What size shear pins to use and how many

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Kehoes23

Kehoes23

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I have been working on a PML 1/2 scale Patriot for a while now, the build is done but I got no primer on it yet, and nothing is drilled yet for the shear pins.
I was thinking of putting four (4) 2-56 shear pins for the drouge and three (3) 4-40 shear pins for the main. Should this be fine or should I just stick with all 4-40 shear pins?
Thanks for your advice:marshmallow:
 
What size diam. is the project?

What size ejection charge?

Any nose weight in the NC? that will make the biggest difference.

How long are your shock cords?

Depending on the answers I will try and recommend. Rarely do you need any on the apogee side.
 
blackjack2564 said:
What size diam. is the project?

What size ejection charge?

Any nose weight in the NC? that will make the biggest


How long are your shock cords?

Depending on the answers I will try and recommend. Rarely do you need any on the apogee side.
Click to expand...
The rocket is 9 feet tall and 7.5 dia.
I'll be using a 30 foot shock cord
About 4 grams of black powder, it's a lot of tube to pressurize
And about 16 oz of nose weight.
I want to use the smaller shear pins for the drouge to prevent drag separation, but not sure if I can use the smaller shear pins on the main.
 
Cross-sectional area of airframe: pi x (7.5/2)^2 = 3.14 x 3.75^2 = 44 square inches.

Pressure separation force = 44 square inches x ejection charge pressure in psi: 1 psi = 44 pounds; 5 psi = 220 pounds; 10 psi = 440 pounds.

from https://rocketmaterials.org/datastore/cord/Shear_Pins/index.php for 3 or more shear screws: each 2-56 nylon screw requires ~21.4 pounds per pin shear force; each 4-40 nylon screw requires ~38.2 pounds per pin shear force. (4) 4-40 nylon screws require ~152 pounds force or 152 pounds/44 pounds/psi = 3.5 psi to shear. Each compartment should also have a small vent hole to prevent ascent pressurization.

I would use enough bp to generate a minimum of 5 psi so that would be my choice for both locations. You also should perform a ground test to confirm separation will occur.

Bob
 
Last edited:
FIRST you need to figure out WHY the shear pins are there (their function).
Figure out how strong they need to be to do that function.
THEN and ONLY THEN think about the charge needed to remove them once their function is complete.

Shear pins hold the rocket together.... "duh", you say....
But you need to figure out how hard it is to hold the parts together.
You need to hold things together during two events: burn out and apogee - plus pressurization if you didn't vent.
Figure out how much force is pulling the parts apart -- how much you need to hold.
Then goto the materials site and figure out what screws are needed to withstand the separating force.
THEN calculate the BP needed to eliminate the shear pins when needed.

MOST PEOPLE SKIP TO THE LAST STEP - do a ground test - and think they have it figured out....

And note: In-appropriately sized apogee charges can lead to huge g-loading at the apogee event -- making the job of the shear pins holding the main that much harder.


In summary: You need to know why you are using the shear pins...size them up, then size the charge to remove them.
 
Last edited:
To add to what Fred said, you did not provide a lot of information about how you might have built or modified your rocket compared to the stock 54 mm PLM kit with a base weight of 16 pound except that you have added 1 pound of nose weight which bring the minimum weight to 17 pounds plus the weight of the burned out motor casing. If the rocket is glassed, the motor upgraded to a 75 mm or 98 mm casing, and/or the recovery system increased to compensate for the additional weigh, the burnout mass could be a lot more. For example a 98 mm 3G burned out motor will weight about 8 pounds, and glassing and upscaled recovery system could push the burnout mass to 30 pounds. If your rocket experienced a peak 5 G deceleration at burnout, that would generate a 150 pound force through the rocket cross-section which is just about where (4) 4-40 shear screws release. This force is nominally in compression, however depending on the ballistic coefficient of the separable components, it could be less, or even in tension for example if most of the drag came from the aft end of the rocket and the nose cone section had a very high density. If the forces at the main or drogue separation points goes into tension you can experience a drag separation unless you use shear pins. This has been discussed in detail on TRF but I don't remember exactly when or in which forum.

Bob
 
Why the nose weight?
Did you glass the rocket?
Rocket that size should weigh about 30lbs depending on the amount of epoxy and hardware used.


I have a 6" G12 tubing rocket just about complete.
I was thinking of using two 4-40 shear pins from Rail buttons dot kom. to hold the NC on.
Don't usually use any for the booster payload connection.

JD

Kehoes23 said:
The rocket is 9 feet tall and 7.5 dia.
I'll be using a 30 foot shock cord
About 4 grams of black powder, it's a lot of tube to pressurize
And about 16 oz of nose weight.
I want to use the smaller shear pins for the drouge to prevent drag separation, but not sure if I can use the smaller shear pins on the main.
Click to expand...
 
The rocket is 9 feet tall and 7.5 dia.
I'll be using a 30 foot shock cord
About 4 grams of black powder, it's a lot of tube to pressurize
And about 16 oz of nose weight.
I want to use the smaller shear pins for the drouge to prevent drag separation, but not sure if I can use the smaller shear pins on the main.
Click to expand...

I have almost exactly the same setup on one of my birds. Have flown it several times under M impulse with four (4) 4-40 shear pins for the main. Haven't used any at apogee separation point and have been to a little over 10,000' with no issues. The main came out early once when both altimeters went off at EXACTLY the same time. Who would have thought? 8 grams of BP gives a pretty good jolt. 4 grams breaks the four bolts every time. I have no weight in my nosecone and it is a LOC 7.5". plastic. I'm using a free bag system that has worked flawlessly.

Hope this info is helpful in making your decision.
 
PLEASE - pay no attention to these "I have a similar setup" posts.
This is no way to choose your shear pins.
You learn nothing -- achieve zero increase in competency.
FIGURE OUT FOR YOURSELF WHAT YOU NEED FOR YOUR ROCKET.

Come back here for a sanity check....but don't just blindly copy what others have done....proper recovery is too important.
 
Yes Fred, That's why I offered only information not advice.
 
Thank you everyone for the information, I'm not going to do what other people have done, I know each rocket is different and each situation is different. I'm going to use this information as a starting point and do ground testing until I feel it's the best setup for my rocket. Thanks again for the information.:cool2:
 
FredA said:
FIRST you need to figure out WHY the shear pins are there (their function).
Figure out how strong they need to be to do that function.
THEN and ONLY THEN think about the charge needed to remove them once their function is complete.

Shear pins hold the rocket together.... "duh", you say....
But you need to figure out how hard it is to hold the parts together.
You need to hold things together during two events: burn out and apogee - plus pressurization if you didn't vent.
Figure out how much force is pulling the parts apart -- how much you need to hold.
Then goto the materials site and figure out what screws are needed to withstand the separating force.
THEN calculate the BP needed to eliminate the shear pins when needed.

MOST PEOPLE SKIP TO THE LAST STEP - do a ground test - and think they have it figured out....

And note: In-appropriately sized apogee charges can lead to huge g-loading at the apogee event -- making the job of the shear pins holding the main that much harder.


In summary: You need to know why you are using the shear pins...size them up, then size the charge to remove them.
Click to expand...


So, you basically have two transition zones that your main parachute compartment shear pins have to be able to hold through... the deceleration of motor burn out and the separation event at apogee.

My question is, how do you determine the amount of force you need to contain? I'm no physicist, so bear with me, but in order to calculate the amount of force applied to the shear pins at apogee, you would need to know the mass of the nose cone and the velocity of the airframe/nose cone when it hits the end of the shock cord during the apogee event, right? How do you do that with any accuracy? How do you calculate the velocity of the airframe/nose cone during the apogee event? Do you make approximations and err to the high side?

I totally agree with you in that it is good to know the forces involved so you can build accordingly, and I confess that I have been guilty of using other peoples suggestions on faith. But, you seem to have some knowledge that would help others avoid unnecessary failures, and I would really like to learn how to do it too. Care to share?

Thanks
 
F=MA is your friend.
You weigh to get the mass of parts on each side if the break points.
There are two break points a conventional DD rocket.

You know the peak deceleration at burn out - worst case is 100% thrust going to zero quickly.
If your rocket pulls 10G's off the pad, then worst case is 10G - 1G or 9G's max -- across BOTH break points at burnout.
You now have M and A, so you can calculate F which is the separation force the shear pins need to withstand.

NOW, granted this is simplified - ignores differential drag which you really want -- the deceleration is all due to drag and fins usually have more drag than the nose, thus pulling apart. You can fine tune your answer knowing this, or you can assume both parts will have drag and ballpark the same and use ~50% max thrust as the estimated acceleration value to model at burn out.

At apogee, you need to BREAK the drogue shear pins but retain the main pins intact.
Let's skip early/late deployment -- those add lots of stress -- and assume you nail apogee.
Let's also (for now) not talk about wind and rockets parts bumping -- let's talk apogee deployment.
YOUR CHARGES create the forces on the main shear pins.
If your apogee charge is "perfectly" sized, it will be somewhat gentle.
If you blow the crap out of things at the top, the shock can be pretty violent.
IT'S IN YOUR HANDS -- a good reason to ground test and get that apogee charge "just right."

What's typical??? For me, 20-ish G's is enough.
I've seen 30's of G's on flights where I guess on charges.
I've heard story of 100G peaks....in high winds.

If you blow the crap out of it and hit 100G's and have a 5# NC, then your shear pins would need to withstand (F=MA again) 500 pounds of force.

SO
- weigh to get the masses.
- assume reasonable acceleration values for the two events
- Use F=MA to find the force you need to hold.
- Pick your pins to hold that force
- Size your charge to reliably, but not over zealously break the pins.
 
Last edited:
FredA said:
F=MA is your friend.
You weigh to get the mass of parts on each side if the break points.
There are two break points a conventional DD rocket.

You know the peak deceleration at burn out - worst case is 100% thrust going to zero quickly.
If your rocket pulls 10G's off the pad, then worst case is 10G - 1G or 9G's max -- across BOTH break points at burnout.
You now have M and A, so you can calculate F which is the separation force the shear pins need to withstand.

NOW, granted this is simplified - ignores differential drag which you really want -- the deceleration is all due to drag and fins usually have more drag than the nose, thus pulling apart. You can fine tune your answer knowing this, or you can assume both parts will have drag and ballpark the same and use ~50% max thrust as the estimated acceleration value to model at burn out.

At apogee, you need to BREAK the drogue shear pins but retain the main pins intact.
Let's skip early/late deployment -- those add lots of stress -- and assume you nail apogee.
Let's also (for now) not talk about wind and rockets parts bumping -- let's talk apogee deployment.
YOUR CHARGES create the forces on the main shear pins.
If your apogee charge is "perfectly" sized, it will be somewhat gentle.
If you blow the crap out of things at the top, the shock can be pretty violent.
IT'S IN YOUR HANDS -- a good reason to ground test and get that apogee charge "just right."

What's typical??? For me, 20-ish G's is enough.
I've seen 30's of G's on flights where I guess on charges.
I've heard story of 100G peaks....in high winds.

If you blow the crap out of it and hit 100G's and have a 5# NC, then your shear pins would need to withstand (F=MA again) 500 pounds of force.

SO
- weigh to get the masses.
- assume reasonable acceleration values for the two events
- Use F=MA to find the force you need to hold.
- Pick your pins to hold that force
- Size your charge to reliably, but not over zealously break the pins.
Click to expand...

Thank you Fred for the physics lesson! :) I appreciate it. Now I can use this to help figure out the pins on my L3 which is similar in size to the OP's project.
 
FredA said:
F=MA is your friend.
You weigh to get the mass of parts on each side if the break points.
There are two break points a conventional DD rocket.

You know the peak deceleration at burn out - worst case is 100% thrust going to zero quickly.
If your rocket pulls 10G's off the pad, then worst case is 10G - 1G or 9G's max -- across BOTH break points at burnout.
You now have M and A, so you can calculate F which is the separation force the shear pins need to withstand.

NOW, granted this is simplified - ignores differential drag which you really want -- the deceleration is all due to drag and fins usually have more drag than the nose, thus pulling apart. You can fine tune your answer knowing this, or you can assume both parts will have drag and ballpark the same and use ~50% max thrust as the estimated acceleration value to model at burn out.

At apogee, you need to BREAK the drogue shear pins but retain the main pins intact.
Let's skip early/late deployment -- those add lots of stress -- and assume you nail apogee.
Let's also (for now) not talk about wind and rockets parts bumping -- let's talk apogee deployment.
YOUR CHARGES create the forces on the main shear pins.
If your apogee charge is "perfectly" sized, it will be somewhat gentle.
If you blow the crap out of things at the top, the shock can be pretty violent.
IT'S IN YOUR HANDS -- a good reason to ground test and get that apogee charge "just right."

What's typical??? For me, 20-ish G's is enough.
I've seen 30's of G's on flights where I guess on charges.
I've heard story of 100G peaks....in high winds.

If you blow the crap out of it and hit 100G's and have a 5# NC, then your shear pins would need to withstand (F=MA again) 500 pounds of force.

SO
- weigh to get the masses.
- assume reasonable acceleration values for the two events
- Use F=MA to find the force you need to hold.
- Pick your pins to hold that force
- Size your charge to reliably, but not over zealously break the pins.
Click to expand...


Thank you Fred,,,,,,,,,,
I just copied this into "My documents".......

Teddy
 

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