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On the first tab the sea level pressure and temperatures are entered allowing the density to be calculated.

From this having entered an altitude the pressure, temperature and density can be calculated at this altitude
The second tab, using the previous tab information calculates the stagnation conditions for a given flight speed 
 
Either Velocity or a Mach number can be entered.
The third tab using the previous conditions, for a given nozzle stagnation temperature and NPR can calculate the nozzle exit area having specified the nozzle mass flow. 
 
 
 
The inverse calculation can be done by specifying the nozzle exit area and the program calculates the mass flow or a nozzle area.
The throat and exit nozzle (fully expanded) conditions are given in detail on the fifth tab.
Typical constants used within the calculations can be altered.

This is accessed from the top menu bar.
All the values entered can be saved and re read back into the program using a simple text file. XML can also be exported.