Shear Pin Failure: Video & Need Advice

Rocket: LOC V2, paper/cardboard regular LOC tube, Stretched to 8’ tall, 8”diameter, 26LB. Dual Deploy, 4—2/56 shear pins, 4 grams of black powder to separate separate/break shear pins for main chute. Weighted nose cone because it’s a V2 (19.7oz that comes with kit). See photo

See video, and the captures of the nose cone separation where the shear pins failed. As the video goes to close up, you will see (going from left to right), the nose cone, the chute inside of the chute protector, and the top section of airframe with altimeter bay. The nose cone obviously came off right at apogee when the drogue ejection charge went off.(this was also confirmed because at 500 feet, I actually saw the main ejection charge fire in the air with a large puff of smoke)

I have flown this before with no problems. BUT, this time, I added 6.7oz to the nose with a GPS and battery unit. This additional weight caused the nose cone to break the shear pins when the drogue chute ejection charge went off.

My guess is the drogue slowed the rocket, the top section went flying and when the shock chord tightened, the force of the jerking sheared the shear pins. With a 120” chute, the rocket took a really long time to come down from 5600 feet.

This worked fine before without the extra weight from the GPS. My guess is, I must have been really close to this happening before without the GPS. And the added weight just pushed it over the edge.

Solution Questions:
Do I add 2 more shear pins of same size; kind of a hassle with 6 shear pins?
Use 4 larger shear pins; or 2 of the same and 2 larger?
Is this just a freak accident and it won’t happen again?
If I add more shear pin strength, I have to add more black powder; am I at risk of damaging the tube at some point of increasing black powder? (yes I would ground test)

https://www.youtube.com/watch?v=wJPeRrhpPJI&feature=youtu.be

Are you using a kevlar shock cord? If so, the jerk forces are higher. Try using nylon or tape loops to absorb shock.

You may also not need so much apogee BP.

How long is shock cord.....making longer MAY be the simple solution.
Smaller charge.

After looking at video. I agree it snapped back after reaching full length.

Did you ground test with new configuration?

blackjack2564 said:

How long is shock cord.....making longer MAY be the simple solution.
Smaller charge.

After looking at video. I agree it snapped back after reaching full length.

Did you ground test with new configuration?

Click to expand...

That's a great and obvious idea. I didn't even think if it. I need to measure it. The cone is really heavy though. Yes, I ground tested it.

Any rocket with substantial nose weight poses this risk. Blackjack is spot on - you need to find the right balance of shock cord length and drogue deployment charge strength so there is not a jarring effect when the payload reaches the end of the cord. I went through this with my 7.5" Bullpup (+/- 11lb nose cone). I eventually found the right balance and never had the problem again.

I agree with lengthening the cord, but given the size of your V2 I would go with 4-40 pins (4 of them). Just be sure to ground test, as you will likely need more powder.

+1 to everyone else...Smaller drogue charge + longer cord would go a long way here. The "blow it up or blow it out" philosophy will not help this situation. Four 4/40 nylon pins are also a good idea but then I would think significantly reinforcing the shear pin holes is needed seeing how it is LOC tubing. Also, using a drogue *may* help dissipate any shock forces - but I am not sure a drogue could open fast enough for that to be a factor.

OF course, with a smaller drogue charge a ground test is definitely in order prior to the next flight.

-Dave

If this is a really dumb question, please forgive me and move on. :blush:

I guess you did confirm that the pins sheared and didn't fall out or get pushed out, right?

I fly a rocket with similar dimensions, BT 7.61" x 8' tall, over 22lbs fully loaded. It also has a heavy FG nose come with 10oz of addl nose weight. The NC's secured to the cargo bay with 4, 4-40 nylon shear pins but only uses 2.5 grams 4f BP for a good deployment. I use a 24" Sky Angle drogue on 25' of 1" kevlar shock cord. We launch on a soft pasture so a 6' Recon main lands roc nice and soft, I enjoy a nice short walk...

I believe if you upgrade to 4-40 shear pins and do addl ground testing with a little less BP, you'll get a better deployment with everything coming out when it should.

Good luck!

Bat-mite said:

If this is a really dumb question, please forgive me and move on. :blush:

I guess you did confirm that the pins sheared and didn't fall out or get pushed out, right?

Click to expand...

LOL, Your computer is plugged in, right? The obvious is what usually kills us. But in this case, I got that one down. thanks though!!!! you just never know

DAllen said:

+1 to everyone else...Smaller drogue charge + longer cord would go a long way here. The "blow it up or blow it out" philosophy will not help this situation. Four 4/40 nylon pins are also a good idea but then I would think significantly reinforcing the shear pin holes is needed seeing how it is LOC tubing. Also, using a drogue *may* help dissipate any shock forces - but I am not sure a drogue could open fast enough for that to be a factor.

OF course, with a smaller drogue charge a ground test is definitely in order prior to the next flight.

-Dave

Click to expand...

good points from everyone. The top of all my rockets are reinforced with a couple of plies of fiberglass cloth and epoxy. In this case, I used one layer of carbon fiber cloth w epoxy. I launch at Lucerne Valley CA, dry lake bed. as close to cement as you can get. So the pins go though that material on the cardboard. Thanks

Spicer007 said:

I fly a rocket with similar dimensions, BT 7.61" x 8' tall, over 22lbs fully loaded. It also has a heavy FG nose come with 10oz of addl nose weight. The NC's secured to the cargo bay with 4, 4-40 nylon shear pins but only uses 2.5 grams 4f BP for a good deployment. I use a 24" Sky Angle drogue on 25' of 1" kevlar shock cord. We launch on a soft pasture so a 6' Recon main lands roc nice and soft, I enjoy a nice short walk...

I believe if you upgrade to 4-40 shear pins and do addl ground testing with a little less BP, you'll get a better deployment with everything coming out when it should.

Good luck!

Click to expand...

Yes. always good to point out that at least one ground test is required; but probably 2. Someone may be reading this post a year from now and need your advice. Thanks

blackjack2564 said:

How long is shock cord.....making longer MAY be the simple solution.
Smaller charge.

After looking at video. I agree it snapped back after reaching full length.

Did you ground test with new configuration?

Click to expand...

Obvious question; "Did I test w new configuration". Uh, no I did not. Learned something new today. Change something, test it. Thanks!

csoechtig said:

Any rocket with substantial nose weight poses this risk. Blackjack is spot on - you need to find the right balance of shock cord length and drogue deployment charge strength so there is not a jarring effect when the payload reaches the end of the cord. I went through this with my 7.5" Bullpup (+/- 11lb nose cone). I eventually found the right balance and never had the problem again.

Click to expand...

Thanks! All of these comments make sense; now. Good thing my rocket didn't blow into Arizona with the 120" chute.

csoechtig said:

Any rocket with substantial nose weight poses this risk. Blackjack is spot on - you need to find the right balance of shock cord length and drogue deployment charge strength so there is not a jarring effect when the payload reaches the end of the cord. I went through this with my 7.5" Bullpup (+/- 11lb nose cone). I eventually found the right balance and never had the problem again.

Click to expand...

What size shear pins did you use and how many? Was the airframe paper or glass

In my 7.5 in rocket....35lbs, I used 4 shear pins...... 4-40's. Airframe is phenolic [PML] wrapped with kevlar 1in. ribbon, 2 layers [hoop] & 1 wrap 5.5 carbon.
Shock cord was 55ft of 1in tubular nylon for apogee & 25ft for main...main has 20ft shroud lines and I figure that in for actual 45ft distance.
My payload is 3ft , charge is 3.75 for 1st.....4.5 for back up. Same for apogee.
No pins on apogee side, just friction fit, by coating coupler with epoxy & sanding till proper fit was attained. Inside of coupler had 2 wraps of glass.

Simple solution would be enlarge your 4..... 2-256 holes for 4-40. You may even get good results with out upping the BP charge.

I've flown over 20 L1 and a few L2 flights with dual deployment in various rockets, some requiring significant nose weight (i.e. Bullpups), some not so much. The one thing that I've found to be CRITICAL to successful flight is GROUND TESTING. But before the testing, start with a good idea about the size and number of shear pins needed and with a good idea about how much FFFFg you will need to do three things: 1) pressurize the tube length/size you are trying to separate from another component, 2) to sheer the number/size pins you have AND 3) push the component and any laundry (chute, cord, nomex) out of the tube being pressurized.

There are great resources available to help with each of these all over the web. A few that I've found helpful are shown below, but there are PLENTY others. I struggle a bit with shear pins, but I've watched only 2 of my flights deploy a main thousands of feet higher than I wanted. My mistake: overcooking the drogue charge BOTH times. The first time was an initial miscalculation on my part, the second time I had a braincramp and dumped the same amount into the drogue charge well as I was putting in the main charge well.

What happens at apogee when that charge fires violently is that the nosecone is shot so hard that the inertia created by the mass of the nosecone (moreso if there is added nose weight) shears the pins when the shock cord reaches the end of its length. Go back to GROUND TESTING and find a good, clean separation of the two components without launching the part across the neighbor's fence or into their home. With any rocket set up at about a 35 degree angle to the ground, I like to see the section being separated go about 1/3 to 1/2 the distance of the length of the shock cord when the charge goes off. Others may like more or less, but this has worked very well for me.

Here are some of the links I like to use to help me determine what size and how much with shear pins and black powder:

Shear Pins and Sizing
https://rocketsnw.com/?page_id=666

Shear Pin Sizing
https://www.feretich.com/Rocketry/Resources/shearPins.html

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

Ejection Charge Sizing
https://www.vernk.com/EjectionChargeSizing.htm

Idea for "cutting" shear pins cleanly for paper airframes
https://rocketry.tungstentech.com/tutorials/shearpins/default.aspx

This last link is interesting, and I modified the idea just slightly, using very thin (.45mm) brass strips about 3/4" x 3/4" (L x W) embedded in AV bays and inside body tubes to create a guillotine effect for shearing nylon pins. I have seen many paper tubes, phenolic tubes, blue tubes, and others get a hole that enlarges with repeated use, to the point that the shear pins won't stay in place. Putting opposing brass "plates" (for lack of a better word) in an area where the shear pin is inserted gives a great cutting effect while the hole stays the proper size over repeated launches.

To do this, I use a Dremel to create a very small impression in the AV bay/nosecone outer side and the body tube inner side where I've already drilled the shear pin holes. Once the location is ready, and I've slightly curved the brass to match the curve of the components, I rough up the brass with coarse sandpaper, then use JB weld to bond it to the components. Once cured, sand the plates and installation area if needed to remove any high spots, then drill holes through the plates using the original drilled hole as a guide. You'll love it! Now your shear pins (screws) will actually screw into the holes..., and they WILL shear as they should!

for one thing it looks like you using Kevlar for shock which has no give to it. Nylon will stretch some. Plus like Jim said longer cord is better.

SammyD said:

I've flown over 20 L1 and a few L2 flights with dual deployment in various rockets, some requiring significant nose weight (i.e. Bullpups), some not so much. The one thing that I've found to be CRITICAL to successful flight is GROUND TESTING. But before the testing, start with a good idea about the size and number of shear pins needed and with a good idea about how much FFFFg you will need to do three things: 1) pressurize the tube length/size you are trying to separate from another component, 2) to sheer the number/size pins you have AND 3) push the component and any laundry (chute, cord, nomex) out of the tube being pressurized.

There are great resources available to help with each of these all over the web. A few that I've found helpful are shown below, but there are PLENTY others. I struggle a bit with shear pins, but I've watched only 2 of my flights deploy a main thousands of feet higher than I wanted. My mistake: overcooking the drogue charge BOTH times. The first time was an initial miscalculation on my part, the second time I had a braincramp and dumped the same amount into the drogue charge well as I was putting in the main charge well.

What happens at apogee when that charge fires violently is that the nosecone is shot so hard that the inertia created by the mass of the nosecone (moreso if there is added nose weight) shears the pins when the shock cord reaches the end of its length. Go back to GROUND TESTING and find a good, clean separation of the two components without launching the part across the neighbor's fence or into their home. With any rocket set up at about a 35 degree angle to the ground, I like to see the section being separated go about 1/3 to 1/2 the distance of the length of the shock cord when the charge goes off. Others may like more or less, but this has worked very well for me.

Click to expand...

I agree with almost everything you've said. The size of the drogue charge is critical and usually doesn't need to be as large as many use. I put my drogue about a foot from the payload and if the drogue is 2 ft long, I only need 3 feet of separation between the payload and fin can to get good drogue deployment. Shooting the two parts to the end of the shock cord is not needed and causes the issues you've described.

The one thing you said that I don't really agree with is

3) push the component and any laundry (chute, cord, nomex) out of the tube being pressurized

Click to expand...

I was in a protracted "discussion" with other on this forum a while back about whether the ejection pressurizes the tube, causing the nose cone to eject and the nose cone pulling the laundry out of the tube, OR does the ejection charge push the laundry out of the tube with the nose cone. The conclusion we came to was that it is a little of both. The thing is, you can't depend on the ejection charge pushing the laundry out of the tube unless it is in a d-bag and/or forms a piston like plug in the BT. Witness all of the G-Force flights that ejected the nose cones but the laundry stayed in the tube. Stronger charges would impart more energy to the nose cone and allow it to pull the laundry out, but if the nose cone is a tight fit or pinned, the pressure will build up and eject the nose cone without applying any significant ejection force to the laundry.

If you think about it, to apply force to the nose cone, the pressure has to build up inside the whole BT, completely around the laundry, before it can force a pinned nose cone out of the BT. As the nose cone leaves the BT and the pressure is reduced, unless the laundry provides a tight plug in the BT, the pressure behind the laundry will dissipate quickly and not push the laundry out of the BT. There will be some pressure difference that will assist getting the laundry out, but not enough to consistently depend on.

I agree with your three item, just not including the laundry in step 3. It's only the component that you need to make sure comes out. If the laundry comes out too, that's an added bonus that you can't depend on unless the component pulls it out.

Notes from the insane asylum

I hope some of you are still watching this thread...

Some observations as I am just about to increase the size of the shear pins.

-First, I have 5 shear pins (not 4); not so evenly spaced. (I don't really remember why I put 5 in there.....)(

-You know how the shear pins usually shear and are stuck in the nose cone shoulder holes? Well, when I went to mess around with the rocket, only 4 of my nose cone shear holes had left over shear pins filling them up. I know its absolutely possible that I only put 4 in cuase I have made other mistakes in the past. That's point 1. Because I loaded a GPS in the nose this time, I did the shear pins in the field. I sort of think I just put 4 in, but no way to know, other than my comment here.

-The rocket has flown 3 other times with 5 smaller shear pins (for sure, cause I put those pins in at the house, before gps unit added), but with out the extra 6.7oz in the nose cone.

- I don't really know at this point if I had 5 or 4 shear pins in this flight with the failure (kind a sounds like that Dirty Harry movie where he doesn't remember if he shot all his bullets, right?)

-Also interesting: My 5 shear pins are not evenly spaced. On the OPPOSITE side of where the largest gap is between the shear pins, the tube appears to be kinda creased on the outside. Yeah, like because the shear pins weren't spaced evenly, the gap side levered pressure on the opposite side. IS THIS REALLY POSSIBLE. I am pretty sure the original tube was in good shape......

Today's Questions:
1) fix the shear pin spacing, ok , yeah.

2) Do I go with 5 of the larger shear pins go down to 4? The circumference is about 24". Again, I have been using 5 of the smaller ones, except maybe on this last flight.

blackjack2564 said:

In my 7.5 in rocket....35lbs, I used 4 shear pins...... 4-40's. Airframe is phenolic [PML] wrapped with kevlar 1in. ribbon, 2 layers [hoop] & 1 wrap 5.5 carbon.
Shock cord was 55ft of 1in tubular nylon for apogee & 25ft for main...main has 20ft shroud lines and I figure that in for actual 45ft distance.
My payload is 3ft , charge is 3.75 for 1st.....4.5 for back up. Same for apogee.
No pins on apogee side, just friction fit, by coating coupler with epoxy & sanding till proper fit was attained. Inside of coupler had 2 wraps of glass.

Simple solution would be enlarge your 4..... 2-256 holes for 4-40. You may even get good results with out upping the BP charge.

Click to expand...

Start over........

Fill holes with epoxy, evenly space 4 new for 4-40's . Drill new, either higher or lower than originals
Lengthen shock cord if needed.

Usually easier to start over than try to fix the existing un-equal spacing.


PS if just paper tubes, you could add a 1-2in. wide band of fiberglass, where the pins are going, would guarantee a clean shear. put a coat of thinned epoxy [10%]on inside of tube where pins go to harden the paper . [or CA]

Or if you don't want to mess with pins anymore, use masking tape shear strips....I do it all the time when I stick a motor in so large the NC won't seat all the way to use my pins....just 1/4 to 1/2in wide strips...same number as pins .



It works very well ........even if it is ugly.
There's more than one way....to skin a cat!

AGREED

blackjack2564 said:

Start over........

Fill holes with epoxy, evenly space 4 new for 4-40's . Drill new, either higher or lower than originals
Lengthen shock cord if needed.

Usually easier to start over than try to fix the existing un-equal spacing.


PS if just paper tubes, you could add a 1-2in. wide band of fiberglass, where the pins are going, would guarantee a clean shear. put a coat of thinned epoxy [10%]on inside of tube where pins go to harden the paper . [or CA]

Or if you don't want to mess with pins anymore, use masking tape shear strips....I do it all the time when I stick a motor in so large the NC won't seat all the way to use my pins....just 1/4 to 1/2in wide strips...same number as pins .

View attachment 261629

It works very well ........even if it is ugly.
There's more than one way....to skin a cat!

Click to expand...

I use clear tape to snug up the nose cone. You only need enough to get a good tight fit two or three wraps depending on how lose it is. Then use your shear pins that you have already. No need for more shear pins.

mikec said:

Are you using a kevlar shock cord? If so, the jerk forces are higher. Try using nylon or tape loops to absorb shock.

You may also not need so much apogee BP.

Click to expand...

I just wanted to follow up on this post as I have tried the tape loops and they have worked great on my last two launches. Just this Saturday I put in six tape loops in the Kevlar shock cord on my Loc Iris. She went to 2714 feet on an I211 and the dual deploy worked perfectly. Four of the six loops tore open, the cord slipped out of one loop and the last loop was still intact after landing. On several previous launches I popped the nose cone off at apogee. I tried shear pins, but they ripped up the tube too much during ground testing, so I never tried them on a launch. So I really like the tape loop method. Thanks for the suggestion mikec.