Shear Pins and Rivets

[Dave Pritts]

Have been flying 2.6" birds on F & G motors for a while. Just flew a LOC 3.1 IRIS on an H100W for L1 certification. Been reading up on HPR topics and would like to have the collective wisdom on when and how to start using shear pins and rivets(plastic) to hold my rockets together. Thanks in advance for any suggestions.

Regards,

Dave P. (Now L1~ Woot)

 

[scsager]

They call it drag-separation.

When the motor burns out the rocket coasts upward. Sometimes the "coasting" lasts 20 seconds or more and the rocket might still be moving at hundreds of MPH. It's during this part of the flight where the (lower) section of the rocket with the fins has more "drag" than the upper section. The "drag" can cause the rocket to separate and the chute could come out when the rocket is still climbing rapidly.

Shear pins are usually 2-56 nylon screws that prevent premature separation. The ejection charge is strong enough to push the sections apart and cut the 2-56 screws.

Plastic rivets are used where you do NOT want the sections to separate. Like when you want half of the AV-Bay to stay with the payload section. Rivits are used because they are easy to remove when you want to take the rocket apart. Personally, I never use rivets. Instead I use #6 stainless steel machine screws and weld-nuts to hold the AV-Bay to the payload section.

Also recommended - Modern High-Power Rocketry 2 - If you are at all serious about High Power, you need to own a copy of this book.

 

[Dave Pritts]

Scott,

Thanks for the reply. I understand the why of shear pins and rivets. I'm interested in the when the weights, velocities, and motor impulses dictate their use for safety purposes. BTW: I just ordered a copy of Modern High-Power Rocketry 2. <Many thanks for the suggestion.

Dave P.

 

[Cameron Anderson]

Anything I friction fit and can easily shake apart, I shear pin. Except cardboard...I don't have luck pinning cardboard. But other people do.

 

[rharshberger]

Anything I friction fit and can easily shake apart, I shear pin. Except cardboard...I don't have luck pinning cardboard. But other people do.

Shear Pins in cardboard airframes takes some extra work to make function properly, most people solution involves brass shear plates installed into the nosecone and airframe with the shear pin going through those.

 

[Cameron Anderson]

Shear Pins in cardboard airframes takes some extra work to make function properly, most people solution involves brass shear plates installed into the nosecone and airframe with the shear pin going through those.

Too much work, that's why I almost exclusively fly fiberglass! Drill, tap, done.

 

[rharshberger]

Too much work, that's why I almost exclusively fly fiberglass! Drill, tap, done.

I don't fly fiberglass unless its cardboard wrapped with glass or CF, I like being able to fly larger rockets on H and I motors, instead of I and J motors. I have two FG rockets and both weigh nearly twice the weight of my cardboard wrapped rockets, FG is tougher and easier to prep and paint, but I fly cardboard almost exclusively so I don't mind that extra half hours to hours worth of work.

 

[David Schwantz]

The more weight you have in the NC for balance, the easier it will separate. Some smaller stuff 13, 18,24 mm, I do not use pins, even though there is a small amount of weight in NC. The friction fit works fine as the motors do not have enough thrust to cause them to separate at burn out. But larger motors, or ST's and Warp 9's can cause separation. I had 3 lbs of led in a Phoenix and one pin still allowed separation on a J250W. Had to add another. when I build a rocket I will put shear pin mounts in line with the fines. If I only need one, then I have 2 extra in case one hole gets, ripped, or plugged.

 

[Dave Pritts]

Thanks David Schwantz for sharing your experiences. That's just the kind of feedback I need to begin thinking about handling drag separation.

 

[manixFan]

Regarding rivets: I don't use switch bands at all. All of my A/V bays are essentially couplers held in place by plastic rivets. I've used them on 38mm diameter on up to 7.5" N powered rockets. The biggest issue with using them is getting the right depth (and of course hole size). If they are too short you can't insert the head, or you have to oversize the hole to allow the prongs to expand. Too long and they don't stay flush against the airframe. I consider them mostly single use, but they are so inexpensive that it's not an issue. You can buy them in packages of 100 in different lengths to accommodate various airframe thicknesses.

Probably the biggest issue for many folks is getting them out without marring the paint next to the rivets. I find a very sharp pocketknife slid under the head works well and with practice can be done without harm to either the rocket or rivet.

Regarding shear pins: pretty much any dual deployment rocket gets shear pins in the nosecone. Otherwise you risk spilling the main when the apogee charge goes off and the upper payload section hits the end of the shock cord. For small rockets I use styrene rod. Larger or heavy nose cones get 2-56 nylon screws. I have an old rocket calc application on my phone that will tell me how much black powder is required to shear a given number of screws. Smaller diameter rockets get 2 screws, larger either 3 or 4, but that is dependent on nose cone weight. I just use a very small hammer to pop in the screws into an appropriate sized hole - much faster than trying to screw them in. I cut the heads off and knock them thru with a punch when I need to replace them.


Tony

 

[Dave Pritts]

Tony,

Thanks for sharing your experience. I understand rivets to keep AV and Payload tubes together. Makes sense.
For shear pins... besides using them in the nosecone do you use shear pins to keep the AV bay and booster tube together as well?

 

[aerostadt]

I have been using rivets for hatches on mid-power R/C rocket gliders with cardboard tubes. I usually use tape on one side as a hinge and a cardboard fixture with a rivet on the other end. This makes a convenient way of installing a charged battery and sealing things up.

 

[TimothyG]

I don't use rivets, bolts and shearpins here. Rivets give me fits during assembly for some reason where threaded components are a breeze. I'm also stepping my shear pins up to 4 size instead of 2. For anything 2.6 or smaller I use #2's still but I'm moving more and more towards bigger and faster stuff so #4's are getting used more. I've had 4 X #2 shearpins shear upon drag seperation after a very aggressive boost on a 98mm Min dia project with lots of base drag.

 

[manixFan]

Tony,

Thanks for sharing your experience. I understand rivets to keep AV and Payload tubes together. Makes sense.
For shear pins... besides using them in the nosecone do you use shear pins to keep the AV bay and booster tube together as well?

I think the only time it makes sense to use shear pins on the booster to A/V is if there is a chance of drag separation. Usually as long as there is a good friction fit between the sections you'll be fine. But if there is a big imbalance in weight or 'dragginess', shear pins may be needed. I use the time honored 'shake test', hold the assembled rocket by the payload bay and lift it off the ground and shake it. If the parts stay together, you should be fine. In my experience I've not had one separate that passed that test.


Tony

 

[TheTank]

This is good info. Im building my first dual deploy rocket. 3” cardboard tube (LOC). I used 3 rivets to hold the payload section to the av bay, and then 3 2-56 shear pins to hold the nose cone on. The pins are a screw in fit and I added thin CA to the holes in the cardboard to strengthen the shearing surface... but am still concerned about tearing the cardboard. As the nose cone is pretty loose, i considered adding some glass “patches” to the inside of the holes in the tube but I am not sure if this would help. Can anyone point me towards info on the brass tube method mentioned above?

 

[OZRoc]

Edited below.

 

[OZRoc]

... Can anyone point me towards info on the brass tube method mentioned above?

Geday,
There is a method of embedding brass shear plates into a plastic nosecone shoulder. Very messy. Decide how many shear places you want. Scrape / grind out depth and area of brass shim material you have obtained from your local hobby store. It pays here to add a slot to the trailing edge of the scrape out so you can bend the brass material 90deg as a keeper for the material. Glue the brass material to the shoulder and drill a hole for the shear pin. Brass material acts as a guillotine for the shear pin.
Have used successfully in the past but much prefer to go fibreglass everything these days.
Cheers,
Mark

 

[Reinhard]

Can anyone point me towards info on the brass tube method mentioned above?

I prefer this method:

https://translate.google.com/transl.../j3/index.php/knowhow/recovery/124-shear-pins

Reinhard

 

[JoePfeiffer]

I'm a little surprised at the size sheer pins people are using. For my 2.2" fiberglass and 2.6" cardboard dual deploy rockets I use two 0-80 pins; that gives a separation force of 34 lbs, which is ample for a four pound rocket.

 

[TheTank]

I'm a little surprised at the size sheer pins people are using. For my 2.2" fiberglass and 2.6" cardboard dual deploy rockets I use two 0-80 pins; that gives a separation force of 34 lbs, which is ample for a four pound rocket.

As I am building my first dual deploy rocket, I was trying to research the different setups for rivets and shear pins and either I just wasnt searching right or there wasnt much out there. What I did find led me down the path to use 3 2-56 size nylon sheer pins on my 3" cardboard tube rocket. I am just hoping it isnt too much where I damage the tube even with hardening it around the holes to create a solid sheering surface. Im second guessing myself now thinking I should have only done two...

 

[o1d_dude]

Have you considered a "shear plate" cut from an adult beverage or soft drink can epoxied to the wall of the tube?

I think I might have done that one or twice.

 

[OverTheTop]

Generally shear pins are needed when there is a fast change in altitude that creates an internal pressure that pushes the NC off the airframe. Probably not needed for moderate velocities and accelerations, and lower flights. A breather hole in the airframe to allow the pressure to equalise does wonders, but when the rate of change of ambient pressure is great there is good reason to use shear pins.

I suspect it will only be really necessary to use shear pins to prevent drag-separation if the rear airframe is really draggy or much larger than the diameter of the NC. Should not be much of a problem in most rockets IMHO.

 

[JoePfeiffer]

Generally shear pins are needed when there is a fast change in altitude that creates an internal pressure that pushes the NC off the airframe. Probably not needed for moderate velocities and accelerations, and lower flights. A breather hole in the airframe to allow the pressure to equalise does wonders, but when the rate of change of ambient pressure is great there is good reason to use shear pins.

I suspect it will only be really necessary to use shear pins to prevent drag-separation if the rear airframe is really draggy or much larger than the diameter of the NC. Should not be much of a problem in most rockets IMHO.

The other thing is (in my case, anyway) the nose cone can work itself loose and separate partway down.

 

[TheTank]

Have you considered a "shear plate" cut from an adult beverage or soft drink can epoxied to the wall of the tube?

I think I might have done that one or twice.

i had thought about it. As loose as the cone is, i may glass the inside of the tube to create that shearing surface using thin cloth (.75 ounce) and thinned resin.

 

[TheTank]

Generally shear pins are needed when there is a fast change in altitude that creates an internal pressure that pushes the NC off the airframe. Probably not needed for moderate velocities and accelerations, and lower flights. A breather hole in the airframe to allow the pressure to equalise does wonders, but when the rate of change of ambient pressure is great there is good reason to use shear pins.

I suspect it will only be really necessary to use shear pins to prevent drag-separation if the rear airframe is really draggy or much larger than the diameter of the NC. Should not be much of a problem in most rockets IMHO.

All that I had read indicated that on a dual deploy setup, you ran the risk of seperation/deployment of the main when the drogue fired so it was best to pin the nosecone.

 

[OverTheTop]

All that I had read indicated that on a dual deploy setup, you ran the risk of seperation/deployment of the main when the drogue fired so it was best to pin the nosecone.

Maybe my setup is different to a lot of others. I blow the NC off at apogee, with a drogue attached. The rest of the airframe stays together. The main chute is in the upper compartment and gets blown into the air at the relevant time. All other airframe connections, apart from the NC shear pins, are screwed.

 

[TheTank]

Maybe my setup is different to a lot of others. I blow the NC off at apogee, with a drogue attached. The rest of the airframe stays together. The main chute is in the upper compartment and gets blown into the air at the relevant time. All other airframe connections, apart from the NC shear pins, are screwed.

makes sense. Im running my drogue in the sustainer and then the main in my upper payload. Being my first dual deploy, im thinking of still running an ejection charge in the motor (several seconds after apogee) just as a backup to fire the drogue if for some reason the altimiter/charge doesnt get it open on the maiden flight (yes, i will be testing beforehand - but i would still rather be safe than sorry).

 

[rharshberger]

makes sense. Im running my drogue in the sustainer and then the main in my upper payload. Being my first dual deploy, im thinking of still running an ejection charge in the motor (several seconds after apogee) just as a backup to fire the drogue if for some reason the altimiter/charge doesnt get it open on the maiden flight (yes, i will be testing beforehand - but i would still rather be safe than sorry).

Running motor back up is not a bad idea nor is it uncommon, as long as the drogue gets deployed the rocket isnt usually damaged too bad if the main fails to deploy.

 

[OverTheTop]

Being my first dual deploy, im thinking of still running an ejection charge in the motor (several seconds after apogee) just as a backup to fire the drogue if for some reason the altimiter/charge doesnt get it open on the maiden flight (yes, i will be testing beforehand - but i would still rather be safe than sorry).

Great idea.