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So I have a system where there is an air tank (0.2Litres @ 3000psi, 293K) and I have a convergent-divergent nozzle attached directly to it. What I want to know is an expression for how the mass flow rate varies with time as the mass in the tank decreases causing the pressure to decrease.

How do I go about setting up the relationship for the mass flow rate? And would I need a similar relationship for the volume in the air tank? I think I would find the pressure simply from P=MRT/V once I have expression for M and V. My guess is the mass flow rate vs time graph would look like an inverse logarithm graph.

I have already related the mass flow rate to the pressure in the air tank and so this would allow me to calculate how the thrust from the nozzle varies over time.

Thanks for any help.