Shear pin failures

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supertaco

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Let me preface this by saying thank you to all those fellow flyers that are educating me on the use of shear pins in another thread.

New questions: Has anyone experienced a shear pin failure? If so, what was the failure mode (e.g. binding, stripping)? And, even in the event of a failure, did the rocket recover in a non-ballistic manner?

Thanks!
 

Bat-mite

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I have heard tales of binding shear pins but never experience it. People swear you should not use less than three, but I have two in some of my smaller rockets.

The more likely failure than non-shearing of pins is shearing them too early, caused by an over-aggressive deployment charge. Suppose you decide to use motor backup to your altimeter for drogue deployment. You run sims that show recommended delay time of 5 seconds, so you add a couple of seconds to the delay, i.e. you drill for 7 seconds.

At launch time, due to wind, or perhaps a slightly over-energetic motor, your get more altitude out of your rocket than anticipated, and apogee occurs two seconds later than expected. Your altimeter charge and motor backup charge deploy at the same time, giving you a combined force that is twice that needed to eject the forward section.

Due to the excessive force created by the overpowered ejection charge, the drogue harness snaps back against the AV bay, and the force causes the nose cone pins to shear at apogee. Now you have your main out at apogee and a long recovery walk ahead of you.

The only way around thus us to not use motor backup, and use redundant altimeters and charges.
 

teepot

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The only failure I've had was a shear pin that didn't shear and was dragged thru the body tube creating a small zipper. I started using brass blades to ensure the pin shears. I switched over to copper foil which is thinner. I don't have to create as big of depression to use the foil.
 

supertaco

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I switched over to copper foil which is thinner. I don't have to create as big of depression to use the foil.
Huh, copper foil? Any specific material? I see copper foil tape; which seems completely easy to use. Great idea.
 

heada

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Stainless steel shim stock.


A single 6"x50" roll will last for dozens of rockets. I cut it into 1"x1" and apply with a very thin coat of epoxy inside the tube. Only needed on cardboard tubes. I also apply between the ribs on a plastic nosecone with a thin coat of epoxy. Doing both the tube and the nosecone act almost like scissors to cut the 2-56 nylon pin. I don't tap the hole for nylon pins. I drill the hole slightly under-size and when I push the pin in, the threads will hold it in place but its easy to push out with an awl after they've been sheared off.
 

Rocketjunkie

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I've had several failures, usually from shearing too early.
For cardboard and Blue Tube, I usually use masking tape for a friction fit. However, if you must use shear pins, use the shim stock. Mark alignment after drilling holes but before taking apart the sections.I do not tap the holes, just use a 5/64" bit for #2 and 7/64" for #4. PML phenolic and fiberglass I always use shear pins with no other hole prep. In any case, you don't want to have the joint too loose or it will shear early and shake out the main. I use the pick up the rocket by the nose and check it doesn't just fall apart. A slow separation is OK but drill a vent hole in each section. If your holes do get buggered up, rotate the the inner section some and drill new holes in the coupler (through the body tube holes). Mark the new alignment. I never use less than 2 pins, just use smaller ones on small rockets. Use a long apogee shock cord to allow the parts to slow down before jerking at full extension.
 

cerving

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I've taken to using #4 shear pins in the main parachute section and #2's in the drogue section. I've had several instances in which the force of the drogue deployment was enough to pop the main shear pins too when I used #2's all around.
 

crossfire

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I have heard tales of binding shear pins but never experience it. People swear you should not use less than three, but I have two in some of my smaller rockets.

The more likely failure than non-shearing of pins is shearing them too early, caused by an over-aggressive deployment charge. Suppose you decide to use motor backup to your altimeter for drogue deployment. You run sims that show recommended delay time of 5 seconds, so you add a couple of seconds to the delay, i.e. you drill for 7 seconds.

At launch time, due to wind, or perhaps a slightly over-energetic motor, your get more altitude out of your rocket than anticipated, and apogee occurs two seconds later than expected. Your altimeter charge and motor backup charge deploy at the same time, giving you a combined force that is twice that needed to eject the forward section.

Due to the excessive force created by the overpowered ejection charge, the drogue harness snaps back against the AV bay, and the force causes the nose cone pins to shear at apogee. Now you have your main out at apogee and a long recovery walk ahead of you.

The only way around thus us to not use motor backup, and use redundant altimeters and charges.
Let me preface this by saying thank you to all those fellow flyers that are educating me on the use of shear pins in another thread.

New questions: Has anyone experienced a shear pin failure? If so, what was the failure mode (e.g. binding, stripping)? And, even in the event of a failure, did the rocket recover in a non-ballistic manner?

Thanks!
In s lot of my FG kits I use 1 pin. Ever has failed me
 

supertaco

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Great tips, every one of them! Getting myself a lifetime supply of rolled stainless steel sheet :) Like the idea of putting bigger shear pins in the main parachute compartment.

I've only ever been a friction fit guy. We sent a two stage rocket, 5" in dia x 14 ft tall and all fiberglass, to over 16000 ft AGL in Black Rock in 1995, it recovered well and it was all friction fit. We also built and flew Son of Godzilla for many years on just friction fit until we loaned it/gave it to our friends who fly it with shear pins. However, I'm feeling the need to actually progress my techniques. So I totally appreciate all of the help with this.

Thanks again!

A single 6"x50" roll will last for dozens of rockets. I cut it into 1"x1" and apply with a very thin coat of epoxy inside the tube. Only needed on cardboard tubes. I also apply between the ribs on a plastic nosecone with a thin coat of epoxy. Doing both the tube and the nosecone act almost like scissors to cut the 2-56 nylon pin. I don't tap the hole for nylon pins. I drill the hole slightly under-size and when I push the pin in, the threads will hold it in place but its easy to push out with an awl after they've been sheared off.
I've had several failures, usually from shearing too early.
For cardboard and Blue Tube, I usually use masking tape for a friction fit. However, if you must use shear pins, use the shim stock. Mark alignment after drilling holes but before taking apart the sections.I do not tap the holes, just use a 5/64" bit for #2 and 7/64" for #4. PML phenolic and fiberglass I always use shear pins with no other hole prep. In any case, you don't want to have the joint too loose or it will shear early and shake out the main. I use the pick up the rocket by the nose and check it doesn't just fall apart. A slow separation is OK but drill a vent hole in each section. If your holes do get buggered up, rotate the the inner section some and drill new holes in the coupler (through the body tube holes). Mark the new alignment. I never use less than 2 pins, just use smaller ones on small rockets. Use a long apogee shock cord to allow the parts to slow down before jerking at full extension.
I've taken to using #4 shear pins in the main parachute section and #2's in the drogue section. I've had several instances in which the force of the drogue deployment was enough to pop the main shear pins too when I used #2's all around.
 

Handeman

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I've taken to using #4 shear pins in the main parachute section and #2's in the drogue section. I've had several instances in which the force of the drogue deployment was enough to pop the main shear pins too when I used #2's all around.
I understand what you are doing, but have you considered #2 shear pins on the NC and a friction fit on the apogee joint and reducing your apogee charge?
With a vent hole or two in the fin can, drag or pressure separation is seldom an issue for the apogee side on non-extreme flights. Even loose slip fits usually work there. Since the apogee charge only needs to open the rocket enough to get the drogue into the air stream, a reduced and minimum apogee charge can work quite well without putting the shock stresses on the nosecone shear pins. That also lets you reduce your main charge some and reduce all the stresses on the recovery system.

Just a thought...
 

Handeman

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Great tips, every one of them! Getting myself a lifetime supply of rolled stainless steel sheet :) Like the idea of putting bigger shear pins in the main parachute compartment.

I've only ever been a friction fit guy. We sent a two stage rocket, 5" in dia x 14 ft tall and all fiberglass, to over 16000 ft AGL in Black Rock in 1995, it recovered well and it was all friction fit. We also built and flew Son of Godzilla for many years on just friction fit until we loaned it/gave it to our friends who fly it with shear pins. However, I'm feeling the need to actually progress my techniques. So I totally appreciate all of the help with this.

Thanks again!
I understand where you're coming from. I've been making changes to what has been working for me for years. Mostly from peer pressure here about what's the best way to do things. It's too early to tell if they really make any differences, but so far, I'm not convinced that the TRF overthink is really worth it.
 

teepot

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Huh, copper foil? Any specific material? I see copper foil tape; which seems completely easy to use. Great idea.
I was looking for real thin brass and found a roll of copper foil. It's plenty thick enough to cut the shear pins. It's .005/36 gauge. I got it off amazon.
 

Bat-mite

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BTW, with FG and CF tubes, you can tap the hole and screw in the pin. Some people hate this because it takes time and effort to screw in the pin, but I always do it. No chance of them falling out, etc. If you do this, make the outer hole a little larger than the tapped hole so that it is easier to align your tubes. Just has to be smaller than the screw head.
 

Dan Griffing

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I have heard tales of binding shear pins but never experience it. People swear you should not use less than three, but I have two in some of my smaller rockets.

The more likely failure than non-shearing of pins is shearing them too early, caused by an over-aggressive deployment charge. Suppose you decide to use motor backup to your altimeter for drogue deployment. You run sims that show recommended delay time of 5 seconds, so you add a couple of seconds to the delay, i.e. you drill for 7 seconds.

At launch time, due to wind, or perhaps a slightly over-energetic motor, your get more altitude out of your rocket than anticipated, and apogee occurs two seconds later than expected. Your altimeter charge and motor backup charge deploy at the same time, giving you a combined force that is twice that needed to eject the forward section.

Due to the excessive force created by the overpowered ejection charge, the drogue harness snaps back against the AV bay, and the force causes the nose cone pins to shear at apogee. Now you have your main out at apogee and a long recovery walk ahead of you.

The only way around thus us to not use motor backup, and use redundant altimeters and charges.
My last launch had a premature motor ejection charge that went in about 7 instead of 14 seconds.

But that was likely from the Aerotech J425R-14A I used having a 2.6 second motor burn time instead of its rated 1.6 seconds.

Ground observers, and my onboard video camera verified the premature motor ejection, while my Eggtimer Proton recorded the motor burn duration.

An experienced rocketeer friend of mine explained that a longer motor burn will cause the motor ejection delay time to be reduced.

When this happens, its not the fault of the motor and dual deployment ejection charges going off at the same time. Things like this can happen whenever motor ejection is used.

I’ve also experienced the shock cord “snap-back” putting about 20g’s on the nosecone (as measured by a nosecone-mounted Altimeter3) and popping the sheer pins to deploy the main chute. This can happen if too much drogue deployment charge is used, or if the time delays on redundant drogue chute charges aren’t set to different values. Still this way is better than not having the drogue deploy at all.
 

Banzai88

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