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Re: [billbooth] Deployment Injury - Perris - 23 November 2006

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I've said it before, again and again...The best "shock loading limiting device" I can think of is Dacron lines. If we all insist on jumping "no stretch" lines, these injuries will just keep happening.



Is there somewhere a chart that gives the measurement of the dynamic characteristics (for an example, a % of working elongation) of the different lines (HMA, Dacron, etc)...

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Below is my post from a different thread. I think it is relevent to this thread, since it might save someone from ending up in this forum...:

Dacron lines - they strech more than Microline and have more friction when the slider grommets slide on them. Here is Performance Designs' opinion on them (from their website):

"Why would anyone want to use Dacron line on their canopy?

Though most people prefer the lower bulk and drag of Microline or Vectran, Dacron is often the best choice for some applications. Dacron is a fairly elastic line, so it gives a little when there is a sharp "spike" to the opening force. This elasticity won't change the really good openings very much, but it can take the edge off those occasional abrupt openings where your packing was a little off or your airspeed was a little high at opening time. Dacron may be preferable in a student operation, where unusual body positions can compound opening issues. Some camera flyers with very heavy helmets also prefer Dacron lines. Older jumpers, who may not want to subject their bodies to hard openings, may want Dacron to help reduce the impact should something get a little out of control at opening time."

Also, Bill Booth made some comments on that issue, too:

"Spectra (or micro-line) is strong and tiny, so it reduces both pack volume and drag , which means you get a smaller rig and a faster canopy. Unfortunately, It has a couple of "design characteristics" (this is manufacturer talk for "problems") It is very slippery (less friction to slow the slider), and stretches less than stainless steel. This is why it hurt people and broke so many mini risers when it was first introduced. Now, I must say that the canopy manufacturers did a wonderful job handling these "characteristics" by designing new canopies that opened much slower than their predecessors. However, the fact still remains, that if you do have a rare fast opening on a microlined canopy, Spectra (or Vectran) will transmit that force to you (and your rig) much, much faster, resulting in an opening shock up to 300% higher than if you have Dacron lines. (It's sort of like doing a bungee jump with a stainless steel cable. At the bottom of your fall, your body applies the same force to the steel cable as it would to a rubber bungee cord, but because steel doesn't stretch, your legs tears off.)"

Moreover, you can opt for lighter, brass slider groomets, instead of heavier, stainless steel slider groommets. Lighter mass equals less momentum (speed) down the lines. BTW, Precision Aerodynamics favors them, saying they contribute to softer openings.

Also, on their website Performance Designs says that "larger Spectres open slower than the smaller models".

So, to sum up, my perfect choice would be large Spectre, Dacron lines, and brass slider grommets. Careful packing and deploying at no faster than 120mph (and not while tracking, especially steep tracking) will help, too. Some people also say that Psychopacking helps, but I am not sure about this.

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...Some people also say that Psychopacking helps, but I am not sure about this.


Being new to the sport and having just had my first lessons in packing, what is Psycopacking?
"I'm not lost. I don't know where I'm going, but there's no sense in being late."
Mathew Quigley

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What type of lines comes standard on tandem rigs?

On all Vector and Sigma tandem canopies, from day one, only Dacron lines. People who would never skydive, often very old, or handicapped, jump tandem. There is no justification, that I see, to risk these peoples lives, just to save a few pounds on a tandem rig.

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On all Vector and Sigma tandem canopies, from day one, only Dacron lines.



When I was packing at Skydive Hollister, I packed up several 1000 pound spectra lined 421s. Was there a short run of them?

When they came back from a re-line with dacron, the tandem masters complained of long snivels. We put so much effort in to slowing down the openings with the Spectra lines that the openings with the Dacron lines scared them.
My grammar sometimes resembles that of magnetic refrigerator poetry... Ghetto

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Arrrrrrrgh. I've had all I can stands and I can't stands no more!
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the fact still remains, that if you do have a rare fast opening on a microlined canopy, Spectra (or Vectran) will transmit that force to you (and your rig) much, much faster, resulting in an opening shock up to 300% higher than if you have Dacron lines.



This quote comes up every 6 months or so and it drives me absolutely batty!

At the risk of being the first one to stand up and say the emporer's nakid, the claim that you can get a 300% higher opening shock is based on fuzzy math.

Spectra has an enlogation of 3.5%
Polyester has an elongation of 14%
That's 300% more elongation not a 300% increase in opening shock.
If that was true, a 3-4 G opening (which is an average opening) with a Dacron canopy could equate to a 12-16 G opening with Spectra (which is no where near average).
A hard 8 G opening would equate to a 32G opening! People would be killed on opening on a fairly regular basis!

The math also doesn't factor in the transmission of force through the riser and into the harness which also elongates on opening, not to mention the elastisity of the canopy itself and we haven't even talked about weave yet.

The problem is that we know, just based on elongation, spectra line doesn't absorb as much shock as Dacron. Where the Voodoo comes in is determining how much it increases the opening shock when put into a system that also has componants that attenuate the shock load. It is difficult because there is so much anecdotal evidence of out of trim canopies being relined with dacron.

I had a Sabre with Specra relined with Spectra. The opening shock reduced greatly. That tells me my previous lines were out of whack, not that the new lines absorb shock better or are somehow magical. There are people out there who reline their canopies with dacron and when it opens softer say the dacron was the solution and don't even mention the trim was out which was why the thing was relined in the first place.

When microline was introduced, ZP was also new on the market. The old stocastic varience that gave the odd hard opening for the first 20-50 jumps until the canopy became porous was replaced with a canopy that hard the odd hard opening every 20-50 jumps for the life of the canopy. Blaming microline for those hard openings is specious.

I'm going to see if I can find a force transducer and data system and I propose that I test the shock loading on spectra vs. dacron.

Dacron my be as incredible at absorbing shock as many beleive or have only an insignificant amount of force reduction.

I agree 100% that Dacron absorbs more shock but I do not believe it to be significant enough to choose it over the advantages of micro line. Non-cascaded dacron has, afterall, been known to stay in the container after the bag leaves. I'll try to find that pic tomorrow too.
I would rather be a superb meteor, every atom of me in magnificent glow, than a sleepy and permanent planet.

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I've said it before, again and again...The best "shock loading limiting device" I can think of is Dacron lines. If we all insist on jumping "no stretch" lines, these injuries will just keep happening.



Is there somewhere a chart that gives the measurement of the dynamic characteristics (for an example, a % of working elongation) of the different lines (HMA, Dacron, etc)...



My experience with dacron has been great. I got rid of my stiletto and ordered a new 150 spectre. I had them put dacron lines on it. On my second jump I deployed and next thing I found my self having to look and see if I had a canopy, asking myself "did it open"? It was that soft. Normally I dont have to look of course. It also took about 15 jumps for the slider to start coming down all the way. In the beginning I even had a hard time getting it all the way down.

What did happen that took some investigating was that the toggle pressure was like a tandem. Prior to going to a stiletto i put 900 jumps on a 135 spectre, so i am very aware of what it should be.
The combination of hard links and the plastic cover would reduce the amount space available for the steering line when the slider grommet was resting on the plastic bumper. I dont pull the slider down past my head so the amount of clearance was to small, especially when the cat eye would try to pass thru. I was afraid of it getting "stuck" when landing. It took of bit of convincing with PD but once they saw it, i was forced to go to soft links with the "hat". All is well now. So fyi, dacron, hard links and plastic guards may not be a good mix.

hopefully not to far off topic, but thought I would share this for others...I love the softer openings with dacron and will never jump anything else. I also put it on an x's canopy for her fused neck and it worked well.

J



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On all Vector and Sigma tandem canopies, from day one, only Dacron lines.



When I was packing at Skydive Hollister, I packed up several 1000 pound spectra lined 421s. Was there a short run of them?

When they came back from a re-line with dacron, the tandem masters complained of long snivels. We put so much effort in to slowing down the openings with the Spectra lines that the openings with the Dacron lines scared them.

Yes, we did a brief experiment years ago, when "micro-Line first came out, with a few canopies. But, as you said, these were all recalled after about 100 jumps, and re-lined free of charge. This brief exposure to microlined tandem canopies proved to me that they were a bad idea.

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This is exactly what I mean.

If you change the size, it affects filling time. Throw in the fact that the Stilleto was designed in the early 90s and the spectre is more recent and you'll have an improved opening characteristic. Also, change the planform you're jumping and you'll have a different opening characteristic.

To ignore those factors and credit Dacron with the change in opening characteristics and you've got a specious conclusion. There is no eveidence that the lines are what softened the opening but many will believe that it was the answer.

I don't understand this.

I'm not trying to take away from your experience, I'm only encouraging you to question your conclusion and look for other possible answers.
I would rather be a superb meteor, every atom of me in magnificent glow, than a sleepy and permanent planet.

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>To ignore those factors and credit Dacron with the change in
>opening characteristics and you've got a specious conclusion.

Well, but keep in mind that changing to Dacron doesn't just change shock absorption characteristics. It changes trim and how trim changes. It changes friction between slider grommets and lines. It changes weight of the bag as it lifts off, and friction against the line stows. All that goes into what an opening "feels" like.

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Well, but keep in mind that changing to Dacron doesn't just change shock absorption characteristics. It changes trim and how trim changes. It changes friction between slider grommets and lines. It changes weight of the bag as it lifts off, and friction against the line stows. All that goes into what an opening "feels" like.


Ok. But how is that some reserve canopies having spectra lines, but still having acceptable openings. even though they are designed to open fast.

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>But how is that some reserve canopies having spectra lines . . .

?? You can get acceptable openings with both sorts of lines. I'm just pointing out that dacron lines are quite different than spectra - it's not just that they're more shock-absorbing.

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I'm going to see if I can find a force transducer and data system and I propose that I test the shock loading on spectra vs. dacron.



You might want to take a look here. http://www.pcprg.com/prginst.htm The link called "Riser Load System". Potvin did hundreds of test jumps like this, but all of them were to compare sliders and packing techniques, not line types.

This type of testing is hard work, and you really need the proper equipment to get good data. An alternate to doing a manned jump which requires less equipment would be an unmanned test drop with a single load cell.

But I encourage you to do it if you have the time and money, because the military and the manufacturers usually keep this data to themselves when they collect it. It would be a very worthy experiment, something almost sure to get you a speaker's slot at the PIA Symposium in 2009.

Another possibility would be static tests on the ground using weights. The load cells could be smaller and less expensive, and you could use a computer based data acquisition unit instead of a special portable data logger.

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In addition, dacron tends to hold its trim better, resulting in softer openings for longer.

I'm sure this is regarding microline...



No. Dacron holds trim MUCH better then Microlione.

Egh, sorry, I mis-quoted there. I know that dacron holds trim better than microline - my fault. My question was regarding the trim characteristics of vectran as they compare to microline and dacron... I've been told that vectran is better at maintaining trim.
I really don't know what I'm talking about.

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Well, but keep in mind that changing to Dacron doesn't just change shock absorption characteristics. It changes trim and how trim changes. It changes friction between slider grommets and lines. It changes weight of the bag as it lifts off, and friction against the line stows. All that goes into what an opening "feels" like.


Ok. But how is that some reserve canopies having spectra lines, but still having acceptable openings. even though they are designed to open fast.

It's a numbers game. Reserves aren't used that often at terminal velocity, so it will take a long, long time before we have a problem with a micro-lined reserve. Remember, no-stretch lines are only a problem in very rare situations. If your canopy doesn't have an unusually fast opening, then what kind of line you have makes no difference. A reserve use only happens every 80,000 jumps, and something around 90% of reserve openings are subtermnal. This means a terminal reserve opening only happens every 720,000 jumps...plus, not very many reserves have no-stretch lines. All this means that the chance of terminal opening on a such a reserve are well over a million to one.

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Uh...I'm sure you mistyped. There's noway reserves happen once every 80,000 jumps! 800 jumps maybe.

Also, are you calling Spectra "no-stretch" or refering to vectran and HMA? I unfortunately don't know the elongation on those two but dacron, spectra and even kevlar stretch to some degree.
I would rather be a superb meteor, every atom of me in magnificent glow, than a sleepy and permanent planet.

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I unfortunately don't know the elongation on those two but dacron, spectra and even kevlar stretch to some degree



Initial modulus (grams per denier) is 100 for Dacron, 550 for HMA, 300 for Spectra, and 600 for Vectran.
You know you have a problem when maggot is the voice of reason at the exit points

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