Shear pin in fiberglassed cardboard?

Hi- I glassed the tubes of my DD test bed, a LOC nuke pro max. My plan is to fly this as much as possible on MPR, so I can gain comfort and experience with dual deploy. I know many folks put brass shims in the NC shoulder or into the tube wall itself to ensure the pins get sheared instead of the cardboard getting ripped. Is a metal shear plate still needed if I have a layer of 6oz e-glass around the tube?

Thanks!!

I would, I recently had to fix a tube because of that same problem. It was a fiber-glassed LOC tube and the shear pin holes were about 1/2in down the body tube. When the charge fired it pulled a good chunk off the top of the tube. However, if you don't want to put a brass insert then you could probably just move the shear pin holes down at least 1 1/2in.

rocketjet787 said:

I would, I recently had to fix a tube because of that same problem. It was a fiber-glassed LOC tube and the shear pin holes were about 1/2in down the body tube. When the charge fired it pulled a good chunk off the top of the tube. However, if you don't want to put a brass insert then you could probably just move the shear pin holes down at least 1 1/2in.

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Thanks! What size pins did you use?

Hmm, I've used shear pins in rockets from 38mm to 7.5", from H to N motors in cardboard, phenolic tubing, and fiberglassed paper and phenolic. In most tubes the shear pins are just below the top of the body tube. I was taught you want the shear pins close the the end of the body tube so the broken pins don't drag over a long area and possibly bind. I've never had any real damage to a body tube other than the hole deforming on plain paper tubes. But that can be minimized by soaking the area with thin CA.

I use the typical nylon screws (2-56 nylon) with larger rocket and styrene rod with smaller ones. I don't think I've ever seen a tube damaged by a shear pin as described by soopirV. It would be interesting to hear more details.

When you think about it the strength of a shear pin should be far less that that of a body tube. I think if it were a bigger issue we'd hear from a lot more folks with damaged body tubes. I'm not saying it can't happen but in the 15 years I've been flying (and helping out at launches) I've never seen it.

Here's a simple test you can try: drill your holes, insert your shear pins, and pull sharply on the nosecone to break the pins. My experience is that you'll get a clean break of the pins with no damage to the tube at all. If the holes do get damaged you can just fill them and drill new ones.

But obviously my experience differs from rocketjet787, so as always, your mileage may vary.


Tony

I have used 1/16" styrene and 2-56 Nylon screws as shear pins in Loc cardboard tubes without problems. One thing that can be a problem is if there is a loose fit between the nosecone shoulder and the body tube; that can cause the shear pin to bend/stretch rather than shear.

manixFan said:

Hmm, I've used shear pins in rockets from 38mm to 7.5", from H to N motors in cardboard, phenolic tubing, and fiberglassed paper and phenolic. In most tubes the shear pins are just below the top of the body tube. I was taught you want the shear pins close the the end of the body tube so the broken pins don't drag over a long area and possibly bind. I've never had any real damage to a body tube other than the hole deforming on plain paper tubes. But that can be minimized by soaking the area with thin CA.

I use the typical nylon screws (2-56 nylon) with larger rocket and styrene rod with smaller ones. I don't think I've ever seen a tube damaged by a shear pin as described by soopirV. It would be interesting to hear more details.

When you think about it the strength of a shear pin should be far less that that of a body tube. I think if it were a bigger issue we'd hear from a lot more folks with damaged body tubes. I'm not saying it can't happen but in the 15 years I've been flying (and helping out at launches) I've never seen it.

Here's a simple test you can try: drill your holes, insert your shear pins, and pull sharply on the nosecone to break the pins. My experience is that you'll get a clean break of the pins with no damage to the tube at all. If the holes do get damaged you can just fill them and drill new ones.

But obviously my experience differs from soopirV, so as always, your mileage may vary.


Tony

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Thanks, Tony! Just one point of clarification- I haven't had any issues, I'm just trying to prevent any. Rocketjet787 reported a failure, which is what I'm afraid of!

soopirV said:

Thanks! What size pins did you use?

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I use 1/16 inch nylon shear pins.

There are various grades of nylon 6/12, 6/6, 6 ect ect some sources even sell different plastic polymides that aren't nylon at all but may be listed as nylon. The different harrdness and shear strength capacities of these variables can make a huge difference in the outcome of your cardboard. As can the fitment of your NC shoulder, lose and sloppy is not good for cutting shear pins. IMHO for all it takes inset brass cutters in your airframe and you'll have it covered.

soopirV said:

Hi- I glassed the tubes of my DD test bed, a LOC nuke pro max. My plan is to fly this as much as possible on MPR, so I can gain comfort and experience with dual deploy. I know many folks put brass shims in the NC shoulder or into the tube wall itself to ensure the pins get sheared instead of the cardboard getting ripped. Is a metal shear plate still needed if I have a layer of 6oz e-glass around the tube?

Thanks!!

Click to expand...

I like your thinking. I did something similar with a Loc Lil Nuke that I extended, and made dual deploy. This was 1999 or so. I flew a bunch of dual deploys with it on G80 and G125. Got the bugs worked out and then hit it with an H180.

I would use nylon 2x56 screws - two to be exact. The brass cutter is not a bad idea. What I would do first though is to harden up the hole in the card board with ca/ super glue.

I think the fg wrapped takes care of any hole reinforcement. .

Well done..let us know how it turns out

Kenny

soopirV said:

Thanks, Tony! Just one point of clarification- I haven't had any issues, I'm just trying to prevent any. Rocketjet787 reported a failure, which is what I'm afraid of!

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Whoops, sorry about that. I fixed my post for future reference.


Tony

I use aluminum inserts as a shear point.

@RFO. Where did you get those aluminum inserts? They look great.

BDB said:

@RFO. Where did you get those aluminum inserts? They look great.

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https://www.apogeerockets.com/Build...s-3-pk?zenid=7772145efd06721ca1fc8974a0db35f7

Random Flying Object said:

https://www.apogeerockets.com/Build...s-3-pk?zenid=7772145efd06721ca1fc8974a0db35f7

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I've seen those, but wasn't sure about the price. do you use 2-56 nylon screws in place of the screws that come with the kit?

soopirV said:

I've seen those, but wasn't sure about the price. do you use 2-56 nylon screws in place of the screws that come with the kit?

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The kit comes with a first set of nylon 2-56 screws, just replace with the same type after first use. For shear pin configuration, mount the threaded insert on the outside of the body tube and mount the countersink insert inside the coupler tube. The edge of the countersink insert will act as a knife to cut the nylon screw. Since the threaded insert is mounted to the outside of the body tube, it will be easy to remove the sheared screw after your flight.

Use the exact opposite configuration with the stainless screws if you want to couple tubes that do not come apart in flight.