Nomex Shock Cord Protector

I've been puzzling the concerns of using elastic shock cords in MPR and HPR. I've been advised of the benefits of Kevlar and steel leaders. I've been advised of the pros and cons of pure Kevlar vs elastic for reducing initial shock to the rocket.

So the benefit of elastic is lower actual shock. The downside is it's burnability. What if I made a tethered pouch of Nomex that would spool the shock cord out at deployment but protect it from the ejection charge. Any thoughts on this idea?

I like to keep things simple and just use Kevlar for all shock cords. You attenuate the shock by bundling the cord. Masking and electrical tape work well. You can see some bundles in the picture.

Also, check this thread....
https://www.rocketryforum.com/showt...-ever-made-a-shock-absorbing-recovery-harness

Titan II said:

I like to keep things simple and just use Kevlar for all shock cords. You attenuate the shock by bundling the cord. Masking and electrical tape work well. You can see some bundles in the picture.

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I prep my shock cords the same way, bundle and tape, I would also add that if you make the shock cord long enough it helps to dissipate energy. This will help to level any energy absorption qualities of elastic over Kevlar in this application.

There's also the weight consideration. Just how much does that Kevlar weigh vs elastic? If it takes that much more Kevlar to equal the dissipation ability of an elastic, then doesn't that start to become a noticeable amount of weight in your rocket?

There's a big difference between MPR and HPR when it comes to shock cords. For MPR, I will use elastic sometimes if the rocket is light enough. But for heavier MPR and all HPR, I usually use nylon webbing, either flat or tubular. My HPR shock cords are very long, sometimes over 20 feet. I will also use a short length of Kevlar attached to the anchor, and tied to the long nylon shock cord. It really depends on the design of the rocket. With my zipperless designs, I generally attach the nylon right to the anchor. For traditional designs, I'll use the short Kevlar and long nylon, with a Nomex blanket to protect the chute and nylon webbing.

https://www.rocketryforum.com/showt...uild-5-Finger-Death-Punch&p=579079#post579079

https://www.rocketryforum.com/showt...uild-5-Finger-Death-Punch&p=579402#post579402

I often use a kevlar shock cord in the fin can and tubular nylon as the center of upper portion of the shock cord. This lived the best of both worlds.

Does the tubular nylon have more shock absorption than the Kevlar?

EXPjawa said:

Does the tubular nylon have more shock absorption than the Kevlar?

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Generally nylon is more elastic than Kevlar, which is one of the reasons why some would choose nylon over Kevlar. The Kevlar has the fire resistance and strength/size ratio qualities over nylon.

So in the case of Kevlar you could do away with a Nomex shock cord protector and even down size the cord, where as with nylon unless you plan on replacing your shock cord often then you need some heat protection at least for the last part closest to where it is anchored where your charges go off.

Given that the ejection charge is calculated correctly and ejection occurs at a relatively low rocket velocity, say 10-15 miles per hour. What loads/forces would you actually expect the shock cord to experience? I'm thinking about smaller medium/high power rockets ranging between 1-2 Kg weight.

Also, how much design tolerance would you apply to those loads in order to cover all unexpected conditions and eventualities?

SO

mpitfield said:

Generally nylon is more elastic than Kevlar, which is one of the reasons why some would choose nylon over Kevlar. The Kevlar has the fire resistance and strength/size ratio qualities over nylon.

So in the case of Kevlar you could do away with a Nomex shock cord protector and even down size the cord, where as with nylon unless you plan on replacing your shock cord often then you need some heat protection at least for the last part closest to where it is anchored where your charges go off.

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Let me ask this: if I were to fasten the shock cord to a eye bolt that was in turn attached to the upper plate of a baffle, is the shock cord protected from the ejection blast, or does it still require heat protection? The assumption in this situation is that the tube is BT70, with the top of the baffle approximately 5 diameters ahead of the engine case.

EXPjawa said:

Let me ask this: if I were to fasten the shock cord to a eye bolt that was in turn attached to the upper plate of a baffle, is the shock cord protected from the ejection blast, or does it still require heat protection? The assumption in this situation is that the tube is BT70, with the top of the baffle approximately 5 diameters ahead of the engine case.

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I always use a bit of wadding with my baffles (most are the 2/3 moon shaped disc style, with 4 discs) with steel leaders attached to the baffle plates,and kevlar shock cords at least 3 times longer than the rocket. The shock cords are Z-folded along a strip of masking tape with approximately a 1/4" between loops(usually 6-8 z-folds), I then place a second piece of tape over the top of the other piece of tape trapping the shock cord between them, it has worked great for me, usually there are one or two loops still taped at recovery. As for wadding being required, probably not necessary but a little never hurts just in case a hot particle makes it through.