Cutting shear pins on paper airframe: Brass shim material?

I have a few "paper" airframe rockets that I'm converting to dual deploy and have purchased a supply of 2-56 nylon bolts and a selection of various sized removable nylon rivets.

In the past I've seen pictures of nose cones with brass shims epoxied to the shoulder to cut the shear pins. Shouldn't there also be similar brass shim material on the inside of the body tube and payload bay?

I picked up some brass shim material (0.005) which is flexible enough to conform to 2.6 and 3.0 body tubes but seems rather thin.

What does everyone else use to cut shear pins on paper airframes?

If your nosecone is particularly light, try not using shear pins at all, just friction fit the nosecone. My smaller diameter rockets that aren't fiberglass, I just friction fit and haven't had the nosecone pop out prematurely yet.

Harden the holes with CA and it is not needed.

cwbullet said:

Harden the holes with CA and it is not needed.

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That was my first thought.

My main concern is preserving the integrity of the body tube and thought I might give the brass shim technique a try.

I will need to give this a try too on Big Boo-tay...

Why do you need shear pins?

As Henry said, you can use friction fit for many rockets. I've got a 4" dia. +10# DD L2 rocket that has never used shear pins. A good friction fit has always worked just fine, even for +Mach L motor flights.

I would suggest looking at the need for shear pins again. I know a lot of folks on the forum say they are needed, but I'm not convinced that is the case, especially for rockets below large L2.

cwbullet said:

Harden the holes with CA and it is not needed.

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+1. I have several 4" rockets with 12+ flights on them and no noticable change in the shear pin holes. One point - if using a plastic nose cone tht has raised rings (e.g. Madcow), make sure to put the shear pin holes through those rings. If not, thre will be a gap between the NC and the BT and the pin won't shear as cleanly.

David

Wizard said:

+1. I have several 4" rockets with 12+ flights on them and no noticable change in the shear pin holes. One point - if using a plastic nose cone tht has raised rings (e.g. Madcow), make sure to put the shear pin holes through those rings. If not, thre will be a gap between the NC and the BT and the pin won't shear as cleanly.

David

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Good point. I haven't thought that far ahead and may well have missed that. Thanks for the tip.

Handeman said:

Why do you need shear pins?

As Henry said, you can use friction fit for many rockets. I've got a 4" dia. +10# DD L2 rocket that has never used shear pins. A good friction fit has always worked just fine, even for +Mach L motor flights.

I would suggest looking at the need for shear pins again. I know a lot of folks on the forum say they are needed, but I'm not convinced that is the case, especially for rockets below large L2.

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Not sure I do. On the other hand, I really don't want to have problems on an L2 flight. I've always relied on friction fit but there were two occasions a couple of years ago when I had a mid-power rocket lose the nose cone from an empty payload bay before reaching apogee. Same rocket both times so it may have been something specific to that rocket. I've had a lot of high power flights since then without incident. Thanks for the testimonial for friction fit of nose cones.

Do you also recommend friction fitting the payload bay to the forward end of the alt-bay? I'm thinking about using rivets for that.

I use Evergreen styrene rod on most of my paper air frames. 1/16 inch diameter in place of 2-56 screws, and .080 in place of 4-40s. No need for metal slicers for these as they shear consistently whereas the nylon screws have more of a tendency to tear than shear in my experience.

When I've used nylon shear pins on paper frames I've cut squares from a Coke can and epoxied them in place to do the cutting.

MarkH said:

...I've cut squares from a Coke can and epoxied them in place to do the cutting.

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Good idea!

This is what you are looking for.

It is a pair of tee nuts made specifically for rocketry body tube. One of the pieces is threaded and the other piece is counter sunk. When the counter sunk piece is used on the outside and the threaded piece is used on the inside of the coupler it makes a perfect rivet replacement. When the threaded piece is used on the outside and the counter sunk piece is placed on the inside of the coupler tube it makes a great cutter for nylon screws.







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Random Flying Object said:

This is what you are looking for.

It is a pair of tee nuts made specifically for rocketry body tube.

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Nice. Where does one find these?

Looks interesting, RFO. What are the dimensions of those things, and where would one buy them?

Handeman said:

Why do you need shear pins?

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Consistency. Shear pin strength is well known and pretty reliable. I used to friction fit everything (90% success rate), but I was always futzing with adding/removing masking tape until it "felt" right. Temperature and humidity also impact the friction fit and the amount of tape needed. After a few flights, my nose cone shoulders were a gummy mess.

I once retrieved a cardboard rocket from the snow, assembled it wet, forgot about it for a few days, the cardboard swelled, and I damn near destroyed the rocket trying to remove the nose cone that had masking tape on the shoulder.

The shear pins are a cleaner, worry-free solution for me.

woferry said:

Looks interesting, RFO. What are the dimensions of those things, and where would one buy them?

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I designed them a while back and final decided to do a small production run of the 6-32 size. If the demand is present I would be willing to do a run of 4-40 and/or 2-56 size.





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Please make them.

accpool said:

Please make them.

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Yep, I believe I promised you some. What size? I would assume most folks would want 2-56?


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About how much per pair? I'd take ten of each size if they're a couple of dollars per pair...

Random Flying Object said:

Yep, I believe I promised you some. What size? I would assume most folks would want 2-56?


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#2-56 for shear pins. #4, #6, #8 for securing avbays

Buckeye said:

#2-56 for shear pins. #4, #6, #8 for securing avbays

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Unfortunately that is several production runs. These parts would be far too expensive to make one by one. I would like to pick the size of my next run based on demand. #2 or #4, which will it be? I already have #6 in stock.


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Random Flying Object said:

Unfortunately that is several production runs. These parts would be far too expensive to make one by one. I would like to pick the size of my next run based on demand. #2 or #4, which will it be? I already have #6 in stock.


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What about starting a thread in the vendor are with a poll about size?

Those are sweet! I have been using PEM nuts but these look much nicer. Good workmanship!

Random Flying Object said:

This is what you are looking for.

It is a pair of tee nuts made specifically for rocketry body tube. One of the pieces is threaded and the other piece is counter sunk. When the counter sunk piece is used on the outside and the threaded piece is used on the inside of the coupler it makes a perfect rivet replacement. When the threaded piece is used on the outside and the counter sunk piece is placed on the inside of the coupler tube it makes a great cutter for nylon screws.

View attachment 251274
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Demand? I would buy them. I demand them.

I would buy #2, #4, #6, and #8 for various uses.

This is what they look like after installation, epoxy fillet and paint.





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I like, I'd buy. I've used some thin Al strips epoxied on the inside tube to cut the 2-56 nylon screw in smaller cardboard rockets. I've had too many friends with large projects get a "shock" deployment of the friction fit main chute at > 10k apogee that results in a lost rocket. One fellow went after an errant rocket with an RDF tracker and due to the changing bearing, correctly surmised someone was moving it in a vehicle. Eventually lost the signal and rocket was never returned.

What's a "shock" deployment? Slightly agressive apogee charge blows the components to the limit of the shockcord and the momentum of the "main" recovery laundry (especially if it's a loose fit) pushes out the nosecone for an unintended apogee deployment

I had a single deploy where the loose fit saved the fincan. It was my L2 rocket I launched at one of the Midwest Power launches. Apparently, I got some grease in the "blowhole" and the motor ejection charge didn't fire. It was a lightweight, LOC tubed 4 inch rocket. The Covert parachute (remember them?) slid forward with the shockcord and pushed the nosecone off while the rocket was still a few hundred feet in the air while in the nose dive. The Covert chute "exploded"/shredded and the shockcord did the obligatory zipper. Thing is the rocket body was knocked sideways and landed flat in nice soft dirt. First thing I checked was the fincan. It was perfectly O. K. Next thing was there was enough room for a coupler repair of the zippered bodytube and since I had an un-opened 4 inch LOC ebay sitting at home, I turned the rocket into a keychain camera carrying, dual deploy sport flyer. Kurt

Random Flying Object said:

This is what they look like after installation, epoxy fillet and paint.

View attachment 251304



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Correct me if I'm wrong but that looks like a fiberglass rocket & coupler.

Are these "grommets" designed to work on cardboard airframes as well?

In any case, I'd be interested in the #2 and #4 sizes if the pricing is right.

Thanks!

o1d_dude said:

Correct me if I'm wrong but that looks like a fiberglass rocket & coupler.

Are these "grommets" designed to work on cardboard airframes as well?

In any case, I'd be interested in the #2 and #4 sizes if the pricing is right.

Thanks!

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The picture further up of the black and white airframe is 4" paper tube, they work great. I used them on a Torrent for both static coupler connection and shear pins.



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Yes, very nice!!

#2 first then #4 later.

Random Flying Object said:

Unfortunately that is several production runs. These parts would be far too expensive to make one by one. I would like to pick the size of my next run based on demand. #2 or #4, which will it be? I already have #6 in stock.


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I'd definitely be interested in these. Are the #6 size currently available? If so, how much are you selling them for? I'd be interested in a production run of # 2 and #4 as well. #2 first though.
Thanks,
Dave