I'm trying to get some theoretical performance numbers for a cold gas thruster using quasi-one-dimensional flow, and I don't know where to start. For combustion engines you would just take a given chamber pressure and the temperature from the combustion, but with a cold gas thruster you have neither of those. Moreover, due to the quick expansion of the gas from the tank, the temperature will be dropping, so my questions are:
What would be the pressure for the thrust chamber in a cold gas thruster? I reckon it's not the actual tank pressure, cause then you wouldn't bother having a converging nozzle at all, just diverging. So, given a tank pressure, how would you calculate the pressure inside the engine?
I suppose you have to take into account the cooling of the gas when expanding into the thrust chamber, but for that you'd need the chamber pressure to calculate the temperature drop. Am I missing anything?
EDIT1: My wording might have been a bit unclear, so as a clarification, the main question is why would you create a thrust chamber and a converging section for a cold gas thruster when you could easily achieve choked flow from the tank outlet itself. Adding to that, in the probable case that I'm missing some important functionality, how would one calculate the chamber pressure if you were to use one?
EDIT2: To anyone that might find this question in the future, here I was assuming the flow from the tank was coming already at mach 1 due to choked flow from the outlet. However, the usage of a regulator removes most, if not all, of that velocity, so you need a nozzle to accelerate the static gas at the outlet pressure.
EDIT3: Looking at it again, why would the inlet pressure be the outlet pressure of the regulator? If you look at that paper, there's an expansion chamber before the actual nozzle where the gas would expand. Then, the pressure would reach an equilibrium dependent on how much gas is leaving the nozzle and how much is being fed, at a pressure lower than the regulator outlet. Am I wrong?