To shear, or not to shear...

What is the consensus about the use of shear pins? I have read many posts on RMR and reviewed the information on ROL. I have seen the recommendations on how many shear pins to use for a given volume and BP charge.

But, what do you use? I am scratch building a 4" rocket with a 38mm motor mount and will have an electronics bay for dual deployment. Would you, or would you not use shear pins for the nosecone in the payload section? If so, what material would you use - nylon screws or styrene rod?

Let the commentary begin...

BSexton,

You will have to make the final determination of using shear pins by yourself, but we can offer a few guidelines. Shear pins are usually used on heavier models to prevent drag seperation of the heavy sections upon thrust burnout. A 4" plastic nosecone would not normally need shear pins, unless a significant amount of noseweight(several ounces to pounds) was added. I normally don't use shear pins on models smaller than 6" diameter, but that doesn't mean you can't try them.

Something else to consider is airframe construction. If your model is paper, shear pins can tear the paper and elongate the hole. To get a good shear, a hard surface such as Phenolic, Glass or Carbon Fiber is normally used as they can "cut" the shear pins quickly and easily without Bodytube damage. A Glassed paper tube may also suffice, but I would put some Epoxy or CA around the inside and thru the shear hole to harden/strengthen the shear point.

As far as types of pins go, many people use styrene rods or nylon 4-40 bolts. I use about 6 nylon 2-56 bolts for my 7.5" models rather than 2 or 3 larger 4-40 bolts. They shear easier and leave less stress around the shear hole.

Hope this helped.

Carl