I am new to this so do not yell at me.. From what I have learned from brother in law (level 3 for many years) is a three inch rocket should not need shear pins, except if dual deploy you would want the nose cone (if set up this way) not to separate from the apogee charge and drag out the main. Not sure if you were talking about nose cone.
Bill
Sounds like an overgeneralization. The diameter of the BT has nothing to do with whether or not you need shear pins.
Think about what a sheer pin does: it holds the parts together until you are ready for them to separate. Sheer pins aren't the only way to hold parts together. The other way is to friction fit them (i.e., make them very tight). Loose tubes require some masking tape to get a tight fit, and this is a pain, because outside temperature has an effect on tube diameter (things swell and contract).
If you can hold your parts successfully together without sheer pins using friction fit, then don't complicate things with sheer pins. However, if your tubes are loose and you don't want to gunk them up with tape, sheer pins are a great option.
There are three things during the rocket's flight that might make the parts separate prematurely. They are:
- air pressure build up in the tubes due to the changing atmospheric conditions as the rocket ascends
- sudden drop in acceleration at motor burnout
- overenergetic apogee ejection charge stretching the shock cord and causing a whip effect on the payload section, which jerks the nose cone out at apogee (this is not catastrophic, but results in a very long walk :wink: )
For bullet one, you can drill a pressure-relief hole in the booster and payload tubes to mitigate the problem
For bullet two, your simulation should give you an idea of your rocket's acceleration at burnout. If the change in acceleration is drastic, then assume your payload section will continue upwards via momentum while the booster starts to fall due to drag. Here is where your rocket may prematurely separate during its acceleration phase, which usually destroys the rocket. This is mitigated by good friction fit, or shear pins in the booster.
For bullet three, again, good friction fit or shear pins in the NC. Hold that NC in place until you are ready for your main deployment charge. This can be mitigated to some extent by making your apogee charge small enough to not stress your harness, but large enough to separate the booster and payload.
Hope this helps.