My Mom is a seamstress and she is making my parachutes for me. What are your thoughts on 100 lb Dacron braided line for a parachute shroud line on a 5 lb rocket ? Do I need higher strength or is 100 lb test enough. Thanks in advance.
Parachute shroud line
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rc dude
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I'm no expert, but I have heard that you should size the strength of your recovery system to handle a 50G shock. So 5*50=250lbs. You should be fine strength wise with four lines or more. I personally use 110lb strength rope that I buy in 100ft spools, but my biggest chute had 16 lines. (Overkill, but more gores looked cooler 
)
I have tried braided fishing line, but it seemed to want to tangle easily, and also tended to hold kinks. Your line may be different than what I bought though.
Let us know how it works out,
Kyle
) I have tried braided fishing line, but it seemed to want to tangle easily, and also tended to hold kinks. Your line may be different than what I bought though.
Let us know how it works out,
Kyle
HeavyLight
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The only experience I have with Dacron was making bowstrings with it. I think some big things are the size of your parachute and how many lines you intend to use. I made a 36" parachute with 8 lines of 150 lb. Kevlar thread for an approximately 4 lb. rocket. That's probably stronger than it needs to be but it has held up under a couple of rough deploys. I don't know if any of that will help you but there's my experience with constructing parachutes so far.
bobkrech
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I suggest 100 G minimum. That's a fast opening +/- 5 seconds from apogee. So for a 5 pound rocket that's >500 pounds total.
4 lines > 125 pounds, 6 lines > 90 pounds, 8 lines > 65 pounds, 12 lines > 45 pounds, 16 lines > 35 pounds, etc.
The rated value for lines is the minimum test (breaking) strength. There is no built-in design factor for lines as there is for rated ropes and lifting gear.
Bob
4 lines > 125 pounds, 6 lines > 90 pounds, 8 lines > 65 pounds, 12 lines > 45 pounds, 16 lines > 35 pounds, etc.
The rated value for lines is the minimum test (breaking) strength. There is no built-in design factor for lines as there is for rated ropes and lifting gear.
Bob
rharshberger
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I make my chutes using #95 paracord, any chute 36" or larger gets a minimum of 8 lines, overkill yes. But I haven't stripped a chute yet, my current chute that should be finished later today is a 60" with 15 shroud line (yep brain fart an odd number of gores) and I reinforce all the gore seams right where they start at the major diameter for about an inch on both sides, each line is stitched about 6" up the seam.
What kind of Dacron line do you have? Several years ago I bought a 3000 ft spool of 150 lb Dacron line from a kite store. It's a wholesale spool of the line used to raise and lower those 1" blinds you buy at most home stores. I've used it on chutes as large as 53". Those chutes were actually the skin from a golf umbrella. They have eight lines about 75" long and I've been using these as the main chute on a 10 lb rocket for about 10 years now. So far I haven't had any issues with them.
If you're going with six gores on a chute for a 5 lb. rocket, those 100 lb Dacron lines should be just fine.
What design are you using for the canopy? Is it just a hexagon flat sheet or are you going to make gores for a hemi or semi-elliptical?
BTW, I highly recommend you get a hot knife or even a soldering pencil to cut rip-stop nylon with.
If you're going with six gores on a chute for a 5 lb. rocket, those 100 lb Dacron lines should be just fine.
What design are you using for the canopy? Is it just a hexagon flat sheet or are you going to make gores for a hemi or semi-elliptical?
BTW, I highly recommend you get a hot knife or even a soldering pencil to cut rip-stop nylon with.
rharshberger
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Another trick for cutting individual gores is to use a template made of 1/8" plywood the shape of the gore and a rotary cutter on a cutting mat. Make sure and put something like self-adhesive neoprene on the cloth side of the pattern to keep the nylon from trying to move, I don't and sometimes one edge of the long seam is longer on one side, not a big deal the chutes still turn out great.
I almost always use a 3/16" or even 1/4" pressboard patterns to make my gores. The reason I use the hot knife is because is melts the edge together and prevents fraying. One of the first chutes I made was just cut with scissors and after being chewed on by the cows in the pasture it landed in (I think they liked the BP residue) I threw it in the wash machine. Every single seam frayed apart and it ended up looking more like a giant hair ball than a parachute. It went from the wash machine to the trash. Ever since then, I've always used a modified soldering pencil as a hot knife for every chute I've made. Just working with nylon pieces that don't fray is worth using the hot knife method IMHO.
It's going to be a round chute approximately 40" 3 colors cut into 6 pie pieces(rip stop nylon) . This is what I plan on using https://www.ebay.com/itm/like/400820028050?lpid=82&chn=ps
That looks like it should work fine. You may want to try this site. Twice the length for only $2.50 more. Ebay isn't always the best buy. https://goodwinds.com/line-winders/braided-dacron.html
Thanks. What kind of thread works best for rip stop nylon?
rharshberger
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A good quality nylon or polyester, doesn't have to be super heavy. My wife when she sews the panels together stitches them together then fold the seam over in one direction and stitches the seam again. FYI if you are making small chutes of less than 48" its worth buying the Thin Mill Ripstop nylon as it packs much better in smaller body tubes, my 24" 6panel made from regular weight RS Nylon is a tight fit in a BT60, my TopFlight 24" thin mill chute packs to about half the space.
Well... If you're going by the 50 gee rule then yes, you're fine with 6 lines of 100 lb. test. But if you look at the 100 gee case, you're using 600 lb. worth of line for a 500 lb. load. That's not much design margin, and basically zero redundancy. Maybe you want to consider 100 gee as having margin build in, but if it's really the load you expect then I'd suggest going to 8 lines.
It's always good to have more capability than you think you need, just in case your estimates are wrong. (That's design margin. They are estimates, after all.) It's also a good idea, when possible, to have adequate capability even with one failure, in this case one broken shroud. (That's redundancy.) So, with 600 lb. capability and an estimated 500 lb. load, you've got 20% margin, which may or may not be acceptable. But with one line broken, your left with 500 lb. capability, so zero margin.
8 lines and one failed would give 700 lb. capability for 500 lb. load, which is 40% margin after failure, and that's comfortable margin.
For those getting ready to tell me "No, the margin needs to be 2x or 4x the anticipated load," that's a safety factor, which is a different thing from design margin.
rharshberger
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That should do the job.
I guess I don'e quite get the point of this. You are using a 100 G rule to define what your design margin should be. 500 lbs for a 5 lb rocket. Why would you then design more margin into the system. Wasn't the 100 G rule what should define the margin? If you're using a 100 G rule to define the design margin, shouldn't he be using slightly smaller shroud lines to lighten things and get back to the 500 lb design limit?
As I said, I don't know the origin of the 100 gee rule, and if it has margin built in then designing for exactly 500 lb (after one failure, meaning no offense to anyone's mom) is fine. But if 100 gee is put forward as a reasonably accurate prediction of the real load, then margin should be added on top of it. In other words, if you're expecting 50 gee and design for 100 then you're fine. If you're expecting 100 gee you should design for at least 120.
rharshberger
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And people have been accused of overbuilding on here before.....lets keep it safe and sane. Maybe design and build based in the results of your sims. No point in building for more than 50G's if the rocket can't do that much.
I would agree, but simulation programs like Rocksim and OR can't realistically simulate the virtually instantaneous shock experienced by the parachute lines. I'm not saying the shock is 100 gee, I'm saying I don't know what it is, I wonder if and how any real data has ever been obtained, and when an estimate is given based on experience, what exactly the number given represents.
Here in this thread we've been told both 50 gee and 100 gee as rules of thumb, but no one has said, and I for one have no idea, whether those are two different estimates of the expected shock, two difference estimates of the expected value plus margin, or one of each. The safest assumption, for lack of any better information, is that the highest plausible figure from someone with more experience that you/I/one is an estimate of actual and to add margin. That way, however, lies madness as a new figure with margin included (like 120 gee) gets passed around until someone else doesn't know if maybe it's an estimate of actual and adds more margin, etc. For all I know, 20 gee might be a good estimate of actual shock, and even the 50 gee figure has margin upon margin from going around this loop.
What we need is data, or at least an explanation of just what the thumb rules are meant to represent.
Here in this thread we've been told both 50 gee and 100 gee as rules of thumb, but no one has said, and I for one have no idea, whether those are two different estimates of the expected shock, two difference estimates of the expected value plus margin, or one of each. The safest assumption, for lack of any better information, is that the highest plausible figure from someone with more experience that you/I/one is an estimate of actual and to add margin. That way, however, lies madness as a new figure with margin included (like 120 gee) gets passed around until someone else doesn't know if maybe it's an estimate of actual and adds more margin, etc. For all I know, 20 gee might be a good estimate of actual shock, and even the 50 gee figure has margin upon margin from going around this loop.
What we need is data, or at least an explanation of just what the thumb rules are meant to represent.
There are ways to calculate shock loads on parachutes. Check Bob Krech's post here and download the Parachute Recovery Systems Design Manual. It has an amazing amount of information and formulas in it.
If you fly DD drogueless and with a long drogue shock cord, you can calculate the actual expected max load on the recovery system by calculating the amount of force/energy in the fin can and motor assembly traveling at drop speed (75 - 100 ft/sec) and use that as the amount of force hitting the recovery system when it comes to a stop in about 0.1 seconds. Since the main chute is designed to drop the whole rocket at about 15 - 20 ft/sec, you can calculate the drop rate with just the upper section (probably 10 - 12 ft/sec) and subtract that from the speed of the fin can. In this scenario, the main slows the upper section to the 10-12 ft/sec while the fin can continues falling at the high speed decent rate. The longer the drogue shock cord, the more time the main has to slow down the upper section and the greater the relative speed differences will be. When the fin can hits the end of the shock cord, that's the actual max force you can expect. You can add margin to that when you design the recovery system.
Most smaller rockets in the 10 lb or less range have quite a bit of margin in their recovery systems already because of the selection of relatively larger components. This is demonstrated by how few fin cans break loose on DD flights when the main opens below the fin cans and they fall a long ways and snap to the end of the shock cords. As the rockets get bigger and heavier, the margins drop and it becomes more likely there will be recovery system failures.
The simplest and lightest way to reduce this shock load is to shorten the drogue shock cord and use a drogue chute that keeps the fin can below the upper section on the way down so there is no hard drop and stop of the fin can after the main opens. That would prevent the free falling fin can from happening, but you still might want to use those figures to calculate the size of the recovery system as your margin.
If you fly DD drogueless and with a long drogue shock cord, you can calculate the actual expected max load on the recovery system by calculating the amount of force/energy in the fin can and motor assembly traveling at drop speed (75 - 100 ft/sec) and use that as the amount of force hitting the recovery system when it comes to a stop in about 0.1 seconds. Since the main chute is designed to drop the whole rocket at about 15 - 20 ft/sec, you can calculate the drop rate with just the upper section (probably 10 - 12 ft/sec) and subtract that from the speed of the fin can. In this scenario, the main slows the upper section to the 10-12 ft/sec while the fin can continues falling at the high speed decent rate. The longer the drogue shock cord, the more time the main has to slow down the upper section and the greater the relative speed differences will be. When the fin can hits the end of the shock cord, that's the actual max force you can expect. You can add margin to that when you design the recovery system.
Most smaller rockets in the 10 lb or less range have quite a bit of margin in their recovery systems already because of the selection of relatively larger components. This is demonstrated by how few fin cans break loose on DD flights when the main opens below the fin cans and they fall a long ways and snap to the end of the shock cords. As the rockets get bigger and heavier, the margins drop and it becomes more likely there will be recovery system failures.
The simplest and lightest way to reduce this shock load is to shorten the drogue shock cord and use a drogue chute that keeps the fin can below the upper section on the way down so there is no hard drop and stop of the fin can after the main opens. That would prevent the free falling fin can from happening, but you still might want to use those figures to calculate the size of the recovery system as your margin.
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