[F-FT] A new anti-zipper design

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OK, I mentioned in another thread that I had just launched a slightly
modified PML D Region Sandhawk yesterday. Here are some details on the 'slight modification' -- which I'm really pretty jazzed about. Bear in mind that this was the first flight of this design and I'm planning to do a number of follow ups, and hope to write up an article for Sport Rocketry or some such magazine.
I had a launch a couple of months ago of my Black Brant Vb, where there was about a 1" long zipper on the tube. I was pretty surprised to see a zipper on the PML Quantum tubing, but I cut the last inch off the rocket and it still flew fine, so no real harm done. However, the zipper got me thinking about what I could do to prevent it, so on the long drive back from the launch I did some thinking. Once I got home, I had a design in mind, but I then did a google search on the anti-zipper methods to see a) what other folks do, and b) if my method had been used before. I wasn't thrilled with the other anti-zipper methods I'd found, and I didn't find my design anywhere else, so I continued with it.
Bear in mind that this method is designed only for piston ejection systems -- which is one of the reasons I really like the PML line of rockets.
My major complaint about the existing anti-zipper methods is that you must drill holes for the ejection gases at the front of the new 'bulkplate', and then must hope that the parachute gets PULLED out of the top end of the rocket. I don't like the possible problems that can introduce, and in my mind you still have the possibility of the zipper occurring from the OTHER end (in the upper tube). Further, you then have to protect against the exhaust gases with a chute protector, which is one more thing to add to the mix.
Here is the 'basic' description of my design (followed by details): I lengthen the piston, cut slots in it, and then shorten the attachment cord so that the last half-inch of the piston cannot come out of the tube. When the ejection charge goes off, the piston is (as normal) blown out the front of the tube, but as the slots emerge from the end of the tube, they vent the ejection gas off to the side. The piston at that point has completed the task of driving the recovery system out of the body tube, and since it stops right at the top of the body tube, it (at that point) acts JUST LIKE the existing zipper systems, in that the stress of the cord is then place around the entire perimeter of the body tube.
An added note is that since the slots run up the side of the piston, it also acts like a shock absorber -- because as the first part of the piston emerges, the gas is at higher pressure. As the piston continues out the end, the pressure is getting lower, so by the time that the piston reaches the end of its travel, the gas pressure has been reduced.
Now, for more details. Instead of the standard piston, I used a tube coupler, which is five inches long. I had four 1/4" wide slots cut the length of the coupler at 90 degrees from each other. This gives a total 'surface area' for the opening of 4 square inches. This is equivalent to a body tube opening of about 2.25", so the area for case to escape is fine.
Additionally, I used a motor-mount centering ring (sized for the coupler) at the bottom of the new 'piston', in order to assure that there's a little more structural strength at the bottom of the piston. Instead of the standard nylon strap that comes with the kit, I used wide tubular Kevlar (which I do normally anyway).
As part of the modification, I had to glue the strap a little bit different than the normal kit instructions call for it. The Kevlar strap is fed through the bottom of the hole to the top, then the D-ring is put on, then the other end is folded back through the hole to the bottom side. The end of the strap is then epoxied to the bottom side of the piston plate -- but note that the difference is that you need to make sure that no epoxy gets into the hole that the strap feeds through.
At this point, the rocket could be used 'normally', in that the piston would eject from the tube, and hang from the Kevlar strap. The next step, though, is what makes it work -- pull the strap of Kevlar up through the center slit, and make sure it's 'tight'. Move the piston such that it has the last 1/2 inch in the top of the body tube (even with slight stretch of the Kevlar), then TIGHTLY tie-wrap the Kevlar where the two pieces come out of the top of the piston. Done correctly, this will prevent the piston from traveling out of the tube any farther. Place the D-ring at the halfway point of the 'loop' of Kevlar that is now coming out of the top, and then tie-wrap it in place, as well.
The beauty of this method is that after the flight, you can cut the tie-wrap, remove the piston, and clean out the built-up crud from the body tube for the next flight.
I should add that I've played around with several methods of attempting to shorten the Kevlar, but the tie-wrap method is both the simplest and 'best' that I've found so far.
My next test flights will experiment with trying to intentionally cause zippers, by using very short (and/or very long) delays on higher-power motors. I'll keep folks posted on the results...
David Erbas-White
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David Erbas-White wrote:

Your technique sounds very familiar to what I tried with a few BT50 modrocs many years ago. I would use a balsa piston/bulkhead that would slide between the MMT and a coupler glued into the top portion of the BT but enough room for the NC shoulder. The shock cord was attached to the top of the balsa piston with a long eye screw that went all the way through the piston for strength. I then made a blow hole in the BT right below where the piston would end up at after deployment. Basicaly it's a floating piston.
On my next PML build I'll have to try your HPR technique out.
Ted Novak TRA#5512 IEAS#75
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Hello David
There was an article on this in an old copy of Tripolitan back around 1987 or 1988 when I first joined Tripoli. If I can find a copy of the article I can scan for you. This guy used the same method only he used holed in the side of the coupler just beneath the bulkhead and let it extend out an inch or so just to vent the ejection charges.
Steve __________________ Steve Naquin TRA 677 L2
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On Mon, 18 Jul 2005 12:23:18 -0700, David Erbas-White

I've done that with a few variations. I used holes at the top if the piston instead of slits. I fiberglaseed the piston because (like you) I was concerned about its strength without he fiberglass. I used steel cable to restrain the piston but I think I might like your method better.
I've tested it with short delays and one or two cases of blowby (thanks for not saying to only use your delays with your motors and closures Jerry) and it survived.
The bottom picture shows it under chute you might be able to make it out. http://www.para520.org/docs/Pic_Pages/Pic_Misc.htm
I think I posted it to ambr once too.
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Phil Stein wrote:

Note that with the method I've outlined, the actual major 'stress' on the shock cord comes from the one end at that is epoxied to the motor mount, and the mid-point of the shock cord where the D-ring is tie-wrapped. There is minor stress from the D-ring to the piston face, but the reason for the strengthening is more for the 'potential' for side-to-side stress. It's probably not strictly necessary, but since I'm not a mechanical engineer I decided that overkill is the best way of handling it <G>.
David Erbas-White

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Can you email a sketch of the cord attachment to the piston?

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Can you email a sketch of the attachment to the piston?

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No, because my drawings are not even legible to myself... <G>
I'll try and take some photos later and post them.
David Erbas-White
Chad L. Ellis wrote:

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On Mon, 18 Jul 2005 14:11:19 -0700, David Erbas-White

I did 2 interlocking U bolt sone on top and one on the bottom.of the piston. I attached two cables to each of the two ubolts attached to the top centering ring which was also glassed.
Conseptually, it sounds like pretty much the same thing. There are some differences in the implementation.
One thing I'd suggest is make sure you have enough area opened up in the piston. I don't know how big your slits are. To me the word slit means they're pretty narrow but that might just be phrasology. 8-)
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I thought of the same exact thing before too, but never too seriously. I stopped short of trying to figure a way to vent the ejection charge.
Cool idea, I hope it works out!
--
Joe Michel
NAR 82797 L2
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David, I have done this type thing on a couple of PML kits and works great. It must be a very strange place inside your mind if your brain works like mine. LOL. Seriously though I was shown this idea by someone else I wish I could give him the credit but I forget his name. I thought of fiberglassing the inside of the piston also to add strength but have not gotten around to trying that yet.

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Hello David
There was an article on this in an old copy of Tripolitan back around 1987 or 1988 when I first joined Tripoli. If I can find a copy of the article I can scan for you. This guy used the same method only he used holed in the side of the coupler just beneath the bulkhead and let it extend out an inch or so just to vent the ejection charges.
Steve __________________ Steve Naquin TRA 677 L2
David Erbas-White wrote:

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Hello David
There was an article on this in an old copy of Tripolitan back around 1987 or 1988 when I first joined Tripoli. If I can find a copy of the article I can scan for you. This guy used the same method only he used holes in the side of the coupler just beneath the bulkhead and let it extend out an inch or so just to vent the ejection charges.
Steve __________________ Steve Naquin TRA 677 L2
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David
I scanned the article for you. Sorry but I'm new to this group and I wasn't sure how to post an image so I am sending you a link to each of the two pages.
http://216.77.188.54/coDataImages/p/Groups/215/215922/folders/156726/1616190TripolitanDec1988Page20.jpg
http://216.77.188.54/coDataImages/p/Groups/215/215922/folders/156726/1616191TripolitanDec1988Page21.jpg
Steve __________________ Steve Naquin TRA 677 L2
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Thanks Steve,
Andrew

http://216.77.188.54/coDataImages/p/Groups/215/215922/folders/156726/1616190TripolitanDec1988Page20.jpg
http://216.77.188.54/coDataImages/p/Groups/215/215922/folders/156726/1616191TripolitanDec1988Page21.jpg
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Definitely very much the same -- the only differences (really) are that I tried to design mine specifically to the PML kits I've been using -- and the way mine is set up, there is no stress (really) on the piston bulkhead, as the major stress is taken up by the strap mounted on the engine mount, up to the D-ring.
David Erbas-White
snipped-for-privacy@bellsouth.net wrote:

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David,
Very good idea, I like the PML kits specifically because of the piston. It will be interesting to see the results of your testing and what damage, if any, develops over the long run. Keep us informed.
Andrew Grippo

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