Zipper repair on large fiberglass airframe?

[AllDigital]

I've got an 8" fiberglass airframe with a 15" zipper, at the top of the fin can, that I am going to attempt to repair. The zipper happened after landing when the fin can dug into the tundra and the main was whipped around by 30mph winds. This is ironic, as the vehicle was coming in ballistic at almost 300fps when the main deployed (and luckily didn't zipper) and gently set the rocket down.

I've opened up the wound (see pics below) and I've got 2.5 oz fiberglass cloth and am using West Systems 105/207 epoxy. I plan to use a piece of coupler, covered in non-stick aluminum foil, as the mandrel and then lay down strips of fiberglass cloth, like patching a hole on a boat. I also will do a few wraps around the top of the fincan as reinforcement. I am worried about the tension forces pulling in and how the repair will survive with BP charge forces pushing out. Any pro tips on how to do a large zipper repair that will survive future ejection charges?

Thanks,

Mike


Coupler as mandrel...

 

[Steve Shannon]

Why do you think that didn’t happen when the chute came out during ballistic descent? That’s the only situation where I have ever seen a zipper like that in a fiberglass body tube.

 

[jimzcatz]

You don't need the coupler or cloth, just put some tape on the inside of the tube and fill the wound with epoxy from the outside. When the glue has cured remove the tape, sand to fit. I have done this before. Easiest repair.

 

[AllDigital]

Why do you think that didn’t happen when the chute came out during ballistic descent? That’s the only situation where I have ever seen a zipper like that in. Fiberglass body tube.

This rocket is a test platform for a new spring-latched deployment system. It was our second test flight out at FAR. We had four cameras on board looking at every inch of the vehicle. We were sure the zipper happened in the air, but when we reviewed the frame-by-frame of the aft cameras the next day we were surprised to see the zipper happened after it landed. It was an extremely windy day and the main chute is 18 feet in diameter. There was an enormous amount of force from the chute whipping around on the ground and the fin can was dug in like a boat anchor.

Here is video: https://vimeo.com/manage/videos/685342633

At the 3 min mark you can see the fin can land softly (24 fps). It is hard to tell with that edited version, but the fin can has no damage. At the 3:30 mark in the film you can see the force of the wind pulling on the fin can and lifting the upper airframe up 20 feet in the air. The fin can also sustained a lot of fin damage. In this flight the surface wind was the culprit.

 

[AllDigital]

You don't need the coupler or cloth, just put some tape on the inside of the tube and fill the wound with epoxy from the outside. When the glue has cured remove the tape, sand to fit. I have done this before. Easiest repair.

I've done that on a smaller rocket, but on this 8" airframe without the coupler acting as a mandrel the airframe constricts inward significantly. It needs a mandrel to hold it open. Also, this aft end of the rocket is used for an emergency chute with remote controlled pyros. These need to work when it is ballistic and the fin can has extra shear screws, so the charges are oversized (4g FFF and 5g backup). The charges create 300+ pounds of separation force. I doubt the seam, with only epoxy filler, can withstand that type of ejection force. ?

 

[Steve Shannon]

I would build it up with fiberglass tape and epoxy over the coupler mandrel like you originally planned. I’d add a little fiberglass pulp to the epoxy and I’d use a rasp to smooth the repair. A little Mylar around the coupler will keep it all from sticking to the coupler or epoxy the coupler in place and change to zipperless construction (as described by Stu Barrett long ago).

 

[jimzcatz]

Understood, yours and Steve's method make the most sense

 

[crossfire]

How about cutting tube where zipper ends. Get a coupler and tube.

 

[Steve Shannon]

How about cutting tube where zipper ends. Get a coupler and tube.

Or even just cut it shorter if there’s enough motor mount. As my friend Dale says, “Old rockets never die; they just get shorter!”

 

[AllDigital]

How about cutting tube where zipper ends. Get a coupler and tube.

Oh yeah, how obvious. That is a much better and easier repair option -- without creating a shorty. I've got extra coupler, I'll get another 18" of air frame and this beast will just weigh a bit more.
The fin can is from a Wildman Jumbo Interceptor, so it already has a coupler connector between the fins. Thanks!

 

[jimzcatz]

Problem solved, good luck with the repair. At the risk of being Captain Obvious and absolutely no disrespect intended, you DO know how to cut a tube square right? Of course you do, what was I thinking!!!!

 

[Hobie1dog]

or epoxy the coupler in place and change to zipperless construction (as described by Stu Barrett long ago).

can you point me to a page somewhere showing a "zipperless construction"

 

[Steve Shannon]

can you point me to a page somewhere showing a "zipperless construction"

https://www.rocketryforum.com/threads/what-does-“zipper-proof”-design-mean.148677/post-1824898

 

[K'Tesh]

Critical thing to remember with zipperless... Make sure that the body tube is attached firmly to the nosecone if you're anchoring the shock cord to the nosecone.

Forgetting that important step lead to this result:



Well... In watching that video again, I forgot another thing... Keep the camera pointed up.

So let me describe what happened...

The up part went fine as you could see (best to watch it full screen, I remembered to point the camera up, and caught the rocket again on screen it just before the video ended) ... The ejection charge fired, and blew the recovery bay off *AND* the nosecone off of it. The parachute was trapped inside the recovery bay. As the rocket fell, the parachute *BARELY* managed to work its way out of the recovery bay, and successfully deploy, just before the rocket landed. So, the rocket was undamaged, but it could have been damaged had that chute not opened, or opened mere milliseconds later. Another problem occured when the camera's battery failed as I was filming that launch. Though there were witnesses to the flight namely Rob Appleton, Wilson Alness, Ed Harrison, and the other members of OROC that were at the Filible’s Folly launch on September 15, 2015, in Sheridan, Oregon. If anyone managed to film that I'd really like a copy of it... I remember what I said JUST AFTER the parachute opened, and the rocket landed milliseconds later... "I *MEANT* to do *THAT*!!!" Truth be told, it was just plain dumb luck.

 

[AllDigital]

Problem solved, good luck with the repair. At the risk of being Captain Obvious and absolutely no disrespect intended, you DO know how to cut a tube square right?

I was planning on using a table saw and a high square fence, since I have a square end already. I use this method on smaller airframes and on all my motors/liners/grains. Is there a better way?

can you point me to a page somewhere showing a "zipperless construction"

I've used tennis balls, but Giant Leap has these "Fireballs" that are much easier to work with. Those have worked very well for me. Ironically, I had two of them on this rocket -- one on top to protect zipper on the upper airframe and one at the top of the motor mount to prevent line wear. Unfortunately, I had not place one at the top of the fin can. The upper ball definitely prevented a zipper when the main opened at 280 fps.

https://giantleaprocketry.com/products/large-fireball?variant=37741121306796

 

[jimzcatz]

Balls are not the well known zipperless design. It incorporates where couplers and bulkheads are placed. I don't use it so I can't explain it very well.

 

[cwbullet]

Your imagination can create limitless designs. I have used small softballs or foam tubes of all types.

You can use:

1. Foam Golf Balls
2. Foam nerf bullets
3. Foam Nerf Darts
4. Pool noodles
5. Pipe insulation
6. Bouncy Balls

Options are limitless. I tend to use softer balls or tubes. They absorbed the force and protect your tubes.

 

[3stoogesrocketry]

I was planning on using a table saw and a high square fence, since I have a square end already. I use this method on smaller airframes and on all my motors/liners/grains. Is there a better way?

I suggest bolting the new 18 inch section of airframe onto the coupler that way if there is another incident , you can simply remove and replace the end .

 

[Steve Shannon]

I was planning on using a table saw and a high square fence, since I have a square end already. I use this method on smaller airframes and on all my motors/liners/grains. Is there a better way?
Snip…

I suggest a sled on your table saw, with a backstop the length of the tube and an endstop against the square end. Place the tube over the retracted blade and then feed the blade up until it’s cutting through the tube, then rotate the tube to cut all the way around the tube circumference. Even if the sled is not perfectly square the tube end will be as square as your square end.

 

[Jimmy Wilson]

I was involved in a group build way back in 1995. It was a 7.5" PML phenolic air frame. We were concerned with zippering of the body tube with the brittleness of the phenolic the main reason. The rocket had five 54 mm mounts which we made long enough to also accept the first generation Aerotech Hybrid motors. We ended up deciding on using what called an "inner tube" We installed a 7.5" coupler in the main air frame for which we made a centering ring for a 6" body tube. and another at the top of the motor mount. We were not using motor ejection so that was not a concern in the construction. We utilized dual deployment so the main chute was in the upper body tube attached to the nose cone and the AV bay and the drouge chute was in the lower tube. The design performed as designed and we were able to fly twice in one day, first flight with 3 K 550. and 2 K-250 motors. It attained an altitude of over 7,000 feet. Gary Rosenfield of Aerotech was at the launch and generously sponsored our second flight of the day to demo the new Aerotech Hybrid motors. It was powered by 5 Hybrid K motors and reached an altitude of over 5,000 feet, and was recovered safely after it's second flight of the day.

 

[AllDigital]

Here she is back to beautiful...