New to using Kevlar shockcord, looking for advice

Discussion in 'Low Power Rocketry (LPR)' started by Bill S, Sep 4, 2019.

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  1. Sep 4, 2019 #1

    Bill S

    Bill S

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    I am going to do my first rocket up using Kevlar shock cord. I had thought to keep the Kevlar short, just below the end of the body tube, to avoid zippering, and attach some elastic to that. I would probably be attaching the Kevlar to the engine mount, and notching the outside of the centering rings and running the Kevlar up the side of the tube.

    What concerns me about this approach is this: What if the elastic suffers damage and needs to be replaced; the end of the Kevlar shock cord won't be protruding out the end of the tube so it'll be hard to access to attach a replacement elastic cord.

    Any other suggestions for a Kevlar noob as well?
     
  2. Sep 4, 2019 #2

    rcktnut

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    Tie a loop in top end of Kevlar, tie elastic to loop.
     
  3. Sep 4, 2019 #3

    GaryT

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    Use Kevlar for the whole shock cord and leave the elastic out. Depending on what size BT your using will determine the size of Kevlar to use. IMO you can't have a SC that's to long, as for zippering you can "try" to prevent it but if you have a rocket coming in ballistic and then pop the main? it WILL zipper no matter what the BT is made of, that is if the SC doesn't break 1st.

    Also if your going to notch the CR, notch the side close to the MM tube. Tie a knot on the under side of the CR and run the extra down the MM tube, apply your epoxy to the knot and the rest of the Kevlar. Again depending on the size of the BT you can run the Kevlar down both sides of the MM tube, enough so the loop extends about a foot out the top of the booster tube.
     
    Last edited: Sep 4, 2019
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  4. Sep 4, 2019 #4

    Andrew_ASC

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    Keeping Kevlar short ends badly. I know this from experience with a short stubby 54mm diameter RB05A L1 fiberglass rocket... They had a rule of thumb about 2-3 times the length of body tube you needed cord. That rule of thumb failed on high power application of Kevlar. What ended up working for a 3.3 pound rocket about 2.5 feet long was 20 ft of Kevlar 3/16” thick. The cord would jerk violently when to short and snap at the glue joint on body tube. The nose and chute would drift away while the body tube would crash when cord was too short. I also had some scratch build 24mm Min diameter CF rocket about 7 ounces and 1.5 ft long use #400 of Kevlar about 9 ft long successfully. I’m still learning too. I find out experimentally any rocket over a pound or using Kevlar does not obey a rule of thumb. When you finalize design ask people in recovery section they will have the experience to help you size the right Kevlar length and thickness.

    Honestly for most low power rockets it’s hard to beat elastic for price. Just use a bunch of Kevlar i’d Recommend at least 15ft for small rocket.
     
  5. Sep 4, 2019 #5

    BABAR

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    Great article on replaceable shock cord.

    https://www.apogeerockets.com/education/downloads/Newsletter338.pdf
    Even Kevlar will burn through eventually, but will last longer than elastic.

    Whatever length of elastic you would use, I would multiply it by at least 1.5. Good thing is Kevlar is usually easier to pack than elastic.

    Also google “apogee rockets model rockets deployment shock cord” for a nice video on using a crochet hook to daisy chain the Kevlar. Makes it much easier to pack, less likely to tangle.
     
  6. Sep 4, 2019 #6

    neil_w

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    Good solid approach, works great. I do it on all my builds.

    1) put a loop at the end of the Kevlar. End of the loop should stop just short of the end of the BT.

    2) if you need to replace the elastic, you simply fish the Kevlar out the back of the rocket (through the MMT), tie on a new elastic, and then feed it back into the rocket. Easy peasy.

    3) if, for whatever reason, the Kevlar isn’t long enough to go out the back, the I have found it is possible to attach elastic to the loop in the Kevlar inside the end of body tubes starting at BT55. Just did this in my latest build. It’s a bit tricky and I need to use tweezers but I got it done fairly quickly. I wouldn't try it at BT50 or below, though it might be possible if you’re extremely dexterous.

    4) @hcmbanjo has documented how to make the Kevlar itself replaceable, on his blog. I haven’t tried that yet myself.
     
    Last edited: Oct 14, 2019 at 3:50 PM
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  7. Sep 4, 2019 #7

    BABAR

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    Great minds think alike.
     
  8. Sep 4, 2019 #8

    Bill S

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    Neil_W, what blog would that be?
     
  9. Sep 4, 2019 #9

    neil_w

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    http://modelrocketbuilding.blogspot.com/

    Essential reading for the LPR builder.

    Here's everything he's said about Kevlar: http://modelrocketbuilding.blogspot.com/search/label/Kevlar
    Here's everything he's said about the replaceable Kevlar mount: http://modelrocketbuilding.blogspot.com/search/label/Replaceable Kevlar Mount

    He (aka Odd'l Rockets) also sells motor mount kits that have the replaceable Kevlar designed in: https://www.siriusrocketry.biz/ishop/oddl-rockets-hdem-55-heavy-duty-motor-mount-kit-1943.html
     
  10. Sep 5, 2019 #10

    Bill S

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    Thanks - that should be pretty good reading.
     
  11. Sep 5, 2019 #11

    Andrew_ASC

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    On the ASP 13mm A kits I did exactly that. I used a alpine butterfly knot on the elastic to create a loop at one end to tie the Kevlar too.
     
  12. Sep 5, 2019 #12

    EvryThngIsAOk

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    Taking a page from the new Boyce Aerospace kits I started to install the Kevlar in a large u shape that extends just past the top of the body tube secured to the engine mount passing thru the centering ring.
    I tie it around the body tube, and pass it thru a small hole in upper ring at the engine tube.
    Then I attach the elastic to the top of the Kevlar loop
    Have had a problem with zippering yet.
     
  13. Sep 5, 2019 #13

    hcmbanjo

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    The Apogee Peak Of Flight article explains it the best:
    https://www.apogeerockets.com/education/downloads/Newsletter338.pdf

    Hard to believe it's been five years since I figured it out!
    The Odd'l Rockets Heavy Duty Engine Mounts (HDEM) with replaceable Kevlar line and heavy wall BT-20 tubing
    are available from most online vendors.
     
  14. Sep 5, 2019 #14

    Bill S

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    I looked over the article in the Apogee Newsletter; awesome stuff. I do wonder though, if anyone has put the tube that encircles the Kevlar on the outside of the centering rings so as to keep the Kevlar away from the exhaust?
     
  15. Sep 5, 2019 #15

    Knuckledragger

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    If its braided kevlar, you can make a knotless loop and replace the SC as needed. Make the loop end first then work backwards to securing it around the motor mount. Do a dry run or three but as a general rule give yourself 4-5x a free end as the size of the loop. If you use a sharpie and mark the tag end at the junction or somewhere on along its length you have a visual indicator if its slipping. I've used this method for tying leaders on flylines for years and had maybe two or three pop loose (too short on the tag end). Plus there's no knot to tangle up on.

    IMG_0055.jpg IMG_0059.jpg
     
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  16. Sep 5, 2019 #16

    BABAR

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    That makes a lot of sense
     
  17. Sep 5, 2019 #17

    neil_w

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    I think in the replaceable Kevlar implementation it would simply be harder to do this way, and if you think about what the back of the rocket would look like, I'm uncertain if the Kevlar would be likely to stay tight due to slack between the loop and the tube.
    upload_2019-9-5_14-12-37.png

    Actually, I guess it wouldn't be very much, maybe not an issue. But running the tube through the outer edge of the rings is definitely a bit harder than gluing it to the motor mount on the inside.


    That said, when I do my permanent Kevlar installations, I run the Kevlar around the outside of the ring for exactly the reason you specify. When everything is glued in, the extra slack isn't an issue.
     
  18. Sep 5, 2019 #18

    BABAR

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    Not much force to loosen it. Little piece of tape should hold it. I am thinking you may need some reinforcement to keep it from "cutting" through the ring when ejection fires. Then again I used cardboard or foam board rings, plywood rings wouldn't be likely to rip
     
  19. Sep 5, 2019 #19

    Bill S

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    Good point. It also occurs to me that it would be harder to glue in the engine mount and keep the glue out of the plastic tube the Kevlar is to go through.
     
  20. Sep 5, 2019 #20

    neil_w

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    Yes. In other words, simple practical considerations
     
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  21. Sep 5, 2019 #21

    gna

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    1. Untie the elastic from the nosecone.
    2. Use something like a bicycle spoke or a crochet hook, and reach up through the motor mount, and hook the kevlar or the elastic.
    3. Pull down through the motor mount.
    The kevlar and elastic will be hanging free out of the rocket. Easy enough to replace the elastic.
     
  22. Sep 5, 2019 #22

    mbeels

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    Or, the lower centering ring (aft) gets the hole next to the engine mount, and the top (fore) centering ring gets the hole at the outside, next to the body tube. The Kevlar runs at a slight angle, but that is no issue. On my last two rockets I used a small tube (the tube that the Aerotech ignitors came in with my 24/40 reloads was the perfect size) to guide the shock cord from one hole to the other. That makes it easier to replace. It also protects the top of the shock cord from glue when glueing in the mount.
     
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  23. Sep 6, 2019 #23

    BRS Hobbies

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    I'm not a big fan of connecting the Kevlar thread to the engine mount since it can take a direct hit from the ejection charge. A much better approach is to connect the Kevlar thread to an ejection baffle or near the top of body tube using an Estes tri-fold mount. If going the tri-fold mount method, then i put a couple of knots in the Kevlar thread and a put a loop on the other end. The parts with the knots gets glued inside the mount and is glued inside the body tube so the loop is just inside the end of the body tube. Then just attach the elastic to the Kevlar loop and it can be replaced as needed.
     
  24. Sep 6, 2019 #24

    Mustang67

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  25. Sep 13, 2019 #25

    Bill S

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    I tested this out on my Interceptor build. I cut a notch in each of the centering rings, on the outside, and epoxied the plastic tube in place. When installing the engine mount, I made sure to not put any glue in the area where the end of the plastic tube would be. Just to be safe, I inserted into the plastic tube a length of 1/16 brass rod. Naturally, despite my effort to avoid getting glue near the tube, some found itself on the tube and bonded the metal rod a bit. All it took to free it was a nudge and now the tube is clear and ready to receive the kevlar.

    I do have some issues with the kevlar loop loosening a bit, as I am not that good at knots. I snugged it down, but it seems to want to loosen up a bit. To help with that, I put some masking tape on top of the kevlar cord to help keep it from loosening up any more and coming off the end of the rocket.
     
  26. Sep 13, 2019 #26

    neil_w

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    Pics?
     
  27. Sep 13, 2019 #27

    Bill S

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    The loose tail is visible on the lower left; the kevlar actually goes under the tape to wrap around the engine mount; its the tail that is hanging out from under the tape.

    DSC01439.JPG
     
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  28. Sep 16, 2019 #28

    STEMdad3

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    Can confirm. Midwest power (last year?) my son and I were flying our Star Orbiters. Mine went up first and came down in two pieces with the elastic melted clean through. Luckily they are so light that the body tube kind of glided down on the fins to a rather smooth landing and the nose cone didn't drift too far. Walked up to Teddy's booth and before we even got there he had the kevlar that we needed in hand, must of saw us coming clear from across the road.

    Back to subject at hand, we use the coffee stirrer through the centering ring method on almost all our LP and MP that aren't min. diam. and it works AWESOME. We've put them up against the motor mount, and out at the edge. Haven't noticed any real difference in performance. Do be careful out at the edge though, as someone mentioned they tend to get glue in the hole when placed there and need cleaned out with a small pipe cleaner before glue sets up.

    If going with only kevlar (which i highly recommend after the Star Orbiter incident) make it LOOOOONG. Star Orbiters are ~4ft tall and I think we made the all kevlar cords for them around 22 or 24 ft total length.
    -Aaron
     
  29. Sep 17, 2019 #29

    readytorock556

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    I usually use 5 times the length of the rocket with Kevlar and 1 1/2 of nylon elastic shockcord, joined with a square knot and with a half hitch on each end or sometimes a bowline to form a loop to tie off the shock cord.
     
  30. Sep 22, 2019 #30

    Bill S

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    The new Kevlar installations on 2 rockets worked fine, no problems at all. I'm going to be retrofitting some of my other rockets. The only thing that is a hassle is when I use superglue to toughen the parachute end of the body tube (Krazy glue brand, which is thin and comes with a brush), it requires a lot of sanding to get rid of the extra thickness on the tube lest the nosecone fit way too tight.
     

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