What I mostly use are 2X56 nylon screws. For my biggest rockets, I use 4X40 nylon screws. There are calculators out there for BP charges and shear pins. Regardless, I highly, highly recommend you ground test. I have seen many recovery failures where the people (especially college teams) rely only on the calculations. I video record my ground tests. A number of times, my specific configuration doesn’t “agree” with the calcs, so I have to adjust and retest. I keep an Excel sheet of my calcs along with test results. This greatly helps when I prep the rocket again. For my backup charges (which I use almost all the time), I increase the BP charge 10-20%. You also want to test with a spent motor in the booster section. And I finally learned to ground test before painting since my ground tests would cause some scratching.
For cardboard rockets, I use mostly brass shims to help cut the pin to minimize tearing the cardboard even though I also strengthen the cardboard hole with CA. (Some of my older rockets had Al shims, but it’s softer so I’ve switched to brass.) Not an issue with FG AFs.With shear pins, I still drill a small hole in each AF section to minimize pressure issues. Oh, I use a minimum of 2 shear pins. Don’t use just one as it can jam with the uneven force. Normally I use 3 and sometimes 4. Depends on calculations and especially how heavy the section is. For example, my GPS sled in the NC makes the NC heavier so the shock of the drogue coming out can cause drag separation of the NC. So the shear pins are to help prevent that. I’ve seen a number of times that the NC pins are stressed with only the drogue charges.
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From TRF….A 50 +/- 3 pound shear load is required to fail (2) 2x56 shear screws. To insure that you develop enough force to shear the screws, you should have an ejection that will develop a 75 pound shear load.A 81 +/- 3 pound shear load is required to fail (2) 4x40 shear screws. To insure that you develop enough force to shear the screws, you should have an ejection that will develop a 120 pound shear load (or about 60 pound shear load for each 4x40 screw.)
