shear pins for fiberglass

I've got some new fiberglass DD kits ordered, what do you recommend for shear pins in 38mm, 54mm, and 3" fiberglass? screws? rivets? what size?

2-56 nylon screws

RKeller said:

I've got some new fiberglass DD kits ordered, what do you recommend for shear pins in 38mm, 54mm, and 3" fiberglass? screws? rivets? what size?

Click to expand...

I use #2-56 for everything. Ground test to determine how many you need. I use just one shear pin in 38mm FG.

Ground test to determine how many you need.

NO -- Ground testing only determines (confirms) the charge size needed to undo [eliminate] the sheer pins.
It tells you NOTHING about how many sheer pins you need. NOTHING!!!

You need to determine the mass retained and the acceleration forces on that mass.
Which leads you to choose sufficient sheer pin strength to hold the mass under the acceleration.
THEN and ONLY THEN can you think about what it takes to remove those pins....and you can test that with a ground test.

This site might help.

https://www.info-central.org/?article=303

I'd go with 2-56 nylon.

I do a minimum of two, because I don't like asymmetrical loads (lends itself to the possibility of binding).

That said, you might want to consider ground testing. I'm glad I did.

Greg

Removing sheer pins is the easy part....
Spend time learning how and why to size them.

I'd hazard to guess - based on responses to threads like this - that >90% of RF members have no clue how to do this.
So many go instantly to ground testing - which is the last step, for confirmation only, that you can undo a function you need to avoid tossing the laundry early.

FredA said:

Ground test to determine how many you need.

NO -- Ground testing only determines (confirms) the charge size needed to undo [eliminate] the sheer pins.
It tells you NOTHING about how many sheer pins you need. NOTHING!!!

You need to determine the mass retained and the acceleration forces on that mass.
Which leads you to choose sufficient sheer pin strength to hold the mass under the acceleration.
THEN and ONLY THEN can you think about what it takes to remove those pins....and you can test that with a ground test.

Click to expand...

Relax, chief. "Ground testing" is not just ejection charges. It means test everything the rocket does on the ground, before you send it in the air.

To figure out how many pins you need, use past experience, do some math, compare to friction fitting, throw the payload as hard as you can, whatever.

In fact, ground testing the drogue charge and deccelerating the nose cone is a test of the shear pin strength.

Don't feel bad, Fred chewed me out about this too...

I don't feel bad. I am correct! :wink:

I might add that the 38mm FG rocket I mention above is an old Blackhawk38 with a super-heavy nosecone. To that I also add a BRB900 tracker. So, the nose weighs a ton, and just one #2-56 nylon screw is enough to hold it all together during the drogue event and descent.

FredA said:

I'd hazard to guess - based on responses to threads like this - that >90% of RF members have no clue how to do this.
So many go instantly to ground testing - which is the last step, for confirmation only, that you can undo a function you need to avoid tossing the laundry early.

Click to expand...

Instead of needlessly flying off the handle, how about enlightening us with your method? My method is to yank hard on the nose cone (while it is on the ground, you know, "ground testing"), and if the shear pins hold, it seems OK in my book. Adequate drogue shock cord length is also important. I like 15' of kevlar on that Blackhawk38. I am 100% successful with my shear pin flights.

On my 4" Darkstar Extreme I use three 2-56 nylon screws and have never had an issue. Lined them up with the three fins for spacing like the Wildman suggested in his directions with the kit.

...Fudd

I am correct!

Of course you are.....seems like everyone is on this forum.....
Whatever.....

FredA said:

I am correct!

Of course you are.....seems like everyone is on this forum.....
Whatever.....

Click to expand...

Yeah Fred, everybody on this forum is always correct!

FredA said:

I am correct!

Of course you are.....seems like everyone is on this forum.....
Whatever.....

Click to expand...

Fred, I understand what you are getting at, but perhaps an explanation would help versus ripping folks for not knowing.

So, the first function of the shear pin is to contain say a nosecone and parachute. Let's expand that and say it is on a dual deploy flight and you do not want to kick the nosecone and chute when the apogee charge pops.

So, you need to know the mass of the nosecone, chute, harness, and associated hardware. You also need to have an estimate of the acceleration of said items. Once in place it is simply force= mass x acceleration.

Once you know the force you need to realize that most 2x56 screws shear at about 35 pound (testing this is a good idea). Pick the number of screws that gives you a total greater than the force of deployment.

Then you go to testing charges.


Mark Koelsch
Sent from my iPhone using Rocketry Forum

timbucktoo said:

This site might help.

https://www.info-central.org/?article=303

Click to expand...

thanks, that's a great help, but now I'm a little worried with how much pressure is required to blow 2 #2 screws in a 38mm rocket.

In general, I think your concern for that size airframe is warranted. To break two screws that shear at 35 pound each requires about 40 psi if my mental math is right.

For a reference point, I have flown 38mm rockets on a single 2x56 nylon screw. Started with two screws, but it was more than I needed.


Mark Koelsch
Sent from my iPhone using Rocketry Forum

RKeller said:

thanks, that's a great help, but now I'm a little worried with how much pressure is required to blow 2 #2 screws in a 38mm rocket.

Click to expand...

Takes me about 1g of black powder to shear 2x 2-56 nylon shear pins in a 38mm rocket pressurizing a 12" long bay.

No Mark, I think it's better that I learn to just keep quiet and let the people who know they are right be "right."

After all, they aren't launching at my field, so what should I care..........

Besides -- it's all there to be found searching. No need to regurgitate it here....