Flame proof cord?

We've just started out with model rockets. Cardboard / plastic home brew rockets with Estes A/B/C engines.
We've been using nylon builders chalk line cord to tie the nose cone, body
and streamer together and have had a couple of the cords burn through - it makes it kind of hard to find the rocket when you watch the streamer alone slowly make it's way back down!
We use recovery wadding or scraps of wool insulation which limits damage to the cord & streamer.
I know there is kevlar cord etc available, but is there a home brew solution - 10lb multistrand fishing line trace? Wool ?
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I'm prety new to this myself but have a couple of thoughts for you:
Natral fibres may be beter and could be soaked in a bicarbonate of soda so make them self extinguishing.
I've used a shot length of picture wire - just enough to have a loop coming out of the top of the tube to which other shock cord can be attched.
Good luck,
Halam

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So... use it. Also, look up the tips and techniques for securing the kevlar inside the rocket. I've been looping around one of the BTs behind a centering ring, either at a transition or around the engine mount. Also, I've learned to use a loop of kevlar, with the knot buried in the epoxy (i.e. nothing to come untied during flight).
For heavier loads, I learned to do a four-strand braid. Start by twisting an inch or so of the *centers* of two strands together. Loop that twist over something (it becomes your loop), then braid the rest of it. Now, feed the non-loop end through a hole in a centering ring, split it into two sets of two strands, and tie the two sets together. Now, run each set around opposite sides of the BT and tie them together on the other side. This is fairly secure on its own, but smear epoxy over the whole thing (behind the CR) to lock it in place.
Make sure the strands are long enough that the loop just sticks out the front of the rocket, so you can tie your shock cord onto it.
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Are Rolling Hitches used for attching shock cord to motor mounts? Has nobody tried or are they proven unsuitlable?
The rolling hitch is used in sailing when pulling along the length of something = its a clove hitch with an extra turn.
Halam
writes:

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Assuming you mean "attaching shock cord to kevlar loops". If you attach shock cord directly to the motor mount, it's likely to get burned during the ejection. For attaching the kevlar to the motor mount or BT, I use a simple single-strand overhand knot to secure the "cable" to itself, then a square knot on the other side of the BT. The epoxy keeps tension on the square knot, which keeps it from untying.
As for the shock cord, it depends on how much force is expected. For my small rockets, I use a Bowline, often with a hitch or two after it to lock it from unraveling. For large rockets, a Flemish Eye Knot (otherwise known as a double figure eight or mountain climbing knot) is preferred.
I think the choice of knots depends on how likely it is that the knot will come undone on its own, not the physical strength of the knot.
Also, the two knots I mentioned won't close on their own like a hitch will. The loop stays open under tension, so it won't crush the chute material.
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I don't think using any sort of hitch is a good idea, unless you epoxy it securely. A better choice, IMO, is either overhand follow through, figure 8 follow through, or a fisherman's knot.
Hitches are usually used in a situation where further adjustments will need to be made, which means they can be untied fairly easily... not something I'd want to purposely design into a recovery system.
Kevin OClassen
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I use kevlar string where the shock cord mount is down near the ejection charge. Then I tie a loop in the kevlar string up just below the plane of the body tube where the nose cone sits. To this loop I tie my shock cord be it nylon or elastic. This keeps the shock cord away from the ejection gasses and the kevlar string below the exit plane of the body tube. I keep the kevlar string below the exit plane of the BT to keep it from being pulled through the side of the BT (Zipper effect). Since the kevlar string is very thin it does not take much to pull it through the side of the tube. However the shock cord is wide and thicker so it won't unzip the tube. By tying the shock cord to the kevlar just below the tube end I get the best of both worlds. Kevlar does not burn and the wider shock cord does not easily cut through the BT. On bigger heavier rockets I actually braid the kevlar string to make it 5 times stronger. Then I do the same thing. Never had a failure in the kevlar nor a tube that got zippered.
Be sure to pull the shock cord and kevlar string out and tight while putting in the wadding to keep them above the wading and protected as much as possible.
I don't have a lot of experience in the High Power arena. So failure may happen to me some time or another. I also had an experienced fellow tell me not to use elastic for HP rockets. He said,"The only thing those are good for is to hold up your underwear". I followed his recommendation and replaced my elasic shock cord with a 30 foot long tubular nylon shock cord. It worked great! Thanks James.
KT
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for higher powered models I would install an eye or U bolt in the last centering ring, then the thick (1/2 inch) kevlar shock cord is tied into a loop (or some people sell pre sewn loop... which is more convienent) and attach it to a climbing hook or those chain link with a screw on it (forgot whats it called) and attach one end to the eye hook or U bolt. Then the other end I would attach nosecone and recovery material, and flameproof cloth. Everything is attached with a bolt of some kind so the recovery material can be easily and quickly disassembled. That way you can have one recovery system for several rocket (assuming they need to use the same kind of chute....) and not have to get super expensive ones for every one of them.
--
TAI FU
"rob" <not snipped-for-privacy@all.com.au> wrote in message
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Many thanks for the replies guys - lot of useful info. I'd go for the kevlar but I've not seen it in any local hobby shops (Western Australia). Most hobby shops have little more than half a dozen kit rockets and a handful of estes motors.
I might use a short length of light gauge multistrand stainless trace wire and go with some of DJD's and Halams ideas to prevent cutting u the side of the tube. cheers guys.
Hey, I just bought a box of 24 assorted Estes engines for $95 australian - say $4 each. Hows this compare with US/UK prices. rob
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You can buy kevlar string off the internet. Do a search for kevlar string or rocket shock cords etc.
KT
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wrote:

Thanks KT - I found that kevlar fishing line is available, I'm going shopping on the weekend, it might work for rocketry & fishing ! cheers rob
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If you can find a shop that is specialized in kites in your area you'll have a good chance of finding kevlar. Here in holland it's available in most kiteshops. flying kites is popular here.
If you really have a hard time finding it you may consider using the thin steel cable used in fishing. Worked for me before I found kevlar and it's available in every tackle and bait shop. Plenty of those in your area I bet. If you are living near the coast that is.
fly 'em high down under, Roland.
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Unless it's changed, I'd skip the steel fishing line. I stopped using it after my first attempts about three decades ago. It kinks and breaks, and it burns from ejection exposure. Somewhere around the 1/16" stuf that NCR used in their mounts it's no longer an issue, but the thin stuf just didn't work for me.
--
Bob Kaplow NAR # 18L >>> To reply, there's no internet on Mars (yet)! <<<
Kaplow Klips & Baffle: http://nira-rocketry.org/Document/MayJun00.pdf
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