sheer pins 101

I am building a L2 attempt that is an tube fin . what about sheer pins ,how are they sized,how do u pick the right size and number thank you

This calculator is a pretty good reference for sizing ejection charges and calculating the number of 2-56 or 4-40 shear pins you should need.

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

First, the word is "shear," which means to cut off (like shearing a sheep). "Sheer" means "transparent."

There was a HUGE thread on this recently, with lots of arguing. In a typical DD rocket, you need shear pins for two reasons. The first is to retain the booster at motor burnout when drag forces take over. Having your rocket drag-separate during ascent usually results in a zipper when the drogue comes out, and of course, kills your altitude.

The second reason is to retain the NC when the apogee charge fires. The apogee charge will also pop the NC unless it is retained.

Please note that there are other means of retention besides shear pins. Masking tape, friction fit, etc. The problem with those is that they can't be measured and reproduced.

Here's a basic summary of what to do:

1. Weigh your fully loaded payload/NC section, coupler to tip. Make sure it is in flight-ready condition (main and harness are in, altimeters and batteries are in, etc.).
2. Weight the NC by itself.
3. Simulate your flight in OR or RockSim.
4. Note the amount of acceleration at motor burnout, and also at the apogee charge.
5. Force = mass X acceleration (F = ma).
6. Assume a worst-case scenario of a sudden stoppage at burnout. So use your max acceleration in G force -1.
7. Multiply this times your mass (or weight) in pounds.
8. That gives you F. Note the shear strength of your pins (for example, a #2-56 6/6 nylon screw has a shear stength of anywhere from 30 - 45 psi, depending on temperature and other factors.
9. Divide F by 30 to get the number of pins.
10. Online BP charge calculators will tell you the number of shear pins the charge can break for the volume of space entered.
11. Repeat using the acceleration at apogee event and the weight of the NC.

That was a nice summation Bat-Mite without getting too technical like couple of other threads recently on this subject. You described how to figure the forces and where to get the data to enter into the equations.

Thanks. I really have been paying attention. :wink:

also, lots of ground testing... make sure it works before sending it up.

my first dd was on a rocket that I wasn't Lvl 2 on.. it helps with the stress, trying not to do too many things at one time.

size wise, is what I have been using to figure out the size of the pins. I use 2-56 on tubes 4in and lower and 4-40 on bigger tubes.. it has worked for me.. you can use 2-56 on bigger tubes it just takes alot more of time.

Fiberglass is easy to put pins into.. cardboard is a bit more tricky, it needs some reinforcing.
i use pins on all my fiberglass rockets...

good luck

Yeah, cardboard tubes need some kid of metal shim glued to the inside of the airframe to shear the pin. MACPerformance kits take nylon screws very well, too. I thread mine.

Bat Mites post was good....

+1 on the ground test - if you are unsure and learning about shear pins.. think of every ground test that you do as a rocket saved from lawn dart, zipper, etc. etc. more ground testing is better.

Bat-mite said:

Yeah, cardboard tubes need some kid of metal shim glued to the inside of the airframe to shear the pin. MACPerformance kits take nylon screws very well, too. I thread mine.

Click to expand...

Glad I saw this. I'm just starting to build a 'custom' tube fin rocket that Mike at MAC Performance put together at my request. Was wondering about how the canvas phenolic material takes to tapping for shear pins. The tapped holes in the Blue Tube I used for my L2 project seem to be holding up well after 3 flights (plus a half dozen ground tests), so I figured the canvas phenolic would do well since it seems to be more rigid.

https://www.google.com/search?q=roc...4.5206j0j7&sourceid=chrome&es_sm=122&ie=UTF-8

I use "thin" ca in and slightly around the holes on standard cardbord airframes. I tap the outer hole and the inner hole is the outside diameter of the pin. Numerous flights show no wear on the holes of ones I have prepared this way.

Bat-mite said:

Yeah, cardboard tubes need some kid of metal shim glued to the inside of the airframe to shear the pin. I thread mine.

Click to expand...

Well, not true, mine shear just fine with a cardboard tube and i believe others will agree. That said some kind of metal shim couldn't hurt and will certainly help protect the integrity of the hole in the tube and therefore the tube itself. I simply reinforced my shear holes with CA and they've held up just fine in ground testing.

I haven't used the metal on cardboard rockets yet..

I have been just using fiberglass.

In the past I have just smashed up cardboard rockets, I gave up and switched to fiberglass.
It has saved me money in the long run, well that's what I tell my wife.

sed6 said:

Well, not true, mine shear just fine with a cardboard tube and i believe others will agree. That said some kind of metal shim couldn't hurt and will certainly help protect the integrity of the hole in the tube and therefore the tube itself. I simply reinforced my shear holes with CA and they've held up just fine in ground testing.

Click to expand...

Same here, and occasionally re-CA the holes and re-tap the cardboard. Have numerous flights on multiple cardboard rockets and all still doing great. The thickness of the materials most companies use are very durable. And even if the holes get too big to re-use, you can always fill them and drill new holes next to them.

Apogee has threaded metal inserts that can be used for shear pins or non-shear coupling.

I saw those, a bit pricy but worth it in the long run. might try them in my next project!

thank you all I will be testing,testing,testing I am glad I live in farm country all so if you look a lot of people cant spell

metro said:

thank you all I will be testing,testing,testing I am glad I live in farm country all so if you look a lot of people cant spell

Click to expand...

Hey I resembles that remark.