Shock Cord Protection

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brewster_rockit

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Hello all,

When I returned to the model rocket hobby after many years away, it seemed like I was having shock cord protection issues that I never remembered from before my hiatus. Sewing elastic shock cords were burning through after just a few flights, while the rubber ones in the kits were fairing somewhat better.

I've heard the term "shotgun ejection charge" thrown about, and I suspect that Estes had reformulated the ejection charge for better ejections during my time away from the hobby (probably due to feedback from customers about ejection failures.)

With that being said, am I alone in my shock cord burnthrough troubles? Should I just double up on the wadding I'm using to protect my shock cords, or is there a technique for installing the wagging I should be aware of? I already place sheets of wadding over the tube and push down to get a good seal where the wadding touches the wall of the body tube. Is it time to move away from sewing elastic (or Quest-style sewing elastic plus kevlar) on my scratchbuilt models?
 
I've used this technique ever since I read that newsletter. I've yet to have to replace a chord from scorching (I really need to get and fly more) but I have needed to replace chords that I messed up.
 
Some people use a sheet of wadding filled with dog barf (cellulose insulation). Some people just use dog barf. Wadding alone is probably the least favorable method.
 
I've been using 70-130lb braided Kevlar/Oval elastic or Stainless Steel Leaders/Kevlar/oval elastic on my LPR & MPR rockets for almost 20years now. Since changing to these set-ups I haven't lost a shockcord to burn through.

below are a couple photos of the Stainless Steel avaition cable and fishing leaders I've been using since the late 1990's. Kevlar from 50-300lbs braided (tubular) can be found in many places but the best place i've found is "TheKevlarStore" on line.

Oval elastic in 1/8" and 1/4" have been specifically produces for shockcord use. I purchase it in 144yd rolls.

Another thing about shocklines. ALL supplied by the manufacturers are Way to short. Generally the Stainless Anchors are about 12-15" long, the Kevlar 36-42" long and the Elastic another 36" - 60" long depending on the overall length of the model. These longer lengths allow the rocket body and Nose cone to seperate and slow down before stretching the elastic out completely which puts very heavy loads on the components and can lead to the "Dreaded Estes Dent" if the Shockcord is made to short.

473p02a_BT-50h-Bt70 MMT & Shock Anchor_01-15-08.JPG

473p02b_Shockcord anchor-100lb Kevlar_01-15-08.JPG

Kevlar Line-b1_28lb to 138lb (128dpi)_06-05.jpg

Kevlar Line-d2_Yellow 70lb Braided (Tubular) Kevlar_11-16-15.JPG

Oval Elastic_.125in x144yd roll ShockCord Material(WholeSaleFabricsStore.com)_05-16-14.JPG
 
I've been using 70-130lb braided Kevlar/Oval elastic or Stainless Steel Leaders/Kevlar/oval elastic on my LPR & MPR rockets for almost 20years now...
I think I half get it, but something puzzles me. The steel chord will never burn through and will be indestructable for all practical purposes in the present application. Should the rest ever need to be replaced for any reason, as long as the top end of the cable is accessible then changing it out while leaving the steel in place is easy.

What I don't get is this: why use kevlar in between the steel and the elastic. The kevlar and steel serve the same purpose, do they not? Both are strong, inelastic, durable materials for attachment to the rocket body, to which the elastic is then attached. So why steel → kevlar → elastic, rather than just steel → elastic?
 
On most rockets (works esp. well with longer rockets, since you have more room) you can install a baffle system...good examples are:

https://www.rocketarium.com/Build/Ejection-Baffles

I think the kevlar string method is usually the easiest/best, just make sure you make it long enough, since there is no stretching to absorb the shock when it reaches the end of the line.
 
Apogee sells kevlar cord for pretty cheap. It's all I use in my rockets now. I tie it off on my engine mount, so that there is no chance of losing the parachute during deployment.
 
Kevlar is now the standard, especially for LPR and MPR...much tougher and more heat resistant than the old rubber shock cords. Because of this it's now standard practice to anchor shock cords to the engine mount rather than near the top like in the early days. Be generous with the wadding and/or cellulose, especially with larger engines. I suspect larger engines come with proportionately larger ejection charges as well. Case in point: I never had any problems until recently when moved from a C11 to a D12 in one of my models. 3 flights in with no shock cord damage until I went with the larger motor; my 120lb kevlar cord fried clean through right where it was anchored to the engine mount. Fortunately it held on just long enough for the rocket to reach the ground and the wadding saved my chute from the worst of it.
 
I think I half get it, but something puzzles me. The steel chord will never burn through and will be indestructable for all practical purposes in the present application. Should the rest ever need to be replaced for any reason, as long as the top end of the cable is accessible then changing it out while leaving the steel in place is easy.

What I don't get is this: why use kevlar in between the steel and the elastic. The kevlar and steel serve the same purpose, do they not?

Sort of, except that Kevlar is lighter and much easier to work with !


Both are strong, inelastic, durable materials for attachment to the rocket body, to which the elastic is then attached.
So why steel → kevlar → elastic, rather than just steel → elastic?

All low-power rockets I've build recently skip the elastic altogether (other than a few older or cheap models), and just go with Kevlar all the way: from MMT attachment, all the way to nose cone.

For high-power rockets, it's tubular nylon or high-strength Kevlar. Also, all the way.


Kevlar is now the standard, especially for LPR and MPR...much tougher and more heat resistant than the old rubber shock cords. Because of this it's now standard practice to anchor shock cords to the engine mount rather than near the top like in the early days.

I think it's also a more durable approach, and a stepping stone towards HPR build practices.
That's precisely how you attach a shock cord on MPR/HPR rockets.

Be generous with the wadding and/or cellulose, especially with larger engines. I suspect larger engines come with proportionately larger ejection charges as well.

Indirectly - yes.
Larger engines (24/29 vs. 18mm) need larger motor mount tubes, which usually go into larger diameter airframes. Therefore, you would need larger ejection charge to achieve the same pressure (psi) during ejection.

Case in point: I never had any problems until recently when moved from a C11 to a D12 in one of my models. 3 flights in with no shock cord damage until I went with the larger motor; my 120lb kevlar cord fried clean through right where it was anchored to the engine mount. Fortunately it held on just long enough for the rocket to reach the ground and the wadding saved my chute from the worst of it.

FWIW, I bought two spools of Kevlar from Amazon: 2000 lbs for MPR rockets, and 250 lbs more low-power ones.
Both get epoxied to the MMT.
Of either ever breaks, for any reason, I can always go back and epoxy them to the airframe wall, aka the old-fashioned way.

YMMV,
a
 
Agree to all the comments about Kevlar. When I returned to the hobby a few years ago I had only heard of it as something in bullet proof vests, not model rockets When I was a kid building rockets it was white glue for an adhesive and rubber shock cords anchored between two slits cut into the top of the body tube. CA was a new invention, and was not in wide use. Kevlar and nomex were yet to be invented.
Nowadays Kevlar is the shock cord standard, due to its heat resistance. Though not bulletproof, it will last much longer than elastic or rubber. Nomex (Aramid) is a fabric that modelers frequently use as a blanket to replace wadding. It is the same stuff used to make race car drivers' jumpsuits: high heat resistance. It can be used over and over and switched between rockets, so it is economical in the long run. What I haven't seen mentioned here is a nomex shock cord protector sold by some vendors. It is a fabric sleeve that slips over your shock cord. Usually used in mid and high power stuff, but no reason it can't be used for low power rockets. Or, the most economical way would be to buy some aramid/nomex fabric and make your own shock cord sleeves and parachute blankets. Or, if you can't sew, have a girlfriend or mom do it for you.
I usually attach small snap swivels between the Kevlar and elastic cord, rather than tying them together. This facilitates easy swapping out of sleeves, chute protectors and even baffles. And of course, I attach a swivel to the parachute and attach that to a loop on the shock cord about 1/3 of the way down. This will lessen tangling of the shock cord with the nose cone weaving in and out during deployment, as compared to attaching the parachute to the nose cone loop or screw eye. It's a tip I learned from Chris Michielsson, a professional model builder. Hope this helps, and welcome back to the sport!
 
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