Another shock cord question

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soopirV

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Hi- I've searched the forum, and am still stuck, hence the new thread. Rule of thumb for shock cord length seems to be 3x the length of your rocket, with a tensile strength of 150x the mass of same. How do you break it up with dual deploy configurations? Is it half and half? Seems like you would want more cord in the main bay, since you'll be coming in at a higher speed than the drogue break, but that seems contradicted by a report on Apogee, that of course I can't find now...any advice?
 

timbucktoo

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I usually use 4-5 times the length of rocket for each harness and usually make the drogue longer than the main.
 

markkoelsch

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I am typically using each harness at 4-5 times total length of the rocket.
 

dhbarr

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I am typically using each harness at 4-5 times total length of the rocket.
I love you one and all, but I'm not hearing a rule-of-thumb limit.

I cannot believe that 5x Big Daddy is equivalent to 5x Mean Machine.

Hope to be proven wrong :)
 

Nytrunner

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Hi- I've searched the forum, and am still stuck, hence the new thread. Rule of thumb for shock cord length seems to be 3x the length of your rocket, with a tensile strength of 150x the mass of same. How do you break it up with dual deploy configurations? Is it half and half? Seems like you would want more cord in the main bay, since you'll be coming in at a higher speed than the drogue break, but that seems contradicted by a report on Apogee, that of course I can't find now...any advice?
When I'm lazy enough to use a rule like that, I try and make the exposed cord 3x the length of the longest section its attached to. (usually with the chute 1/3 away from the smaller piece, although I may try Davidmecann's technique of attaching the nose at 1/3 and the main at the end when i build my next dual-deploy)

Personally, I think the 5x length is overkill and can lead to bigger shocks when the booster or payload falls all the way to the end of a crazy long cord. (after saying that I'm going to take cover before the bullets start flying)

150 times the weight of the rocket? I haven't heard that much.
 

timbucktoo

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Long harnesses are quite advantageous at apogee. Have seen many NC's come off when the drogue was too short.
Also, a long harness just might let you get your rocket back from the tree gods without the need of a chainsaw.
 

Igotnothing

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Altimeters measure pressure change, not velocity. So apogee deployment could be ripping along at several hundred knots of airspeed in a fairly horizontal orientation. Make your drogue(less) cord heavy and long. Main can be half the length and strength.
 

soopirV

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When I'm lazy enough to use a rule like that, I try and make the exposed cord 3x the length of the longest section its attached to. (usually with the chute 1/3 away from the smaller piece, although I may try Davidmecann's technique of attaching the nose at 1/3 and the main at the end when i build my next dual-deploy)

Personally, I think the 5x length is overkill and can lead to bigger shocks when the booster or payload falls all the way to the end of a crazy long cord. (after saying that I'm going to take cover before the bullets start flying)

150 times the weight of the rocket? I haven't heard that much.
How else do you determine the length to start with?
 

Nytrunner

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How else do you determine the length to start with?
When I was doing IREC, we started with the ejection force from our charge -> momentum of separating pieces and their associated energy -> compliance of tubular nylon/length to dampen that energy (I prefer to protect nylon instead of using kevlar). Of course, I was an engineering student before I was serious about rocketry, and that thought process carries over heavily into how I design. What makes shock cords hard is that the answer is usually different for every rocket, and after ejection the behavior becomes rather nonlinear. Which means after you've worked through a solution a couple times (a valuable exercise), you begin to see trends and can start making assumptions*.

*3x the longest section is an approximation using tubular nylon thats stretchier than kevlar (which I don't use)

**on the same vein, I dislike overzealous ejection charges for the same reason as crazy long shock cords
 

OverTheTop

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I tend to the shorter lengths of harness, probably around 3x or thereabouts. Tubular nylon exclusively as it has more give than Kevlar.

I do z-fold each harness so that the energy of the ejection doesn't all get dissipated at the full extension point. I think it helps get the airframe coming around in line with the NC for when the final snap does happen, reducing my chance of a zipper.
ZFold.jpg
Further details here:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=4612&start=78
 

soopirV

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When I was doing IREC, we started with the ejection force from our charge -> momentum of separating pieces and their associated energy -> compliance of tubular nylon/length to dampen that energy (I prefer to protect nylon instead of using kevlar). Of course, I was an engineering student before I was serious about rocketry, and that thought process carries over heavily into how I design. What makes shock cords hard is that the answer is usually different for every rocket, and after ejection the behavior becomes rather nonlinear. Which means after you've worked through a solution a couple times (a valuable exercise), you begin to see trends and can start making assumptions*.

*3x the longest section is an approximation using tubular nylon thats stretchier than kevlar (which I don't use)

**on the same vein, I dislike overzealous ejection charges for the same reason as crazy long shock cords
Not sure what IREC is, but I could pick up on your engineering background! I'm in the biomedical field, so my comfort-zone is in experimentation, and the "rules of thumb" are great ways for the non-mathematically inclined to get close to something to test! I do understand how you're approaching it, and while I would be hard-pressed to figure it out on my own, it makes perfect sense! I would love to be able to accomplish the following: Measure charge, calculate all the resistance/friction and force-absorption and whatnot, and end up with a ground test that results in the nose coasting to a stop at the exact end of the shock cord...would be so cool. Instead I fall back on my training- "well, that didn't work, let's try this!" and it's just nice to have a line to start from.
 

soopirV

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I tend to the shorter lengths of harness, probably around 3x or thereabouts. Tubular nylon exclusively as it has more give than Kevlar.

I do z-fold each harness so that the energy of the ejection doesn't all get dissipated at the full extension point. I think it helps get the airframe coming around in line with the NC for when the final snap does happen, reducing my chance of a zipper.
View attachment 316860
Further details here:
https://forum.ausrocketry.com/viewtopic.php?f=6&t=4612&start=78
I've become a fan of the z-fold as well! I haven't checked your link yet, but your secondary tape-wraps (around the individual loops) caught my eye- it makes sense that that would absorb additional energy after the stack has split, and I'm wondering if that was determined to be beneficial by results, or if you're taking a protective approach?
 

Nytrunner

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I do z-fold each harness so that the energy of the ejection doesn't all get dissipated at the full extension point.
Blast, I forgot to mention z-folding.
Got caught up in calculations again.....

Like most things in rocketry, there are different approaches.
Some like longer cords meant allow the pieces to expend their energy, some like shorter cords that are designed to dissipate energy.

I'm in the latter camp **ducks back behind shelter to hide from the long-cord camp**
 

paulm

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There are several good rules of thumb in model rocketry. One is that the length should be 10x the width or more, or 10 calibers or more. When this is true, the shock cord rule of 3xL is also true. For the Big Daddy, L<<10d, so you must use good experience for shock cord length. I would never use less than 6 ft.
 

OverTheTop

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I'm wondering if that was determined to be beneficial by results, or if you're taking a protective approach?
Apologies in advance to any long harness advocates. I dislike long harnesses and think they are unnecessary. YMMV :)

Having said that, there needs to be some way to dissipate the energy and limit the snap at the end that might floss the airframe. I thought it through and thus use z-folds. I would like to get a camera on the deployment to confirm how I think it is behaving.
 

rharshberger

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What no one seems to be stating is whether we are talking tubular nylon or tubular kevlar. A TK harness will need to be longer than a TN, because TN has an elasticity that TK doesn't.
 

soopirV

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What no one seems to be stating is whether we are talking tubular nylon or tubular kevlar. A TK harness will need to be longer than a TN, because TN has an elasticity that TK doesn't.
I was asking about TN, good point!
 

markkoelsch

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I fly all tk, and have since about 1999-2000.

I also fly primarily at a site that has some trees so really long harnesses can be a significant help in getting rockets back.
 

soopirV

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Twice as long as the tallest tree is a good starting point...
lol, I'm very thankful to fly out of a desert scrub-land! We've got some trees, but the biggest worry is a string of telephone poles not too far away.
 

Handeman

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I'm not sure there is a good rule of thumb for shock cord length.

If you believe "blow it out or blow it up " and drogueless recovery, then i think you need +5 times rocket length for both shock cords and lots of prayers. The length will compensate for the too large of charges and the shock involved at deployment. Without a drogue to keep the fin can above the payload, the long cord will cause extreme loads on the anchors when the main chute opens and stops while the fin can fall the full length of those cords and hits bottom very hard. This isn't usually an issue with smaller rockets, but as you get bigger and heavier it can bite you.
Watch your recovery. The longest part of the flight is usually the fall from apogee to main deployment. You can get a ballistic flight profile after the apogee charge opens the rocket. When the payload comes down point first and drags the fin can, it's ballistic. Maybe not as fast as if the rocket was together, but it's probably a lot faster then you designed for. When that happens, if the fin can doesn't hit and collapse the main after it opens, it drops to the end of the shock cords. The longer the cords, the more the main slows while the fin can falls and the greater the difference in speed when the fin can finally hits the end of the cords. Of course when the main opens and the fin can doesn't foul the main or break off the cord when it hits bottom and everything comes down together, the LCO usually says "Great Flight", everyone claps and nobody pays any attention to the fact the flight was really a near miss.

If you use a reasonable apogee charge and a drogue chute to keep the payload above the fin can the whole way down, you can use shock cords that are 2 times the rocket length or less and not have any issues.

The idea of a cord length as long as the highest tree is kind of tongue in cheek and doesn't work most of the time. Every time I've landed in a tree, it's draped across the tree and the nose cone is just as high as the fin can. You would have to have a cord at least 2 times the tree height to have any chance of aiding recovery and that is only if it lands in a lone tree. If it's a cluster of trees or a woods, it just drapes across more trees and makes the whole recovery process tougher.

As you can see, my opinions on recovery don't follow the majority so take them any way you want.

Good luck and I hope you have lots of great flights.
 

Nytrunner

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The logic in Handeman's excellent explanation is also part of why I favor shorter cords.

Without doing energy calculations and whatnot, (for tubular nylon) having each cord section 3x the length of the longest piece attached to it makes for a good amount of total cordage that's more packing friendly than 5x total length. Try Z-folding it and you can quickly develop your own recovery routine.
 

o1d_dude

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OneBadHawk sells pre-cut TN shock cords in 20' and 25' lengths which accomodates most of my rockets. Did I mention that the loops are sewn? One loop at each end and one for the chute attachment if desired. If you really want to go Cadillac style, Ted will sew heavy duty swivels into shock cords to your specifications.

...that reminds me...I need to put an order in...
 
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