Alternate Shock cord materials beside Kevlar?

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AfterBurners

AfterBurners

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I was giving this some thought the other day and was wondering if "Bungee" cord material would make a good alternative to the standard shock cord material? I know the most common would be nylon or Kevlar, but there seems to be "no give" and literally causes "shock / stress to the rocket" but with an elastic type cord I think personally it would work better or perhaps have a portion of the recovery harness with elastic and then either nylon or Kevlar. As far as heat is concern I'm sure there might be a way of treating material or maybe use a Nomex sleeve protector for it. Either way it might prove to be rather efficient and cost effective. I just think the amount of money spent on a one piece of Kevlar, of course depending on size can be rather expensive and probably in most cases from what I've seen on this forum "Overkill".

I mean is Kevlar really necessary for a recovery harness when there's other materials like nylon and corded elastic?? I think having a recovery harness that does not provide any "give" would be harder on the rocket's components and mounting attachment point and over time would cause failure to that particular stress point especially if you miscalculate the velocity at deployment and it's more likely to zipper your rocket's air-frame in the process, of course you can imagine this stress point will be greatly jeopardized if you had a heavy rocket, but of course I'm assuming most of us take the time to accurately determine the correct motor ejection / delay for each of our rockets.

I wouldn't say going without a Kevlar recovery harness is being cheap, but again really why use a material that literally will created more "shock" on your rocket? Some will argue just buy an exaggerated amount and again that might work, but how much are you willing spend. You could buy a couple motors for the price of a 30 foot piece of Kevlar.

Another thought? Maybe attaching a small amount of elastic to the chute as a leader line to absorb the shock as well?
 
A bungee cord would not make a good recovery harness. Elastic recovery harnesses can cause rocket sections to slam together when they spring back.
 
Elastic material does stretch, but it also stores energy as it stretches. That energy has to go somewhere, and it usually causes rocket parts to bang into each other as that stored energy is dissipated. Hence, the old saying "elastic belongs in your underwear not in rockets". With kevlar or tubular nylon there are tricks to mitigate "shock".
 
+1
Add to that: Proper length of your recovery harness as well as several folds along the length that are lightly bound with tape around the folds. This serves to dissipate energy as the harness is extended. Has worked very well for me for a long time.
 
The correct answer to the several questions asked by the OP: "it depends"

I have used this stuff in a couple MPR/HPR rockets

https://www.paracordplanet.com/2-5mm-shock-cord-top-colors/

It is extensible -- it elongates under tensile stress. It also exhibits a significant amount of elastic hysteresis -- that is to say it would be useless as a slingshot. It packs small, and it stands up well to flame (not fire proof, but very slow to burn).

I have only used it in rockets where the separated parts of the rocket are very different in mass -- where the recovery pops off a light and not (very) aerodynamic nose cone. I had a plan to pair it with a velcro shock absorber for this -- but I chickened out. I have a more elaborate design for this elastic+velcro harness on the drawing board (literally) right now. Fingers crossed, it will fly in April.
 
Hey thanks guys for your input. Much appreciated! I'm just always looking for different ways of doing things that may or may not always be best. I just thought I would throw it out there. What about a portion of the harness some type of low elasticity cord as jlabrasca mentioned above with maybe Kevlar as an anchor perhaps? Anyway all great responses! Thank You!
 
jlabrasca said:
The correct answer to the several questions asked by the OP: "it depends"

I have used this stuff in a couple MPR/HPR rockets

https://www.paracordplanet.com/2-5mm-shock-cord-top-colors/

It is extensible -- it elongates under tensile stress. It also exhibits a significant amount of elastic hysteresis -- that is to say it would be useless as a slingshot. It packs small, and it stands up well to flame (not fire proof, but very slow to burn).

I have only used it in rockets where the separated parts of the rocket are very different in mass -- where the recovery pops off a light and not (very) aerodynamic nose cone. I had a plan to pair it with a velcro shock absorber for this -- but I chickened out. I have a more elaborate design for this elastic+velcro harness on the drawing board (literally) right now. Fingers crossed, it will fly in April.
Click to expand...
jlabrasca said:
The correct answer to the several questions asked by the OP: "it depends"

I have used this stuff in a couple MPR/HPR rockets

https://www.paracordplanet.com/2-5mm-shock-cord-top-colors/

It is extensible -- it elongates under tensile stress. It also exhibits a significant amount of elastic hysteresis -- that is to say it would be useless as a slingshot. It packs small, and it stands up well to flame (not fire proof, but very slow to burn).

I have only used it in rockets where the separated parts of the rocket are very different in mass -- where the recovery pops off a light and not (very) aerodynamic nose cone. I had a plan to pair it with a velcro shock absorber for this -- but I chickened out. I have a more elaborate design for this elastic+velcro harness on the drawing board (literally) right now. Fingers crossed, it will fly in April.
Click to expand...

I like the cord and for the price it seems pretty reasonable. I might buy some and try it out. I think a combination of this cord and Kevlar would be a good fit.
 
Might not be a bad idea to do a burn test on anything experimental you try out. Make sure it doesn't burst into flames, etc.
 
jlabrasca said:
https://www.paracordplanet.com/2-5mm-shock-cord-top-colors/
Click to expand...
I wonder if that would make a good alternative to braided elastic for LPR shock cords. They seem to have it in a lot of sizes. Because it is round it will presumably weigh more than flat braided elastic at the same size, but then you probably don't need the same size with this stuff. Biggest question of course is durability when subjected to ejection charges.

Not sure how to calculate required size. Flat braided elastic doesn't generally come with a tensile strength rating, so hard to determine equivalence. The paracord is listed at 100 lb for 1/8", 210 lb for 1/4". And they have 3/16" which presumably would fall in the middle.
 
neil_w said:
I wonder if that would make a good alternative to braided elastic for LPR shock cords. They seem to have it in a lot of sizes. Because it is round it will presumably weigh more than flat braided elastic at the same size, but then you probably don't need the same size with this stuff. Biggest question of course is durability when subjected to ejection charges.

Not sure how to calculate required size. Flat braided elastic doesn't generally come with a tensile strength rating, so hard to determine equivalence. The paracord is listed at 100 lb for 1/8", 210 lb for 1/4". And they have 3/16" which presumably would fall in the middle.
Click to expand...

Tensile strength under static loading isn't really that meaningful a discriminator for our application.

That said, a double length of Singer oval sewing elastic has held my BT50 Wac Corporal (solid balsa nosecone) together through 5 launches -- including two on first generation Q-jets with the flame-thrower/shot-gun ejection. I will guess that the small diameter bungee would be at least as resistant to failing under jerk.
 
The whole point of Kevlar is its burn resistance. All of those elastic things are subject to burning/melting if too close to the ejection charge.
So most configurations are Kevlar at the bottom where most of the hot ejecta occurs, and then the Kevlar is tied to a much longer nylon or elastic cord higher up.
The last kit I worked on (Madcow 29mm DX3) came with some pretty heavy Kevlar that ties to nylon cord. For my other kits I stopped using Estes mounts, and started using engine mounted shock cords. I use 100# Kevlar around the base of the motor mount. It started out with this, but I make my own slightly different modifications to it: https://www.apogeerockets.com/education/downloads/Newsletter338.pdf

100# Kevlar is ... heavy kite string. I double it up, but its still pretty small. And not expensive. I think I paid about $8 for a big spool of it that will be more than enough for a long time.
Please note: all of this information relates to mid-power builds (24mm, 29mm). I'm not L1, so I don't need anything heavier than this. yet.
 
Bungee was coming commonly used in the ‘80s and early’90s. Done right it works but is bulky, heavy and wears out quickly. Not heat resistant.
 
Kevlar with kevlar sewn loops make the best shock cords. They can take a direct BP hit with no damage. They are VERY strong. I typically use 15' to 20' for 4" rockets and 25' to 40' for 6" and larger rockets. Keeps the parts well separated during descent. I would avoid anything elastic or anything that will burn.
 
Tubular nylon from LOC. Wash it after each use and it will last for years. I have a set that is about 5 years old and has MANY flights on then....still looks great. Strong and inexpensive. Even better if you use a cord protector....which I also wash each time I use it.
Andrew
 
Yes Kevlar is more expensive....but I ALSO have a kevlar set (for dual deploy) that I bought from top flight in 2013. Again, I simply wash it after each use/launch and it still looks almost new. I move it from rocket to rocket. I paid more up front but between my set from LOC and this set....I have not bought shock cord in a LONG time.....Kevlar is the best value and performance combination for your money. Just make sure you make t a bit longer that all.
Andrew
Happy New Year!!!!!!
 
Early on in my rocketry career, before I wanted to invest a great deal of money in my builds, I followed Larry Brand's advice and used paracord for my shock cords.

Now at least 11 years later, those shock cords are still in use. Bear in my mind the cord used was real paracord and not the imitation stuff sold at gas stations and Big 5 stores.
 
Cmac said:
Ooh nice! With my two boys home from school we have been/will be building lots of LP rockets. We have vowed "no more rubber bands, folded paper shock cord mounts or plastic/cotton thread chutes.
Click to expand...
I was working on my Magg this morning, so here's a picture of it. The coffee straw runs along the BT on the outside, Kevlar string tied around the motor tube. The other end ties to the shock cord, just short of zipper length.

IMG_20200327_110000801.jpg
IMG_20200327_110047522.jpg
 
In the 80's-90's it was common to use shock cord. I had many rockets that used it. One was my (about the year 1990) Aerobee 300, flew it on a cluster of 3-K550 motors. Used 5/16" dia cord. Worked just fine. To give an idea of the 300 size, the small rocket in the background pad is an EZI-65.

I use Kevlar now, that also works fine.

img151.jpg img163.jpg img154.jpg img20200121_09243372.jpg
 
AfterBurners said:
I just think the amount of money spent on a one piece of Kevlar, of course depending on size can be rather expensive and probably in most cases from what I've seen on this forum "Overkill".
I mean is Kevlar really necessary for a recovery harness when there's other materials like nylon and corded elastic??
Click to expand...

Kevlar has become dirt cheap these days.
You can buy 200 ft spool of 100lb braided Kevlar for $10-11.
That's $0.0018 per foot, or half-a-cent for a typical 3-foot rocket application:
https://www.amazon.com/gp/product/B01A9SUACA/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1&psc=1

The reason I like it is that it's strong, and it doesn't burn.
And it's dirt cheap.

AfterBurners said:
I think having a recovery harness that does not provide any "give" would be harder on the rocket's components and mounting attachment point and over time would cause failure to that particular stress point especially if you miscalculate the velocity at deployment and it's more likely to zipper your rocket's air-frame in the process, of course you can imagine this stress point will be greatly jeopardized if you had a heavy rocket, but of course I'm assuming most of us take the time to accurately determine the correct motor ejection / delay for each of our rockets.
Click to expand...

After years of flying, I haven't had a single recovery failure or airframe damage event that can be attributed to Kevlar shock cord.
That statement holds true for Estes / low-power, mid-power, and high-power rockets.
I've had nylon harness burn or singe considerably, I've had (Estes) elastic cords break completely, but Kevlar has been bullet proof (pun intended).
I now use Kevlar exclusively.


AfterBurners said:
I wouldn't say going without a Kevlar recovery harness is being cheap
Click to expand...

I would.
Optimizing 1/2 of one cent ($0.005) per low-power rocket is kinda pointless, to me.

AfterBurners said:
Another thought? Maybe attaching a small amount of elastic to the chute as a leader line to absorb the shock as well?
Click to expand...

Elastic will break.
That's how I lost multiple nose cones and rockets, until I switched to Kevlar.

YMMV.
 
I prefer nylon. Kevlar extends about 3% to break, nylon about 10%. The energy to break is about the same as the Kevlar has 3x higher loads. This requires stronger anchor points. So even though the Kevlar is stronger, the shock loads are higher. Kevlar does have better heat resistance. If you want smaller than 1/2" nylon, you can get it here https://snowshoe.com/collections/repair-relace/products/snowshoe-lacing-by-the-yard or if you need a lot :) https://snowshoe.com/products/500yd-spool-snowshoe-lacing-techdeck
 
afadeev said:
Kevlar has become dirt cheap these days.
You can buy 200 ft spool of 100lb braided Kevlar for $10-11.
That's $0.0018 per foot, or half-a-cent for a typical 3-foot rocket application:
https://www.amazon.com/gp/product/B01A9SUACA/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1&psc=1

The reason I like it is that it's strong, and it doesn't burn.
And it's dirt cheap.



After years of flying, I haven't had a single recovery failure or airframe damage event that can be attributed to Kevlar shock cord.
That statement holds true for Estes / low-power, mid-power, and high-power rockets.
I've had nylon harness burn or singe considerably, I've had (Estes) elastic cords break completely, but Kevlar has been bullet proof (pun intended).
I now use Kevlar exclusively.




I would.
Optimizing 1/2 of one cent ($0.005) per low-power rocket is kinda pointless, to me.



Elastic will break.
That's how I lost multiple nose cones and rockets, until I switched to Kevlar.

YMMV.
Click to expand...
Emma's Kites FTW (For The Win!)

I've been using their 2000lb test cordage for the past couple of years and have no complaints.
 

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