black powder on kevlar

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watermelonman

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I am starting to work on my first dual deployment system. Is there any issue at all with black powder firing in a compartment with my kevlar shock cord? I imagine not, since I never even worried about elastic shock cords with direct motor ejection.
 
That's pretty much the point of kevlar in shock cords. Yes, it's stronger/lighter, but its used as much for fire resistance as anything else. Technically, you CAN burn/wear through it, but in most rocket applications it will hold up to ejection blasts just fine. If you've never had an issue with elastic shock cords burning, you REALLY have nothing to worry about with kevlar. A given kevlar cord will last dozens if not hundreds of times longer than a similar sized elastic cord.

s6
 
That's pretty much the point of kevlar in shock cords. Yes, it's stronger/lighter, but its used as much for fire resistance as anything else. Technically, you CAN burn/wear through it, but in most rocket applications it will hold up to ejection blasts just fine. If you've never had an issue with elastic shock cords burning, you REALLY have nothing to worry about with kevlar. A given kevlar cord will last dozens if not hundreds of times longer than a similar sized elastic cord.

s6

Perfect. Thanks!
 
As others have said Kevlar is light-years better then straight elastic, nylon or other lines. BUT and a Do Say BUT! Kevlar is only Heat Resistant. it degrades a little each time it is subjected to temperatures exceeding 900°f, Our BP motor Ignition temp is 909°f and goes up from there. Not to mention the long after burn of most BP motors AFTER the ejection charge has expended.

There are a couple things that can extend the lift of our Kevlar Motor mount tied Shock-cords.

* Always route the Motor mount wrapping Kevlar line to the OUTSIDE edge of the model Forward centering ring wedged between Inner airframe body tube and centering ring edge. This at least moves the Kevlar as far away from the ejection burst and after flame as practical.
* A ablative or heat retarding sleeve can be added to the first 3 to 6" of the shock-line, Nomex or Fibreglass braided sleeveing work really well or a cut off Q-Tip stem will protest the average LPR model 70-100lb Kevlar for many flights.
* Braided rather then Twisted Kevlar has a bit better chance against the heat effects. Still Degrades but the braid helps limit the damage locally.
* Use a slightly heavier Kevlar braid then needed. Personally I now use 70lb-kevlar in micros, 70-100lb in LPRs and 90-130lb in MPR's up to about 3.3lbs to add a little extra life to the lines.

* Most important change I've made in the last decade: Add Stainless Steel beading wire, Small diameter aviation cable or braided stainless fishing leaders to my rocket motor mounts as a Flame Proof shock-cord anchor. Generally these anchor lines only need to be 6-10" long to take most of the heat away from the Shock-line. Spliced eye to eye fishing leaders in 30lb strength are generally heavy enough for even MPR models up to 3.3lbs. .018" dia. 49strand Stainless beading wire with tiny crimp sleeves make loops for 3 to 5" micro models. 7x19strand .024" and .038" Stainless aircraft cable (twisted Wire Rope) with crimp sleeves for LPR and MPR models. All can usually be transferred to new builds as these shock-Cord anchor systems are pretty much indistructable.
 
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Just to add, I take my DD kevlar cords and hand wash them using dishwashing detergent, flush with clean water, then hang to dry in the shade. Helps to keep them clean, and reduces the smell as I store my rockets in the house. Which, incidentally, is another reason I like fiberglass rockets is I can hose them out to clean the BP residue and burned dog barf off.
 
kevlar is quite flame resistant unless it gets a contaminate on it: discovered on a ground test that where a bit of rocket expanding foam had gotten on the kevlar that location on 700# line was about 80% severed by burn through.


Sent from my iPhone using Rocketry Forum
 
kevlar is quite flame resistant unless it gets a contaminate on it: discovered on a ground test that where a bit of rocket expanding foam had gotten on the kevlar that location on 700# line was about 80% severed by burn through.

Sent from my iPhone using Rocketry Forum

Did the foam burn and create more heat? or you think some chemical reaction between foam and kevlar accelerated by the heat?
 
There is a reaction that occurs- I am not qualified to explain it. A little bit of Public Missiles Expanding Foam got onto my Kevlar harness by mistake. Did not think it would matter. I ground tested black powder charges and got a surprise. No damage nearest charge- just soot, but if you look in far left of photo the cord is nearly burnt through where the foam was. This cord would have failed.
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What happened is probably quite simple. When a charge goes off, portions of still burning powder fly. One or more of the hot burning particles probably hit the foam. Then it was held up against the kevlar and nearly burned it through.

It is not the hot gases that do in most recovery systems. It is the burning particles. Trap the, and even the nylon Shute can deal with the short burst of heat as the gases blow by. That is why dog barf put in loosely works. The barf stops the particles while letting the expanding gases go by.


Kirk
 
What happened is probably quite simple. When a charge goes off, portions of still burning powder fly. One or more of the hot burning particles probably hit the foam. Then it was held up against the kevlar and nearly burned it through.

It is not the hot gases that do in most recovery systems. It is the burning particles. Trap the, and even the nylon Shute can deal with the short burst of heat as the gases blow by. That is why dog barf put in loosely works. The barf stops the particles while letting the expanding gases go by.


Kirk
Tangentially, this is also the reason why you want to use the finer 4FG BP instead of coarser grains, or BP substitutes or smokeless powder. Smaller particles burn out faster, but bigger particles and slower chemistries burn longer. Big particles that hang around longer and stick to the recovery system components will cause more damage because of the increased contact time.

Bob
 
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