Shock Cord

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JoshLewy32

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So its about that time in my rocket build that I am ready to buy some shock cord. Usually I use 1/2" nylon that I buy off of amazon or the rocketry warehouse. This time around though while searching through amazon I found some 3/16" shock cord that is almost like bungee rope. My question is, has anyone really used this kind of rope before, and if so how were the results? My line of thinking is that since this bungee rope stretches to 100% its original length (if you have 2 feet packed in at launch then when under stress it will stretch to 4 feet) you will be able to save some weight and a lot of shock cord. Here is the link for the type of rope I found.

https://www.amazon.com/gp/product/B0084EFESM/?tag=skimlinks_replacement-20
 
Last edited by a moderator:
Bad idea because of the recoil/snap back factor. After it stretches out, it has to un-stretch; this will cause your rocket parts to slam back together and damage will result.

Most mid- and high-power rocket people prefer more rather than less and we call it recovery harness.
 
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JoshLewy32 said:
So its about that time in my rocket build that I am ready to buy some shock cord. Usually I use 1/2" nylon that I buy off of amazon or the rocketry warehouse. This time around though while searching through amazon I found some 3/16" shock cord that is almost like bungee rope. My question is, has anyone really used this kind of rope before, and if so how were the results? My line of thinking is that since this bungee rope stretches to 100% its original length (if you have 2 feet packed in at launch then when under stress it will stretch to 4 feet) you will be able to save some weight and a lot of shock cord. Here is the link for the type of rope I found.

https://www.amazon.com/gp/product/B0084EFESM/?tag=skimlinks_replacement-20
Click to expand...

It actually won't stretch all that much as the parts are fairly light, if you pull bungee with your hands you will see the difference. Its also fairly heavy for its size.
 
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llickteig1 said:
Bad idea because of the recoil/snap back factor. After it stretches out, it has to un-stretch; this will cause your rocket parts to slam back together and damage will result.

Most mid- and high-power ticket period prefer more rather than less and we call it recovery harness.
Click to expand...

+1
Elastic shock cords are not good. Short elastic shock cords are even worse. Instead, use a longer length of nylon or Kevlar. For low power rockets it can be very lightweight material, like Kevlar string.
 
And with kevlar or nylon wrap up 3-4 ft in a zigzag pattern wrap a layer of masking tape around .I usually have 4-6 wrapped pieces on the bottom section and 2 on top of the paracute. As it comes out it will pull apart and absord the load.
 
JoshLewy32 said:
So its about that time in my rocket build that I am ready to buy some shock cord. Usually I use 1/2" nylon that I buy off of amazon or the rocketry warehouse. This time around though while searching through amazon I found some 3/16" shock cord that is almost like bungee rope. My question is, has anyone really used this kind of rope before, and if so how were the results? My line of thinking is that since this bungee rope stretches to 100% its original length (if you have 2 feet packed in at launch then when under stress it will stretch to 4 feet) you will be able to save some weight and a lot of shock cord. Here is the link for the type of rope I found.

https://www.amazon.com/gp/product/B0084EFESM/?tag=skimlinks_replacement-20
Click to expand...

When you fly that stuff in a rocket and the rocket reaches apogee and separates
and that material stretches to double it's relaxed length what happens next ?????

Try this.......

https://onebadhawk.com/light-kevlar.html

Teddy
 
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Nathan said:
+1
Elastic shock cords are not good. Short elastic shock cords are even worse. Instead, use a longer length of nylon or Kevlar. For low power rockets it can be very lightweight material, like Kevlar string.
Click to expand...

1++

However many kits will ask for/include elastic shock cords. The shock cord I usually get has a elastic thread with Kevlar over it, it hasn't really affected me all that much but it could still possibly damage your rocket on deployment. Only use nylon on low power rockets because on larger motors the nylon will burn away and your parachute won't deploy. Nylon doesn't need a fire to burn, just the heat of the ejection charge will weaken/burn it away.
 
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RocketryBill said:
1++

However many kits will ask for/include elastic shock cords. The shock cord I usually get has a elastic thread with Kevlar over it, it hasn't really affected me all that much but it could still possibly damage your rocket on deployment. Only use nylon on low power rockets because on larger motors the nylon will burn away and your parachute won't deploy. Nylon doesn't need a fire to burn, just the heat of the ejection charge will weaken/burn it away.
Click to expand...

Yes, nylon is susceptible to heat damage. However, if you put measures in place to protect it (Nomex sleeve, wrap in chute protector, etc), it is a viable option. Also, every one of us should inspect the recovery harness before/after each flight.... no matter what material it's made of.
 
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If you want a little more stretch you can crochet the cord, which will pull out and reduce shock, or to make a permanent slightly stretchy cord you can utilize kevlar with a portion crocheted (say 3 feet) which is then fed back through the loop to form a permanent section that does not pull out. This section will be slightly elastic, but combined with the rest of the cord crocheted that pulls out it gives a rather gentle (in comparison) deceleration compared to just straight kevlar. This can be done with nylon (550 paracord for instance) as long as the bottom is protected (masking tape, kevlar sleeve etc).
 
kjkcolorado said:
Yes, nylon is susceptible to heat damage. However, if you put measures in place to protect it (Nomex sleeve, wrap in chute protector, etc), it is a viable option.....
Click to expand...

I've found that blue painter tape works very well for protecting nylon harness, especially the knots on the quick links, etc. It protects the nylon and then comes off much easier then standard masking tape when it does show signs of where and it's time to replace it.
 
rharshberger said:
I do the same thing on all my rockets with kevlar or nylon recovery harnesses, but instead of tubing I use blue masking tape, so far it has worked very well.
Click to expand...

I like the tubing because it's essentially permanent in with the shock cord, I have yet to have any of the tubes rip apart.

It must work very well at absorbing shock, because the last one, sometimes two 'loops' (nearest the BT) are usually still intact.
 
Handeman said:
I've found that blue painter tape works very well for protecting nylon harness, especially the knots on the quick links, etc. It protects the nylon and then comes off much easier then standard masking tape when it does show signs of where and it's time to replace it.
Click to expand...

Excellent suggestion. I do this too as well as use it for screw terminals and the all thread and nut on the end that hold the E-bay together. Anything I don't want powder residue on. Works great.
 

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