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  1. #1
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    11th October 2016
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    Shock cord mounting

    I know there are a bunch of different ways to mount it on the motor mounts.
    If I want to put it through the centering rings, does it need to be on both sides or can I do it on just one?
    It says in the instructions for my rocket that it relies on the motors forward closure retention eye bolt for attachment of the shock cord. Is that only on bigger motors? Anybody have pictures?
    Does the way I want o do it work though? Shock cord on one side epoxied to the mmt and the centering rings?
    Thank you for your time!

    Last edited by Ccolvin968; 23rd December 2016 at 07:05 PM.

  2. #2
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    If I use Cesaroni motors, they don't have a forward closure do they? How exactly would that work?


  3. #3
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    Quote Originally Posted by Ccolvin968 View Post
    I know there are a bunch of different ways to mount it on the motor mounts.
    If I want to put it through the centering rings, does it need to be on both sides or can I do it on just one?
    It says in the instructions for my rocket that it relies on the motors forward closure retention eye bolt for attachment of the shock cord. Is that only on bigger motors? Anybody have pictures?
    Does the way I want o do it work though? Shock cord on one side epoxied to the mmt and the centering rings?
    Thank you for your time!
    I've mounted many shock cords epoxied along one side of the motor mount. It works just fine.
    Smaller CTI motors do not have a forward closure that can accommodate an eye bolt.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  4. #4
    Join Date
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    if this is for your 4" fg L2 project, you might consider running a Y type mount with Kevlar on opposing sides of the motor mount tube.
    Rex
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    L2-competitor 3, AT J350W, 8/27/2016, Bong, 2557'
    my youtube channel http://www.youtube.com/channel/UC0gB...?feature=watch

  5. #5
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    How strong does it need to be?




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  6. #6
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    Yeah, it's for my L2 build.
    I did the Y type mount for my L1 rocket. I just don't like the way it makes the loop.
    I have no idea why, but I like the look and how neat one line is rather than two with a knot and a big loop.
    If I do only one side, will that be detrimental to my rocket in the long run? Or it it personal preference?
    Thanks for all of the suggestions!
    Corzero, that's a pretty impressive mmt!
    I don't think I'll go that far for L2. :-)

  7. #7
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    Attaching it on one side only will not be detrimental.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  8. #8
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    Good to know.
    Thanks Steve!

  9. #9
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    The closure eyebolt method is also great for 54mm and up. Cesaroni has an accessory forward closure with eyebolt thread for 54mm, and the 75/98 hardware has eyebolt threads standard. The situation is similar for Aerotech. I kind of like it because it reduces the amount of rigging inside your rocket a fair amount and streamlines construction. Some of the Madcow 79/98 kit instructions are for the eyebolt method. I did it for my L3 and was quite happy.
    Dave Cook
    NAR 21953 L3 - TRA 1108 - DART San Diego

  10. #10
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    I'll take a look.
    I'll be flying my 4 inch Tomach on 54 more often than 75 for cost savings.
    I'll have to get both if I were to do it that way, but it sounds like it will be worth the extra cash.

  11. #11
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    I'll take a look.
    I'll be flying my 4 inch Tomach on 54 more often than 75 for cost savings.
    I'll have to get both if I were to do it that way, but it sounds like it will be worth the extra cash.

  12. #12
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    As caveduck mentioned, a threaded forward closure is available for CTI 54mm. You didn't mention how heavy your rocket would be, but on those windy days when you want to stay low, a small J or large I in 38mm might be what you want to fly, but that wouldn't be possible with CTI if you go with the threaded forward closure method as your only way to mount your shock cord.

    The other thing with using threaded forward closures is, you can't use motor ejection backup to your DD. Some people like that, especially if they are flying single altimeter DD. I've never used the motor backup so I won't say one way or the other but it is another thing to consider.

    The forged eyebolt in CORZERO's pics is also something I don't usually do, but it is certainly a viable method. My only suggestion is to fit the eyebolt and CR into the body tube before you epoxy everything. The pics in CORZERO's post look like the eyebolt might not fit inside the BT. Check clearances with shock cord through the eyebolt before permanently installing.

    I used the tubular Kevlar epoxied to the MMT method in my L3. I used a piece long enough to be epoxied to both sides of the MMT and the top loop extend about 6" out of the BT. I then fed the loop through and around a 30Kg man-rated ball bearing climbing swivel as my attachment point for the shock cord.

    BTW, the swivel pictured in the last pic of CORZERO's post is the type I don't recommend. I used one of those one time and it came back stretched and distorted and not swiveling any longer. I would only recommend true ball bearing swivels. YMMV
    Handeman

    TRA #09903 L3 :D 3/29/2015

    "If you don't use your head, you have to use your feet!" my Dad

    Tripoli Central Virginia #25 - BattlePark.org

  13. #13
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    Quote Originally Posted by Steve Shannon View Post
    Smaller CTI motors do not have a forward closure that can accommodate an eye bolt.
    Yes, but you can make one for 38mm. I learned this trick from one of Crazy Jim's posts.

    Lift the cap and pour out the ejection powder from the well. Carefully thread an eye bolt (3/16", I think) into the touch hole in the bottom of the plastic well. Fill the well with epoxy.

  14. #14
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    I've aways been a little concerned about kevlar being installed directly in the line of the hot gasses from the ejection. I've read that it can, and does erode over time.

    To that, I've seen designs that have the shock cord protrude beyond the aft CR, and then end in a loop that gets wrapped around the motor casing/motor retainer. After a flight, you can pull it back to inspect the cord, then from the front pull it back into position. Why don't more people use this method for rockets that are too tight for an eye bolt or U bolt?

    Last edited by K'Tesh; 30th December 2016 at 03:33 AM.
    Dreaming of making the rockets I dreamed of as a kid (and then some).


    NAR L1 Cert flight: Sheridan, Oregon, USA. Sept. 19, 2015. Flew Deep Space OFFl on an I357T-14A Blue Thunder

  15. #15
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    I've never used a y-type harness. I always use an eyebolt, eyenut, or u-bolt in my MPR and HPR builds.
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    NAR 91107, Level 2

    I really, really hate bugs.

  16. #16
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    Quote Originally Posted by qquake2k View Post
    I've never used a y-type harness. I always use an eyebolt, eyenut, or u-bolt in my MPR and HPR builds.
    And for long body tubes that are hard to reach into, you can use my U-Loop attachment point to allow for inspection...

    Dreaming of making the rockets I dreamed of as a kid (and then some).


    NAR L1 Cert flight: Sheridan, Oregon, USA. Sept. 19, 2015. Flew Deep Space OFFl on an I357T-14A Blue Thunder

  17. #17
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    Quote Originally Posted by K'Tesh View Post
    And for long body tubes that are hard to reach into, you can use my U-Loop attachment point to allow for inspection...]
    I like that. Are your loops knotted or sewn?
    NAR 91107, Level 2

    I really, really hate bugs.

  18. #18
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    Quote Originally Posted by qquake2k View Post
    I like that. Are your loops knotted or sewn?
    So far, I've only used knots, but sewing would work too.
    Dreaming of making the rockets I dreamed of as a kid (and then some).


    NAR L1 Cert flight: Sheridan, Oregon, USA. Sept. 19, 2015. Flew Deep Space OFFl on an I357T-14A Blue Thunder

  19. #19
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    If you are not going to be using the motor ejection at all, I see no reason to have kevlar on your MMT. Use it where your harness will connect to the A/V bay. My level 2 build (currently in progress) uses something like K'Tesh's "U Loop" method. Except I sewed a swivel in place of the ring. I have the ring, and want to use the ring, but seems the swivel is a better option.

    I have built a few with the kevlar attached directly to the MMT. One side, both sides, wrapped around, whatever suits my fancy at the time. I like the idea of a Y Harness to help distribute the load, but don't believe it is necessary.

    But I am far from an expert.
    Nothing is foolproof to a sufficiently talented fool.
    TRA #16513
    Level 1: Danger Close ---AT H123W to 1240'--- 29 OCT 2016

  20. #20
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    Quote Originally Posted by CORZERO View Post
    How strong does it need to be?




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    Got any more pictures of that primered rocket in the back?
    TRA Las Vegas, L2

  21. #21
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    All of the removable/replaceable shock cord methods are OK, but I don't use any of them. My experience, and this is just my experience, is with a Public Missiles Callisto. It has a 1" nylon shock cord from the motor mount to the piston. I've been flying it since 2003. I flew it on a cold day a couple years ago and the BT broke in half on landing. That gave me a chance to inspect the shock cord close to the MMT. It was a little melted on the surface, but still strong and solid and I expect it to last another 10 - 12 years. I didn't record how many flights it had the first 5 years, but I would estimate the total flights as of today in the 75 - 100 range.

    Because of that experience I use a Kevlar loop epoxied onto the MMTs and skip the added weight of metal parts like eye bolts and u-bolts or the extra complications of removable or replaceable shock cords. I expect the shock cords and mounting to outlast the rocket.

    Of course, that's just me, YMMV
    Handeman

    TRA #09903 L3 :D 3/29/2015

    "If you don't use your head, you have to use your feet!" my Dad

    Tripoli Central Virginia #25 - BattlePark.org

  22. #22
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    Quote Originally Posted by Handeman View Post
    All of the removable/replaceable shock cord methods are OK, but I don't use any of them. My experience, and this is just my experience, is with a Public Missiles Callisto. It has a 1" nylon shock cord from the motor mount to the piston. I've been flying it since 2003. I flew it on a cold day a couple years ago and the BT broke in half on landing. That gave me a chance to inspect the shock cord close to the MMT. It was a little melted on the surface, but still strong and solid and I expect it to last another 10 - 12 years. I didn't record how many flights it had the first 5 years, but I would estimate the total flights as of today in the 75 - 100 range.

    Because of that experience I use a Kevlar loop epoxied onto the MMTs and skip the added weight of metal parts like eye bolts and u-bolts or the extra complications of removable or replaceable shock cords. I expect the shock cords and mounting to outlast the rocket.

    Of course, that's just me, YMMV
    I agree. A good nylon or Kevlar mount will last longer than the rocket. I also avoid heavy metal links and use good ol' knots to make connections. A bowline is strong and can be undone fairly easily, if needed.

    The only cords than burn through for me are Estes rubber bands!

  23. #23
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    Quote Originally Posted by Handeman View Post
    All of the removable/replaceable shock cord methods are OK, but I don't use any of them. My experience, and this is just my experience, is with a Public Missiles Callisto. It has a 1" nylon shock cord from the motor mount to the piston. I've been flying it since 2003. I flew it on a cold day a couple years ago and the BT broke in half on landing. That gave me a chance to inspect the shock cord close to the MMT. It was a little melted on the surface, but still strong and solid and I expect it to last another 10 - 12 years. I didn't record how many flights it had the first 5 years, but I would estimate the total flights as of today in the 75 - 100 range.

    Because of that experience I use a Kevlar loop epoxied onto the MMTs and skip the added weight of metal parts like eye bolts and u-bolts or the extra complications of removable or replaceable shock cords. I expect the shock cords and mounting to outlast the rocket.

    Of course, that's just me, YMMV
    The black straps that PML uses on the hot gas side of their pistons is not just nylon, but a blend of materials that chars slightly on the outside to protect the strap from melting. I have some that are quite stiff as a result, but continue to serve well and I've never melted one.
    The colored strap on the outside of the piston is nylon. I've seen instances where people accidentally swapped the two during construction with a result of melting through the nylon.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  24. #24
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    Quote Originally Posted by Steve Shannon View Post
    The black straps that PML uses on the hot gas side of their pistons is not just nylon, but a blend of materials that chars slightly on the outside to protect the strap from melting. I have some that are quite stiff as a result, but continue to serve well and I've never melted one.
    The colored strap on the outside of the piston is nylon. I've seen instances where people accidentally swapped the two during construction with a result of melting through the nylon.


    Steve Shannon
    I was not aware there was a difference, I thought it was the size. Do you know what the added material is?
    Handeman

    TRA #09903 L3 :D 3/29/2015

    "If you don't use your head, you have to use your feet!" my Dad

    Tripoli Central Virginia #25 - BattlePark.org

  25. #25
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    Quote Originally Posted by Handeman View Post
    I was not aware there was a difference, I thought it was the size. Do you know what the added material is?
    For some reason polyester and cotton comes to mind, but I could be completely wrong. I'll see if I can find the reference. It might have been in their FAQ.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  26. #26
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    Quote Originally Posted by Steve Shannon View Post
    For some reason polyester and cotton comes to mind, but I could be completely wrong. I'll see if I can find the reference. It might have been in their FAQ.


    Steve Shannon
    Here it is, although they just say nylon blend:
    Piston Strap Heat Resistance/Nomex Protectors
    We sometimes get questions about whether the piston strapping needs to be protected from ejection heat. First, the strap is a nylon blend, not pure nylon, so it's relatively resistant to the short-term heat of an ejection charge. Regarding strap protection, PML has sold more than 20,000 kits over the past 10 years. That's probably 100,000 flights assuming only 5 flights per kit (which is probably substantially too low, but for the sake of argument that's what we'll use). We've only had two piston straps returned because of burn-through damage. That's a 0.00002% failure of piston straps due to heat damage. Clearly, customer experience has shown there’s no need to add piston strap protection; it only adds unnecessary weight and cost.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  27. #27
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    I have run a bit of heat shrink tube over the 6-8" of Kevlar closest to the MMT on my past couple of builds to help protect it from the ejection gasses.

    I like the idea of running the shock cord all the way out the end of the rocket. My only thought is that you'd have to have some kind of tube to run it through to facilitate easy reinstallation, which in turn would give ejection gasses a place to escape. Perhaps a small dab of RTV where the cord comes out on the aft end will seal things up since it isn't likely that you'll pull it out after every flight.
    NAR 98735
    L1 6/20/2015
    L2 2/13/2016

  28. #28
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    I've had success wrapping the first couple feet of my nylon or elastic lines in ductape. Inexpensive, easily replaceable, and no noticeable adverse side effects yet.

    The brief instant of high temperature from the charge just doesn't hang around long enough to degrade the tape.
    "I'm at least 70% confident about whatever I say (90% of the time)"- college me

    NAR 101195
    Level 1: Big SAM, 9/10/16

  29. #29
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    +1 on the duct tape. I've been using this too. I have a Kevlar cord protector, but it's not big enough to fit over the knot at the end of the cord. I tape the exposed part of the cord with tape and add a little more every year or two.

    Handeman

    TRA #09903 L3 :D 3/29/2015

    "If you don't use your head, you have to use your feet!" my Dad

    Tripoli Central Virginia #25 - BattlePark.org

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