Shoud line question

[xxxREKxxx]

Tell me about parachute shroud lines. (OK, Please) How does the length affect the performance. Are there any rules of thumb. You see my other big passion is hiking and climbing and seeing's how I'm such a cheap B, I make a lot of that gear also. I was looking at the huge piece of ripstop in the barn and thought, Hmmmm?
I could just measure the lines from a commercial job but I would like to know the reasoning behind the design.
Thanks,
Bob

 

[AndyC]

1.5 x the chute diameter (for round/hexagonal/octagonal) chutes is the optimum. Less than this and the chute doesn't adopt its fully-inflated (most efficient) shape. Much more than this and you are just adding bulk and the risk of tangling, for no real gain.
These rules vary slightly for polyconical chutes, where the shroud length is dictated by the angle at the skirt of the canopy. But for most rocketry chutes 1.5 it is!!

Hope this helps,

Andy.

 

[powderburner]

andyc is correct in the rule-of-thumb that he reported, but to be just a bit more specific:
1.5xD refers to the length of the exposed part of the line from the edge of the canopy to the point where the lines are gathered.
If you are making 'chutes with the lines running continuously over the top of the canopy and back down the other side then you will obviously have to add in an extra D.
The number of lines is a whole nother critter, and I don't remember seeing any recommendations or rules-of-thumb for that one. This is purely opinion/gut feel (no quantitative data for substantiation) but I think you reach the optimum number of shroud lines somewhere around eight to twelve. Less than that and your parachute begins to get a lot of distortion around the edges. More than that and you just end up adding complexity and weight. Anyone else out there have any better info?

 

[OccamMD]

Is Kapton cord what you all use for shroud lines or shock cord? If so, where can I get some? I wanted to put it on a LOC kit I just finished?

Thanks so much.

 

[shockwaveriderz]

kapton is a plastic...what you may mean and want is kevlar shroud lines....

https://www.pratthobbies.com has it in various sizesand I beleive there is a TRF member here who also sells it in various sizes....

 

[cls]

for shrouds on smaller chutes like garbage bags up to 24" or so, use "upholstery thread" from the fabric store.


if you want to make your own chute from ripstop nylon, Cliff-bob sez go for it! it's really easy!!

$20 will get you a special foot for your sewing machine that will make the job of "French overlap" hemming real easy. you don't have to end up with a ragged edge with strings falling off.

for shrouds on mid-sized chutes, you can use polyester twine or cotton. I like polyester a little better because it stretches the same as ripstop nylon. both 1/16" and "#2" (looks like 3/32"?) polyester "drapery cord" at the sewing store is quite sufficient for mid to large size chutes.

astoundingly the upholstery thread is too heavy for hemming ripstop nylon, and for tacking the shrouds on. it will rip big holes in the ripstop. instead, use nylon monofilament thread, or even regular nylon thread.

I was dubious about the heat ability of nylon thread but it seems to be OK, especially the monofilament.


hope this helps. if you make a chute then you have to post a picture, ok!??

 

[AndyC]

Best stuff for shrouds on smaller chutes is dyneema - a form of nylon line. It's very soft and thin, and incredibly strong. You can buy it on a reel from kite shops, in various strengths. Only thing to watch is that it has a very low melting-point, so needs to be well protected whilst inside the rocket. Not the cheapest stuff either, but it's what they use on NASA chutes, and on most paragliders too.

Andy

 

[xxxREKxxx]

Thanks for the answers. Those are good points to start from. I've got 18" with 6 upholstry thread shrouds in the works and I'll post pics. My wife's sewing machine is set up for the thin stuff. She'll even hem it for me sometimes.
Bob

 

[xxxREKxxx]

Here it is hanging

 

[cls]

nice hem! yer wife is a keeper

looks like you tied the shrouds through the fabric & around the hems? is it threaded through a buttonhole there, I can't see? dunno how well that will last. let us know!

I usually lay the shroud on the canopy, laying from hem towards the center, about 4", and overstitch on that. holds great and doesn't tear the fabric.

there's more than one way to skin the cat though.

 

[xxxREKxxx]

That's a good suggestion on the hem. I'll do something like that on future chutes. I put the thread through with a blunt needle so as to not rip the cloth. I'll run it on an old bird for a test first.
Bob

 

[mkmilion]

https://www.rocketreviews.com/tool_spherical_parachute_patterns.shtml


https://nira-rocketry.org/Document/LeadingEdge/LE2000MayJun.pdf

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