Shock Cord Length Question

Discussion in 'High Power Rocketry (HPR)' started by Ccolvin968, Oct 14, 2016.

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  1. Oct 14, 2016 #1

    Ccolvin968

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    As a few of you may have seen, I recently finished my Formula 75 for my level 1 at the end of the month.
    Some pictures I've seen show extremely long shock cords and some show shorter ones.
    I epoxied both ends to my motor mount and ran the front of the loop through the nose cone like that.
    I have about 2.5 feet between the fuselage and the nosecone. The chute is tied on about 12 inches behind the nose cone along with the chute protector. Is this too short or should I be okay?
    Thanks!
     
  2. Oct 14, 2016 #2

    Banzai88

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    Typical shock cord length is 2x-5x rocket length beyond the top of the tube.
     
  3. Oct 14, 2016 #3

    AfterBurners

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    You can never have a shock cord that is too long....I would stay on the 5x
     
  4. Oct 14, 2016 #4

    NateLowrie

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    The answer depends on a host of factors like the forces involved, what stresses is the system designed to take, do you need to minimize weight because you are going for extreme performance and when and what speed are you planning on deploying. The advantage of a longer shock cord is you put less stress on the harness attachment points because the separation forces are spread out over a longer timeframe. The disadvantages are that longer cords are proportionally more difficult to manage and there is a weight and space penalty.

    If you are looking for a good rule of thumb, 15 ft cord for 38mm through 54mm diameter rockets, 25ft cord for 3in and 4in diameter, 50 ft for 6in, and at least 75ft for anything larger. Your cord will work in theory but I would advocate at least a 15ft length.
     
    Last edited: Oct 14, 2016
  5. Oct 14, 2016 #5

    dford

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    I built a Formula 75 in July for my L1 also. I did the same thing. Epoxied both ends of 1/2" flat nylon to the motor mount through upper centering ring. It hung 5 feet out of the airframe. After it cured I added 10 more feet. I mostly wanted shock cord protectors on the initial cord and to be able to play with the rest if I wanted to do DD on it later. I also flew on an H155ST. It's almost a shoe in for a cert flight. I've launched larger motors easily in the F75 with this setup and have used all of the provided black powder without an issue.
     
  6. Oct 14, 2016 #6

    Danh

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    Shock cord should be longer than the tallest tree where you will be flying...
     
  7. Oct 14, 2016 #7

    Ccolvin968

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    Huh... guess I better add some then.
    If I add more length can I use a different material?
    I have the 4 ft. Of Kevlar right now.
    Could I add 1/2 inch tubular nylon rated at 1000 lbs?
     
  8. Oct 14, 2016 #8

    NateLowrie

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    Shouldn't be a problem as long as it's attached securely..
     
  9. Oct 14, 2016 #9

    blackbrandt

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    Agreed. I may go overkill, but I fly 40 feet in my formula 75, and about 70 feet total in my Wildman Darkstar Jr (sum of main and drogue).
     
  10. Oct 14, 2016 #10

    dr wogz

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    my rule is at least 3x the rocket length.. 5x is better.

    One other note that may help in choosing material: it should have a strength of 50x the rocket weight (or better). So, a 3lb rocket should have a shock cord rated at 150lbs..
     
  11. Oct 14, 2016 #11

    Ccolvin968

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    Good to know.
    Just to be sure, the nylon won't have an issue with the heat correct?
     
  12. Oct 14, 2016 #12

    Bat-mite

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    Depends. Nylon is not flame-proof, but it will melt. Use a shock cord protector or some dog barf.
     
  13. Oct 14, 2016 #13

    NateLowrie

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    Or do the cheap thing and wrap some painters tape around the first few feet. The reality is exposure of nylon shock cord to charge is not nearly as much of an issue as exposing the ripstop nylon of a parachute to the charge. Position it above your kelvar cord and you should be fine.
     
  14. Oct 14, 2016 #14

    bclark989

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    Bend Knots!

    Pick your poison on that page. I use double fisherman knots.

    And Nylon does get brittle after multiple exposures to ejection gases if it isn't properly protected. I would strongly recommend a Shock Cord Protector. It also won't hurt to inspect that length of nylon prior to each flight. If it has any good size sections that look like plastic and not cloth, replace it.
     
  15. Oct 14, 2016 #15

    cerving

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    I think the shock cord length depends more on the size and weight of the rocket than it's length. The idea is that you want it to be long enough so that when the nose pops (or payload bay, if you're breaking there) the shock cord substantially extends but not necessarily 100%. What you don't want is for the charge to blow off the nose with so much force that it stresses the shock cord mounts. For MPR-size or smaller Level 1 rockets I'll typically use 10' or so, for larger L2 rockets I'll use 20' at least. The chute should be mounted close to the break, in any case, so it's guaranteed to not get stuck in the tube if the shock cord doesn't spool out all the way. For your Formula 75, I'd go 20', tying the chute 1' from the nose is fine.
     
  16. Oct 14, 2016 #16

    Banzai88

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    Bottom line is you're looking for 12-20 feet beyond the top of the airframe. It's a nose blower, whooooosh.....pop. I'd use 1/4" Kevlar and only need a nomex blanket for the parachute burrito. If you're committed to anything else, make sure that you protect it from the blast (flame) with nomex AND the heat with some barf.
     
  17. Oct 14, 2016 #17

    BrAdam

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    +1. I did this on my L2 rocket. Both ends of both cords. Nylon shows no melt or burn after several ground tests and flight. Tape held up well but will need to be replaced after exposure to 4-5 charges. Oh, and my shortest cords are 20' on my smaller mid power rockets. About 30 on h-j.
     
    Last edited: Oct 14, 2016
  18. Oct 15, 2016 #18

    Ccolvin968

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    Alright! So I just fixed my issue. I now have 4x the rockets length of shock cord.
    The bottom half is Kevlar and the top half of half inch tubular nylon rated to 1000 lbs.
    I will use barf to insulate all of the shock cords from the ejection heat.
    I already have the chute protector attached. As well.
    Thank you all for your help and advice!
     
  19. Oct 15, 2016 #19

    Ccolvin968

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    I also ordered that shock cord protector.
    I'll double it up with dog barf just to be safe!
     
  20. Oct 15, 2016 #20

    JP Morgan

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    Before the launch, club members I'm sure will go over everything with you too.

    Sounds like you're in great shape!
     
  21. Oct 15, 2016 #21

    Igotnothing

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    Here in the NW that would be maybe 150 feet.

    Alternative - tree climbing gear!
     
  22. Oct 15, 2016 #22

    rharshberger

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    Depends on which side of the Cascades your on, at our launch site outside of Pasco there is only a few trees around a house near the launch site and the wind doesnt usually blow that way. We did have a flyer last year try and land one in that houses pool, the only thing that saved it was a tree. ☺
     
  23. Oct 17, 2016 #23

    Handeman

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    You absolutely can have too long of shock cords. I've watch several failure of recovery systems that were directly attributable to too long of shock cords.

    It seldom happens with motor ejection because the rockets are smaller and the materials are relatively strong, but a chute attached near the nose cone that is sized to the weight of the rocket will stop the nose cone in mid air with a speed of only a couple of ft/sec while the rocket body and spent motor fall to the end of the shock cord. It's the shock of hitting the end of the cord that is essentially anchored to a chute that isn't moving that can cause failure. Most times the rocket will be moving horizontally as it tilts over when the ejection happens, but once in a great while it happens to stay vertical and pop the nose cone and chute near apogee, the fin can will drop straight down with almost no drag to slow its acceleration. Might only be one flight in 50, but that one flight could be the first one.

    DD rockets are much more susceptible to too long of shock cords. I watched a L3 fin can drop to the end of its cord and never even slow down or bounce when the 3/8" u-bolt was ripped out of the 3/4" aircraft plywood CR and the fin can continued down ballistic. It's not the ejection charges that you have to build the recovery systems to handle, it a drop the full length of the shock cord. I think a good test would be to attach the shock cord to a tower, parking garage, or other solid object and then drop the fin can straight down. If it can handle the shock when it hits the end of the cord, you are good to go. Just remember, a shorter cord equals less shock.
     
  24. Oct 17, 2016 #24

    mikemech

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    And shorter than your main deploy altitude.
     

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