Shock Cord Alternatives

[CameronMakesRockets]

Hello All!

I am finally working on my Estes 65 Anniversary rocket build and, as usual, the kit came with an elastic shock cord. I have heard stories of older models having the elastic cord becoming brittle over the years and I want this kit to last a while. Are there any good alternatives for the elastic shock cord on smaller models like the Blue Sapphire? Would the inner strands of Paracord be good substitute? Any other alternatives that you have tried?

I have also been thinking of a system that would allow the chosen shock cord to be replaced easily. Any thoughts on this as well?

Thanks!

 

[bjphoenix]

There is lots of discussion here on both subjects.
I use 100# kevlar in my small rockets, about 8' to 10' length. I have not had to replace one.

If you thread kevlar through holes in the centering rings and anchor it around the back end of the motor tube you can easily cut it loose in the future and thread through another piece. I have read of people using stainless steel fishing leaders mounted to the engine mount then changing to kevlar near the front of the body tube. The kevlar part can be changed out if needed.

 

[Sooner Boomer]

https://www.rocketryforum.com/threads/shock-cord-length.172185/post-2345595

 

[Back_at_it]

I've moved to using 200# Kevlar in my smaller rockets as the permeant attachment. This will connect to the motor mount and extend up the tube stopping just before the top. I'll then attach 1/8" elastic as the shock cord. I've never need to replace a piece of Kevlar in a rocket but the elastic is expendable. The elastic needs to be replaced every 10-12 flights as long as I don't forget to add wadding

https://www.amazon.com/gp/product/B08R8PSNST/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B0868LBPHM/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

[gldknght]

For medium sized rockets, I've had really good success with Parachord. I anchor it through holes in the forward centering ring. I almost always use "dog barf" for recovery wadding, so the flame from the eject charge doesn't directly hit the parachord. I've never had to replace one yet.

Use a length at least 5 times longer than the overall length of the rocket.

 

[BEC]

Back before the dawn of time (well, the early to mid 1960s) Estes used Sig contest rubber, which was intended for model airplane use, as the shock cord material. I have one model I built back then whose shock cord is still good (it’s over 50 years old now). I also have some Sig rubber from the early 1970s at the latest, and it’s fine.

Of course you can’t get that stuff any more. The closest to it is FAI Sport Tan rubber from FAI Model Supply. I have begun substituting this in my builds (including in my Blue Sapphire). It’s too early to tell how well it will last and I’ll not live to see if the Blue Sapphire’s shock cord is still good in 2073 (I’d be 118 years old then) but it feels good, stretches MUCH further than the white flat stuff that comes in Estes kits (which is fine until it dries out) and yet with less force, size for size.

I’m using 1/16 inch wide FAI Tan in my contest models I’m building for NARAM this year, and will start using 3/32 in BT-20 and BT-50 models in lieu of the 1/8 inch or so white stuff supplied in Estes kits.

Oh, and AC Supply carries some of this, so they are another source than FAI Model Supply.

I’m moving away from Kevlar, even as part of a shock cord. It either burns through or wears through after a couple dozen flights.

Larger/heavier models such as some have discussed above require other approaches, I think.

 

[mh9162013]

I use Kevlar towards the bottom of the rocket, and attach it to a removable baffle for easy replacement and inspection. If the baffle can't be removed, I'll still attach it to my baffle, but take more precautions to help the Kevlar cord last longer (thicker cord, heat shrink cover over the knot, etc.). But on the end of the Kevlar, I'll attach some elastic that comes with the typical Estes kits, so I have a hybrid shock cord (about 2/3 is Kevlar and 1/3 is elastic that can easily be replaced as needed).

 

[86mustang408w]

I've been using 135 lb braided kevlar cord for all my LPR, from Rocketryworks. I attach it to the motor tube, under the hook or centering ring. Really makes a nice recovery and is perfect for LPR, its kevlar so it's not gonna break. Its been fail proof so far, and a cheap alternative to that rubber band they give you. I can post a pic a setup if you like.

 

[BEC]

The kevlar will burn through.... typically in 20 flights give or take 5. In one extreme case I had a kevlar shock cord (Rocketman small Miss Riley) burn through on the second flight.

That may or may not be a concern to you.

 

[Hobie1dog]

I've been replacing all the old elastic and rubber shock cords in the 40 year old rockets I was gifted lately with new white elastic I buy at the Hobby Lobby store. I always use the dog barf insulation they have at the launches. I have 18 rockets in my room that I've never launched yet.

 

[86mustang408w]

I've been using 135 pound test shock cord from RocketryWork on all my LPR attached to the motor tubes, and then braided nylon for the bigger ones. With hardware from Coles(eyelets, quicklinks), they have nylon braided cord(1000lb) 1/8" for .11 a foot. Just got to look around!

 

[jmasterj]

The kevlar will burn through.... typically in 20 flights give or take 5. In one extreme case I had a kevlar shock cord (Rocketman small Miss Riley) burn through on the second flight.

That may or may not be a concern to you.

I have also had burn-through of unprotected small kevlar attached right next to the motor mount of LP rockets. Some heatshrink on the first couple inches may help, but I don't have the data to back that up.

Tiny 100# kevlar isn't really comparable to 4mm 1800# or 1/4" tubular kevlar in flame resistance.

 

[86mustang408w]

I got about 2000 yards of free kevlar "Mule Tape", going to use that with my kevlar line/quicklink, thanks all!!

 

[86mustang408w]

Perfect!! Thanks all again!!

 

[nict]

The kevlar will burn through.... typically in 20 flights give or take 5. In one extreme case I had a kevlar shock cord (Rocketman small Miss Riley) burn through on the second flight.

That may or may not be a concern to you.

I had my first kevlar burn through last weekend. I hadn't started keeping flight records when I start flying that one, but I think it had 10-15 flights on it. It was a BMS school rocket with 100# kevlar. I'll not be using the 100# any longer.

 

[techrat]

For a thin rocket like the Blue Sapphire, as others have mentioned, stick with high strength kevlar. However, as you go up in diameters and into heavier rockets, I feel that kevlar alone leads to "zippering" your body tube, so I end the kevlar below the edge of the BT (usually looped back down to the motor mount), and then I use something else, either tubular nylon, or even paracord, as it is very cheap and works well.

 

[afadeev]

I have a growing collection of discarded Estes rubber chords (15+), if anyone wants them. Free for the asking, you pay for shipping.

Are there any good alternatives for the elastic shock cord on smaller models like the Blue Sapphire? Would the inner strands of Paracord be good substitute? Any other alternatives that you have tried?

Braided Kevlar for me, 100% of the time on LP and MP rockets.
150# Emma Kites braided Kevlar for LP rockets.
700# Emma Kites braided Kevlar for MP rockets.
Kevlar harnesses (flat or tubular) of various widths (from 1/2 to 1") for HP rockets.

The kevlar will burn through.... typically in 20 flights give or take 5. In one extreme case I had a kevlar shock cord (Rocketman small Miss Riley) burn through on the second flight.

Any material will eventually burn through. Especially if you over-pressurize your airframe during ejection, or achieve FWD closure burn through RMS mis-assembly (ask me how I know).
Estes rubber will go first (BTDT).
Nylon will go next (BTDT).
Kevlar will last.
Kevlar cord protected with Kevlar sheath/sleeve will last the longest.

I have also been thinking of a system that would allow the chosen shock cord to be replaced easily. Any thoughts on this as well?

It's a consideration for HP rockets that you plan to fly 12+ times.
Not as much of an issue on LP rockets because:

• You are unlikely to fly them that often (you have more LP rockets in the inventory then anything else, and keep building new ones, right?)
• Something else will break with LP rockets first (fins, flimsy paper airframe, etc, etc).
• Something else will go wrong with LP rockets more often, necessitating a rebuild (nose cone will fail to eject, rocket will get stuck in a tree, etc.).
• Lastly, it's light enough, that even a hard landing will be repairable, and you can always re-attach a new shock cord using Estes 3-fold method. BTDT.

HTH,
a

 

[lakeroadster]

I have also been thinking of a system that would allow the chosen shock cord to be replaced easily. Any thoughts on this as well?

​

 

[86mustang408w]

The kevlar will burn through.... typically in 20 flights give or take 5. In one extreme case I had a kevlar shock cord (Rocketman small Miss Riley) burn through on the second flight.

That may or may not be a concern to you.

Its not to me, if the rubber bands Estes gives me lasts until they just get brittle from old age, which is about 10 years. I don't see why kevlar line wouldn't be better. If your burning through shock cords it could or may be another issue, now that would concern me.

 

[bjphoenix]

I use the thin 100# (or 150#) for LPR, and connect it near the front of the tube. I mostly fly fairly long rockets so the kevlar connection is away from the immediate ejection gases.

The method in post #18 allows for easy replacement of the kevlar but OTOH puts it right at the point of maximum fire from the ejection and continued burning of the delay charge. (Watch slow motion videos of Estes engines being fired.) I've read that you can do something similar but the part that is inside the rocket should be stainless steel fishing leader. Well I don't have any of that so I just use kevlar.

I have updated some old rockets to use kevlar somewhat similar to post #18. Obviously if the rocket is already built it won't have a conduit embedded in it. I used a long thin drill bit to drill through both centering rings, then I taped the kevlar to the end of a piece of stiff wire and fed it through the holes. Once through the holes then loop the back end around the motor mount tube and continue to the front of the tube.

 

[4regt4]

For a thin rocket like the Blue Sapphire, as others have mentioned, stick with high strength kevlar. However, as you go up in diameters and into heavier rockets, I feel that kevlar alone leads to "zippering" your body tube, so I end the kevlar below the edge of the BT (usually looped back down to the motor mount), and then I use something else, either tubular nylon, or even paracord, as it is very cheap and works well.

I put a foam ear plug over the Kevlar right where it makes contact with the end of the body tube. I've NEVER had a zipper. Had a very late (4 seconds late) ejection on a HP rocket, altimeter showed ~120fps at ejection. No zipper.

Hans.

 

[neil_w]

I've moved to using 200# Kevlar in my smaller rockets as the permeant attachment. This will connect to the motor mount and extend up the tube stopping just before the top. I'll then attach 1/8" elastic as the shock cord. I've never need to replace a piece of Kevlar in a rocket but the elastic is expendable.

Same. I use 1/8" elastic for 18mm rockets, or rockets with BT50 or smaller chute compartment. For 24mm rockets with >BT50 I use 1/4" elastic. The elastic can take considerable damage from ejection charges, particularly if you have a partial ejection, but I haven't yet had one break before I've had a chance to replace it.

Haven't yet had Kevlar burn through, because I have too many rockets and not enough launch opportunity, so no one rocket gets that many flights. I think my most-flown is somewhere around 6-9 flights. A lot of my scratchbuilds have motor tubes that extend considerably forward of the motor block (usually due to design considerations), which I think should protect the Kevlar at least somewhat. I think I'm generally using 135 lb braided Kevlar (#346).