Performance Rocketry 4-inch Nike Smoke (Part 4 - Drilling av-bay vent holes and installing shear pins)

Figure 1 - Drilling shear pin and av-bay vent holes

Then final task before painting is to drill the vent holes for the altimeters and also some smaller holes for shear pins. Figure 1 shows the use of the Dremel for drilling the smaller 1/16" shear pin holes. The larger drill press was used to drill the 1/4" av-bay vent holes visible on the right.  I've explained the reason for the av-bay vent holes but not the use of shear pins, so let me remedy that.

High power rockets are often built with stronger heavier materials than model rockets and the nose cone on the Nike Smoke is a perfect example it's built entirely from fiberglass and with the 10oz of weight in the tip, it probably exceeds 2.5lbs.  When the rocket is launched it will gain speed and momentum quickly under the boost of the motor.  When the motor burns out the rocket will begin to decelerate rapidly.  Since there is more drag on the airframe and fins that portion of the rocket will want to decelerate faster than the more streamlined nose.  This phenomenom can lead to premature seperation of the nosecone from the airframe.  To prevent this, small shear pins are inserted through the airframe and nose cone shoulder that will "lock" the nose cone to the rocket body through the coast phase of flight.  Because the shear pins are small, they will ultimately give way ("shear") with the much larger force of the ejection charge when its time to deploy the recovery mechanism. 

Figure 2 - 1/16" styrene rod (from Evergreen)

 

Some people prefer to use small nylon screws for shear pins, and they work fine for many people.  My own opinion is that properties of nylon tend to make it tear rather than break in shear.  Plus nylon screws can be a tad pricey when you figure your going to go through 3 or 4 every flight.  What I like to do is fabricate my own shear pins out of 1/16" styrene rod.  Styrene is more brittle than nylon and will therefor break cleaner in shear with less force.

Figure 3 - Making shear pins is easy



Making shear pins is a snap.  All you need is a package of 1/16" (or whatever diameter you want your shear pins to be) styrene rod, a piece of brass strip, a piece of wood (so you don't put burn marks in your kitchen table) and a small candle.  Heat up the tip of the brass strip on the candle, place it on the wood, and lightly push a piece of the styrene rod onto the hot metal.  The styrene should melt and fan out to make a small head (like a nail).  Lift the rod, cut to the desired length and repeat.



Figure 4 - A finished shear pin

Figure 4 shows a close up of a finished shear pin.  Depending on the size of your model you may wish to use slightly larger styrene rod.  It all depends on the forces you expect to encounter.  If you need more strength, you have the option of using larger dia rod or increasing the number of pins.  You will want to consider the use of shear pins when you calculate your ejection charge size.  Most of the calculators I've seen out there have inputs for number and size of shear pins used.

Figure 5 - It's easy and inexpensive to make your

own shear pins

Using the method described above, you can make dozens of shear pins in a single evening for pennies.