Shear pin and BP sizing

I am going to be starting some ground tests in the next couple of weeks and wanted to get a better idea on where to start. Shear pins are new to me, and up until this rocket I have successfully used static ports to vent any differences in pressure, but now that the rockets are getting bigger and faster I need to take the next step into the shear pin realm.

Initially I thought of 4-40 nylon shear pins, not sure why, but then I was told by another flier to go with 2-56...below is the setup

I m not sure if velocity plays a factor in the decision but for what it is work RockSim has it at a max velocity of 1300MP or 1.7Mach

Body Tube:
ID = 2.165"
OD = 2.295"
Filament wound carbon fiber centerless ground / smooth (I believe it is ProLine 54mm tubing)

Drogue section:
6" long, contains a 15" drogue/w 3" nomex, and a 20' Kevlar 11/32 harness. It is tight and when I say tight I mean when it is packed it tries to force the sections apart by about .25". I can easily push it down and screw 4-40 shear pins in. Packing volume of the drogue is 4.3"^3, not sure about the shock cord and the 3" Nomex.

Main section:
12" long, contains a 48" main/w 4" nomex, and a 20' Kevlar 11/32 harness. It is also tight, same as above, and again I can easily push it down and screw 4-40 shear pins in. Packing volume of the main is 26"^3, and again I am not sure about the shock cord and the 3" Nomex, but it would be very similar to above.

So the two questions are do I stick with 4-40 or skip to 2-56, obviously the answer will effect the BP requirements. I did some quick calculations and based on the rough number of requiring about 40lbs of force for a 4-40 shear pin for a 20in^3 volume the BP charge should be about .25 grams.

Okay now throw it at me :bangpan:

Is this for your CF Tomach? If so, you could start with what I used for mine, although your Tomach is a little longer. I used 3x 2-56 shear pins in each compartment, with 1g of 4f BP for the main chute, and 0.5g for the drogue. I will probably use .6g for the drogue in flight though, just for a little more separation. You can see the videos of these tests in my Tomach thread.

rc dude said:

Is this for your CF Tomach? If so, you could start with what I used for mine, although your Tomach is a little longer. I used 3x 2-56 shear pins in each compartment, with 1g of 4f BP for the main chute, and 0.5g for the drogue. I will probably use .6g for the drogue in flight though, just for a little more separation. You can see the videos of these tests in my Tomach thread.

Click to expand...

Yes it is the Tomach. For some reason when I built it I drilled and tapped 4-40 holes for teh shear pins. Now that i am getting closer to ground testing and reading other threads,your included, plus talking to some other TRF members, I am thinking 2-56 is the way to go. but before I epoxy in some 2-26 clinch nuts to secure the shear pins I figured I would see if I could get some of the more experienced and smarter TRF members to weigh in. Have you ever used shear pins before? This is my first shear pin build so it's a bit foreign to me. I saw the videos, I had to laugh at the reaction when your test went off.

I'm sure others will pop in here and say for X tube, I use Y number of Z-sized pin. And those may work.

Personally, with shear pins, I go about it a little differently. First, I ask: How much mass is being pinned? Then, once I know that, I'll multiply that by expected G-forces (I've seen 20 by accident, so I use 40 for a safe margin... 100 or more isn't uncommon, for some fliers). Now I know how much I'm trying to hold back. Then, I take that figure and divide by the shear strength of the pins, and from there, that should tell you a number to use. Then I'll size my charge to break that many pins (usually at 150% of combined shear strength to start).

For example, Let's say I've got a nosecone, shock cord, and pilot chute connected to a free bag in which is the main, which is connected to a length of cord. So I will weigh everything, and arrive at a figure. Let's say...
2500g. (I always use grams in rocket weight; don't ask me to change it for this example). 2500g at 40G= 100000g. That's a lot of zeros, so 100Kg--that's 220lb. 220/35 (figure I use for #2-56) = 6.285. I've never seen 0.285 of a shear pin, so I'm going to call that 7. 7 #2-56 just seems like more drilling than I want to do, so let me check #4-40... 220/60 = 3.666. Again, fractions go up to next whole, thus, 4. 4 seems a lot more reasonable than 7. To break the 4 shear pins at a value of 150% of shear strength, I'd need to better 360lb.

Now, I've got my pin number, and my goal for shear, time to measure the chute compartment. Let's say it's 5.38 x14". To make 360 lbf+ in such a compartment, I'd need a minimum of 2.6g. This is where I'd start ground testing...


Later!

--Coop

mpitfield said:

Yes it is the Tomach. For some reason when I built it I drilled and tapped 4-40 holes for teh shear pins. Now that i am getting closer to ground testing and reading other threads,your included, plus talking to some other TRF members, I am thinking 2-56 is the way to go. but before I epoxy in some 2-26 clinch nuts to secure the shear pins I figured I would see if I could get some of the more experienced and smarter TRF members to weigh in. Have you ever used shear pins before? This is my first shear pin build so it's a bit foreign to me. I saw the videos, I had to laugh at the reaction when your test went off.

Click to expand...

I've never used shear pins before, and I'm not an experienced member by any means, so take my advice with caution. My Tomach was my first shear pin rocket too, as well as my first fiberglass rocket. As for the reaction, are you talking about the voices in the videos? That was my family, and ground testing was new to all of us.

For your rocket, to cut to the chase, I would look at two 2x56 screws. I would start at about .8-1 gram BP and see how that goes in a ground test.

I'm with Mark here: don't go overboard on shear pins and test to get the right amount of BP.

My Electronic Recovery video has a section on this (where I use a single pin made from 1/16" styrene).

Coop said:

I'm sure others will pop in here and say for X tube, I use Y number of Z-sized pin. And those may work.

Personally, with shear pins, I go about it a little differently. First, I ask: How much mass is being pinned? Then, once I know that, I'll multiply that by expected G-forces (I've seen 20 by accident, so I use 40 for a safe margin... 100 or more isn't uncommon, for some fliers). Now I know how much I'm trying to hold back. Then, I take that figure and divide by the shear strength of the pins, and from there, that should tell you a number to use. Then I'll size my charge to break that many pins (usually at 150% of combined shear strength to start).

For example, Let's say I've got a nosecone, shock cord, and pilot chute connected to a free bag in which is the main, which is connected to a length of cord. So I will weigh everything, and arrive at a figure. Let's say...2500g. (I always use grams in rocket weight; don't ask me to change it for this example). 2500g at 40G= 100000g. That's a lot of zeros, so 100Kg--that's 220lb. 220/35 (figure I use for #2-56) = 6.285. I've never seen 0.285 of a shear pin, so I'm going to call that 7. 7 #2-56 just seems like more drilling than I want to do, so let me check #4-40... 220/60 = 3.666. Again, fractions go up to next whole, thus, 4. 4 seems a lot more reasonable than 7. To break the 4 shear pins at a value of 150% of shear strength, I'd need to better 360lb.

Now, I've got my pin number, and my goal for shear, time to measure the chute compartment. Let's say it's 5.38 x14". To make 360 lbf+ in such a compartment, I'd need a minimum of 2.6g. This is where I'd start ground testing...

Later!

--Coop

Click to expand...

Hi Coop, thanks for the reply. Truthfully I had to read it a few times to get my head around it, but applying your thought process I believe also leads me to 2-56 shear pins which is helpful.

rc dude said:

As for the reaction, are you talking about the voices in the videos? That was my family, and ground testing was new to all of us.

Click to expand...

Yes it was the ground crew reaction that was funny. When are you going to launch yours?

markkoelsch said:

For your rocket, to cut to the chase, I would look at two 2x56 screws. I would start at about .8-1 gram BP and see how that goes in a ground test.

Click to expand...

Thanks Mark, as I mentioned in reply #3, the holes were drilled and tapped to 4-40, and there are also three of them. However I have been hearing 2-56 a lot so I am going with them by epoxying some 2-56 self clinching nuts behind the inside airframe for the shear pins to grab onto, otherwise they just fall out. BTW if the altitude is 20K what if any adjustments should I take for the amount of BP used for the drogue?

JohnCoker said:

I'm with Mark here: don't go overboard on shear pins and test to get the right amount of BP.

My Electronic Recovery video has a section on this (where I use a single pin made from 1/16" styrene).

Click to expand...

I will watch your video John, surprised I didn't see it already...thanks.

Another thing to note when using shear pins is to use them as push in pins only. Don't screw them into the airframe or backing nut. You can clamp the airframe to the coupler with more force than you expect and this can lead to unreliable performance.

Edward

It's a small rocket. Use 2 2-56 pins, The fancy math may say otherwise, but I'd say something went wrong with the calculations. My wildman Jr has well over a hundred flights, everything from H all the way to J600 (8288 feet) Just speaking from experience. That's the way I set them up. 3 inch rockets and up get three pins and I don't step up to 4-40 until 6 inch. And I'm known to do some crazy stuff (I love me some N10000).
Amount of BP? Small rocket probably won't need much but don't forget that AT and CTI put 1.3 grams in the motor. (just saying you might need to use a plugged motor or scrape out the bp on a CTI) Gotta ask how you measure your bp, can you do hundredths reliably on the field? Otherwise I'd start at the next tenths and work up looking for a good separation, not just a little pop. Don't forget that you can't simulate air pressure from forward flight while on the ground.

good luck

rick

On Saturday, I launched a 4" FG rocket weighing about 13 - 16 lbs. loaded (twice). I used two 2-56 pins in the nose, and two in the booster. I had no separation issues. I used 2.5cc of BP in the booster end, and 2.2cc in the nose end. I would call two successful launches "proof of concept."

Quick update.

Because I had 3 holes @ 120 degrees, and was concerned about binding using just two of them, as some recommended, I installed 2-56 self clinching nuts behind each hole and did a test with 3 2-56 pins. I configured it as ready-to-fly using .75g, no issues it popped perfectly. I only had time to do one test which was the apogee event "drogue" which is 2"x8" (25ish cubic inches) minus the Kevlar harness, hardware, nomex blanket and simulated chute. Because this is a higher alt flight planned on this rocket I am going to add a bit for margin and go with 1g.

The main is about a 2"x13" (40ish cubic inches) of volume, minus deployment so I am going to test with 1g as well.

I will be doing the 1g tests in the next couple of weeks.