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Shock chord idea.

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Senior Space Cadet

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I've been using Kevlar chord for my parachute shock chords. I attach it to the engine mount, so it's really secure. But it doesn't have much give, so you are supposed to use a lot of it, which can be a real pain. I have some 1/8" bungee chord that works great, as a shock chord, on larger rockets, but is too stiff and bulky for small rockets.
I have a little boating experience. It occurred to me that something used for docking boats might help in this situation.
Obviously this one would be too big for a rocket. What I'm proposing is taking a short length of bungee chord, tying the Kevlar chord to one end, spiral wrap some around the length of the bungee, tie the Kevlar chord to the other end, then super glue the knots at each end.
Maybe I should have tried this myself before posting, but with the seasons changing, who knows when I'll get to launch again.
Another thing to consider is marine bungee. The bungee I have is 1/8" that I got from Apogee. Marine bungee is of really high quality. Not sure if there is any difference in heat resistance.
 

o1d_dude

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Using a bungee cord in or as your shockcord harness will give an “Estes dent” you wont soon forget.
 

dhbarr

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Could make sense to add back some give, especially if contained within a nomex sleeve for both flame resistance and keeping chutes from binding.

Be aware that CA isn't the best choice for Kevlar, as it tends to make it brittle and wear.
 

Senior Space Cadet

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Using a bungee cord in or as your shockcord harness will give an “Estes dent” you wont soon forget.
"Estes dent"? Not sure what that would look like. I've used it on BT-60 and BT-80 rockets with no problem, except the heat seems to stiffen up the outer sheath.
 

dr wogz

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"Estes dent":
A semi-round indent in soft balsa nose cones caused from the nose come being 'fired' out the top end of the body tube at ejection, and then reaching the end of the [short] supplied Estes elastic shock cord, and then coming back to try & re-seat itself in said body tube..

Long-ish shock cord tend to be the norm, so that the NC's energy can dissipate. The general rule of thumb is 3x the BT length. but it depend on how heavy the NC is, and how mush ejection charge you have.

I have put a length of Kevlar inside the BT, then attached a length of elastic (Estes style) then either added a bit more Kevlar or just tied on the NC.
 

jrap330

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I've been using Kevlar chord for my parachute shock chords. I attach it to the engine mount, so it's really secure. But it doesn't have much give, so you are supposed to use a lot of it, which can be a real pain. I have some 1/8" bungee chord that works great, as a shock chord, on larger rockets, but is too stiff and bulky for small rockets.
I have a little boating experience. It occurred to me that something used for docking boats might help in this situation.
Obviously this one would be too big for a rocket. What I'm proposing is taking a short length of bungee chord, tying the Kevlar chord to one end, spiral wrap some around the length of the bungee, tie the Kevlar chord to the other end, then super glue the knots at each end.
Maybe I should have tried this myself before posting, but with the seasons changing, who knows when I'll get to launch again.
Another thing to consider is marine bungee. The bungee I have is 1/8" that I got from Apogee. Marine bungee is of really high quality. Not sure if there is any difference in heat resistance.
yes as i suggested tie an elastic to the kevlar ..use kit's cord or go to a fabric store and buy elastic.....
 

dr wogz

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Oh, and while I get the snubber method, I feel an LPR rocket is too small, and you'll just end up with a loop in the shock cord; which (in my opinion) will add another variable towards a failure (being tangled, tied, bound, etc..)

go long on the Kevlar, or go Kevlar & a piece of elastic. Whatever the method, make sure it's long enough to allow the NC's energy to dissipate once ejected.
 

jrap330

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G. Harry Stine had this in "Handbook of Model Rocketry"... similar in principal
Have to be a boy scout or fisherman..to figure out how to splice that in. Bless Mr Stine.
 

jrap330

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Try the Crochet method.
Tim at Apogee has a nice video.
The tighter you pull the knots the more friction is created. BTW you can do it without a crochet needle.

Yeah..spent a few minutes every time you want to launch...besides folding chute...packing wading and putting igniter and engine into tube.......too much time....
 

n27sb

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Yeah..spent a few minutes every time you want to launch...besides folding chute...packing wading and putting igniter and engine into tube.......too much time....
Actually not to bad to prep once you get the hang of it. Eliminates tangles and creates friction upon release eliminating shock.
 

lakeroadster

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Another option is to use two appropriate sized chutes.... One for the nose cone, another for the rocket. That way the nose cone and rocket aren't attached to each other at recovery.

Or if the weight allows, us a streamer for the nose cone... and a chute for the rocket.
 

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"Estes dent"? Not sure what that would look like. I've used it on BT-60 and BT-80 rockets with no problem, except the heat seems to stiffen up the outer sheath.
aka in balsa nose cones " Estes Smile", when the shock cord snaps the nose cone against the mouth of the air frame hard enough to cut into the balsa nosecone leaving a smile shaped dent in it.
 

Banzai88

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Try the Crochet method.
Tim at Apogee has a nice video.
The tighter you pull the knots the more friction is created. BTW you can do it without a crochet needle.

I use this method extensively in my fiberglass 29mm powered rockets.....works amazingly well. Over a hundred flights, have not had one single failure or 'tangle' or had any parts bang together since I started using this method.

The key element with kevlar recovery harnesses is using 'enough'. If you're not sure what that is, put a LOT of it in and ground test. After a few tests, you'll know what a good average for that rocket is. Most rarely, if ever, test like that and simply opt for 5x body length and hope......or buy whatever 25foot piece suits their fancy.

If newtons were more expensive or altitude, or enjoyment were harder to achieve, you'd see more people testing like that instead of launching X feet of extra cargo/recovery harness.
 

rklapp

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I like Kevlar but if it extends too far out of the tube (more than six inches), it tends to zipper the tube. I've been getting really good at tying a slip knot with the Kevlar to the elastic and the MM tube.
 

rharshberger

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I like Kevlar but if it extends too far out of the tube (more than six inches), it tends to zipper the tube. I've been getting really good at tying a slip knot with the Kevlar to the elastic and the MM tube.
I only use kevlar to the mouth of the airframe and put a loop as close to the mouth as possible, a square of tape around the kevlar usually does a good job of preventing zippers at the mouth of the airframe and just below the loop, from the loop onward I use small diameter nylon cord 1mm or 1.5mm cant remember which, nylon has some stretch. To protect the nylon I put the wadding/dog barf under the nylon and then zfold the nylon with single thickness tape to hold them and it works very well so far, no zippers or "Estes Smiles"....yet. Nylon has some stretch to it.
 

rklapp

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I only use kevlar to the mouth of the airframe and put a loop as close to the mouth as possible, a square of tape around the kevlar usually does a good job of preventing zippers at the mouth of the airframe and just below the loop, from the loop onward I use small diameter nylon cord 1mm or 1.5mm cant remember which, nylon has some stretch. To protect the nylon I put the wadding/dog barf under the nylon and then zfold the nylon with single thickness tape to hold them and it works very well so far, no zippers or "Estes Smiles"....yet. Nylon has some stretch to it.
I'm interested in a photo of this set up.
 

o1d_dude

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I'm interested in a photo of this set up.
Here’s the snout of my Cherokee-H showing the kevlar at full extension. A ”weak” overhand knot enclosed in heat shrink tubing...it’s a cardboard rocket and the materials will fail before the knot.

Have not decided what I will be adding to the end of the base harness but I have kevlar and white nylon harnesses from One Bad Hawk. Also some unused black nylon harnesses from LOC.

Have been doing my shock cords like this following a conversation with Crazy Jim quite a few years ago.
 

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rklapp

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Here’s the snout of my Cherokee-H showing the kevlar at full extension. A ”weak” overhand knot enclosed in heat shrink tubing...it’s a cardboard rocket and the materials will fail before the knot.

Have not decided what I will be adding to the end of the base harness but I have kevlar and white nylon harnesses from One Bad Hawk. Also some unused black nylon harnesses from LOC.

Have been doing my shock cords like this following a conversation with Crazy Jim quite a few years ago.
Awesome. This was my first attempt with an 18mm rocket which failed so need to practice more.

C3267CA6-337F-400E-BBAA-8CF451D6DD9A.jpeg
 

o1d_dude

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I dont use elastic on any of my rockets anymore, and as the rubber or elastic shock cords wear out they get replaced with kevlar.
In effect, we eliminate an inevitable repair of rockets we now build.

More launch time.
 

Mike Haberer

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I use heat shrink tubing on the kevlar where it exits the airframe. I start with a small diameter and build up in layers using larger diameters until I have at least 3 layers, sometimes more. I buy the kevlar in bulk in different sizes for different rockets. The Kevlar is such light weight and virtually indestructible that it's the only thing I use now. I just finished a downscale Big Bertha to a 13mm minimum diameter Micro Bertha. It uses very light kevlar from OneBadHawk for the shock cord. Not even bothering with heat shrink tube on this featherweight. The only reason I'm using a recovery method at all is to be able to find it. Rocksim is projecting almost 900' on an A10 (for a 8 inch long rocket). I bought a 500' roll of mylar bird scare tape on Amazon and cut a 1/4" wide by 12" long streamer from it. Added small squares of gorilla duct tape at once end of it and used a hole punch to make a hole to tie the kevlar to. I probably should buy some 1/4A's for the first launch :-o.
 

ep29030

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Another approach: fold your kevlar and wrap with rubber band(s). They absorb energy as the Kevlar is pulled out.
 
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