How many 2-56 Nylon Shear Pins on a 4'' rocket

How many 2-56 Nylon Shear Pins on a 4'' blue tube rocket

Never done it before, using them on my L1 & L2 build. Have a drill and tap set for them.

I'd say 3. Then test your charges. You probably should epoxy a little brass sheet on the inside of the body tube where the holes will be, then drill through those and insert the pins. This will help keep the blue tube from getting deformed too badly over time. Others with more experience may say this is unnecessary.

Kevin

I agree with 3. However, shock cord length and ejection charge size are equally important. You can use the "right" number of pins and with a short shock cord, rip out the pins from the cord reaching its end before bleeding off enough energy. I am an advocate of extra long cords for this reason. And again, proper ejection charge size. Ground test.

GRIFFIN said:

I agree with 3. However, shock cord length and ejection charge size are equally important. You can use the "right" number of pins and with a short shock cord, rip out the pins from the cord reaching its end before bleeding off enough energy. I am an advocate of extra long cords for this reason. And again, proper ejection charge size. Ground test.

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Spoken like a TAP

Have 30 feet of 1" wide Kevlar from OnebadHawk so it's a proper quality product.

Rockets is a 38mm, 4" dia, 66"
Long with a 12" payload bay plus nose cone.

Am using motor for ejection charge.


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GRIFFIN said:

I agree with 3. However, shock cord length and ejection charge size are equally important. You can use the "right" number of pins and with a short shock cord, rip out the pins from the cord reaching its end before bleeding off enough energy. I am an advocate of extra long cords for this reason. And again, proper ejection charge size. Ground test.

Click to expand...

Agree, with Griffins advice, shock cord lengths are a very important part and should not be over looked. Definately size your charges appropriately and ground test!.

I use 3 on all my 4" rockets, Blue Tube and cardboard. Rather than put in a brass sheet I do the following: My nose cones are molded plastic with the raised ridges on the shoulder. The shear pin holes are drilled to go through the aft ridge (that way there is no gap between the NC and the BT where the shear pin is located). The shear pin holes in the BT are soaked with thin CA 3-4 times to thoroughly saturate and harden the BT in that area. That is it. I have never had a problem with deformation/elongation of the holes in even the cardboard BT, and one of them has 15 flights on it, plus 3 ground tests.

I agree with what was said before- ground test to verify cord length and charge sizing. I require 3 successful ground tests in a row before I'll fly a given setup. There is always the chance that a single test is an outlier.

David

I use 3 of the 4-40s on my 5.5 inch diameter rockets, leaving the 2-56 for my smaller diameter models. They shear fine, assuming proper charges. I never bothered with brass sheets to assist in shearing, although that is certainly a good option. For my 6 inch and above fiberglass I use 3 of the 6's.

Shear pin requirement calculation

https://rocketsnw.com/?page_id=666

I use the same number of pins as fins, and line em up, either 3 or 4. Simple, works fine. Agree on the CA hardening in blue tube (though I only applied CA once) - I have one 4" BT rocket this way with 3 2-56 screws with > 10 flights, no problems with the holes. The NC is plastic, though a bit thicker than an aerotech NC - no reinforcement/brass sheets used, no problem. Tapping is not necessary, just drill the hole so it takes a bit of effort to push the screws in - you don't want them loose. Ground test. With 25+' of cord, you should have no problem, unless you put a lot of weight in your NC. I did this once on a PML kit (4" patriot) when I first started out with DD: since its a bit short, and I built too aft-heavy, had to add weight to the NC and used the stock cord (10-15'?) - 2 1/16" styrene pins were not enough, 4 were.

How much are you trying to hold against how many G's?

Determine strength needed to hold that much and divide by holding strength of pins.

Calculate charge to shear that amount.

Ground test.


Later!

--Coop