Shear Pin sizing for fiberglass 5.5" rocket

[tammaraki]

I'm finishing up my L3 scratch built cert rocket and I'm waffling on shear pin sizes. I was originally thinking 4 4-40s on the booster and nose cone. After doing some reading on the forum I was thinking maybe 4 2-56s. Some people say that even that is excessive.

The rocket weighs about 24 lbs. without engines. Nose cone weighs around 2 pounds, it just has the recovery hardware and a Featherweight Tracker.

 

[Tech 68]

On my 5 inch L3 I used 2@2-56 pins for the aft section at apogee deploy and 3@4-40 pins for the nosecone. All ground tested fine but to "hedge my bet", I "Z-folded/taped" the livin' daylights out of my aft shock cord. My biggest fear was that apogee deployment force would shear the nosecone, but all was well.

Having test sheared my pins, my apogee pins sheared at roughly 20 lbs each, and the mains let go at 42.5 lbs each. My aft pins, being my own "concoction" sheared at a slightly lower value than stock black 2-56 screws, which I also tested, getting 24.5 lbs @ shear.

 

[DragonRocketry]

My own personal rule has been 2-56 up to 4", 5" and up I use 4-40. You should be asking this question with your TAP/L3CC. No matter what we say, they may recommend something else.

 

[FredA]

You should be asking this question with your TAP/L3CC. No matter what we say, they may recommend something else.

I beg to differ:
You should tell your TAP/L3CC how you came to your decision on choices and that shouldn't be "I asked TRF."
If they object, a great learning discussion should follow.

L3 is about demonstrating YOUR competence.

 

[rocketgeek101]

On my 6" Ultimate Wildman I used 3 4-40 pins for the nosecone and 2 4-40 for the booster. Has worked fine on two flights thus far.

 

[tammaraki]

This is just for my test flight which my L3CC didn't care if I did it or not. My findings and reasoning will be added to my project doc.

I know what the numbers say, they say 4-40s is overkill. I was just looking for examples of what others have done.

Thank you for your responses!

 

[Rocketjunkie]

2 #2's at apogee separation point. 3 #4's at main separation. It has to withstand the jerks, etc. at apogee separation and descent. Also, use a *lot* of apogee shock cord. This will give time for the ejection forces to dissipate. It will also give time for the parts to slow down. Also size the drogue to stabilize the rocket and keep the main chute compartment above the booster. Better to use a little too much ejection charge than not enough. I've been a TAP since 1997 and have seen lots of successes and failures.

 

[StreuB1]

In my Demon 150 (6.25" thick wall FG) I use 3x#2 at the booster to AVBay/NC (apogee) and 3x#4 for the NC (main)

For my Demon 5 (5" std wall FG) I use 3x#2's and then 4x#2's in the same orientation as above, both are HED.

I agree with Tom (above), "Either blow it out or blow it up." Within reason, of course. Purely for reference, I use 3.6g (3F) and 2.2g (3F) in the Demon 5 (drogue/main respectively), and 4.5g (3F) and 2.0g (3F) on the Demon 150. Yes, the Demon 150 takes LESS powder than the Demon 5 to blow the cone at main.

Gathering your kevlar harness into single wrap taped bundles of 5 z-folds, 6" long, on 20" centers, does a really good job as a progressive shock arrestor for both the apogee deployment as well as the main deployment. Helps harness those forces on larger rockets that require larger shear forces.

 

[jahall4]

2 #2's at apogee separation point. 3 #4's at main separation. It has to withstand the jerks, etc. at apogee separation and descent. Also, use a *lot* of apogee shock cord. This will give time for the ejection forces to dissipate. It will also give time for the parts to slow down. Also size the drogue to stabilize the rocket and keep the main chute compartment above the booster. Better to use a little too much ejection charge than not enough. I've been a TAP since 1997 and have seen lots of successes and failures.

^^^^^^^^^^^^ Very good advice ^^^^^^^^^^^^^

I vary number of #4s between my drogue and main sections in 4" rockets and above. For example (qty 2) #4 in drogue and (qty 4) #4 in main. As I often say harness length is the "great redeemer". Here is a ground test of just that configuration.



Also, @Rocketjunkie mentions "keep(ing) the main chute compartment above the booster (fin can)":

 

[jkovac]

On my 6" Ultimate Wildman I used 3 4-40 pins for the nosecone and 2 4-40 for the booster. Has worked fine on two flights thus far.

I followed your build thread closely when working on my own L3 build (thank you!) and I also used 2 4-40 for the booster and 3 4-40 for the nose cone. Worked great.

OP, probably goes without saying, but ground test whatever you choose.

 

[tammaraki]

Out of an abundance of caution I went with 4 4-40s on the nose and 3 4-40s on the engine section. L3 accomplished at Brothers OR. 12250 feet on an M1340!