Dual deploy questions

jjoos99

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I’m setting up dual deploy for a loc weasel that has bt60 size tube. How many shear pins should be installed? I am assuming pins should be installed where the eBay installs into the main body tube and also at the nose cone? My neighbor laser cut a sled for me that I designed off the 60 size apogee design. Ive seen pictures of eBays that have two powder cups on one end. Is this for redundancy? I’m assuming that a cup is needed on both ends of the bay.
thanks
Jeff
 

Kelly

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I normally use this spreadsheet to size shear pins and BP charges:


I use shear pins at any joint that is intended to separate, though the joint that separates to deploy the main (usually the NC) is probably the more important one, as it is subject to the force of ejection shock, whereas the one which deploys the drogue (usually the lower joint) is 'only' subjected to drag separation and internal pressure separation.
Two powder cups on one end would be for redundancy; if you're doing the typical dual deploy setup you would need at least one cup on each end of the ebay (unless you're relying ONLY on motor ejection charge for drogue deploy, in which case you wouldn't need one on the aft end of the bay).
 

David Schwantz

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Depending on your NC weight, you may not need a shear pin. But if you do, I would use only one on that size rocket. But you will need rivets or a steel screw where the Avbay goes into the payload section so it does not separate. When you say cup, I am assuming you are a mean a charge well. On that size tube space is a premium. I would use a vial instead on both sides of Avbay.
 

jjoos99

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Thanks for the replies. The kit does have a pretty heavy plastic cone. So with this small of a rocket would one pin at the cone and one at the ebay/lower section be enough? The shop I work in sells the #2 pins.
thanks
Jeff
 

cerving

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You'll need to add some kind of metal shims on the nose cone and tubing, shear pins don't work very well in cardboard or thermoplastic. OR, you can use a few pieces of Scotch tape... two or three strips about 3/8" wide should do the trick. Ground test it, of course...
 

Rocketclar

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I use thin brass shims to help shear the pins and they work great. Here's a pic of one on a 54mm tube. I epoxy them on the inside and hold in place with c-clamps. Of course I rough up one side of the brass before gluing. I also file all the ends to ensure no rough edges. Then I check for any roughness and file accordingly. I put CA in the hole on the tube side to strengthen it. IMG_20210117_123547417.jpg Also, ground test! Today I ran a couple of tests for a new rocket and the calculated numbers were no where near enough to work!
 

Kelly

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I use masking tape (applied longitudinally, across the break point) for paper tubes. One inch width of masking tape has about the same breaking strength as a single #2-56 nylon screw, so it also works well for smaller rockets that don't need a full set of screws.
 

b.wieting

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I’d think the brass shim method should be most robust. I myself have had good success with a simple body tube thru hole strengthened w CA (and tapped 2-56 in NC shoulder), tho only on a few flights so far. Btw, for me, about twice as many ground tests as flights.
 

Zeus-cat

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You can fold and tape your lower shock cord so that it absorbs some of the shock and doesn't pop the nose off. I use this method with LOC tubes for H and I motors. Just fold the shock cord and wrap it in tape. Repeat along the shock cord as much as you can.
 

The_Quacken

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I use thin brass shims to help shear the pins and they work great. Here's a pic of one on a 54mm tube. I epoxy them on the inside and hold in place with c-clamps. Of course I rough up one side of the brass before gluing. I also file all the ends to ensure no rough edges. Then I check for any roughness and file accordingly. I put CA in the hole on the tube side to strengthen it. View attachment 516730 Also, ground test! Today I ran a couple of tests for a new rocket and the calculated numbers were no where near enough to work!
Do you think that aluminum tape would work similarly to these because that could be a much easier option
 

Handeman

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Aluminum tape won't work in place of brass shims. The type of metal isn't as important as the thickness. It has to be thick enough to shear the nylon screw without deforming the metal. You can use aluminum from beer or soda cans, that will usually work, but might still be too thin and soft. Soup/coffee cans work well.

If you were building a fiberglass rocket, no question, use shear pins, no shims required.

For a cardboard tube rocket, you have to remember why shear pins are being suggested in the first place. The whole rocket is a system and changing one thing effect most others. I'm suggesting you use friction fit instead of shear pins.

1. The shear pin(s) between the av-bay and the booster are only there to prevent drag separation. On most rockets, a piece of masking tape to add a little friction fit will work just a well, or better, than a shear pin. The apogee charge only needs to be enough to open the rocket and get the drogue chute into the air flow. You don't need a big charge that is going to send the upper section slamming into the end of the shock cord. Blow it out or blow it up is the worst method for apogee charges IMHO. Using too large of apogee charge means you need heavier shock cords, heavier eye/U-bolts, more shear pins, and more likely to see damage to your rocket. Using shear pins on the apogee joint also means you need a bigger charge to shear the pins and tends to increase the shock at the end of the cord, which means you need more pins on the nose cone, heavier shock cords, etc. It's all a system.

2. The shear pins at the NC are there to prevent a main chute deploy at apogee when the forward section hits the end of the shock cord after the apogee charge goes off. A small, reasonable apogee charge reduces the need for an extra number of pins. The more pins you use to prevent deployment at the shock hitting the end of the cord, the larger the main charge has to be to shear the pins. The bigger the charge, the more items have to compensate for that. It's all a system.

YMMV, but all of my DD cardboard tube rockets use only friction fit, no shear pins. I never used a shear pin until I built my L3 fiberglass rocket. I know the TRF groupthink is that you have to use shear pins, but you don't. I've had +100 successful DD flight on cardboard tube rockets using friction fit and you can too. Friction fit isn't hard and just as safe as shear pins IMHO. The only fail point is an apogee deployment of your main. You just have to learn how to do the friction fit and not use big apogee charges. Just do a few DD flights at lower altitude, incase you get a main deploy at apogee, until you are confident with your friction fit. Then go for it!
 

The_Quacken

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Aluminum tape won't work in place of brass shims. The type of metal isn't as important as the thickness. It has to be thick enough to shear the nylon screw without deforming the metal. You can use aluminum from beer or soda cans, that will usually work, but might still be too thin and soft. Soup/coffee cans work well.

If you were building a fiberglass rocket, no question, use shear pins, no shims required.

For a cardboard tube rocket, you have to remember why shear pins are being suggested in the first place. The whole rocket is a system and changing one thing effect most others. I'm suggesting you use friction fit instead of shear pins.

1. The shear pin(s) between the av-bay and the booster are only there to prevent drag separation. On most rockets, a piece of masking tape to add a little friction fit will work just a well as a shear pin. The apogee charge only needs to be enough to open the rocket and get the drogue chute into the air flow. You don't need a big charge that is going to send the upper section slamming into the end of the shock cord. Blow it out or blow it up is the worst method for apogee charges IMHO. Using too large of apogee charge means you need heavier shock cords, heavier eye/U-bolts, more shear pins, and more likely to see damage to your rocket. Using shear pins on the apogee joint also means you need a bigger charge to shear the pins and tends to increase the shock at the end of the cord, which means you need more pins on the nose cone, heavier shock cords, etc. It's all a system.

2. The shear pins at the NC are there to prevent a main chute deploy at apogee when the forward section hits the end of the shock cord after the apogee charge goes off. A small, reasonable apogee charge reduces the need for an extra number of pins. The more pins you use to prevent deployment at the shock hitting the end of the cord, the larger the main charge has to be to shear the pins. The bigger the charge, the more items have to compensate for that. It's all a system.

YMMV, but all of my DD cardboard tube rockets use only friction fit, no shear pins. I never used a shear pin until I built my L3 fiberglass rocket. I know the TRF groupthink is that you have to use shear pins, but you don't. I've had +100 successful DD flight on cardboard tube rockets using friction fit and you can too. Friction fit isn't hard and just as safe as shear pins IMHO. The only fail point is an apogee deployment of your main. You just have to learn how to do the friction fit and not use big apogee charges. Just do a few DD flights at lower altitude, incase you get a main deploy at apogee, until you are confident with your friction fit. Then go for it!
Thanks I may try friction fit at first I’m not quite building cardboard or fiberglass it’s the canvas phenolic tubes so I’m not entirely sure where on the scale this is but I am trying to push some serious flights on this as the rocket should be able to fly on low h through L so it’s sorta hard to tell where in the range of building methods I should go
 

Handeman

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Thanks I may try friction fit at first I’m not quite building cardboard or fiberglass it’s the canvas phenolic tubes so I’m not entirely sure where on the scale this is but I am trying to push some serious flights on this as the rocket should be able to fly on low h through L so it’s sorta hard to tell where in the range of building methods I should go
Totally understand.

Sounds similar to my L2 cert rocket, I211W to L motors (1100 ft to 9500 ft.). I stayed with friction fit on that for 50 flights before retiring it.

I would suggest try friction fit first at low altitudes until you have the deployment part down, along with the drogue chute sizing. The apogee to main deployment drop is the longest part of any high flight so getting the drogue sized and control of that part of the flight is probably the hardest to get right. I usually end up doing 3 - 5+ flights getting that part right.

If the friction fit doesn't work for you, you can try the shims and shear pins, but that may result is damage if not done right from the start. I see it as a progression. Test flight, test flight, test flight... It's part of the fun!

Good luck.
 

The_Quacken

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Totally understand.

Sounds similar to my L2 cert rocket, I211W to L motors (1100 ft to 9500 ft.). I stayed with friction fit on that for 50 flights before retiring it.

I would suggest try friction fit first at low altitudes until you have the deployment part down, along with the drogue chute sizing. The apogee to main deployment drop is the longest part of any high flight so getting the drogue sized and control of that part of the flight is probably the hardest to get right. I usually end up doing 3 - 5+ flights getting that part right.

If the friction fit doesn't work for you, you can try the shims and shear pins, but that may result is damage if not done right from the start. I see it as a progression. Test flight, test flight, test flight... It's part of the fun!

Good luck.
Appreciate the advice! Thanks
 

jjoos99

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I am close for my first dd flight. Did my first pop test on my loc weasel that is a bt60 size tube and weighs right at a pound without the motor. My question is I have d rings at my sled but wondering what cord is best to connect to them being so close to the charge cup? My current set up is Kevlar coming out of the main body attached to 1/4in elastic which connects to the sled. Guessing I shouldn’t use the elastic so close to the charge cup? On the other end of the sled I have the nylon shock cord that came with the kit. What is everyone’s recommendation? Also for a rouge on this smaller 1 lb rocket should I use a 12in chute or streamers?
thanks for the advise
Jeff
 
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waltr

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Elastic and ejection charges are not good. May get one flight but may burn the elastic on first flight. The DD charges are closer to the cords than most motor eject charges so higher heat and more flame.
I like Kevlar for about a foot from the AVbay's charges. Then the LOC Nylon works well but like a small Nomex blanket on the Kevlar to protect the nylon. Think of the Nomex as wadding that stays on the cord and is reusable.

I highly recommend either doing taped bundles or braid the nylon cord. This absorbs energy during deployment and helps prevent zippers and/or harness failures.

12in chute is way too big for a Drogue. I use a 6in TF chute as drogue on a 2.2 pound LOC IRIS which is about perfect.
Did you run sims and check predicted descent rate on drogue and main?
I do use a streamer on my smaller DD rockets for the drogue and have used nothing except breaking the rocket in two for fast descent.
 
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