Fiberglass Tube warped from fin slotting

[mtnmanak]

Does anyone have any suggestions on dealing with this?

This is a 6" diameter fiberglass tube for a WM Darkstar Ultimate. You can see in this photo, the tube flattened out where the slots were cut, so on that end of the tube, it is not round anymore and the CRs don't fit. You can really see the flattening in the lower right of the picture.



For reference, You can see on the other end of the tube, it is still round:




I have run into this issue before, but never this extreme. I tried using a large hose clamp to force it back into round, but that didn't do anything.

I supposed I can grind 3 flat edges on the CRs if it comes to that.

If anyone has any good suggestions, I am willing to give it a try!

 

[tsmith1315]

My first thought is to apply heat. But, is that delamination?

 

[Steve Shannon]

Put the centering ring in tilted 90° and then pull it flat.

 

[mtnmanak]

Put the centering ring in tilted 90° and then pull it flat.

That is my go-to method normally, but I have to figure out how to slide 4 CRs into this tube. I can cant the first one on the MMT, but after that, they all pretty much need to go in straight. The flattening occurs all the way down the fin slots, not just at the opening of the tube.

 

[teepot]

Maybe more clamps or ropes starting at the middle and working back. Each one making it more round.

 

[WILDMANRS]

Does the ring go in the other end of the tube?
just want to make sure the rings are right before we go any further.
This is nothing unusual with slots that long.

 

[mtnmanak]

Does the ring go in the other end of the tube

Tim - Yes, it fits in the forward end no problem. Just emailed you some pics.

 

[WILDMANRS]

Tim - Yes, it fits in the forward end no problem. Just emailed you some pics.

I’m at a launch this weekend I will check email Sunday night

 

[Steve Shannon]

That is my go-to method normally, but I have to figure out how to slide 4 CRs into this tube. I can cant the first one on the MMT, but after that, they all pretty much need to go in straight. The flattening occurs all the way down the fin slots, not just at the opening of the tube.

Can you use a coupler and push them in from the other end? Sooner or later you’ll have to get some epoxy in there also.

 

[tsmith1315]

Or extend the rear slots to the end of the tube, build the fin can on the bench with only the rear fins in place, then add the front fins after that's mounted.

 

[dhbarr]

Do you have a ~7in ball?

 

[mtnmanak]

Do you have a ~7in ball?

Like a bowling ball (I don't have one)?

The coupler idea has some merit, although I only have the one coupler for the ebay and I am a little concerned that if I try to shove it all the way down the tube and it gets stuck in there, I will end up with a bigger problem than I started with.

Or extend the rear slots to the end of the tube, build the fin can on the bench with only the rear fins in place, then add the front fins after that's mounted.

This is definitely an option, although I would rather avoid it, if I can. I have done this in the past with other projects, but have always been of the opinion this weakens the aft end of the rocket. My concern is that this rocket will definitely launch on an N at some point - that is a lot of thrust and pressure at the rear end of the rocket. I honestly have no idea if cutting the slots open would degrade the aft end integrity to the point of concern. If I do end up having to go down this path, I would glass the aft end heavily.

Lot of great suggestions here - thanks all for taking the time to give this some thought and make some constructive recommendations!

I will wait for Tim to get back to me at the beginning of the week to see what he suggests.

 

[dhbarr]

Like a bowling ball (I don't have one).

Inflatable.

 

[Tech 68]

Can you use a coupler and push them in from the other end? Sooner or later you’ll have to get some epoxy in there also.

And when each one is correctly located, use hose clamps to apply force there on the tube.

 

[mtnmanak]

Inflatable.

Will have to see if we have a soccer or basketball that will work. Interested to see if a ball will exert enough pressure to expand the fiberglass and true up the tube. Whether it works or not, fun experiment!

 

[mtnmanak]

I have been trying a bunch of these suggestions. I have found a couple ways to temporarily get the tube in a state where I can get the CRs in, but the tube is so warped that the pressure of getting it back to round is starting to crack the tube:



I don't think this is a normal situation and I will have to let Tim make a call on how to fix it.

 

[troj]

That crack won't present any problems. You're going to have a centering ring there, along with through the wall fins. Lots and lots of reinforcement.

That said, I agree - wait for Tim to get back and chat with him about how to approach it. He's got a lot of experience with these and will know how to get you going.

 

[dman007]

Can you stick a blood pressure cuff inside and keep it in there while you warm it in hot water

 

[mtnmanak]

That crack won't present any problems. You're going to have a centering ring there, along with through the wall fins. Lots and lots of reinforcement.

That said, I agree - wait for Tim to get back and chat with him about how to approach it. He's got a lot of experience with these and will know how to get you going.

Can you stick a blood pressure cuff inside and keep it in there while you warm it in hot water

The operative word in my post above is "starting" to crack. While the current crack is not a structural problem, I stopped trying to manipulate the tube to avoid real damage while I wait for Tim to get to me.

 

[afadeev]

Or extend the rear slots to the end of the tube, build the fin can on the bench with only the rear fins in place, then add the front fins after that's mounted.

This is definitely an option, although I would rather avoid it, if I can. I have done this in the past with other projects, but have always been of the opinion this weakens the aft end of the rocket. My concern is that this rocket will definitely launch on an N at some point - that is a lot of thrust and pressure at the rear end of the rocket. I honestly have no idea if cutting the slots open would degrade the aft end integrity to the point of concern.

This may also be the first compelling argument for a thrust plate/ring:
https://alwaysreadyrocketry.com/product/sc-precision/

I will wait for Tim to get back to me at the beginning of the week to see what he suggests.

That was my first idea - warranty claim on the defective part!
a

 

[Steve Shannon]

Can you stick a blood pressure cuff inside and keep it in there while you warm it in hot water

I might not understand your suggestion. I think you’re suggesting that the warm water might soften the epoxy and make it more pliable. If that’s what you mean, I think you’re mistaken. Epoxy is a thermosetting resin, not a thermoplastic. If you heat it to a temperature where the epoxy becomes soft (which is much higher than you would want to expose a blood pressure cuff) you have destroyed the epoxy.

 

[mtnmanak]

Good read on Tg vs Tm:

https://www.jpkummer.com/sites/defa... Glass Transition Temperature for Epoxies.pdf

 

[mtnmanak]

This may also be the first compelling argument for a thrust plate/ring:
https://alwaysreadyrocketry.com/product/sc-precision/

Already CNCed one (mainly, I like aluminum thrust plates so I can tap them and easily move the expensive 98mm retainer around). It is also way cheaper than the ones you can get online - This thrust ring is about $10 worth of aluminum vs. the $60-$80 online (although, admittedly, theirs are much prettier). And, if I plan ahead, I can cut the center disc out to either be (in this case) a 2.6" bulkhead or a 2.6" thrust plate. So, effectively, I get two expensive aluminum parts for about $5 each.

 

[Chris_H]

I do not see a problem with this. The tubing is relatively flexible in the dimension you are pointing out. I would not expect it to be perfectly round at all. The aft centering ring can be checked with calipers to see if it is close to round. If it fits too loosely, then the airframe will keep some of that shape. It looks like the centering ring will only fit with an interference fit, and it may need to be gently relieved on it's circumference to fit into the end of the airframe. A round, well fitted (snug) centering ring will give shape to the airframe and will remove most of the out of roundness. With a centering ring just fore of the slots, and one just aft, any out of rounds will be minimized to not much, guessing less than 0.050" at most. When you sand the fillets with a round block, assuming that you will, this will clean up one of the lines.

You could squeeze a centering ring in at either end of the slots and then measure with a straight edge to see how much bulge there is, I doubt there will be much.

I bet there is a lot of out of roundness across the board in this tubing if one were to actually set up and measure, mostly all within usable spec, not really noticeable, and with minimal adverse effect at the end of the day.

The centering rings should fix what you are looking at. If I saw that, I would do what I always do, check the roundness of the ring, fit is snug, and send it.

 

[JohnCoker]

I had always assumed slight fiberglass oval-shape was due to stacking during post-cure or transportation. I also haven't worried about it and just let the CRs push it back into round.

The fact that yours occurred only after slotting is interesting because it suggests that there are internal stresses that got relieved when the tube's integrity was broken.

 

[Chris_H]

I had always assumed slight fiberglass oval-shape was due to stacking during post-cure or transportation. I also haven't worried about it and just let the CRs push it back into round.

The fact that yours occurred only after slotting is interesting because it suggests that there are internal stresses that got relieved when the tube's integrity was broken.

Yes, looks like more tension toward the inside surface in the tube being discussed.

Roundness is only one element. Straightness is another. Not much is absolutely perfect. Probably less than that even...

Roundness is provided by verified round well fitted centering rings.

 

[mtnmanak]

I am by no means an expert in Fiberglass tubing or building rockets. So many people on this forum have far, far more experience than I do (which is why I put questions like this up on the board to get great feedback).

However, to-date, I have built 64 High Power Rockets. 15 in the past 10 months alone, ranging from 29mm minimum diameter "darts" to some fairly large 8" diameter beasts. I have encountered warped FG tubes many times. Most of the tubes are a bit out of shape and you have to finagle the CRs in.

This tube was flat on 3 sides and about a 1/4" off or more on each of those flat areas. Shoving a CR in place did not push the tube into round, it just pushed the flat parts out.

John - I suspect you are right, it is probably just a defect.

Tim is going to trade the tube out for me so he can take a look at it and see what went wrong.

 

[Chris_H]

I am by no means an expert in Fiberglass tubing or building rockets. So many people on this forum have far, far more experience than I do (which is why I put questions like this up on the board to get great feedback).

However, to-date, I have built 64 High Power Rockets. 15 in the past 10 months alone, ranging from 29mm minimum diameter "darts" to some fairly large 8" diameter beasts. I have encountered warped FG tubes many times. Most of the tubes are a bit out of shape and you have to finagle the CRs in.

This tube was flat on 3 sides and about a 1/4" off or more on each of those flat areas. Shoving a CR in place did not push the tube into round, it just pushed the flat parts out.

John - I suspect you are right, it is probably just a defect.

Tim is going to trade the tube out for me so he can take a look at it and see what went wrong.

I just received an Ultimate Wildman kit today. I now see that there is no way that a centering ring is going to flatten out that shape.

Enjoy your build when the new tube arrives!

 

[mtnmanak]

Got my new tube last week and got the old tube back to Tim so he can do a post-mortem.

New tube is excellent. Normal amount of touch-up on the CRs required. Getting through the build nicely, will post it to my Dark Star thread when I get a chance (documenting a build takes 2-3 times longer than the build itself!)

As usual, Tim backs up his parts and his rockets - always happy with the Wildman customer service!

 

[rfjustin]

Got my new tube last week and got the old tube back to Tim so he can do a post-mortem.

New tube is excellent. Normal amount of touch-up on the CRs required. Getting through the build nicely, will post it to my Dark Star thread when I get a chance (documenting a build takes 2-3 times longer than the build itself!)

As usual, Tim backs up his parts and his rockets - always happy with the Wildman customer service!

Always good to hear positive customer experience!