Those pesky shock cords.

Discussion in 'Low Power Rocketry (LPR)' started by Doug Holverson, Mar 3, 2020.

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  1. Mar 3, 2020 #1

    Doug Holverson

    Doug Holverson

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    I guess that I'm going to have to relearn something simple at my old age. I had more shock cords break off in the last season than I had in all the previous years in this hobby going back to '73. So what is the best way to mount a shock cord in a little BT-5 or BT-20? These are going to be for little work horse rockets like pop-pods.
     
  2. Mar 3, 2020 #2

    ebruce1361

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    BT5 is a difficult size to work with in my opinion. For BT20 through BT60 tubes, I just do the old fashioned two-fold paper mount with elastic folded in it and glued to the inside of the body tube with a hefty slathering of wood glue under, in, and on top of the paper mount. As for the BT5s, I suggest cutting a small slit in the side perpendicular to the tube for the shock cord to thread through, and then anchor the end of the shock cord to the outside of the tube. I know Estes kits often had a plastic ring that incorporated a launch lug that would slide up the body and fold the shock cord under it and be glued in place there. (See Estes Gnome) If you don't have such a ring available, maybe wrap a strip of paper smeared with glue around the body with the protruding shock cord folded underneath?
     
  3. Mar 3, 2020 #3

    BEC

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    My go-to for small tubes (well, actually most tubes) these days is a length of Kevlar, put in with a small version of the good old Estes tri-fold mount. In contest models I'll just use three feet or so of Kevlar only. In sport models maybe a foot or so, tied to the kit's elastic/rubber shock cord or a couple of feet of same if it's a scratch-built model.
     
  4. Mar 3, 2020 #4

    mbeels

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    What kind of shock cord material are you using?
     
  5. Mar 3, 2020 #5

    CoachSteve

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    My play for BT-5s is kevlar thread instead of elastic - folds up tiny in a tiny Estes style paper fold over and glue in tube using a stick to press it in if your fingers are too big. BT 20 - standard 1/8 elastic, fold and glue - easy peasy
     
  6. Mar 3, 2020 #6

    BABAR

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    How cosmetic does it have to be? This works well for sport rockets:

    For BT-20s and BT-5s, I routinely punch a tiny hole in the rocket body just aft of where the nose cone shoulder will go (this keeps it as far away from the ejection charge as possible, but doesn't block putting the nose cone in.) I make a loop of Kevlar cord and wrap it around the OUTSIDE of the rocket, feeding the loose end through the hole and out the front. I pull the loop tight and wrap a piece of Mylar tape around the loop for cosmetic purposes and to hold it in place.

    What I LIKE about this is that I don't have any trifold mount or anything else on the inside of the tube to "catch" my chute or streamer as the ejection charge tries to push everything out the front, AND it keeps the cord away from the ejection charge as well. Yeah, you have a little bump on the outside of the rocket, for a sport rocket isn't going to hurt aerodynamics much (and the metallic Mylar tape IMO adds color and makes rocket easier to find!)

    Another option is motor mount attachment. The cord being by the ejection charge is one of the major problems with motor mount attachments of shock cords. @hcmbanjo also has a nifty trick for shock cord placement that allows easy access for checking for frayed/burned cords and replacement, link here

    https://www.apogeerockets.com/education/downloads/Newsletter338.pdf

    Somebody recently posted a trick for getting trifold mounts "flat" against the inside of the rocket, sticking it in place and then partially inflating a long skinny balloon in the body tube lumen. Haven't tried it, but sounds slick.
     
  7. Mar 3, 2020 #7

    John Taylor

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    For small light rockets I throw away the rubber band and substitute a long length of elastic. I protect the bottom portion with duct tape if needed from heat.
    For larger rockets I use a long piece of tubular nylon sheathed by the ejection charge by nomex. Attachment methods vary by weight of the rocket.
     
  8. Mar 3, 2020 #8

    Greg Furtman

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    I don't understand why they exit the kevlar string from the motor mount closest to the motor. It would be out of harms way if the notch were on the outside of the motor mount centering rings. That's what I do.
     
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  9. Mar 4, 2020 #9

    BABAR

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    I agree that makes more sense.

    Kevlar is not as flameproof as many people think.
     
  10. Mar 4, 2020 #10

    ebruce1361

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    The main reason kevlar cords are anchored to the motor mounts is because that area is structurally the strongest in the booster section with all the centering rings, fins, and glue going on. So for larger rocket where a mount simply glued to the inside of the body tube could be torn off, that kind of mount makes sense. But for small rockets that fly on 13mm and 18mm motors, it really isn't necessary.

    As for the flame resistant qualities of kevlar, I've never had a problem with it in low and mid power rockets, but for bigger rockets, a steel cable leader that takes the worst of the ejection blast is a better option.
     
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  11. Mar 4, 2020 #11

    neil_w

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    I do that too, for the same reason. I don't know how much difference it makes but I figure it can't hurt.
     
  12. Mar 4, 2020 #12

    ebruce1361

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    @Greg Furtman
    Disregard half of my previous comment. Maybe I'm not fully awake yet, and I thought you were wondering why kevlar cords are anchored around motor mounts in the first place. Now I see you're talking about the kevlar cord exiting the tube so it's out of the way of the ejection blast. Carry on.
     
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  13. Mar 4, 2020 #13

    Greg Furtman

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    More coffee!! :)
     
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  14. Mar 4, 2020 #14

    ebruce1361

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    WIN_20200304_09_13_42_Pro.jpg
    COFFFFFFFFFFFEEEEEEEEEEEEEEEEEE!!!
     
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  15. Mar 24, 2020 at 4:34 PM #15

    Doug Holverson

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    I was wondering if I was using inferior quality kevlar that keeps burning through? Can anybody recommend a good brand of the stuff that doesn't?
     
  16. Mar 24, 2020 at 4:46 PM #16

    o1d_dude

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    Lately I’ve been using the 2000 lb stuff from Amazon. Comes in 50’ rolls from an outfit called Emma’s Kites.

    It’s a braided cord about 1/8” thick and is somewhat stiff. Runs under $20 for the roll. No failures yet.

    Disclaimer: Mid Power and small High Power rockets only.

    My “serious” rockets use thicker kevlar I buy from Bay Area Rocketry. I use a y-harness epoxy’d to the motor tube that ends a few inches below the top of the fin can and attach a tubular nylon shock cord from there on out. One Bad Hawk, of course, so all the loops are sewn.

    EDIT: Sorry. Just realized this was a Low Power question. My bad. Check Amazon for lower test rated cordage from Emma’s Kites. Comes in a wide variety of weights and diameters.
     
    Last edited: Mar 24, 2020 at 4:54 PM
  17. Mar 24, 2020 at 4:46 PM #17

    BEC

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    Well....if it's really Kevlar, there's only one brand....

    But my experience with BT-50-based LPRs that attach, say 100-150 lb. test Kevlar to the motor mount, is that somewhere between 15 and 25 flights it will burn through. That's what I'm doing what I posted earlier in this thread. I have not had a burn-through failure since, even on models with two and three times that number of flights.

    It's also a way to return burn-through victims to flight with a little work using a stiff toothbrush and then some sandpaper wrapped around a dowel to clean a spot on the tube where you can glue in a tri-fold on a well-flown model.
     
  18. Mar 24, 2020 at 4:53 PM #18

    Greg Furtman

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    @Doug Holverson , mid-February we had our annual Rocket Boy weekend at a friend's cabin here in NW Wisconsin. We launch from a frozen lake. I had more shock cords break that day than maybe in the last 3 years. After inspecting them I think that I did not have enough wadding in the body tube which allowed the hot ejection gases to hit the shock cords. I had failures on both elastic & Kevlar shock cords. :confused:
     
  19. Mar 24, 2020 at 4:55 PM #19

    Greg Furtman

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    I wonder if the foil faced duct tape would provide more protection?
     
  20. Mar 24, 2020 at 10:36 PM #20

    mbeels

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  21. Mar 29, 2020 at 5:48 AM #21

    caveduck

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    For small tubes I use a flat non-folded cardstock mount with 3 holes or slits - old Centuri style - and some Kevlar or good sewing elastic or contest rubber woven through the holes. 1/8" rubber is too fat for BT5. Watch out for cheapo elastic and rubber that fail quickly. The white Estes rubber is terrible because of the additives; trash it and use good stuff or Kevlar. Get a half pound of fantastic rubber for $17 from FAI Model Supply and never buy shock cord again :) . A few inches of elastic/rubber and then Kevlar gives some anti-zipper protection.

    I glue the mount in with a very thin layer of 5-minute epoxy and use a dowel wrapped with plastic wrap to press it in place while it cures. It's up close to the top of the tube and will not burn through for a long time. If it ever fails it can be sanded out with the same dowel and replaced. I also like Chris M's tube-through-the-MMT-rings trick for larger models that aren't so weight sensitive. The cord will burn through faster but is trivially replaced.
     

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