Why would one NOT want to place the shear pins as far forward as practical?

I’m surprised I have not found more about this on the forums. Maybe a post or two, so if I’m overlooking something that answers my question directly please point me on the right direction.

Seems that there is some consensus to place the pins as far back on the shoulder as possible, reason being that if the pin fails to shear the less time it spends dragging against the shoulder and the nose would still come out.

I’m dubious of that conclusion because in order for that to happen the pin would have to come out of the hole in the nose cone shoulder and deform the body tube and/or nose cone shoulder. My instinct is that would take more force than the ejection charge is capable of overcoming. And, even if the cone exited the chute it could get caught on those same bent shear pins.

It would seem to me that it is more likely that the aft position of the shear pin holes could actually end up venting ejection gasses before the shoulder has exited the tube. If the pressure is marginal for whatever reason it could mean a failure of the main to eject.

I usually put mine on the first ring around the shoulder if the NC has the thin rings to adjust the fit, or within about 1-1.5" of the bottom of the NC shoulder. On a properly sized shear pins and no gap between airframe and shoulder they will shear cleanly. There is no advantage I can see to moving them closer to the NC Airframe joint, I think it really comes down to personal preference.

Thanks Rich for responding. When you say 1-1.5 that's in what diameter air-frame?

jahall4 said:

Thanks Rich for responding. When you say 1-1.5 that's in what diameter air-frame?

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I pretty much only use shear pins in 38mm and above, mainly its 3" and up for that number (1-1.5") as smaller diameters can have shorter shoulders. My main reason for shear pins is repeatable results, with the launch site temperature variations we get in the Northwest (Eastern WA area) friction fits can be a bit squirrely. Really as long at the pins shear properly they can be put anywhere with no change in function (if someone else disagrees with this observation please post).

38mm motor?

What about using pins a Dual Deploy 1.6" air-frame?

jahall4 said:

38mm motor?

What about using pins a Dual Deploy 1.6" air-frame?

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I could have said 1.6" airframe, but for some reason my brain was thinking about a blue tube version of the Loc Weasel that I am working on as a minimum diamter build, it will use shear pins and the shoulder for it is only 2" long so maybe I will just put the shear pins in the middle.

So... are pins on a 1.6" airframe pretty common? Size #2s? How many? Surely no more than 3?

jahall4 said:

So... are pins on a 1.6" airframe pretty common? Size #2s? How many? Surely no more than 3?

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I only use them on fast moving 1.6" rockets, and the size is 2-56 and usually (always for me so far) 2 pins 180 degrees apart. Even with vent holes I don't trust a rocket that will see over Mach as staying together as the air pressure changes, but then again I overbuild or overcompensate.

I put them as far back as I can. I want them as close to the explosion. I think when they are aft, the nose cone is less likely to get cockeyed if one breaks first, which would cause the nose cone to get stuck and not break the other pins. I generally go 2-56 for 4" airframes or smaller, and 4-40's for larger. I will hold the booster section to the coupler tube in a DD setup with shearpins if I am going to go transonic.

EeebeeE said:

I want them as close to the explosion. I think when they are aft, the nose cone is less likely to get cockeyed if one breaks first, which would cause the nose cone to get stuck and not break the other pins.

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I was thinking that same thing too until I realized that the rear of the nose is always at the same location in that tube no matter where the holes are so the pressure is the same.

This kit I'm working with is a 1.6 Kraft paper (Madbow Kit) so I'll probably never attempt anything approaching mach, although the sim says its possible. My only concern for using shear pins to keep the main from deploying along with the drogue.

Any one concerned with using 2 pins instead of 3? any problems?

jahall4 said:

I was thinking that same thing too until I realized that the rear of the nose is always at the same location in that tube no matter where the holes are so the pressure is the same.

This kit I'm working with is a 1.6 Kraft paper (Madbow Kit) so I'll probably never attempt anything approaching mach, although the sim says its possible. My only concern for using shear pins to keep the main from deploying along with the drogue.

Any one concerned with using 2 pins instead of 3? any problems?

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2 pins are all thats really needed on a 1.6" airframe, but make sure to reinforce the holes with CA or some other method. The holes will elongate if the are not reinforced, even with CA reinforcement. Some people also like metal shear plates to protect the holes which I believe are a good technique on paper tubes.

Certainly using CA.

I have seen talk about shear plates, but installing them inside a small a cardboard tube sounds next to impossible since the plates would have to be recessed. Maybe a specially crafted/modified blind nut, thoughts?

I put mine right in the middle. I like symmetry. I think if you ground test, you are 99.9% sure that your pins are going to break and this is a moot point.

Bat-mite said:

I think if you ground test, you are 99.9% sure that your pins are going to break and this is a moot point.

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Certainly, just some due diligence to increase the chance for success.

EeebeeE said:

I put them as far back as I can. I want them as close to the explosion. I think when they are aft, the nose cone is less likely to get cockeyed if one breaks first, which would cause the nose cone to get stuck and not break the other pins. I generally go 2-56 for 4" airframes or smaller, and 4-40's for larger. I will hold the booster section to the coupler tube in a DD setup with shearpins if I am going to go transonic.

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The body tube gets pressurized evenly. Being "close to the explosion" is irrelevant.

It doesn't matter where these pins are placed. As mentioned, it is a moot point as long as your BP charge does the job to eject the nose and laundry, per your ground test. 1, 2, 3, or 4 pins all work.

Buckeye said:

... 1, 2, 3, or 4 pins all work.

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So... you have never seen a problem with the pins unevenly distributed as would be the case with just 1? No problem with the cone binding?

jahall4 said:

So... you have never seen a problem with the pins unevenly distributed as would be the case with just 1? No problem with the cone binding?

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I have only used 1 2-256 pin in 38mm airframes , going against the so -called rules , since the beginning.

1 was all that what needed according to calculations and that's what was implemented.
These are fiberglass with smooth fitting, cones. Not to tight, nor loose.
I see no reason to use 2, & have to increase BP to shear them.
I have 1 paper rocket that I wrapped a strip of glass around the top of tube where shear-pin hole is.
It also functions fine. I have never had any problem with NC's getting stuck or "cocked" in tube.
I am not the only one doing so, several of my flying buddies do the same.

blackjack2564 said:

...1 was all that what needed according to calculations...

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How are you Calculating? Mass of the cone easy, but what about the acceleration (or deceleration as the case may be) of the cone?

blackjack2564 said:

I have 1 paper rocket that I wrapped a strip of glass around the top of tube where shear-pin hole is.

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Didn't occur to me, I was thinking shear plate on the inside not a reinforcement plate on the outside, thx! Should be easy to do. Did you sort of feather or flare the strip into the tube? Picture?

jahall4 said:

How are you Calculating? Mass of the cone easy, but what about the acceleration (or deceleration as the case may be) of the cone?



Didn't occur to me, I was thinking shear plate on the inside not a reinforcement plate on the outside, thx! Should be easy to do. Did you sort of feather or flare the strip into the tube? Picture?

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Mass of chute, cords,quick-link nomex & cone. All can move when apogee is over zealous. This was 10oz.......considering a force of 35 G's....that would be 22lbs. My pins are 24-25lb shear. And that's only if things go south really bad.

First I wrap tube with glass, let cure, sand & prime.
Then mark with line.
Cut a 1/2in. section of tube off. Cut doesn't have to be perfect cut, cause you use line as key mark later.
Add/glue section to NC shoulder and feather joint with epoxy [thickened] and credit card. Pull epoxy around the joint with card to feather/fill.
On small projects just use JB Weld, larger ones warrant thickened epoxy. [3-4-6in and up]
You will have perfect fit.



I only have pics of fiberglass job I did. Same technique used on paper rocket.
I do this on all high performance project for that " perfect' fit.

blackjack2564 said:

...considering a force of 35 G's...

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So...where is the 35Gs coming from? and are you wrapping the entire tube?

Not sure where 35G comes from, either. A simple test is to prep your rocket and then ground test the apogee charge. Size your shear pins so that the nose cone stays on during the decceleration or shock cord jerk. Throw your payload bay around the backyard and swing it around your head. If the cone stays on, you are probably good!

I use 1 #2-56 pin in 1.5", 2 pins in 2.56", 3 pins in 4", and so on.

Do you know if the 35G accounts for pressure differential?

Forces from pressure differential and venting are minuscule compared to a 35g shock. See: https://home.earthlink.net/~david.schultz/rnd/pressurelag/parachute.html and attached Excel implementation of Dave's calcs.

View attachment Payload Pressure Calc.xlsx

dixontj93060 said:

Forces from pressure differential and venting are minuscule compared to a 35g shock. See: https://home.earthlink.net/~david.schultz/rnd/pressurelag/parachute.html and attached Excel implementation of Dave's calcs.

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I would think so, that's why I'm interested in where 35Gs came from.

So I can "plug" RockSim output into the spreadsheet and it will show force in lbs on the bulkhead nose? ... and if I'm only concerned with altutude won't the spreadsheet work as is for any payload section? All one needs to do is edit dimensions in the yellow cells correct?

jahall4 said:

I would think so, that's why I'm interested in where 35Gs came from.

So I can "plug" RockSim output into the spreadsheet and it will show force in lbs on the bulkhead nose? ... and if I'm only concerned with altutude won't the spreadsheet work as is for any payload section? All one needs to do is edit dimensions in the yellow cells correct?

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Jim (CJ, blackjack2564) should clarify himself, but I am sure that he took a nominal reading from a simulation and multiplied by 5, say 7gee on Rocksim goes to 35gee for a significant recovery event.

Regarding use of the spreadsheet, yes, enter the yellow cells for your particular rocket. This will characterize the force your rocket will experience through flight due to pressure build up in the airframe bays (booster, payload, etc. each run separately).

acceleration from bp charge at apogee from data file.
My charges were way higher than motors.Some over 75G's
I dialed back.....LOl

dixontj93060 said:

yes, enter the yellow cells for your particular rocket. This will characterize the force your rocket will experience through flight due to pressure build up in the airframe bays (booster, payload, etc. each run separately).

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Then qty(1) #2-56 is more than adequate for a 1.6 air-frame even if there is NO vent hole because the force on the nose appears to be no more than about 16 lbs. Sound correct?

jahall4 said:

Then qty(1) #2-56 is more than adequate for a 1.6 air-frame even if there is NO vent hole because the force on the nose appears to be no more than about 16 lbs. Sound correct?

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Well, very likely. I don't know the last time I used vent holes for 2" airframes or less.

dixontj93060 said:

Forces from pressure differential and venting are minuscule compared to a 35g shock. See: https://home.earthlink.net/~david.schultz/rnd/pressurelag/parachute.html and attached Excel implementation of Dave's calcs.

Click to expand...

Thanks, Great resource!

After fiddling with Simply complex’s spreadsheet and some actual, but informal testing, I’m preliminarily concluding 3 points about the use of shear pins in sub-sonic small diameter rockets (e.g. 1.6&#8221:

1) Even 1 pin is overkill since it typically takes 25 lbs to shear it and the pressure delta is never going to exceed 16 lbs at 20,000’ even when the cute bay is NOT vented (from the spreadsheet). Note @ 5000’ the bulkhead force is ONLY 5 lbs.
2) That a paper airframe must be reinforced with something stouter than CA or it will still elongate
3) That a paper airframe does not shear the nylon pin cleanly in that there is a propensity for the head to snap off before the thread shears

Are my conclusions reasonable?

So assuming the cone does not have a lot of ballast, that would have lots of inertia at drogue deployment, why not use a small piece (or two) of Scotch tape on the outside?