I'm doing TARC and since there's a time range, optimizing the parachute size is extremely important. However, based on calculations, my rocket needs a 21 inch chute to hit that range. The problem is that all the parachutes I find around that size are way too heavy because the cloth that they employ is way too thick.
So instead, I've decided to use two parachutes. One of them will be 15". I have yet to decide on the other size, but I think it should be from 12-15 inches in diameter.
The problem is that I have no idea how to calculate how the parachutes interact, so I was wondering if there was anyone out there with extensive dual-chute experience who could shed some light on the topic.
Does the drag just depend on the net area? As in, two 15" would equal to a 21" due to the area calculated from radius? Or is it more complicated due to the aerodynamics caused by interactions with the parachute slip stream caused by another parachute?
I've also heard that the main reason people use dual chutes is redundancy? Is that true? To clarify, I'm thinking of attaching one parachute to the fin section and another one to either the shock cord or the sections with the payload bay and nose cone.
Thanks for your help.