Yup you are absolutely right. Glad I'm doing a build thread
I've been thinking about size+quantity of shear pins for the nose cone, and I'm leaning towards 3 4-40 nylon pins.The nose cone is going to be something like 2.25 pounds with hardware and tracker all installed. I want the pins to hold up to 50Gs, which should be pretty conservative considering my drogue is small for this size rocket (15"), my drogue shock cord is 25' of tubular nylon and I will have redundant altimeters firing the drogue at apogee. The only real way I could get remotely close to 50Gs would be if the drogue shock cord tangled badly during apogee separation, and even then I'm hoping it wouldn't hit that. This means the pins need to hold 112.5lbs of force. Rocketmaterials.org tells me that I'll achieve
115lbs on average with 3 4-40 nylon shear pins. In one test these failed at 99lbs, so if I wanted to be super conservative I could go with 4 of them. That feels like overkill though, and even with a 99lb shear that would survive 44Gs.
So, to get 112.5lbs of force plus a 50% safety margin on a 4" rocket I need 13.4PSI from the main ejection charge which should be fine, though I did the math out and this force applied to a 2.25lb nose cone will accelerate it at 75Gs! Hopefully my home brew tracker mount holds
In practice I'll ground test until I have a reasonably energetic separation of the nose cone and then have my backup main charge be sized 20-25% larger.