shock cord control

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bjphoenix

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I've written about my use of 10' long light kevlar (turns out I bought 100# strength) in my LPR, and by LPR most of mine are BT55 and BT60 size but still very light. That's a lot of "string" to stuff into a tube a few inches at a time and I've also worried about tangles which I've had one time out of about 10 flights. I tried to think of something to wrap the cord around that would still allow it to unroll easily. Last year I came up with the idea to use a spool so I made a prototype out of cardboard. I cut 3 rings and fit over a small piece of 18mm motor, the OD is about 1" so it would fit easily into a BT55.

I put a small piece of twist tie on it so I could theoretically switch from rocket to rocket but it would otherwise be captured by the shock cord. I put the spool at the midpoint of the shock cord and wrap both sides of the cord around it at the same time. Essentially when the cord tries to straighten one piece pulls one direction, the other piece pulls the other direction, the spool spins and reels out the cord (second photo below). I've flown it twice and it worked great. (The cord in the photo is actually single strand kevlar bowstring material, somewhere between 50# and 75# actual strength and very thin, still in use in a couple of my rockets.)

This one was too difficult to make, I think I could just cut a couple of disks instead of 3 rings and put a couple of pieces of balsa in the middle as spacers. I don't think it requires 2 compartments, I think it would work with just one compartment.

I will do more experiments but so far it is looking good. I will make a bigger one for MPR, maybe 1.75" diameter for my rockets.

IMG_4350b.jpg

IMG_4351b.jpg
 

BABAR

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Novel solution, I am afraid if I did it I would be more likely to tangle or snag the cord.

Tim Van Milligan did a video I have attached where he did a crochet series of loops, I have attached the video, just jump to 8 minutes.

I have tried this and it works well, it does take a bit of time but it is pretty reliable and really helps you manage long thin shock cords.

 

waltr

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On thinner cords I do the chain braid like Tim Van Milligan shows in that video. Works great but does take a little bit of practice to do the chain stitch.
 

rocketman4h

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Tim from Apogge has a great video on how to manage the long shock cords so they don't tangle. crochet a shock cord

I have use this method from 18mm to 38mm MD rockets and wow does it work. and no tangles or extra weight. I've gotten quick with just my fingers. Give it a go
 

GlenP

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10' - wow that is a long cord for a typical small LPR rocket, I don't think my Mean Machine cord is that long. I have not ever used one that long before, maybe more like 3' to 4' at the most for LPR. For the even smaller min dia 13mm rockets, I can fit a good length of Kevlar by using the over-under wrap on a dowel or pencil, then carefully insert that into the tube and slide the pencil out. But my simple method would probably not scale up well to 10' long cords like that.

To prevent tangles from that spool as it releases cord from both sides, you might look at how weed trimmer line spools feed, maybe to get some inspiration. They have an inner spool that feeds through a hole in an outer cover, so the spool is more contained, but not sure if you want to add any more weight to what you have. Just an idea to consider, maybe you can come up with something like that, or string guides for the cords on either side of the spool? Not that I have a fully developed idea here, just a suggestion to think about. [EDIT - or like a spool of dental floss, you know?]

But lots of votes already for the crochet method, that is a good idea also.
 
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BEC

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That (the crochet method) looks like it would work amazingly well. I will have to try this.

On long all-kevlar shock cords in competition models I have made loops by wrapping around, say, three fingers, then taping the middle together with 1/4 inch masking tape, then stuffing the resulting bundle in the model...but sometimes the tape doesn't let go.
 

Mike Haberer

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Another example of the "crochet" method. I have use the Sinnet Chain in the past. It works very well. The youtube demonstrates it well....
 

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boomtube-mk2

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Climbers use that method to keep their ropes in check and available for easy deployment.
 

icyclops

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Wow, seems like a lot of work. Think I will stick with using the flat sleeve technique. Just stuff the extra cord in the long sleeve for management and it also helps protect the cord from heat….never had one fail yet….but I don’t do 10’ cords either.
 

rklapp

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That (the crochet method) looks like it would work amazingly well. I will have to try this.

On long all-kevlar shock cords in competition models I have made loops by wrapping around, say, three fingers, then taping the middle together with 1/4 inch masking tape, then stuffing the resulting bundle in the model...but sometimes the tape doesn't let go.
Sorta like this?

 

Sooner Boomer

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I learned this "crochette" method when I was about 10 and learning how to sail. It's quicker/easier/faster than tying a sheepshank. I use it for a lot of stuff, including extention cords. I use it to *store* shock cords in the rocket, or to keep them out of the way when building. I do not fly with this method. I use tape and make "Z"s in the shock cord. I wrap one, two, or three loops of tape aroun each "point". The more tape, the harder it is to break. If everything goes perfect, I have one loop, with one single wrap of tape on it.

 

BEC

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Sorta like this?

For my small models what I do is simpler than that. I did see that figure-8 method demonstrated by someone else working the RSO table at NSL this year. That said, I may have to try this. It should work even with much smaller shock cord material even if I can't just then stuff my hand inside the body tube to pack it like she did. I'm trying to get a long sock cord inside a BT-5 tube instead.

I have just been looping the shock cord around three fingers then putting a single small bit of 1/4 inch masking tape across the middle of the resulting coil. I really should take a picture as I'm having a heck of a time describing it.
 

Crazyrocket

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I learned this "crochette" method when I was about 10 and learning how to sail. It's quicker/easier/faster than tying a sheepshank. I use it for a lot of stuff, including extention cords. I use it to *store* shock cords in the rocket, or to keep them out of the way when building. I do not fly with this method. I use tape and make "Z"s in the shock cord. I wrap one, two, or three loops of tape aroun each "point". The more tape, the harder it is to break. If everything goes perfect, I have one loop, with one single wrap of tape on it.
I do the same thing, except I usually Z fold 2 or 3 times and then put about 2 - 3 wraps of 1" masking tape around one of the ends. Depending on the length of the shock cord, I typically have about 4 or 5 bundles that I then place in the body tube. Never had any issues.
 

bjphoenix

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I've done more tests now and created a few more illustrations. The first image shows how the spool fits in the general layout of the recovery gear.
IMG_4365b.jpg

The photo below shows the next version, tested last weekend in a couple of BT55 and BT60 rockets. Both rockets used 10' shock cords of 100# braided kevlar. Both launches went well, the spool did what it was supposed to do. It was also easy to roll up before the launch and easy to pack in the rocket.
IMG_4355b.jpg


This one was built from a piece of 1/4" x 1/2" balsa- I cut a short piece and roughly sanded it into a cylinder 1/4" thick and just under 1/2" diameter. Then I cut out 2 cardboard disks just over 1 inch diameter and glued them on. I drilled some holes through it and ran a wire tie from a load of bread through the holes to hold the spool to the shock cord. Launch day was the first time it was tested and it proved to be marginal for holding all 10' of shock cord, it needs to be a little bit larger. I place it at the middle point of the 10' shock cord and wind both sides of the cord onto it at the same time so when it unrolls it unrolls both sides simultaneously.

Full disclosure- someone at the launch told me that they had seen something similar to this in Sport Rocketry magazine a couple of years ago, and that they thought someone was making a commercial product like this for highpower rockets. I'm reporting on my ideas and my experiences with this. I had the first idea about 2.5 years ago, maybe longer. I built the first prototype in the first photo just over 2 years ago and flew it then. I had the idea to build a slightly bigger and simpler version and that is what I just tested. As a disclaimer I did not see the article in Sport Rocketry so these ideas are completely my own. Likewise the person authoring the article in Sport Rocketry did not see my idead so what was written was not in any way related to or influenced by me. If the article was written 2 years ago then the author likely had the idea before I did. I just wanted to get that out of the way. I am surprised that I have not seen someone else build this kind of thing before now.

Yesterday I started building a larger one for MPR, to hold 20' of 500# kevlar. It's a little harder to fit in a 2.6" tube. I decided I will try it in the back yard before flying it.
 

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GlenP

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Also kind of reminds me of a two-stringed Yo-Yo.
The side spool "discs" shaped into something like a mini version of a Duncan Butterfly would probably work.

Glad you are having success with your testing. Thanks for sharing the concept.
 

Lee

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That looks like a great idea which I'll have to try.
 

bjphoenix

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Also kind of reminds me of a two-stringed Yo-Yo.
I tried to make it more complicated than it needed to be. For a long time I was trying to develop concepts in my head and getting something to reel out from both sides was stumping me. My first prototype had the 2 separate sections so that one string could wind around one area and the other string could wind around the other area. Turns out both strings can wind around the same area at the same time, because theoretically they should also unwind together. When I realized that it became a very simple concept.
 

Lee

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What about the shroud lines - spooling them also?
 

Sandy H.

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I've done more tests now and created a few more illustrations. The first image shows how the spool fits in the general layout of the recovery gear.
View attachment 476415
The photo below shows the next version, tested last weekend in a couple of BT55 and BT60 rockets. Both rockets used 10' shock cords of 100# braided kevlar. Both launches went well, the spool did what it was supposed to do. It was also easy to roll up before the launch and easy to pack in the rocket.
View attachment 476416

This one was built from a piece of 1/4" x 1/2" balsa- I cut a short piece and roughly sanded it into a cylinder 1/4" thick and just under 1/2" diameter. Then I cut out 2 cardboard disks just over 1 inch diameter and glued them on. I drilled some holes through it and ran a wire tie from a load of bread through the holes to hold the spool to the shock cord. Launch day was the first time it was tested and it proved to be marginal for holding all 10' of shock cord, it needs to be a little bit larger. I place it at the middle point of the 10' shock cord and wind both sides of the cord onto it at the same time so when it unrolls it unrolls both sides simultaneously.

Full disclosure- someone at the launch told me that they had seen something similar to this in Sport Rocketry magazine a couple of years ago, and that they thought someone was making a commercial product like this for highpower rockets. I'm reporting on my ideas and my experiences with this. I had the first idea about 2.5 years ago, maybe longer. I built the first prototype in the first photo just over 2 years ago and flew it then. I had the idea to build a slightly bigger and simpler version and that is what I just tested. As a disclaimer I did not see the article in Sport Rocketry so these ideas are completely my own. Likewise the person authoring the article in Sport Rocketry did not see my idead so what was written was not in any way related to or influenced by me. If the article was written 2 years ago then the author likely had the idea before I did. I just wanted to get that out of the way. I am surprised that I have not seen someone else build this kind of thing before now.

Yesterday I started building a larger one for MPR, to hold 20' of 500# kevlar. It's a little harder to fit in a 2.6" tube. I decided I will try it in the back yard before flying it.
I didn't see the article either, so no comments on that, but I like the ingenuity of the idea and think it is cool.

I would assume it would easily spin and unwind and there would be a bit of a jerk at the end once the cords are at full extension. Did you observe that or is there an element of the concept I misunderstood? I think that is one reason people like the z-fold and tape method is that the energy can be regulated near the final moments of extension. Then again, 10 feet with light weight parts may mean the energy is pretty well dissipated already.

Cool idea!

Sandy.
 

bjphoenix

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I would assume it would easily spin and unwind and there would be a bit of a jerk at the end once the cords are at full extension. Did you observe that or is there an element of the concept I misunderstood? I think that is one reason people like the z-fold and tape method is that the energy can be regulated near the final moments of extension. Then again, 10 feet with light weight parts may mean the energy is pretty well dissipated already.
I did not notice any sudden jerk. Everything seems to come out somewhat in slow motion and the parachute opens before the shock cord even becomes tight. To observers it appears that the body has separated from the chute and nose cone, then when the body reaches the end of the cord it is obvious that they are connected. People in mid power and high power do this routinely- long shock cords of relatively unyielding material. I think the concept is that with longer cords the pieces slow down before reaching the end of the line. I just built some cords of this type for my mid-power rockets for G motors, I used 20' of kevlar. It takes a fairly large spool to hold 20' of 500# kevlar so I'm going to do some backyard experiments before flying. My thought is that I put a nail in the side of my storage shed and hook one end of the cord there, fasten the other end of the cord to a scrap of wood and throw it across the yard to see how the spool behaves in a violent event.
 

bjphoenix

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I built a larger one to use with 500# kevlar in mid power rockets and decided I would test it before flying it. This one has 20' of kevlar with quick links each end, and a spool that is about 1.5" diameter and 1.5" wide. I wound it up, hooked one end to a fence post in my back yard, hooked the other end to a block of 2x4, and threw the block of wood across the yard. Half of the cord tangled. It seemed that as the cord unwrapped it was getting wrapped around itself and that caused a tangle. I tried wrapping with one string on one end of the spool and one string on the other. I did this and the throw test twice, and no clogs. My first prototype had a divider, my second didn't and in LPR it survived at least 6 launches with no problems, but now I think the bigger one needs a divider to keep the 2 strings separate. That will be my next experiment.
 

bjphoenix

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I was talking to someone at a recent launch and he convinced me to try the "figure 8" method. This seems to work well and is quick and easy to do. I was excited about the spool idea but it was a lot of trouble and wasn't working as expected so I'm going to try the figure 8 method for awhile.
 

bad_idea

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I've been using the figure 8 since I got back into rocketry this year and quickly discovered how prone thin kevlar is to tangling, but it's been difficult to get consistent release of whatever I use to hold the figure 8 in place.

Previous launch, a small rubber band broke and released the shock cord easily without tangling. Today, exactly the same rubber band tensioned in the same way (doubled) failed either to break or otherwise release exactly the same shock cord. Fortunately there was enough free cord to permit the parachute to deploy, and there was no damage to the rocket despite the effectively ultra-short shock cord, but I'm thinking next time I may just use blue tape.

Crochet may be the ultimate answer. I get to feeling lazy when I see how much work it is though. :D
 

bjphoenix

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I did 5 launches yesterday. First I removed my spool and did the figure 8 in 10' of thin (100#) kevlar. I had horrible tangling, a huge mess, and I haven't even attempted to untangle it yet. Second I used my spool and it worked perfectly. Third launch was a smaller model that my spool wouldn't fit in so I did the figure 8 again and it worked well. Fourth launch I just stuffed the kevlar in there like I used to do and it worked well. Fifth launch was an older rocket that has a long piece of 1/4" cotton sewing elastic in it and it worked well. This is an Estes Silver Comet kit-bashed into a V2. Considering the 1 out of 2 tangle in the figure 8 I will try some new spool versions next.
 
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