# Finding the flow rate of a gaseous oxidizer flowing through a solid fuel (and they're combusting!)

I'm wondering, is there a way to calculate the volumetric flow rate or mass flow rate of oxygen flowing through a solid fuel (ex. paraffin wax) and combusting with it?

After the combustion, the flow is choked at a converging-diverging (De Laval) nozzle, and the mass flow rate of the gaseous products of combustion can thus be easily found. The output pressure of the gaseous oxygen is also known (recorded from gauge at regulator). Lastly, the pressure at the choke point is also known.

I've been looking at using the Hagen-Poiseuille equation (modified version for compressible fluids) $$Q_2 = \frac{\pi R^4}{16\mu L} \cdot \left(\frac{p_1^2 - p_2^2}{p_2}\right)$$ but I don't know what pressure to set as $$p_2$$, among other things.

Any help would be appreciated,

Thanks :)

Also, if anyone's wondering, the thing oxygen is flowing through is indeed a hybrid rocket motor.