My Removable Kevlar shock cord mount

[McKailas Dad]

This is an idea I had to be able to easily replace my Kevlar...

It was built as a TTW, the rear CR was glued in at final assembly.

It worked flawlessly in my ~3 scratch build (the UFC), till it came in ballistic and totaled it.

At least I can re-use (all of) the Kevlar!

[edwinshap1]

all the stress of recovery is on the bolt, so i'm guessing you can't use it on a high stress rocket?

for general use this is an awesome design, thanks

[chadrog]

all the stress of recovery is on the bolt, so i'm guessing you can't use it on a high stress rocket?

for general use this is an awesome design, thanks

Looks to me that all of the stress is on the knot, which is butted up against the aft centering ring. The loop is around the bolt in case the knot is too small.

[dixontj93060]

I use this method on most of my low power kits; it works well.

[qquake2k]

This is an idea I had to be able to easily replace my Kevlar...

It was built as a TTW, the rear CR was glued in at final assembly.

It worked flawlessly in my ~3 scratch build (the UFC), till it came in ballistic and totaled it.

At least I can re-use (all of) the Kevlar!

Very clever idea, but aren't you concerned about ejection charge pressure loss through the tube?

[McKailas Dad]

Very clever idea, but aren't you concerned about ejection charge pressure loss through the tube?

I had considered that, too. It was fairly snug with the Kevlar doubled over on itself.

Unless (I just thought of this) that the 'blow-by' through the tube was the demise of my rocket?! Oh, say it ain't so.

The autopsy revealed the ejection charge did go off, but weather or not it was too long a delay, hit the ground, then ejected, is unknown.

Yes, Chad is correct. The stress is on the knot, the bolt is for back-up, although doesn't seem necessary. It's part of my retainer, anyway.

[dixontj93060]

Very clever idea, but aren't you concerned about ejection charge pressure loss through the tube?

Most every rocket I build has small pressure relief holes in each section of airframe to guard against popping an NC or payload section during rapid ascent. Small exposure through this type of shock cord mount is no worse.

[MarkII]

That tube tunnel through the centering rings is brilliant and it gives me an idea.

Why not just tie the lower end of the shock cord around the motor itself?

Construction
1. Create a short Kevlar tie to go around the motor.
2. Put a small loop in one end of a length of steel cable.
3. Pass the other end of the steel cable through a neoprene or rubber stopper, and then create another loop at that end after it emerges out of the top of the stopper.
4. Tie or loop your Kevlar harness through this upper loop.

Installation
5. Drop the Kevlar-cable-stopper combination down through the top of airframe, and with the help of a rod, push the cable through the tube tunnel through the centering rings. Alternatively, you can attach a string to the cable, pass that through first, and then use it to pull the cable down through the tunnel.
6. When the cable is pulled all the way through, the stopper gets pulled into the top of the tunnel, plugging it. The stopper should be sized to fit well, but does not have to be wine-cork tight.
7. Tie the lower end of the steel cable to the motor with the Kevlar tie.

Operation
8. When the ejection charge fires, the gas pressure will, if anything, push the stopper more firmly into the top of the tunnel, plugging it even more securely. There will be no gas leak through the tunnel.
9. When the Kevlar shock cord/harness extends, it will not pull out of the rocket because its lower end is tied around the part of the motor that emerges out of the aft end of the mount. The deployment and full extension of the shock cord may loosen or pull the stopper out of the tunnel tube, but that is OK because by then a gas-tight seal of the tunnel will no longer be needed.

Post-Flight
10. Untie the lower end of the cable from the motor afterward. The shock cord can then be extracted from the rocket so that it can be examined for damage or even used in another rocket.

[pick1e]

I did a similar thing with my low power Estes Laser Lance. I can thread the Kevlar through the motor mounts with a piece of piano wire.

I made a loop to go around the motor after the ejection charge was powerful enough to pull the stopper knot right through both motor mounts. That's why the rocket looks a little beat up now

[briannajon3]

I think that this idea really suites for the law power kits only..

[Pem Tech]

Very neat.....
Thanks for sharing guys! Will have to give this a try.

[T-Rex]

I kike it.. keeps me from having to buy a bunch of kevlar for my LPR fleet, just transfer it from one to another... Thanks for sharing!

[McKailas Dad]

It worked well for me. The motors I used in this rocket (3"dia. Scratch, ~13oz. w/ 18" nylon chute), ranged from E18 to F24, and a few of the delays were really, scary long.

I believe this (removable) setup will survive (strength wise) well beyond a zipper, or any other catastrophic failure.

That knot would have to pass through the tube, and three, epoxied, centering rings to fail. That just ain't gonna happen. I think you'll shred the chute before you have this fail.

Not to mention its gotta break the steel bolt first....

I usually used small pieces of tape to make 3 or 4 bundles of the Kevlar to take up the shock, and there were no signs of wear or 'pull through' of the knot.

T-Rex, thats part of the reason I did this, its very easy to swap between rockets...


I have my Wildman Drago set up to be built like this, too. Think I should?

Other than sharp (fiberglass) edges, how would it fail?

[KevinDunn]

That knot would have to pass through the tube, and three, epoxied, centering rings to fail. That just ain't gonna happen. I think you'll shred the chute before you have this fail.

I am looking at this idea for my Wildman Darkstar 4 DD. I think an oysterman's stopper knot would provide a very strong means of attachment, as strong as any knot anywhere else on the harness. The rear CR is fiberglass, and I can't really see the knot pulling through. My only hesitation is the thought of the knot getting hot on an HPR rocket. My rear CRs get pretty sooty. Perhaps the knot could be covered with a little JB weld as a kind of heat shield.

I also thought about pushing a bolt into the tubular kevlar, so that the kevlar grips the threads of the bolt by the fingertrap principle. The head of the bolt would be larger than the hole in the CR.

Anybody used something like this for HPR?

[Nerull]

I am looking at this idea for my Wildman Darkstar 4 DD. I think an oysterman's stopper knot would provide a very strong means of attachment, as strong as any knot anywhere else on the harness. The rear CR is fiberglass, and I can't really see the knot pulling through. My only hesitation is the thought of the knot getting hot on an HPR rocket. My rear CRs get pretty sooty. Perhaps the knot could be covered with a little JB weld as a kind of heat shield.

I also thought about pushing a bolt into the tubular kevlar, so that the kevlar grips the threads of the bolt by the fingertrap principle. The head of the bolt would be larger than the hole in the CR.

Anybody used something like this for HPR?

Putting adhesives on webbing is generally considered a bad idea that negatively impacts the strength.

If you're using kevlar, it should be fine. Kevlar decomposes at 800-900F, which would destroy the resin holding your centering rings together anyway. It takes hundreds of hours at 320F to even slightly weaken kevlar, though long term exposure to temps above 400F will weaken it rapidly. Nylon melts at 450F and should never be exposed to temps above 200F in regular usage.

For the curious, JBWeld is heat resistant to 500F.

[McKailas Dad]

Well, this rocket has been repaired for quite a while now, has nearly 30 flights, and the Mount is still holding with no issues.

That reminds me, I have to pick up more of that small tube (1/4" launch lug IIRC)


Kevin, did you try this on your Darkstar?

Layne, how 'bout you? (It may even be Llama proof! )

[Pem Tech]

Well, this rocket has been repaired for quite a while now, has nearly 30 flights, and the Mount is still holding with no issues.

That reminds me, I have to pick up more of that small tube (1/4" launch lug IIRC)


Kevin, did you try this on your Darkstar?

Layne, how 'bout you? (It may even be Llama proof! )

Between New Destruction Manuals and the back orders Haven't had the time to try the mount out yet. Maybe soon....