Good shockcords

For my team's TARC rocket, we're planning on using tubular Kevlar, for the entire shockcord, but I don't know if the tubular Kevlar will stretch enough to absorb a considerable amount of energy. This worries me because the shock from ejection might effect the eggs. I don't know how much of a beating eggs can take because I've never egg lofted once in my life and I don't want to take a chance. The tubular Kevlar will be coming from Commonwealth at an 1/8" or 1/4" thickness. Should we just use Kevlar for the bottom portion of the shock cord to to take the heat and then tie on tubular nylon or regualar elastic for the rest of the shock cord? Any advice would be greatly appreciated.

you could purchase some kevlar bungie. Make a small loop out of it that will obsorb some of the shock

Ben

Tubular nylon isn't exactly "stretchy" in the way your underwear elastic is, either. It has a little give. A very little.

I'd suggest you experiment with your egg on the ground. Pack it up like it will be in the rocket, and see what it takes to break it.

If the jig is up when your egg breaks in the competition, and you are worrying about breaking it because of your choice of shock cord material, you haven't packed it well enough. That egg needs to be protected from everything up to a lawn-dart, in my opinion.

If you don't like Kevlar, get a nomex shock cord protector for the tubular nylon.

You say "considerable energy". How forceful is this ejection event? :ahhhhh:

N

JJ94

Your rocket will weight between 1 and 3 pounds. I would simply go to my local fabric shop (JoAnns) and buy 1/2" braided elastic instead of, or in addition to, the kevlar. JoAnns sells it online for $1.60 in 4 1/2 yard lengths which is just about the right length for a 3 pound rocket. https://www.joann.com/joann/catalog.jsp?CATID=cat2846&PRODID=xprd315869

Bob

I may go with getting Kevlar (either 1/8"-1/4" tubular or strap Kevlar) and tieing that to the eye bolt in the forward CR and tieing some elastic strap to the other end of the Kevlar. That seems like the easiest, cheapest, and most reliable way out of this situation for now. I'd like to hear more ideas. The Kevlar bungee seems like a good idea, but I'm a bit skeptical. GLR says the Kevlar bungee's strength is the same as normal Kevlar and it can withstand the same heat as normal Kevlar can, but I don't know if that's true.

Forget the elastic, Just Use 1/8 Braided Kevlar and make the SC Long! The longer the Better.

GaryT said the magic word....Long Shockcord.

Braided 1/8" or 1/4" tubular Kevlar either are more then sufficent for what your trying to do. the trick is to have enough lenght to allow the payload (Egg) section to slow down as the chute opens. Keep in mind the opening snap of the chute is just as hard a Shock as the jolt they get from ejection

The shock cord that we were going to use was 20 feet of tubular Kevlar. That should be long enough to take most of the shock away, atleast it should be with my experience. Is there any cons about having a shock cord too long? The longest shock cord I've ever used is about 4-5 feet, and 20 feet is 4 times more than what I'm used to using.

FYI. If anyone wants to stick with old school tri-fold Estes Rubber band method Guillows makes 96" x 3/16" x .03" white 'rubber threads' for balsa planes. The same composition as Estes shockords , just 8' long.

https://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXK537&P=0

1/2" Elastic will be plenty for your application. I use 10' of it on my Four "E9" Clustered LOC Starburst. It takes the heat just fine and when it gets to bad I will just replace it with some new stuff. So far I have recorded 10 (40 ejection charges) flights on it and it is still going strong.

GIJoe said:

1/2" Elastic will be plenty for your application. I use 10' of it on my Four "E9" Clustered LOC Starburst. It takes the heat just fine and when it gets to bad I will just replace it with some new stuff. So far I have recorded 10 (40 ejection charges) flights on it and it is still going strong.

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I'm not worried about strength very much at this point. I know that the Kevlar and elastic will take a strong beating, I'm just wondering which combo to do. I know I will get TK but I want to know if I should use TK for the entire shock cord or use TK for the anchor and then tie on TN or elastic to the end. I want to go the cheapest route but I also want to be sure that shock from ejection is absorbed (and maybe I can rig the chute so the shock from it opening is also absorbed). Right now, I may just buy TK and use the 20 feet for the entire shock cord, but that can always change.

You asked about problems with longer shockcords- they can get tangled and that's a pain. What I do with my MPR that have long cords is to loosely crochet the long cord down to a shorter length before placing it in the rocket above the chute protector. This makes it easier to handle, and acts as a small energy absorber as the nose cone pulls the loops out of the crochet section.

You can do either full TK if you want, or use a section of TK and elastic (ala Quest kits) if it suits you. Your rocket, your choice.

kj

zog43editor said:

You asked about problems with longer shockcords- they can get tangled and that's a pain. What I do with my MPR that have long cords is to loosely crochet the long cord down to a shorter length before placing it in the rocket above the chute protector. This makes it easier to handle, and acts as a small energy absorber as the nose cone pulls the loops out of the crochet section.

You can do either full TK if you want, or use a section of TK and elastic (ala Quest kits) if it suits you. Your rocket, your choice.

kj

Click to expand...

What does 'crochet' mean? And how do you do it?

I "think" it is also called "daisy chaining." It is done like this: take your shock cord and put a simple loop in it. Then, next to your first loop, make a second loop. Pull the "trailing edge" of the second loop into the body of the first loop. Make another loop and pull it in to the previous. Keep doing this until you get to the other end.

It should reduce your shock cord length to about 1/3. Make sure your loops are the same size and are all turned in the same twist and direction.

I looked through several google images but didn't find much.
Edit:
Oops! Found this:
https://images.google.com/imgres?im...4&prev=/images?q=daisy+chain+knot&gbv=2&hl=en

Do the loops become undone once a load is placed on the shock cord?

Added drag from the rocket without a nose cone should take care of the force of the ejection. Unless your ejection time is too short or too long. Ten feet of 1/2" elestic will be the cheapest and should be plenty. A Kevlar anchor somewhere in the body tube will hold it all in place. Good luck with TARC.

I would not tie the elastic to the to the TK. The shock cord is as strong as its weakest link. Which would be the elastic and or the knot. I would go the bungie route and tie a loop in the TK about 2 feet from the nose cone and tie a 1 foot section of elastic from the loop to the nosecone in parallel with the TK.

bunch up a short section of kevlar, and put a section of elastic spanning the stretch so the kevlar has a place it can stretch but u still have the full strength of the kevlar (think about the back packs with the elastic shock absorbers)

jj94 said:

Do the loops become undone once a load is placed on the shock cord?

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Yes, it's kind of a loose knot. I learned to tie up horses with a knot like that. From the horse's side he pulls against the post, from my side if the horse gets wild I can pull out the entire knot with one pull. you don't want a horse to buck your knot tighter while your finger is trying to loosen it (ouch). Not hard to show you in person but I don't know how to describe it...

jj94 said:

For my team's TARC rocket, we're planning on using tubular Kevlar, for the entire shockcord, but I don't know if the tubular Kevlar will stretch enough to absorb a considerable amount of energy.

Click to expand...

It has been awhile since I bought it, but I used to go to REI and get round "bungee-like" cord in a variety of sizes, some down to about 1/8" in diameter. Most climbing shops will have similar. The outer jacket seems to help vs. flat elastic that may get some scorching or blackening over time.

Also, I tend to change them when they seem to start looking dirty -- it's cheap insurance to keep fresh shock cords in your rockets.

Finally, for really long shock cords, you can bundle the cord by wrapping it like ribbon candy, and putting a loose rubber band around it. Or, wrap with a small piece of paper, and secure the wrap with a little piece of masking tape. It will then pull apart from each end to its full length. Just do not make it too tight! Test it manually for release.

I learned how to crochet while working the night shift at a hotel years ago. Here is a link with the basic steps, instead of using a hook, you use your fingers, and keep the loops big enough for you to be able to reach though to get the next bit of cord. Also you don't need to pull the loops tight between links.

Topics 3 and 4 show what I use to tame long cords.
https://learntocrochet.lionbrand.com/cgi-bin/faq-search.cgi?store=/stores/eyarn&faqKey=108&V2=

And here is a photo of a cord that is crocheted. To continue I'd reach through the loop on the right and pull the cord through making another loop.

kj

Peartree said:

Yes, it's kind of a loose knot. I learned to tie up horses with a knot like that. From the horse's side he pulls against the post, from my side if the horse gets wild I can pull out the entire knot with one pull. you don't want a horse to buck your knot tighter while your finger is trying to loosen it (ouch). Not hard to show you in person but I don't know how to describe it...

Click to expand...

View attachment HQ0.JPG

for lower powered rockets i've used frangable links, which is basically making a loop in the shock cord and then taping across the loop. with several of these, the shock cord stetches abit slower. used it many times with kevlar cord instead of elastic and i have not had a problem yet, quite often there are a few links left.