Shear pins on a 8" dia bird.

I’ve been using this Ejection Charge and Shear Pin Calculator for my fiberglass projects and its recommendations have always been confirmed in ground testing.

https://www.rimworld.com/nassarocketry/tools/chargecalc/index.html

However, I’m working on an 8” dia project that may end up using #6 maybe even #8 shear pins if one was to simply scale up the hardware used in smaller projects. But, this calculator does not make a recommendation of pins larger than #4. So, this raises the obvious question… Is it best practice NOT to scale up the hardware and instead use more pieces? So for instance if I use 3 or 4 (#4) nylon screws for pins on a 4” dia. airframe does this mean best practice is to use 6 or 8 (#4) on a 8 dia. project?

IOW is best practice to “scale up” (size) or “scale out” (count)?

One could crunch some numbers, maybe even perform some material testing, but I expect one would end up simply confirming the conclusions and experience of others and to a certain amount their own common sense.

My 8in. has 3ft payload....Skyangle cert3xx chute...50ft of 1inch TN and NC weighs 5.5lbs.

1 shear pin size 4-40 aircraft grade nylon has shear rating of 38-40lbs. I use 4 pins and 4.5 grams of 4f bp with 5.5grams backup.
So I have roughly 150lbs of holding.
Years ago I did all the calcs with G-force and weights...blah..blah ..blah.

Now I have a log book with all size airframes and what I used, so now just look it up and go.
Only time I use #6 is on 12in. airframe
I use 3 pins size 2-256 on 4in frames... #4 is overkill unless your NC weighs a ton. Fewer pins ...less BP to separate...more gentle on recovery gear.

Here’s a table I got from CJ which is quite useful:

Thanks for posting that...looked all over in my files and could not find.

This material data chart is what I used to figure my use.

blackjack2564 said:

My 8in. has 3ft payload....Skyangle cert3xx chute...50ft of 1inch TN and NC weighs 5.5lbs.

1 shear pin size 4-40 aircraft grade nylon has shear rating of 38-40lbs. I use 4 pins and 4.5 grams of 4f bp with 5.5grams backup.
So I have roughly 150lbs of holding.
Years ago I did all the calcs with G-force and weights...blah..blah ..blah.

Now I have a log book with all size airframes and what I used, so now just look it up and go.
Only time I use #6 is on 12in. airframe
I use 3 pins size 2-256 on 4in frames... #4 is overkill unless your NC weighs a ton. Fewer pins ...less BP to separate...more gentle on recovery gear.

Click to expand...

This....
I rely on the experience of others. All the maths in the world are always approximations at best. Standing on the shoulders of giants is my preferred route.
...
You hear that, CJ?! I called you a giant... LoL! In the most respectable way possible.

timbucktoo said:

Here’s a table I got from CJ which is quite useful:
View attachment 388161

Click to expand...

Great chart, but I was hoping to see this type of information for #6 and one thing I have never seen is anything providing guidance for the number and size of pins required to keep the nosecone in the rocket understanding that number could be in a huge envelope. The only thing we know is that the figure is somewhere in between the weight of the nose and tension created by the weight of the nose just before the harness breaks.

I use three #8-32 on my 7.5" and 8" FG birds. I drill 1/8" holes, tap the coupler with a #8 tap, and then redrill the airframe hole slightly larger for ease of alignment.

As long as your deployment charge produces enough pressure to overcome the shear point of the combined screws, you're fine. Doesn't matter whether it's 20 #2-56, ten #4-40 or three #8-32. Just run the numbers.

Personally, with my bad eyes and bad fingers, I prefer less and bigger.

Bat-mite said:

I use three #8-32 on my 7.5" and 8" FG birds. I drill 1/8" holes, tap the coupler with a #8 tap, and then redrill the airframe hole slightly larger for ease of alignment.

As long as your deployment charge produces enough pressure to overcome the shear point of the combined screws, you're fine. Doesn't matter whether it's 20 #2-56, ten #4-40 or three #8-32. Just run the numbers.

Personally, with my bad eyes and bad fingers, I prefer less and bigger.

Click to expand...

Absolutely,

My current SWAG is somewhere in between using #4 or #6...

I regularly use qty 2, #4 in 4” birds for a total pin x section area = 0.01038
So if we simply double that for an 8” total area = 0.02076
If I use qty 3 #6 total area =0 .02343 (#4 equivalent 4.5 pins roughly)

So…. I’m tempted to drill and tap qty 6 #6 everywhere for steel or nylon allowing me to add 3 more shear pins for a total of 6, total area = 0.04686 (#4 equiv. 9 pins)

Based on this calculator: https://www.rimworld.com/nassarocketry/tools/chargecalc/index.html

I can blow 5 #4 (equiv 3 #6) pins using 4g of 4F

OR 9 #4 (equiv 6 #6) pins using 7g of 4F

Both seemingly reasonable figures for a 8” dia rocket using an 8g Charge wells.

jahall4 said:

Great chart, but I was hoping to see this type of information for #6 and one thing I have never seen is anything providing guidance for the number and size of pins required to keep the nosecone in the rocket understanding that number could be in a huge envelope. The only thing we know is that the figure is somewhere in between the weight of the nose and tension created by the weight of the nose just before the harness breaks.

Click to expand...

In the Death From Above video on Youtube, Gerald mentions the shear force on a 6-32 pin is 100 pounds. I looked up ratings for pins for my upcoming 7.5" rocket and saw 4-40 pins are about 60 pounds. The plan for my rocket is three 2-56 pins for booster to payload and three 4-40 pins for the nose cone.

As I just found out the hard way, it is also important to not overkill the drogue ejection charge relative to the shear pins. On the maiden flight of my stretched V-2 containing a 10 pound NC, the four 4-40 pins sheared at drogue charge firing at 2,600+ feet. Both main parachutes- for the nosecone/camcorders and the rocket body- deployed and necessitated a longer-than-necessary recovery walk.

I'm conducting a ground test of my lesser drogue charge later today. I'm also going to use close to the maximum number of 4-40 shear pins for my main charge.

MIke Momenee
TRA#12430 L3

My 8" rocket uses three 4-40 screws to hold in a 7ish or maybe 9lb nosecone (don't recall...). Haven't had a main at apogee event yet (fingers crossed...). The key is sizing the apogee charges to just separate the rocket. Gravity will do the rest. My apogee charge is 5.5 grams and the main is 6.5 grams. I have lots of empty space in the main tube though.

jmmome said:

As I just found out the hard way, it is also important to not overkill the drogue ejection charge relative to the shear pins. On the maiden flight of my stretched V-2 containing a 10 pound NC, the four 4-40 pins sheared at drogue charge firing at 2,600+ feet. Both main parachutes- for the nosecone/camcorders and the rocket body- deployed and necessitated a longer-than-necessary recovery walk.

I'm conducting a ground test of my lesser drogue charge later today. I'm also going to use close to the maximum number of 4-40 shear pins for my main charge.

MIke Momenee
TRA#12430 L3

Click to expand...

Shear pins have two jobs: 1) shear when desired; 2) prevent separation until desired.

It's a good lesson for all reading -- you can put three #2-56 shear pins in a 100# rocket, and by golly they'll shear! But will they hold the rocket together when you don't want them to shear? No way!

There are three forces that act on the rocket that may cause premature separation:

1. Motor burnout vs. inertia. When the motor burns out, the rocket is no longer under thrust. This causes deceleration. However, inertia causes the rocket to keep moving upward. This can cause separation at the nose cone (single deployment) or AV bay (dual deployment). The shear pins need to be of sufficient size and quantity to prevent this separation.
2. Air pressure inside tubes greatly exceeds atmospheric pressure at altitude. Heavy air from the earth's surface is trapped inside the tubes; as the rocket ascends, the pressure outside the rocket decreases. Thus, the air inside the rocket tries to force its way out (nature hates a vacuum), causing premature separation.
3. The apogee charge creates enough acceleration at apogee that the nose cone separates from the payload bay (this is a dual deployment problem only).

How do we mitigate these risks?

For #2, drilling pressure relief holes in the tubes is the easiest way. If this is impossible, then the shear pins must be strong enough to hold the parts together.

For #1 and #3, the shear pins must be strong enough to hold the parts together, but weak enough to shear when desired. So, how do you know?

In this case, F = MA is your friend.

The numbers below correspond to the numbers above;

1. Run a sim that plots acceleration. Find out what the acceleration rate is at motor burnout. In the worst case scenario (which is helpful to plan for, but unlikely to actually occur), the acceleration suddenly falls to 1G at burnout. Supposing for our example that the acceleration just before burnout was 16G, then when the boosters stops, the force on the forward part of the rocket is 15G. Take the weight of everything forward of the booster, let's say 20 lbs., and multiply that times the acceleration. Thus, F = 20 * 16, or F = 320 lbf. From this, you know that your shear pins need to have a combined shear strength greater than 320 lbf. If a #4 nylon screw has a shear strength of 60 lbf, then you would need six of them. Now, knowing that the acceleration of the booster does suddenly drop to 1G at burnout in any normal scenario, you should be fine with four #4 screws.
2. Use atmospheric charts to determine the pressure differential at apogee, and select shear pins to handle the differential.
3. Same as #1, except this time the inertia on the nose cone is supplied by the apogee ejection charge. A BP charge calculator will tell you how many lbf you are generating with your charge, so the shear pins need to be able to retain the NC when the drogue charge fires. Use the same F = MA to find out the shear strength needed.

From the examples above, you now know how many pins to use to prevent premature separation, but will your charge be sufficient to break them when needed?

That question is answered by your BP charge calculator. The lbf of the BP explosion is given, and it needs to be greater than the combined shear strength of your pins for that part of the rocket.

No doubt someone will be along to correct my mistakes, but if you have any questions, post them here or PM me.

No mistakes , but the acceleration (and hence force) created by the apogee charge (#3) is minimal (maybe even negative for the nose) compared to the main payload bay reaching the end of the harness (very positive) and the nose wanting to continue on because of its inertia.

jahall4 said:

No mistakes , but the acceleration (and hence force) created by the apogee charge (#3) is minimal (maybe even negative for the nose) compared to the main payload bay reaching the end of the harness (very positive) and the nose wanting to continue on because of its inertia.

Click to expand...

True.

Bat-mite.........beautifully presented.

Although I have launched several 6" dia. rockets, none had a NC which weighed nearly as much as the V-2. I did not take that into consideration when I used four 4-40 shear pins to secure it in place- the same number as I used for the other 6" dia. rockets.

I didn't ground test the charges for this rocket, and that clearly was a mistake. Sometime we (I) forget the basics, like "GROUND TEST!!".

Mike Momenee
TRA#12430 L3

jahall4 said:

No mistakes , but the acceleration (and hence force) created by the apogee charge (#3) is minimal (maybe even negative for the nose) compared to the main payload bay reaching the end of the harness (very positive) and of course easily mitigated with adequate harness length.

Click to expand...

Bat-mite said:

True.

Click to expand...

And I know you known this John, but for others who may be lurking ;-), this is why you want to use as much harness as you can get into the rocket. I know good kevlar harness is expensive, but one might say it is forgiving of a multitude of sins, some of them whoppers like overly aggressive ejection charges.

And I had a 50 foot drogue harness.............clearly not enough for my dynamite-sized charge.

MIke Momenee
TRA#12430 L3

FWIW... for years I use 4....4-40 shear pins on all my rockets that are 7.5 or 8 in in diameter.Over the years it has served me well.

Only exception was my 6 inch V-2 with all the gear stuffed in NC.
The NC weighed 9lbs. On that one I also needed [4] 4-40 pins.

What rocket is this we are talking about?
9 pins seems a bit excessive if normal situation. All calculators are just guidelines, nothing replaces actual flight experience.
When I first began flying I used several of them.
I found they were close on smaller diam. up to 4 in. But after that varied wildly from what was actually needed.

At least you have the common sense to try and figure this stuff out, hats off to ya for that,&good luck with your project.

It will pay off in the long run to keep a log of each rocket you build. Diameter-weight-charge sizes-# & size of pins used-size of drogue and main-length of shock cords.
Mine even contains simple flight info. Motor used-alit.ude of each altimeter and wind.

Most rockets are pretty close and now I can just refer to my log when building a new one for basic info AND pick motors for field/flying conditions.

That was very nice of you CZ...thank you. [blush]

blackjack2564 said:

FWIW... for years I use 4....4-40 shear pins on all my rockets that are 7.5 or 8 in in diameter.Over the years it has served me well.

Only exception was my 6 inch V-2 with all the gear stuffed in NC.
The NC weighed 9lbs. On that one I also needed [4] 4-40 pins.

What rocket is this we are talking about?
9 pins seems a bit excessive if normal situation. All calculators are just guidelines, nothing replaces actual flight experience.
When I first began flying I used several of them.
I found they were close on smaller diam. up to 4 in. But after that varied wildly from what was actually needed.

At least you have the common sense to try and figure this stuff out, hats off to ya for that,&good luck with your project.

It will pay off in the long run to keep a log of each rocket you build. Diameter-weight-charge sizes-# & size of pins used-size of drogue and main-length of shock cords.
Mine even contains simple flight info. Motor used-alit.ude of each altimeter and wind.

Most rockets are pretty close and now I can just refer to my log when building a new one for basic info AND pick motors for field/flying conditions.

That was very nice of you CZ...thank you. [blush]

Click to expand...

Yep, I log and document everything, almost to a fault because I'm always trying something new, driven by a digital checklist (Microsoft Notes is the Bomb for this) that includes preflight objects, post flight activities, and conclusions. On this bird I'll even be taking pictures and making video of field prep if for no other reason to demonstrate how its done. I'm finding out that college team don't always do this and its costing them.

jahall4 said:

Absolutely,

My current SWAG is somewhere in between using #4 or #6...

I regularly use qty 2, #4 in 4” birds for a total pin x section area = 0.01038
So if we simply double that for an 8” total area = 0.02076
If I use qty 3 #6 total area =0 .02343 (#4 equivalent 4.5 pins roughly)

So…. I’m tempted to drill and tap qty 6 #6 everywhere for steel or nylon allowing me to add 3 more shear pins for a total of 6, total area = 0.04686 (#4 equiv. 9 pins)

Based on this calculator: https://www.rimworld.com/nassarocketry/tools/chargecalc/index.html

I can blow 5 #4 (equiv 3 #6) pins using 4g of 4F

OR 9 #4 (equiv 6 #6) pins using 7g of 4F

Both seemingly reasonable figures for a 8” dia rocket using an 8g Charge wells.

Click to expand...

BTW... forgot to mention that by drill/taping 6 holes (in a hexagon) I can use 2, 3, 4, or 6 pins and still maintain symmetry.

Bat-mite said:

1. Run a sim that plots acceleration. Find out what the acceleration rate is at motor burnout. In the worst case scenario (which is helpful to plan for, but unlikely to actually occur), the acceleration suddenly falls to 1G at burnout. Supposing for our example that the acceleration just before burnout was 16G, then when the boosters stops, the force on the forward part of the rocket is 15G.

Click to expand...

One mistake

No, the inertial separation force is a function of the difference in mass between the sections and the acceleration (deceleration) after burnout. The acceleration before the event doesn't matter.

jderimig said:

One mistake

No, the inertial separation force is a function of the difference in mass between the sections and the acceleration (deceleration) after burnout. The acceleration before the event doesn't matter.

Click to expand...

So, subtract the mass of the booster from the mass of the forward section?

Bat-mite said:

So, subtract the mass of the booster from the mass of the forward section?

Click to expand...

The force trying to separate the rocket at burnout is given below. If you just consider inertia forces you can set R=1
This force can be - or + depending on the mass ratios. If the mass of the lower section is more than 1/2 the mass of the total rocket then the joint is in compression after burnout. No shear pins necessary.

Fsep = a [ M / (1+R) - M1 ]

Where:
a = max deceleration
M = total mass of rocket (mass NOT weight)
M1 = mass of lower section
R = drag ratio

What's your definition of drag ratio for this case?

Drag ratio, R, is the ratio of the upper CdA to the lower CdA. (where upper is above the break).

Great! That's what I was predicting but I wanted to make sure

blackjack2564 said:

...
Only time I use #6 is on 12in. airframe
I use 3 pins size 2-256 on 4in frames... #4 is overkill unless your NC weighs a ton. Fewer pins ...less BP to separate...more gentle on recovery gear.

Click to expand...

I have not used shear pins yet. I am guessing 2-256 pins are nylon screws? Is there a difference between 2-256 and 2-56 nylon screws? When searching TRF, there are probably 100 references to 2-256 pins.

Can someone clarify?

Would these work in a "standard" 4" fiberglass nosecone/air frame?

https://www.mcmaster.com/97263A705/
Thank you,
Zeke

Im willing to wager he really meant 2-56. Those McMaster screws will work fine

The chart CJ referenced as well as an answer to the OP's original question about #6 nylon screws can be found here:

https://web.archive.org/web/2013102...rials.org/datastore/cord/Shear_Pins/index.php

Zeke Johnson said:

I have not used shear pins yet. I am guessing 2-256 pins are nylon screws? Is there a difference between 2-256 and 2-56 nylon screws? When searching TRF, there are probably 100 references to 2-256 pins.

Can someone clarify?

Would these work in a "standard" 4" fiberglass nosecone/air frame?

Click to expand...

Those will definitely work, although I like to order the Philips head versions from McMaster - that way you can use either type small screwdriver you have on hand and I am no longer dexterous enough to keep the little straight slot screwdriver in the slot while driving it in, so when I use those, the paint around the shear pin holes has a bunch of cuts and chips from where I gouged it with a screwdriver. If they made them in torx, I would get them

---
That's a great tip on the Phillips heads. After reading through this thread, GROUND TESTING will definitely be needed before I am comfortable screwing my rocket together.

For reference, here is the Phillips version of the 2-56 nylon screws.

https://www.mcmaster.com/94735A707-94735A124/
Thanks again,
Zeke