Dual Deployment: Shear Pins, Rivets, or Nothing

[Alby]

So I'm working on a rocket that will have dual deployment. The setup will be a drogue parachute between the booster and AV-Bay
and the Main Parachute between the AV-Bay and Nose cone. Or should this setup be reversed with the drogue closer to the nose cone
and the main closer to the booster?


My question is do I need shear pins to secure the AV-Bay? I'm asking because I'm thinking that if I fire the drogue chute, the deceleration
will pull on the cord tied to the AV-Bay and yank it out. That in turn will pull out the main chute at apogee vs. a defined lower altitude.
Are my concerns valid? Do I need shear pins or rivets to secure the AV-Bay for the drogue and main chute deployments?

 

[Antares JS]

You want your main on top. Presumably it's heavier than your drogue and we all know it's advantageous to stability to put heavier things on top.

You seem to be asking about your avionics bay separating from your payload tube containing the main parachute, in which case absolutely, positively fasten it somehow and do not rely on friction fit. I prefer to use metal screws with some kind of insert, but the plastic rivets are fine. Anything you don't want to come apart must have some kind of positive retention.

I am confused about your mention of shear pins - shear pins are typically thin nylon screws put in to secure something that you DO want to come apart later. This sounds like you're talking about something you DON'T want to come apart.

That said, shear pins are not absolutely necessary, but I would highly recommend them on fiberglass rockets and larger rockets. On smaller dual-deploy rockets, you can get away with friction fit for the joints that you DO want to come apart.

 

[Alby]

You want your main on top. Presumably it's heavier than your drogue and we all know it's advantageous to stability to put heavier things on top.

You seem to be asking about your avionics bay separating from your payload tube containing the main parachute, in which case absolutely, positively fasten it somehow and do not rely on friction fit. I prefer to use metal screws with some kind of insert, but the plastic rivets are fine. Anything you don't want to come apart must have some kind of positive retention.

I am confused about your mention of shear pins - shear pins are typically thin nylon screws put in to secure something that you DO want to come apart later. This sounds like you're talking about something you DON'T want to come apart.

That said, shear pins are not absolutely necessary, but I would highly recommend them on fiberglass rockets and larger rockets. On smaller dual-deploy rockets, you can get away with friction fit for the joints that you DO want to come apart.

Thanks Antares. That is exactly what I needed to know. I might use screws like you suggested and secure the AV-Bay to the upper tube holding
the main parachute and shear pins for the lower section (drogue) and nose cone.

 

[teepot]

I would recommend shear pins to hold the nose cone on. Other wise it could come off when the drouge chute is deployed. It happened to me on my first dual deploy attempt and I had a mile walk. And as Antares said secure the av bay to the main payload bay with screws or rivets

 

[Bat-mite]

If this doesn't make sense, talk to someone before you do too much work.

 

[Alby]

I would recommend shear pins to hold the nose cone on. Other wise it could come off when the drouge chute is deployed. It happened to me on my first dual deploy attempt and I had a mile walk. And as Antares said secure the av bay to the main payload bay with screws or rivets

Thanks for the tip Teepot. Makes sense the nose cone and drogue side should have shear pins. I ordered a stack of shear pins so I'll be
sure to utilize them before my flight. I just need to calculate the BP needed to shear them.

Do you have any recommendations on shearing with a cardboard tube? I figured the cardboard might not be ideal for shearing, so I was
thinking of using epoxy to glue some thin strip of metal (eg: A coke can) to be the point where the hole is made so it can be the shear/cutting point.
But I'll take any recommendations.

Thanks,

 

[Alby]

If this doesn't make sense, talk to someone before you do too much work.

View attachment 462100

Bat-mite:

If I run into you at MDRA ESL 268 in a few weeks, you can double check my work. Assuming I get everything ready by then. ;-)

-Alby

 

[DragonRocketry]

Thanks for the tip Teepot. Makes sense the nose cone and drogue side should have shear pins. I ordered a stack of shear pins so I'll be
sure to utilize them before my flight. I just need to calculate the BP needed to shear them.

Do you have any recommendations on shearing with a cardboard tube? I figured the cardboard might not be ideal for shearing, so I was
thinking of using epoxy to glue some thin strip of metal (eg: A coke can) to be the point where the hole is made so it can be the shear/cutting point.
But I'll take any recommendations.

Thanks,

Use some CA to stiffen up cardboard around the holes. thin metal works to. Also keep the holes as far back from the edge as you can. If to close to the edge it will cut through the tube instead of shearing.

 

[Zeus-cat]

I don't use shear pins any more since they damaged my LOC cardboard tubes. Instead, I Z-fold my lower section of shock cord to absorb the excess energy at apogee deployment. It works well for me.

I use rivets to hold the upper section of tube to the av-bay. Before doing that I did pop off the upper section of tube rather than the nose cone.

 

[Handeman]

I've never used shear pins in cardboard tubing. Too much damage and repair work.

Heavy friction fit can work on the nose cone, if you don't use too large of apogee charges and minimal friction fit on the booster section. It's the amount of force the payload section hits on the end of the apogee shock cord with that will determine how much you need to secure the nose cone. Shear pins between the av-bay and booster section will require a larger apogee charge to shear the pins and will increase the force the payload section hits the end of the apogee shock cord with, thus increasing the need for shear pins on the nose cone. It's a vicious circle. Before you know it, you have heavier U-bolt, quick links, shock cords, etc. and have add pounds to your rocket. The whole thing is a system and has to work together. Change one thing and you have to change other, usually not for the lighter option.

 

[David Schwantz]

Actually, everyone is correct. You can use them or not. I did not see what the rocket is?? Weight?? If your NC is fit properly and there is no weight in it, no shear pins may be needed. Again, if your charges are correct and the shock cord is long enough, none may be needed. But, if your NC is heavy, due to balance, then shear pins are required. I do not use plastic rivets myself. I like to use steel button heads, 2-56 are enough for most rockets. I also use shear pins on the booster to Av bay joint.

I'm asking because I'm thinking that if I fire the drogue chute, the deceleration
will pull on the cord tied to the AV-Bay and yank it out.

For the most part the main comes out at apogee due to BP charge to high or the shock cord not long enough, and then it "yanks" the main out. Usually not due to deceleration. That is normally drag separation at motor burn out.

 

[JoePfeiffer]

The other reason to put the drogue lower is if you somehow get substantial horizontal velocity (weather-cocking, off-center nozzle....) it avoids zippering your body tube. This arrangement is called "zipperless" for exactly this reason.

 

[JoePfeiffer]

Even with cardboard, I've found that if the rocket's going to come down several thousand feet under drogue, there's a tendency for the nose cone to wiggle loose. Sheer pins are more reliable.

 

[DeWain]

Actually, everyone is correct. You can use them or not. I did not see what the rocket is?? Weight?? If your NC is fit properly and there is no weight in it, no shear pins may be needed. Again, if your charges are correct and the shock cord is long enough, none may be needed. But, if your NC is heavy, due to balance, then shear pins are required.

Admittedly, I have yet to launch my first dual-deploy rocket, but it sound like David is correct. I'm building a *mid* power dual-deploy rocket with cardboard tubes and balsa nosecone (coated in finishing epoxy), and I am planning to friction fit the AV bay and nose cone. Not only could shear pins do a number on cardboard tubes, the shoulder of the balsa nose cone almost certainly would be ripped by them. Given that this will be a relatively light rocket, I suspect that friction fit will be fine.

So construction techniques depend a lot on the weight of the rocket, and the weight of each section of the rocket. If I had a short midpower rocket with a substantial amount of nose weight, I would not try to friction fit. Mine is an upscale of the Estes Javelin (Mini Brute), so no nose weight is needed for stability.

 

[David Schwantz]

Hi DeWain, good luck with your project. Just so you know for a possible future build, Shear pins will work with cardboard. I like to put brass plates on the inside to act as a "shear plate". As far as a balsa cone, with it being solid, it could be hard to remove the sheared off threaded part. But, if you hollowed it out and installed some thing on the inside such as a small piece of ply, then you could remove the sheared off portion.

 

[Kelly]

Do you have any recommendations on shearing with a cardboard tube?

I've had problems with shear pins in cardboard, for the reasons you would think, and so I've started using masking tape instead of pins. I'm not talking about a friction fit - I mean placing tape over the joint, on the outside of the rocket, which needs to break before separation can occur. Masking tape has a tensile strength of about 27 pounds per inch of width, so one inch of tape gives just a tad less holding power than a #2 nylon screw. I cut 3 strips of tape, about 3 inches long and summing to the total width I need, and place them equally spaced around the joint, with the long direction running axially. Works well, doesn't require fiddling with metal plates/hardeners, and the cardboard doesn't get munged up by the screws. As always, do ground testing to be sure you're using the right separation charge.

 

[Alby]

Hi DeWain, good luck with your project. Just so you know for a possible future build, Shear pins will work with cardboard. I like to put brass plates on the inside to act as a "shear plate". As far as a balsa cone, with it being solid, it could be hard to remove the sheared off threaded part. But, if you hollowed it out and installed some thing on the inside such as a small piece of ply, then you could remove the sheared off portion.

Is your shear plate on the inside wall of the main body tube?

 

[David Schwantz]

Yes it is. Goes between the main tube and Av bay tube.

 

[Alby]

Yes it is. Goes between the main tube and Av bay tube.

Great. Thanks for the tip.

 

[Antares JS]

Pricey, but work well and durable.

https://www.apogeerockets.com/index...earch_in_description=0&keyword=tube+fasteners

 

[Alby]

Pricey, but work well and durable.

https://www.apogeerockets.com/index...earch_in_description=0&keyword=tube+fasteners

Expensive indeed. But I went ahead and ordered 3 packs (Nose Cone / AV-Bay Upper / AV-Bay Lower) and some extra packs of shear pins.
I had 96 points with Apogee, so that pretty much covered my shipping and taxes. ;-)

 

[#gobabygo]

Expensive indeed. But I went ahead and ordered 3 packs (Nose Cone / AV-Bay Upper / AV-Bay Lower) and some extra packs of shear pins.
I had 96 points with Apogee, so that pretty much covered my shipping and taxes. ;-)

Alby, did you order these tube fasteners using shear pins to hold the rocket together for dual deploy? have you used them yet?

 

[StreuB1]

I've used styrene rod and nylon screws as shear pins in cardboard for 20 years and never once had a problem with it.

You should stiffen the cardboard with fast CA glue in several applications so it forms an acrylic/fiber composite area. Then carve out a small area about 1/64" deep in the inside of the cardboard tube. Cut out a round piece of soda can and epoxy this in the carved out section of the tube. Once cured, install your mating NC or Ebay and then drill your shear pin holes. The soda can, as others mentioned, acts as a shear plate and takes the load of shearing the pins. The CA saturated cardboard substrate is very dimensionally stable and will stand the test of time and provide long life. I've never had a shear pin hole wallow out.

 

[Alby]

Alby, did you order these tube fasteners using shear pins to hold the rocket together for dual deploy? have you used them yet?

I sure did GoBabyGo. These fastener shears are awesome. I love them and ordered several more. The only catch is the halves on the outside of the tube could break free after the shear pin is sliced. So I recommend using a dab of epoxy (I used Rocket Poxy) around the outside lip of the fastener to make sure it doesn't come off.

 

[#gobabygo]

I sure did GoBabyGo. These fastener shears are awesome. I love them and ordered several more. The only catch is the halves on the outside of the tube could break free after the shear pin is sliced. So I recommend using a dab of epoxy (I used Rocket Poxy) around the outside lip of the fastener to make sure it doesn't come off.

Awesome awesome information, thank you!!