Threaded shear pin installation

[Sandy H.]

I'm in the process of rebuilding my PML Callisto which suffered a lawn dart due to too little ejection charge to shear the shear pins. In the process of reworking the salvageable components, I note that if the shear pins function as intended, they are a pain to remove from the coupler/nosecone.

I installed 2-56 nylon screws with a clearance hole in the the body tube and a tapped hole in the coupler and/or nosecone, depending on the location. When they do shear, they are left in the tapped hole with no head anymore. In the end, I stuck the tip of a sharp Xacto knife and just spun them the rest of the way in. It really wasn't that bad, but I could see over time that this has a pretty good chance for damage.

When you install nylon screws for shear pins, do you tap the coupler and leave a clearance hole in the BT or tap the BT and leave clearance in the coupler?

Other options or suggestions for easy extraction are welcome.

Thanks,

Sandy.

 

[Handeman]

I haven't used shear pins yet, but I would tap the BT so the head stays with the threaded part and makes for easy removal.

I have a Callisto that is build stock and I have never used shear pins and have never found I needed them. Just curious as to what might be different about your's that you're using shear pins in it?

 

[kandsrockets]

I'm in the process of rebuilding my PML Callisto which suffered a lawn dart due to too little ejection charge to shear the shear pins. In the process of reworking the salvageable components, I note that if the shear pins function as intended, they are a pain to remove from the coupler/nosecone.

I installed 2-56 nylon screws with a clearance hole in the the body tube and a tapped hole in the coupler and/or nosecone, depending on the location. When they do shear, they are left in the tapped hole with no head anymore. In the end, I stuck the tip of a sharp Xacto knife and just spun them the rest of the way in. It really wasn't that bad, but I could see over time that this has a pretty good chance for damage.

When you install nylon screws for shear pins, do you tap the coupler and leave a clearance hole in the BT or tap the BT and leave clearance in the coupler?

Other options or suggestions for easy extraction are welcome.

Thanks,

Sandy.

I tap my body tube and make the hole in the coupler/nose cone tight but not tapped so I can just use a small pick to push out the left over pin.

 

[boris katan]

Shear pins are typically used for the parachute compartment in a dual deployment setup. This helps keep the chute compartment closed when the drogue is deployed.

For a single deployment rocket, friction fit should be all that is needed.

 

[Austin]

The threaded 2-56 shear pins that I use are not screwed in to a tapped hole, but rather the hole is large enough to just push them in place. I use CA to tighten the hole so the pin fits snug. It also hardens the hole and makes it stronger and sharper for a clean shear. If you use more than two shear holes, offset one of the holes another 1/2" so the locations are "keyed".

 

[Sandy H.]

OK, good. I just did it wrong the first time by tapping the coupler. It does make more sense to not have them tapped into the coupler and instead into the BT. I guess the bottom line is that there are options for repair for the existing ones when/if they do become damaged and I can do the next ones differently.

For reference, I converted my Callisto to DD, which is why I am using them at this time. Its really about getting some experience with DD, even though it has only flown on MPR motors as of yet.

Hopefully this weekend, I'll get another launch.

Thanks for the help.

Sandy.

 

[roadkill]

I prefer to use 1/16" smooth styrene rod
for shear pins, drill holes just big enough to
push it in about an inch, cut almost flush to
body tube with pliers and your ready to rock...

...post flight just grab the rest of the rod and use
it to clear the holes... Cheaper, faster, easier....
2 per joint on 3" and 3 per joint on 4" tube has proven
to work well, but you gotta ground test to dial in proper
ejection charge...

 

[snowman2]

When you install nylon screws for shear pins, do you tap the coupler and leave a clearance hole in the BT or tap the BT and leave clearance in the coupler?

I do not tap anything. The hole is large enough to push the pin into place with a finger. I put a piece of tape over the head to ensure it stays in place. The head normally comes off when the tape is removed after flight. The piece in the cone is pushed into the cone with a small screwdriver.

If you don't like little pieces of pin rattling in the cone then drill a small hole in the cone's bulkhead so they can fall out. Put a piece of masking tape over the hole for flight. Better yet, if the cone is wide enough, epoxy a ring into the cone's shoulder and attach a bulkhead with screws. You then have the option of using the cone for extra payload space, such as for a GPS transmitter.

 

[Handeman]

OK, good. I just did it wrong the first time by tapping the coupler. It does make more sense to not have them tapped into the coupler and instead into the BT. I guess the bottom line is that there are options for repair for the existing ones when/if they do become damaged and I can do the next ones differently.

For reference, I converted my Callisto to DD, which is why I am using them at this time. Its really about getting some experience with DD, even though it has only flown on MPR motors as of yet.

Hopefully this weekend, I'll get another launch.

Thanks for the help.

Sandy.

If you have converted it, that would explain the shear pins.

How did you do the conversion? I might consider doing the same thing. How does it fly on the MPRs (G64?). That's all my Callisto has flown on so far. Right now the only DD rocket I have is my L1. I'm planning on a cert flight with a Loki I110 in January. The Calisto will get an H shortly after that.

 

[Sandy H.]

For the conversion, I went to the PML site and reviewed the page on CPR conversions. Specifically, I cut the tubes where they said to. The e-bay is a 6" long coupler with a 2" long BT in the middle. I made the bay with a 1/4"-20 threaded rod, some couplers and eyes through the middle. I welded the pieces here and there to make the assembly work how I thought I wanted it to. The sled was thin ply and had a middle ring to support the battery. Rev 2 would be different, but it was an acceptable first try.

The first and only flight was nice for the boost phase, but I didn't add anymore BP to shear the pins and was flying motor deploy only for the first flight. Result was a lawn dart that wrecked the front half and messed up the e-bay phenolic tubes. I'm most of the way done with the rebuild and hopefully will fly with the club this weekend.

I'll post more with the new arrangement if it is successful.

Thanks to all for the shear pin suggestions.

Sandy.

 

[Juerg]

Shear pins are working reliably only if there is a proper "shear".
Nylon screws just in the airframe is calling for trouble because there is no clearly definded shear plane, thus resulting in largely varying shear forces.
We do have a nice pictorial online on "how to do it right":
https://alrs.argoshpr.ch/Argos/knowhow/recovery/shearpin
(German only, but the pictures should talk for themselves)

Juerg

 

[KDRaven]

nice pictures, I am thinking on adding shear pins to my 2.1" rocket to the nose cone thats using DD but have not got the pins yet and built the ejection charges to test it out. I did test with out shear pins and 18 grains (not Grams) of 3fBP was a bit much but with shear pins it might tame down some or I'll just lower the amount of BP.

 

[Juerg]

nice pictures, I am thinking on adding shear pins to my 2.1" rocket to the nose cone thats using DD but have not got the pins yet and built the ejection charges to test it out. I did test with out shear pins and 18 grains (not Grams) of 3fBP was a bit much but with shear pins it might tame down some or I'll just lower the amount of BP.

Shear pins won't necessarily tame things down. They are not destroying energy but just allowing for pressure build-up. Potentially ejection will get even more violent.
Use the calculation formula to check the amount of BP.

Cheers

Juerg

 

[Handeman]

Shear pins won't necessarily tame things down. They are not destroying energy but just allowing for pressure build-up. Potentially ejection will get even more violent.
Use the calculation formula to check the amount of BP.

Cheers

Juerg

I was under the impression that the use of shear pins was to prevent the nose cone and thus the main recovery from deploying when the drouge deployed in a dual deployment system. I wasn't aware they were used to build pressure for deployment. Does that mean there are cases where shear pins may be needed in a single deployment rocket?

 

[jrcox]

I was under the impression that the use of shear pins was to prevent the nose cone and thus the main recovery from deploying when the drouge deployed in a dual deployment system. I wasn't aware they were used to build pressure for deployment. Does that mean there are cases where shear pins may be needed in a single deployment rocket?

I use shear pins sometimes on single deploy rockets, simply to have a solid connection and a moderately loose coupler. If you are using something to keep the nose cone on, then install a metal screw or one of the quick / removeable plastic rivets. The idea is to break the shear pin under a specific load and to keep the parts together until it is time to separate.

I like Juerg's idea with the brass tubing to create a shear surface, in the past I have used thin brass plate epoxied to both surfaces (coupler and airframe) and drilled a hole thru the plates. The tubing looks to be easy to install and reliable.

John

 

[Sandy H.]

If you have converted it, that would explain the shear pins.

How did you do the conversion? I might consider doing the same thing. How does it fly on the MPRs (G64?). That's all my Callisto has flown on so far. Right now the only DD rocket I have is my L1. I'm planning on a cert flight with a Loki I110 in January. The Calisto will get an H shortly after that.

Here is the link to the ebay design. As I say in the thread, it isn't perfect, but I think it is sufficient for the first/second revision.

https://www.rocketryforum.com/showthread.php?p=505614&posted=1#post505614

I think the points about using the brass tube reinforced shear pin mounting is interesting and would provide a reliable method for attaching shear pins. I kind of think it is overkill for the application, but what do I know, so far I am 0 for 1 with shear pins, so I should listen more than speak for sure! It is a nice method and pretty sexy as far as an installation goes.

Thanks for the tips.

Sandy.

 

[boris katan]

I epoxyed a thin 1/2 inch square piece of aluminum sheet on the outside of the coupler, and another on the inside of the body tube. Then drilled through them for shear pin. Creates a sharp shearing edge. Used two of the 2-56 Nylon screws for a four inch dual deployment setup. Works great.

Only did this for parachute compartment in dual deploy setup. Not for drogue compartment, as that blows first. Also wouldn't bother with shear pins for single deployment. Why bother when some masking tape does fine for a light friction fit.

 

[Juerg]

I was under the impression that the use of shear pins was to prevent the nose cone and thus the main recovery from deploying when the drouge deployed in a dual deployment system. I wasn't aware they were used to build pressure for deployment. Does that mean there are cases where shear pins may be needed in a single deployment rocket?

You are right, shear pins are used to hold things in place until the moment when separation is wanted.
But it's in the nature of things that pressure has to build up before the shear-load of the pins is met. So it's a side-effect (and if or if not ejection is getting more violent because of that is just guessing).

Juerg