Attaching shock cord to motor tube

Discussion in 'Beginners & Educational Programs' started by David H Lee, Jul 14, 2018.

  1. Jul 14, 2018 #1

    David H Lee

    David H Lee

    David H Lee

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    Hi all, I am working with some beginner students on a TARC competition rocket. One question we ran into during the build has to do with attaching the shock cord to the motor mount. The students want to know whether we should just fit the shock cord through the tiny space there is between the motor tube and the centering ring, or to drill a hole in the centering ring to run the shock cord through.

    Thanks for your help.
     
  2. Jul 14, 2018 #2

    K'Tesh

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    6 one way... Half a dozen the other.

    Personally, In these kinds of situations, I'm a fan of extending the shock cord so that it extends down past the back of the rocket, and has a loop that is placed (read: taped) around the motor. Routing this with a straw or a similar tube, allows the shock cord to be drawn down and inspected or replaced as necessary. For that reason, I'd drill or cut a small notch.
     
  3. Jul 14, 2018 #3

    K'Tesh

    K'Tesh

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  4. Jul 14, 2018 #4

    blackjack2564

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    Been glueing shock cord to MM tube for years.
    For motor eject the first 3 ft. need to be Kevlar,
     
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  5. Jul 14, 2018 #5

    lakeroadster

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    Welcome! Here's a video for you...
     
  6. Jul 14, 2018 #6

    Alan15578

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    I use a variety of of methods. For the OP I would recommend this method with the staw, with one addition. Put the top hole hear the outer edge of of the centering ring, keeping the cord farther away from the ejection charge. When you prep the rocket, pull the cord taught and push a wad of flameproof wadding past it all the way down to the motor. This will prolong the life of the cord.
     
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  7. Jul 15, 2018 #7

    BABAR

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    Alan makes a great point above about putting the hole as far lateral from from casing as possible.

    My biggest problem with this is that motor mount attachment of the cord places the shock cord in the worst possible place relative to the ejection charge. If you have ever seen videos of a rocket motor in a static test stand, you know that even after ejection there is a flame (kind of like a big candle flame) that comes out the forward end of the motor casing for a few seconds, so for every launch you are not just blasting an ejection charge over the cord, you are actually "toasting" it for a few seconds. I have thought about whether it would be worth it to do a NAR paper looking at just how far this "flame" effect extends forward, and how it changes with different motors and also particularly with minimum versus non-minimum diameter rockets. Chris's idea of making it easier to swap out the cord makes a lot of sense. I have gone to just guessing that the effect goes about 4 inches forward of the front of the casing, poking my shock cord through here and extending it forward, wrapping the cord AROUND the outside of the rocket, and covering it with Mylar tape. I haven't had any burn throughs with this technique, and the tape seems to hold pretty well so I haven't had any "zipper" or structural damage for the attachment site. My rockets are sports type, often unpainted, so while it isn't cosmetically ideal (you get a ridge under the tape on the outside of the rocket where tape goes around the loop) it isn't a problem for me.

    One solution may be to use a piece of heat resistant tube (maybe a small carbon fiber tube) in place of Chris's Q Tip that extends FORWARD of where you think the engine flame is going to reach.
    This allows you to replace the cord if needed, but also hopefully puts the exposed area of the cord away from where the heat is going to be the worst.

    anyone have ideas other than Carbon Fiber tube for this? Need something cheap, small, light, and relatively flame resistant.
     
  8. Jul 15, 2018 #8

    lakeroadster

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    The problem though with a static test is that it is not replicating actual in flight conditions. One would speculate the actual scenario is more like a chimney effect due to air passing across the open body tube.



     
    Last edited: Jul 15, 2018
  9. Jul 18, 2018 #9

    BABAR

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    great vids (yours?)

    I agree that if you put the motor in a body tube, won't be exactly the same, but I don't buy your chimney idea, as there is never a constant flow of gas going from front to back. My experience with cord burn throughs (Kevlar and otherwise) has been that it looks like the burn through is just in front of the motor casing. So at least to me, unless you are using a completely flame proof shock cord-- which Kevlar is NOT (example, I've seen der MicroMeister's pics of a short wire fishing leader), to somehow protect the first 3 or 4 cm of the cord just in front of the motor casing. Alternatives are to not attach it at all to the motor mounts (use the old Trifold method, poke a hole and loop it through, tying it off outside the rocket) with the attachment will forward of the motor mount. Or use Kevlar and something like Chris Mitchelson's (aka HCMBanjo) method and replace it every 5 or so flights. Or put something around the cord to protect that exposed area.
     
  10. Jul 19, 2018 #10

    lakeroadster

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    No, not mine, just found them on YouTube.
     
  11. Jul 19, 2018 #11

    Bat-mite

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    I always mount my harnesses with a quick-link to a U-bolt or forged eyebolt, so that it is replaceable. Also, I like to clean them sometimes.
     

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