Shear Pins

[marvSRG]

I'm thinking about using shear pins in an upcoming two stage project to ensure the connection between the stages...I may as well go ahead and use them for nosecone retention, too w/dual deployment. I know the concept of them, but could someone give me a run thru on how to do them? I originally just thought small nails would work, but when I helped out Tom Binford with his Nike Smoke @ O-Berg last year, I noticed that they were the size of small nails, but he used a screwdriver to put them in, so I assumed they were small screws. I wasn't paying attention in full detail, though, so I'm not sure exactly how it was done. They may have been nylon screws, too. At any rate, could someone give me a walk thru on this?

 

[Ryan S.]

2-56 nylon screws is what you use. What you do it drill holes big enough to fit the screws on eihter side of your airframe right through you nosecone. Then take off the nosecone do everything that is needed and when you are finally putting the nosecone on stick the pins through the holes to hold it on. When the rocket reaches apogee the force of the charge will shear them and you rNC will come off

 

[marvSRG]

Thanx, Ryan. More questions.... Will the pins just be able to be pushed thru, though? Or will you need to use a small screwdriver to put them in, b/c that's what I believe I saw. Also, why don't the threads hinder the nosecone movement? You'd think they would.... It's obvious they work, though...I'm just wondering why.

 

[BlueNinja]

You can also use styrene rod in the same way.

 

[Ryan S.]

just shove the pins through. How would the threads hinder NC movement I am not sure what you mean

 

[edwardw]

It depends on your airframe size but you can use 4-40 nylon pan head screws or 2-56 pan head screws. I buy mine from McMaster Carr in the 1" length. Then you just trim down do sized and insert into the airframe. I usually do three pins so that there is less chance of binding, ie, nose con jamming if you only use one or two pins. Has worked for me and many people in my club.

Edward

 

[daveyfire]

I use two pins on opposite sides of the cone on 4" birds and four pins on 6" and 7.5" birds. For plastic NCs and cardboard tubes, I also cut out a shallow recess and epoxy in a thin brass strip to cleanly cut the pin on ejection. This avoids bending the pins and tearing the cardboard or elongating the holes in your nose cone. Works like a charm to keep drag separations from happening and to keep that danged nose cone in place until main deployment

 

[marvSRG]

Ryan...I had the idea that the screws would bind up and keep the nosecone on because I thought that they were literally screwed in to retain the cone and I was thinking that they were actually "blown out" by the ejection charge. Since they were screws, I figured that due to their threads, they couldn't be "blown out." But by the way David explains it, it sounds like they are actually broken when the ejection charge goes off. If this is true then I get it...they couldn't be popped out anyways because the threads hold them in there for that purpose...most likely because the pressure change from within the rocket would literally pop them out if they weren't bound by the threads. So they're bound by the threads to keep from being blown out by the pressure, and then when the ejection charge goes off, it's powerful enough to literally break them, allowing the nose/stage/whatever to come loose. Therefore also showing why nylon is the material of choice. This is because nylon can be easily broken with the force of an ejection charge. That would prove fatal with metal screws. Whew. Am I right on this now?

Thanx!

 

[Rocketjunkie]

Marv,

The Smoke uses 3 4-40 x 1/4" pan head nylon screws. They just press in and the holes are not threaded. I was probably using the screwdriver to work one in if the nose was slightly misaligned.
A #2 nylon screw shears at 35 lb., a #4 at 70 lb. If using shear pins in LOC tubing, you need to reinforce the tube at the holes, soaking in CA and redrilling the hole works. I also have the fit snug but not too tight.

 

[edwardw]

You have the right idea, the pins 'shear' clean off where you insert them. Most of the time I still have the heads on the rocket body, don't know where the rest goes.

Edward

 

[Ryan S.]

yeah if you take alot of peoples bigger Nosecones you wil notice there is stuff inside, these are the bottoms of the sheared of screws