shock cord- kevlar plus rubber?

[bjphoenix]

1. I've watched rockets with long kevlar or nylon shock cords- the nose cone comes off with some velocity but it plus the laundry slows down before hitting the end of the shock cord.
2. I've launched LPR with stock rubber shock cords where the nose cone went to the end of the cord, stretched it out, then snapped back and damaged itself and/or the body tube.
3. I launched one of my own rockets with relatively long kevlar and a lightweight nose cone that reached the end of the cord and failed the snap swivel. That rocket was using a streamer and it didn't slow down the nose cone enough.
I've thought about this before but now I'm building an Estes Bullpup with a good amount of weight in the nose cone. I'm concerned that the nose cone may have high velocity when it hits the end of the shock cord. I'm wondering if it would be worthwhile to use a long piece of kevlar but also putting a normal Estes rubber shock cord in the system. If so then the next question would be what is the best way to connect a rubber cord to a thin kevlar cord.

 

[teepot]

Ditch the rubber and use just Kevlar. My guess would be about 10'. Longer is always better. Put your chute two or three feet from the nose cone.

 

[SolarYellow]

Some have suggested putting a length of elastic in the shock cord as a limiter, but still rely on the strength of the Kevlar. So tie the elastic to two points on the Kevlar shock cord that are farther apart than the length of the elastic. It will stretch the elastic to slow down the nose cone more gently, but ultimately, the Kevlar is holding both pieces together.

Probably even better is an energy absorption method that doesn't return the energy like a spring. Lots of people like to z-fold the shock cord into multiple small bundles and tape those with blue masking tape. Some do it, but use small rubber bands.

 

[mh9162013]

Use a long piece of Kevlar...maybe something that's 5-7 times the length of the rocket.

Or try using a shorter piece of Kevlar...maybe something that's 3-4 times the length of the rocket, but has a bit of elastic built in, but only in a way where when the elastic is fully stretched, the bulk of the force will be exerted on the Kevlar, like this:



Where the red is the Kevlar and the blue is the elastic. I believe this is what @SolarYellow described in his above post.

 

[cbrarick]

solar yellow is right, z folds and tape on kevlar only
meets the kiss principle
I'm not a fan of complicated or having multiple potential failure points

 

[bjphoenix]

Where the red is the Kevlar and the blue is the elastic. I believe this is what @SolarYellow described in his above post.

I've also thought of this and it would be easy to do.

solar yellow is right, z folds and tape on kevlar only
meets the kiss principle
I'm not a fan of complicated or having multiple potential failure points

I might experiment with this but it would be a little tedious to put together with 150# kevlar.

Ditch the rubber and use just Kevlar. My guess would be about 10'. Longer is always better. Put your chute two or three feet from the nose cone.

That's what I do now. For my rockets up to BT70 I use 10' length of 150# kevlar. These rockets average around 2' long so that is in the ballpark of 5 times the rocket length. I clip the parachute to the nose cone. Almost always at deploy the chute opens fairly close to the rocket, slows down the nose cone, the body continues to fall as the cord unrolls as if there was a separation then everything comes down together. I think the key thing is having the chute connected so it slows down velocity of the nose cone. I did a launch a couple of years ago with the streamer connected to the shock cord with a loop allowing it to slide freely along the cord. There was nothing to slow down the nose cone so it continued traveling at a good velocity until it hit the end of the cord and failed the snap swivel. This was with the fairly light nose cone that Estes uses for BT60. I was thinking I might have a much more severe condition with my Bullpup nose cone with about 0.4 oz added weight.

 

[lr64]

If rubber can survive the ejection charge, so can nylon. Or maybe even lycra, A stretchy cord doesn't have to be as strong as a rigid one like Kevlar, and puts less load on the attachment points. And it's less tangly and complicated than a rubber piece spliced in. Rather than doing the latter, a bit of bungee cord would be more reliable, but I'd bet not as reliable as nylon. Maybe a foot or two of kevlar, to withstand heat, tied to a generous length of nylon? Maybe using a sheet bend with an extra turn on it, with heat shrink over just the area where the knot is.

Recommend braided nylon seine twine. Not because I've used it in a rocket, but because I've seen it take a lot of abuse. You can get it up to 600 lb test! However, after a safety factor, it shouldn't be any stronger than the rocket and the attachment points, because then it will stretch less and stress everything else more.

For all I know, there's a new, stretchy synthetic that can take higher temperatures and stretch even more than nylon (40 percent, I think, when new).

P.S. We used to get our winch line from Memphis Twine.

 

[teepot]

If you use a long enough Kevlar shock cord the nose cone will be barely moving when it gets to the end of it's length. Anything elastic will cause the nose cone to snap back if it's not long enough too. But it's your rocket fly it the way you want. I would add that a swivel on the shroud lines where it meets the nose cone will stop the chute from twisting it's self into a knot. My suggestions are just suggestions.

 

[Colorado Mike]

My Bullpup has an all kevlar cord that measures 40 times the diameter of the rocket. I attach the chute about 10 inches from the nose cone. Have not had any issues at all.

 

[ep29030]

With Kevlar on HPR, I do Z folds, then put rubber bands over the folds, so they get broken as the Kevlar unfolds, and bleed off some of that ejection energy. You might consider having the nosecone come down on its own parachute.

 

[lr64]

If you use a long enough Kevlar shock cord the nose cone will be barely moving when it gets to the end of it's length. Anything elastic will cause the nose cone to snap back if it's not long enough too. But it's your rocket fly it the way you want. I would add that a swivel on the shroud lines where it meets the nose cone will stop the chute from twisting it's self into a knot. My suggestions are just suggestions.

There's elastic and there's elastic. Braided nylon doesn't return nearly as much of the energy as rubber or nylon monofilament would. Of course, monofilament is really annoying in other ways, too.

 

[Daddyisabar]

What matters is that you look really really cool using manly high power materials and techniques in your little LPR Bullpup. Can you crochet your super long kevlar line to get it to fit? Oopsie, there goes your cool card.

Oh well. I have seen and done many horrible things. Tying two god awful Estes rubber shock cords together to get more length, worked so many times but soooo uncool. Using that C6 7 purple lable motor because it was the biggest number and must be the most powerful. Flying in wind with no spill hole, getting the dreaded Estes smile that amused the old dudes. The horror.

It's a hard knock life for the LPR flyer. Back to taping broken shrould lines and plastic chutes, messy baby powder and packing dog barf. Being seen in the fabric department at Hobby Lobby buying braided underwear elastic. Tri fold mounts. Cool card long gone. Never to return. So .

 

[K'Tesh]

And then there's my little essay on the value of using rubbers...

 

[Rocketholic]

@bjphoenix:

Been doing a bit of reading here and there and would like to offer the following for LPR motor ejection (A-E typical Estes motors with a 16 oz or less rocket):
1) Make sure your nose cone is a "soft" fit in the body tube. If it's a snug or tight fit you risk it not coming off at all or acting like a pistol round. I like mine able to fall out or close to it if I turn the rocket over.
2) Yes, longer cord is better. Also, a flat cord offers a little more wind resistance upon deployment.
3) For the modified/non-stock "kits" that I have I do like to use the rubber band they put in the kits. I attach it to the end of the several feet of Kevlar chord hanging out of the rocket if I'm not using the stock fold-and-glue. The gumby does help with minimizing the shock load to the attachment. This however depends on how well your ejection charge is timed with the apogee of your trajectory. High speed deployment: all bets are off!
4) 1/4" elastic cord works great too. I had an E26 powered good sized rocket (~17 oz) that used Kevlar cord with a few feet of elastic cord on the end and a 28" nylon chute at the end of that. I watched it "boing" a few times when the chute fully opened, and the elastic cord did what it was supposed to do. Worked great.

I realize I'm new to this forum but figured my experiences would at least be decent data points.

Once you get into heavier builds and larger motors the size/length of the shock cord and methods to mitigate shock load become more important.

 

[kjhambrick]

<<snip>>

For all I know, there's a new, stretchy synthetic that can take higher temperatures and stretch even more than nylon (40 percent, I think, when new).

@lr64 --

Are you saying there is a new synthetic line that is more stretchy and withstands heat better than nylon ?

Do you know the name of the material ?

Thanks

-- kjh

 

[4regt4]

I've used a parallel Kevlar/elastic setup on all of my HPR rockets. The elastic maxes out at twice it's relaxed length (I.e. a 3ft length of elastic stretches out to 6ft). I bundle/z-fold the Kevlar into smaller bundles with either tape or small rubber bands holding them, and bundle the elastic separately. I generally do 3ft of elastic paralleled with 6ft of Kevlar, then another 6ft of Kevlar.

Here is where it surprised me: Not all of the Kevlar bundles pull out of their wrapping. Meaning that the elastic doesn't stretch all the way out and is doing all the work. I figured that the elastic would slow things down as it stretches out to its max, and then the Kevlar in parallel takes over. Not so.

Over 50+ flights done this way, I've had no failures. And I've had some Aerotech "bonus delays" where the ejection was 4 or 5 seconds after apogee. Never had a zipper.

On MPR, mostly out of laziness, I do Kevlar -> elastic -> Kevlar. Not in parallel. It's worked well. And, no, I've never had any sort of "snap back" damage.

On LPR, it's often Kevlar to just out of the body tube, then whatever rubber or elastic is supplied with the kit for the rest of the distance. Because it's easy.

Hans.

 

[lr64]

@lr64 --

Are you saying there is a new synthetic line that is more stretchy and withstands heat better than nylon ?

Do you know the name of the material ?

Thanks

-- kjh

Nope. I was saying that I didn't know there wasn't. I checked the melting temperature for Lycra, though. According to the following source, Lycra melts at 480 degrees F.
https://www.aeroskinworldwide.com/p... fabrics can,is comprised of 85% Polyurethane.
This source says 425 for Nylon, but I've seen higher numbers quoted. Not sure if the same flavor used for rope and twine, though. Note that the "service temperature" is lower. I assume the same applies to Lycra.

This paper has somewhat more pessimistic data for Lycra, so maybe we're not there yet:
https://www.academia.edu/48379728/Thermal_properties_of_elastic_fibers

E-glass has something like twice the elongation at break as Kevlar does, and S-glass has more than that. You can get braided tapes and even, I think, twine, but I don't know how they hold up to mechanical abuse.

 

[lr64]

What matters is that you look really really cool using manly high power materials and techniques in your little LPR Bullpup. Can you crochet your super long kevlar line to get it to fit? Oopsie, there goes your cool card.

Oh well. I have seen and done many horrible things. Tying two god awful Estes rubber shock cords together to get more length, worked so many times but soooo uncool. Using that C6 7 purple lable motor because it was the biggest number and must be the most powerful. Flying in wind with no spill hole, getting the dreaded Estes smile that amused the old dudes. The horror.

It's a hard knock life for the LPR flyer. Back to taping broken shrould lines and plastic chutes, messy baby powder and packing dog barf. Being seen in the fabric department at Hobby Lobby buying braided underwear elastic. Tri fold mounts. Cool card long gone. Never to return. So .

How could I have missed this post? Thank you for making me smile.

Wasn't the Estes "shock cord" just model airplane rubber? Or maybe they moved on to something else after that. The pertinent facts about model airplane rubber being that it could stretch a whole long ways, and that it ages and wears. Gluing it in the way they had you do was an admission that the rocket wasn't expected to last very long.

That fabric elastic goes bad, too. The other day, when my favorite belt needed another hole, I dug up an old pair of suspenders. Within an hour, my pants were down around my knees.

Anyway, the phrase "Kevlar shock cord" seems like an oxymoron to me.

If someone wanted to go high tech, I could imagine a telescopic shock cord using hollow line with another line stuffed inside it, with lots of friction between them and some way to limit the travel. Left to the reader is how to keep it from grabbing like one of those finger traps, and how to reset it. One could also imagine a little reel with a device to set the drag as is done with fishing reels. I wonder if it wouldn't be just as light as the heavy Kevlar required to withstand the loads that cause zippers.

 

[bjphoenix]

If you use a long enough Kevlar shock cord the nose cone will be barely moving when it gets to the end of it's length

Yes but for a 1.3" diameter nose cone that weighs 1/2 ounce how long would that need to be. I know that a much lighter 1.6" diameter nose cone needs more than 8'. (The 1.3" nose cone is heavier because it has weight added to it.)

What matters is that you look really really cool using manly high power materials and techniques in your little LPR Bullpup. Can you crochet your super long kevlar line to get it to fit?

I can and sometimes do that but it isn't foolproof and is a bit fiddly for me to do at the launch field if there is a little bit of wind. For 1.6" diameter rockets I use a spool that I made and wind the 10' cord around it. It seems to be mostly reliable.

 

[Azamiryou]

If someone wanted to go high tech, I could imagine a telescopic shock cord using hollow line with another line stuffed inside it, with lots of friction between them and some way to limit the travel.

Or the same idea, but low tech: anything that adds friction to bleed off energy while letting the cord spool out. Maybe a super munter hitch on a D ring? Or two cords tied around each other like a sliding knot adjustable bracelet? Or even just a few wraps around a D ring? Or — so it doesn't have to rub a surface — a wrap around a roller with adjustable resistance.

Quicker to reset than crochet or Z folding. But would it quickly wear out the shock cord?

Might be worth experimenting with.

 

[teepot]

Yes but for a 1.3" diameter nose cone that weighs 1/2 ounce how long would that need to be. I know that a much lighter 1.6" diameter nose cone needs more than 8'. (The 1.3" nose cone is heavier because it has weight added to it.)

I can and sometimes do that but it isn't foolproof and is a bit fiddly for me to do at the launch field if there is a little bit of wind. For 1.6" diameter rockets I use a spool that I made and wind the 10' cord around it. It seems to be mostly reliable.

Even for a rocket that small and light I would still use Kevlar rather than rubber or elastic. When I got back into rockets I used elastic for a while. Then I discovered Kevlar. Kevlar takes up less space than elastic and now every rocket has a Kevlar shock cord regardless of it's size or weight.

 

[lr64]

Even for a rocket that small and light I would still use Kevlar rather than rubber or elastic. When I got back into rockets I used elastic for a while. Then I discovered Kevlar. Kevlar takes up less space than elastic and now every rocket has a Kevlar shock cord regardless of it's size or weight.

The smaller and lighter they are, the less likely they will be damaged by shock loads by "shock cords" that don't stretch.

 

[teepot]

never mind.

 

[bjphoenix]

The smaller and lighter they are, the less likely they will be damaged by shock loads by "shock cords" that don't stretch.

I've noticed that with a parachute attached to a nose cone that is not weighted the nose cone stops before getting to the end of my 10' cord so there is no shock loading on the cord. I was wondering what to do with a weighted nose cone that is now 4 times the weight. I haven't had enough experience with streamers attached to the nose cone to know if they are significantly different with an unweighted nose cone. If the streamer is not attached to the nose cone that is when I've had the problem. Let me add- this is all with reasonably accurate delays. If the delay is pretty far off it might be a different story.

 

[Pem Tech]

NO! No rubbers! They degrade and inevitably fail. We use five feet of KEVLAR thread in all our LPR kits. I use even more in some of my larger personal builds. This gives the NC and recovery setup time to slow enough to not cause damage (rebound etc.). To prevent zippering wrap a bit of tape around the cord where it contacts the edge of the BT.
Ages ago I grew tired of prepping a rocket for launch and finding the rubber SC was completely useless.
You will find no rubbers in our workshop.

 

[SolarYellow]

E-glass has something like twice the elongation at break as Kevlar does, and S-glass has more than that. You can get braided tapes and even, I think, twine, but I don't know how they hold up to mechanical abuse.

Off the top of my head, the elasticity and strength of Nomex compared to Kevlar are roughly in inverse proportion. So although it has lower strength, Nomex has the stretchiness to keep peak shock loads down, and flame resistance is similar between them.

 

[DirkTheDaring]

There is no such thing as too long a shock cord.

 

[Onebadhawk]

Yes please.
Use rubber bands for shock cords..
lol.

When you're done with that I sell my 1/8" Tubular Kevlar cheaper then Emma kites on ebay.

Teddy

 

[OverTheTop]

I tend to put a small pilot chute on the NC. Once the nose is off the pilot pulls out the main.

 

[Ronz Rocketz]

Use enough Kevlar that the rocket reaches the ground before the chute deploys.

Seriously, Kevlar is awesome. However, I must have two left thumbs because half the time with LPR, the Kevlar turns into a rat's nest. Elastic doesn't ball up on me like Kevlar. That said, I only use Kevlar with MPR because I've snapped the elastic on a Star Orbiter.