, which therefore means that heavy spacecraft lose less velocity rising through the atmosphere and that the can orbit longer at a given altitude. Because of economies of scale, heavier spacecraft cost less per unit mass than lighter spacecraft; however, their total cost is still higher than lighter spacecraft.

In atmospheric spacecraft, multi stage spacecraft and spacecraft cost mass is used to calculate empty mass, fuel mass and frontal area.

Empty mass = mass / empty mass ratio

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

Frontal area = pow( ( mass / 13,000 ), 0.6666 )

Frontal area is then used to calculate the deceleration of the rocket at a given airspeed assuming a coefficient of drag of 0.8 obtained from Model Rocket Drag Analysis, NAR R&D Report.

drag = 0.5 * 0.8 * frontal area * airspeed^2

deceleration = drag / mass
 

In pumped rocket and rocket cost mass is used to calculate empty mass ratio, fuel flow and interface mass.

interface mass = 13,000 * mass / effective tensile

fuel flow = mass * liftoff acceleration / exhaust velocity

empty mass ratio = ( mass - payload mass ) / empty mass
 
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