Just had a quick question on kevlar thread strength.

I'm in the process of restoring/retrofitting some LPR recovery systems and was curious about the strength of thread to use. I've been seeing that most people use the 130-135# stuff for what I'm doing. Is that an overkill? I also have some of the thinner 90# thread and that crap is hard to break. Do the ejection charges on a rocket with a motor size of C6-7 or lower really produce enough force to break this 90# stuff? Has there ever been a test on this type of thing?

It's not the force - it's the ability to withstand the repeated high temperatures and still hold up. Keep in mind that, while kevlar can tolerate high temps, it's not entirely impervious - it will degrade over time. So you want enough thickness there to maintain enough strength even after it's been degraded.

Think of it as being a frayed rope. You want the reduced diameter rope to still be strong enough to do the job.

In short, the initial strength is not the main concern. It's the residual strength after repeated ejection events that matters. Otherwise, you'll be replacing this one too soon.

I'd use the 135#, or maybe even go up a step from there (~190#, IIRC).

Doug

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Ahh... that makes good sense. I got 3 done (2 Estes Rouge's and an Aero-Hi) with the 90 that I had. I doubt they will get a lot of launches anyway. From here on out tho I will use the heavier stuff.

Thanks for the info!

What kinda temps are we looking at during ejection, just for curiosities sake?

Someone else will need to answer that

I want to say the kevlar is good to 700°F, but don't quote me on that. More importantly, it's not necessarily the temp, but the total heat transfer (ie, calories or BTUs). That is, it's not just how hot it gets, it's also how long it stays hot.

It's sorta like how total impulse is the the area under the thrust curve, but in this case, it's the area under the temperature curve. (The total heat, in calories, is the area.)

For example, hypothetically, the cord might withstand 1 second at 700°F with little damage, but be more greatly damaged by exposure to 650°F for 2 seconds. So it's a combination of peak temp and total time that affects the life of the cord. And, quite frankly, the engineering can get into 40 shades of gray, too tedious for most folks, including this engineer

I just go by feel (TLAR) and put as much cord in as I can without it being too stiff or taking up too much space. I have four or five sizes of kevlar at the house, and use whatever is available that looks like it's up to the task. That's a rough idea of how I engineer it

BTW, one key, IMO, is not overstressing the cord. When a chute opens, if the rocket is not at top dead center, the blossoming chute can impart quite a jerk on the recovery harness. You can dampen that jerk by adding some tape loops to the cord. This reduces the chance of breaking the shock cord (or getting a zipper or shredding a chute).



HTH.

Doug

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YORF member #11

Now that the war(s) are ending, where do you buy bulk braided kevlar at the lowest available cost?

BP ejection temps are about 1500 F but the exposure time is pretty short and any material has a heat soak time. Some materials like kevlar, paper, phenolic, wood, tend to char before burning and can withstand short exposures measured in seconds before being an issue.

Whether flame is present or not does not matter.

Jerry

Hell even the 90# stuff has to be better than that old thin rubber cord that Estes used don't it?

The advantage to rubber is that, since it stretches, it dampens the jerk, so it never takes the full load of a hard deployment. The down side, besides it becoming brittle over time, is that it stores the energy of being stretched out - it doesn't dissipate that energy as would the tape being torn.

As a result, there are many times when you get damage to the rocket as the stretched rubber pulls the rocket crashing back into the nosecone. I have seen plenty of dented (balsa) nosecones from this.

Doug

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YORF member #11

Rubber
Elastic
Kevlar
Cotton

Each have their benefits and procedures.

Kevlar promotes zipper/slit

Elastic is stronger and longer lasting than rubber

Rubber is dirt cheap

Cotton is slightly flexible but mainly cheap and strong and readily available.

Kevlar with a piece of shrink-wrap where it intersects with the tube or wide ribbon shaped and with masking tape held deployment loops installed on each flight is probably best overall, but for low hassle and labor elastic or cotton is the way to go.

Jerry

You could also use a removable shock cord anchor. I designed this one (built and tested by Dr. Roy F. Houchin II, see pages 6 - 8 of this issue of the Apogee Components "Peak Of Flight" newsletter: http://www.apogeerockets.com/educat...wsletter231.pdf ). This removable shock cord anchor can be retrofitted into just about any rocket, with no modification to the rocket being required. It allows the Kevlar and/or the elastic (or rubber, or cotton) cord(s) to easily be inspected for wear and degradation between flights. For two-part Kevlar/elastic or Kevlar/rubber shock cords, the use of this anchor permits the elastic (or rubber) to be tied to the Kevlar cord *below* the top edge of the body tube, which prevents the Kevlar cord from "zippering" the edge of the body tube at ejection.

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That removable shock chord anchor is way cool!