Kevlar shroud lines

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mkquinn

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Hey y'all,

I'm working on my first scratch-built cert 1 rocket, and I'm ready to make the parachute. I plan to go to <a href="https://www.seattlefabrics.com/nylons.html" TARGET="resource window">Seattle Fabrics</a> for some rip-stop.

I would like to use Kevlar for the shroudlines, but am unsure how heavy the thread should be. I've been looking at <a href="https://www.thethreadexchange.com/" TARGET="resource window">The Thread Exchange</a> and was overwhelmed by the selection, so I'm not sure which size to buy.

BTW - The bird is about 3 lbs, 5 feet tall with 29mm MMT and G10 fins.

cert1.jpg
 
The size #346 is 150 lbs tensile strength. They've got plenty in stock too, which has been unusual for them lately. You want the bonded...which this is. Preferably braided, which this is not. It will still work.

I recently bought a spool of braided Dacron fishing line which I will be trying on next batch of nylon chutes. Dacron, while not quite as high in heat resistance as Kevlar, will still stand up very well, and has many of the same properties as Kevlar. The braided lines will be easier to sew directly to the nylon fabric. I bought my Dacron in the sporting/fishing section at Walmart. Was around 15 dollars for <del>300</del> 500 yards. That's a lot of shroud lines. Mine is 50 lbs. since I do LPR, but it would probably hold up fine for this bird. But I know they make a higher tensile strength if that is a concern.
 
After searching the forum on shroud lines there was a lot of concern about melting, so this concerns me. Fishing line seems like a natural choice as long as you can match the strength with what is required. Is there a "tensile strength of shroud line per pound of rocket" table out there?
 
Here's a pic comparing Kevlar and Dacron. The Dacron is not your typical fishing line. Doesn't have a "plastic" feel like your typical mono filament or braided line. It's softer and more flexible, yet it still has low stretch, abrasion resistant, high tensile strength, and fairly heat resistant.

The Kevlar shown in the pic is roughly 120 lb. I have been flat braiding my shock cord leaders(LPR) with this stuff, and it works out wonderfully, although I could easily use a single strand and get away with it. I have used the same 120# Kevlar for my shroud lines up to 24 inch, and I believe it could work for chutes probably twice that size.

As I said earlier, I think the Dacron will be much easier to sew directly to the nylon fabric, and give a better "lock" once it's stitched to the fabric. There is always a chance of melting anytime your talking about nylon, but there are some that will hold up fairly well to heat. Your ripstop fabric probably has a lower melting point than the Dacron shroud lines....just something to consider.
Kevlar is some tough stuff and has a very high heat tolerance, but I think at times, especially when talking shroud lines, it's an overkill. By all means use it if you are concerned with melting... but good packing, wadding/chute protection should prevent it no matter what you're using.
 
Forgot the pic....

The Kevlar is natural, bonded, size 300, from Thread Exchange.
The Braided Dacron is Power Pro brand and is 50 lb. from Walmart.

Is there a "tensile strength of shroud line per pound of rocket" table out there?

Don't know of any, but it sounds like a good idea. Typically, the attachment points and knots will fail before the line does.

Kevlar-and-Dacron.jpg
 
Typically, the attachment points and knots will fail before the line does.

Good point. I believe it's recommended to run the shroud lines over the seams of the chute for added strength, which could help there. I like the fishing line pics, I'm not a big fisherman so I didn't realize they made line like that.

So the question is, if I use 150 lb line on a 3 lb rocket, is this overkill? I was planning a 4 shroud line chute design. I'm just having a hard time fully understanding the possible forces on a chute at high power speeds.
 
It's the G's you pull from the deceleration upon opening the chute that puts the strain on the lines. Ideally, you will be either at apogee or under drogue when the chute opens. In my limited experience, I've never seen or experienced a broken parachute line of any material.
 
That's good to hear! I'm getting the feeling I may be overly concerned about shroud strength.
 
As long as the lines are not going to burn and the attachment is secure, everything should be ok. Make sure to pay appropriate attention to the shock cord length, strength, and attachment as well.
 
IMHO you'd be hard-pressed to find a better suited shroud line than some of today's high-tech braided fishing lines like PowerPro. They are designed for ultimate tensile strength in a minimum diameter which makes 'em perfect for parachute shroud lines. They are not designed for high-heat but like Foose said, they'll hold up at least as well as the rip-stop will. So whatever you use, you're still going to need to properly protect everything from the heat of ejection gasses.

If you should go with some sort of braided fishing line, pay extra close attention to your attachment method. Most braided lines have a very slick surface and standard fishing knots (like the clinch knot) will instantly slip with most braided fishing lines - I only use the palomar knot with braided lines (like PowerPro). Here's a good off-topic index of fishing knots: linky

So if you use these lines on your parachutes, don't just sew the straight line into the chute seams. Tie a couple knots in the end of the line, then zig-zag sew the knots tightly into the chute so the knots cannot pull through.
 
That knot page in awsome, I'm saving that. I've never been good with knots.

OK, to extend the topic just a little: I'm getting great advice for shroud line material :D, but as to attachment: I was planning to crimp a grommet to the corners of the chute at the seams and attach the lines there. Any problems with this design? I'm picturing a "modified" square chute design with the seams/attachment at the corners. I'm not good at drawing so I can't provide a pic.
 
I'm not super thrilled with using grommets. I tried them on some of the first chutes I made and just wasn't comfortable with them. The grommets I used have "teeth" on one side that wound up tearing the fabric in one case. The other thing I feared was the friction of the metal wearing the shroud lines down over time or could just picture the teeth sawing away at the lines. Never happened, just speculation and worry. I know others that have used the grommets and they seem to work fine.

I was happier sewing ribbon loops to mine, but the best method is sew the lines directly to the chute. That's why I suggest some sort of woven or braided line with a decent sized diameter. It makes it much easier to sew the lines to the fabric.
 
I agree with Foose. I don't really see any benefit to using grommets - unless you just like how they look. Don't get me wrong, if that's your reasoning, then it's the right answer for you. In my opinion, they just add weight and more possible failure points and they also take up more room. So I'm not trying to change your mind, just showing you some options.

Here's a shot of the last parachute I made - a 12" hemi. The single zigzag stitch is the seam between panels, the heavy zig zag over and over and over is the stitching that holds the shroud lines in place. I put a couple knots in the shroud lines before seating them into the seam so there's just no way these things are going to pull out. I haven't tested the theory, but I would be pretty confident that a grommet would pull out of the fabric before this attachment method would fail. I also think it looks very clean.

chute_shrouds.jpg
 
I agree with Foose. I don't really see any benefit to using grommets - unless you just like how they look. Don't get me wrong, if that's your reasoning, then it's the right answer for you. In my opinion, they just add weight and more possible failure points and they also take up more room. So I'm not trying to change your mind, just showing you some options.

Here's a shot of the last parachute I made - a 12" hemi. The single zigzag stitch is the seam between panels, the heavy zig zag over and over and over is the stitching that holds the shroud lines in place. I put a couple knots in the shroud lines before seating them into the seam so there's just no way these things are going to pull out. I haven't tested the theory, but I would be pretty confident that a grommet would pull out of the fabric before this attachment method would fail. I also think it looks very clean.

It looks great, I appreciate the pic. My wife is the seamstress, so she'll be intersted in this method. She is the one dying to make a parachute for me, as long as I come up with a design for her ;)

No particular love of grommets, but the point against them are worth considering.
 
I have been using nylon cord the 1/16th size for my larger chutes. Been sewing them to umbrellas for the chutes and been having good luck with them.
the last one I set up with loops sewn to the chute. I found a cotton woven webbing at wal-mart in the sewing section it is 1/2" wide and comes in a roll
9' long. I cut it in to 3" strips folded it over and then did a box X stich pattern on it to the material and it looks like this. I got the idea from
foose4string after seeing some of his chutes.
I also have been using spiderwire fishing line for my LPR chutes. it is braided and I like the way it ties and is light weight and flexable. I have the 30# stuff.

loop.jpg
 
Good point. I believe it's recommended to run the shroud lines over the seams of the chute for added strength, which could help there. I like the fishing line pics, I'm not a big fisherman so I didn't realize they made line like that.

So the question is, if I use 150 lb line on a 3 lb rocket, is this overkill? I was planning a 4 shroud line chute design. I'm just having a hard time fully understanding the possible forces on a chute at high power speeds.

I've been using 50lb kevlar of shroud lines on 30 and 36in hemi Chutes for a very long time. I've yet to snap or pull out a single line.


I have burned off a number of tubular nylon and decon lines, so i've completely stopped using them on my homemake chutes. Because I buy Kevlar in Rolls from places like the Thread exchange and others I don't see any need to "skimp" on shrouds for my chutes. Competition chutes are now made with 13lb kevlar two stand twisted thread from 600yd rolls. At 3 cents a yard it just doesn't make any sense to burn off or break shrould lines of other materials.
Same with all my rip-stop nylon or real Silk chutes. Why waste the time on cheaper line with I don't have to;) 6 to 48" Nylon Hemis use 13, 26, 50 or 100lb kevlar line. 100lb only on model that are intended to max out that 3.3lb LMR limits.

Someone mentioned using fishing lines that are super thin like "Spiderwire" or some of the "No stretch" super lines. Well there is a Super down side to them. They ALL weaken with Age and exposure to HEAT. I love spiderwire for BASS'IN but Hate it for chutes. the slightest heat and SNAP! Kevlar is no longer used as fishing line:( Wish is was, that braided Stren 70lb yelow Kevlar was JUST Great for shock lines... If you really hunt you can sometimes score a few rolls from the bigger Fish & tackle shops in DeepSea green, but they are few and far between now.

Grommets, hard tabs and even fabric loops are just something else to break, wear thru or hang-up in the lines or tubes.
Keeping the Chutes as clean, sleek and slick as possible is the best advise I can pass on.
Sewing the lines directly into the canopy seams has always made the most sense to me. I use a simple overhand loop sawn directly into the seamd between gores to keep my shrouds permanently anchored, So far after 23+ years, they're still 100% secure.

Chute Fab-b-sm_Shroud attachment_02-10-04.jpg

X-Form-b-sm_12in RipStop Nylon_06-02.jpg

Plastic Hemi chute-c-sm_3pic Pg_01-12-00.jpg
 
What type of shoud line material do you use? How does it relate to weight and power of your rockets?

I use 1/4" flat nylon cord. The largest chute/rocket combo to date was a 12 foot parachute over a 6" X 12 foot rocket weighing in at 40 lbs.

I hope you didn't take my comment as a jab. I think its great that you have chosen Kevlar. I on the other hand did not as I believe it to be overkill.

If your recovery system is designed well, Kevlar is not needed.


1/4" Flat nylon cord:
https://www.thru-hiker.com/detail.asp?PRODUCT_ID=MG109
 
I hope you didn't take my comment as a jab. I think its great that you have chosen Kevlar. I on the other hand did not as I believe it to be overkill.

Not in the least. I've really been appreciating everyone's angle, and needed more examples from experienced flyers such as yourself. I like to hear what works and doesn't work from everyone. :cool:
 
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