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  1. #1
    Join Date
    18th January 2009

    Madcow/RW Formula 54 Recovery Question

    I typically use kevlar for the y-harness of my rockets and epoxy the tails along the sides motor mount tube. To the eye of the y-harness I then at a shock cord of tubular nylon.

    Most recently for my original Wildman Jart I simply cut an appropriate length of 3.5mm kevlar tubular braid and epoxied one end along the side of the motor mount tube and will attach the other end to the nose cone at the appropriate time. No tubular nylon in the recovery train.

    I'm thinking about doing the same thing for my Madcow/RW Formula 54...running one 12' length of 3.5mm kevlar cord from the nose cone to the motor mount. Total weight of the kit parts at this point is 19 ounces and I anticipate all up weight coming in at less than two pounds.

    I keep hearing it's a bad idea to use kevlar for the entire shock cord due to its lack of elasticity. On the other hand, I have seen it done.

    What's our consensus here?

    Last edited by o1d_dude; 26th October 2017 at 07:15 AM. Reason: typo
    Cranky Kong, L2 Certified Rocket Monkey
    "But we were promised jet packs!"

    Total Impulse in 2017: 605.77 Ns (Equivalent to an 89% I motor)
    Total Impulse since 2010: 13,872.11 Ns (Equivalent to a 35% N motor)

  2. #2
    Join Date
    10th July 2007
    Melbourne, Australia
    My personal preference is for TN, but either way should work. Maybe consider z-folding the harness with tape to take some of the shock out of the extension?

    Kevlar is strong, but it only stretches 6-8% before breaking.

    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  3. #3
    Join Date
    22nd January 2017
    North Florida
    The key to using tubular Kevlar is to size your deployment charges so that it's enough to extend the strap but not enough to pull it tight.
    NAR# 103244
    Level 1 - PML AMRAAM 2
    Level 2 - Madcow Adventurer 3

  4. #4
    Join Date
    18th January 2009
    Savannnah, Ga
    I concur, having done exactly the same thing. No issues after several years of use.

    Smaller lightweight rockets go by their own set of rules in my play book. I would not do it on bigger heavier models, but it makes things so much simpler just gluing one cord on and that's all. Key being using a long enough cord & you're doing that. Gets my seal of approval...LOL

    I have found that the thinner Kevlar does seem to have a bit of "forgiveness" [stretch]. I do wrap a bit of tape around the kevlar where it contacts the airframe edge, to keep it from fraying after many flights.
    Jim Hendricksen
    L-3 Tripoli 9693
    [ICBM, Orangeburg,SC R.I.P.] - QCRS ,Princeton ILL - MDRA , Price Maryland - Woosh, Bong Wisconsin- ROCC, Charlotte NC , ICBM Camden SC
    "Made" member of Chicago & Carolina Rocket Mafia exact science.......but not exactly !!!

  5. #5
    Join Date
    26th January 2010
    Northern California
    I've only ever used all Kevlar on small rockets. On MPR and bigger I've always put an attachment point on the forward centering ring, usually an eyebolt or u-bolt. I attach a short length of Kevlar to that and then attach flat or tubular nylon to that. I tend towards very long recovery trains, and bundle them using small rubber bands. It is my belief, and it is only belief, that the "un-bundling" of the shock cord, and its length, absorb a lot of the energy of the ejection sequence. Another issue for me, other than elasticity, is price. Nylon webbing is less expensive than Kevlar. Finally, I believe that the relatively wider nylon webbing is less apt to cause zippers.

    I have no scientific data to back up my beliefs. When I became a BAR, and started advancing towards mid and high power, I picked up a lot of tips and tricks here on the forum. For instance, when I was building my LOC IV (my first high power capable build), it was recommended to me to use an eyebolt as the recovery attachment. I liked that idea, and the ability to be able to replace the harness should it ever become necessary. By epoxying the harness to the motor tube, should it ever fray or break, you're limited in your replacement options. On 4" and above airframes, I can simply reach in to disconnect or reconnect the harness. On smaller airframes, I put a loop on the end of the Kevlar and use tools such as finger style pickup tools to put the loop through the attachment point, and pull the rest of the harness through the loop. Of course with my zipperless designs, the aft attachment point is exposed, but the forward is usually a bulkhead in the forward airframe.

    I realize that eyebolts and u-bolts add weight, as do quick links. But being Captain Low-N-Slow, that's not an issue for me.
    Last edited by qquake2k; 25th October 2017 at 06:53 AM.
    NAR 91107, Level 2

    I really, really hate bugs.

  6. #6
    Join Date
    19th January 2009
    Stafford VA
    All great advice! I agree with all of it. The comments from Keisling about sizing your ejection charges is what I consider key. Not just for Kevlar shock cords, but for all shock cords. It's easy to do with DD, but even motor ejection should adjust ejection charges. I have a PML rocket with a piston that only needs 0.5g of BP. The Hobbyline 29/40-120 loads come with 0.75g and the HPR 29mm loads come with 1.4g. I always adjust them by only using 0.5g for that rocket.

    You need to find the right size charge for your rocket and then what type of shock cord you use and how long it has to be becomes a much less critical questions.


    TRA #09903 L3 3/29/2015

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

    Tripoli Central Virginia #25 -

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