transonic

I put together a video of my third and final attempt to launch a supersonic Apogee Aspire. I guess transonic is *technically* the most accurate description but supersonic has a better ring to it! I lost the rocket in tall grass, and therefore lost the altimeter, but I found I could do a lot...

Does anyone know the mass flow rate for the HyperTek L550? I'm working on a transonic build and needed to ask this. I know that the equation for the mass flow rate is the density of fluid * velocity * cross-sectional area, but how do I find velocity and cross-sectional area? Thanks so much!