Initial burning surface area is calculated by:
A=T[4 pi D2/4]
Then by A=T[2 pi D2/4] + 2 pi dO[lO-lOC]+2 pi dI [lI-lIC-mb]
Where:
A is burn area at time t
T is a "texture" term defining width of burn front and a constant for each propellant type. This is always greater than unity and is controlled by granule configuration, inhibition layer etc.