Excessive gap between motor tube and CRs

[Dane Ronnow]

I purchased a 29/BT-80 motor mount kit from Apogee, along with an Aero Pack 29L retainer. The fit of the CRs and retainer base on the tube seems excessive (compared to the fit on 24mm MMTs I've built), with two full wraps of blue painters tape around the tube still allowing the CRs and retainer base to slide on. My guesstimate on the gap is not quite 1/16", but a very fat 1/32".

I know that glued joints are supposed to be tight. How would you fill the gap? The tape works, but will Elmer's glue (for the CRs) and JB Weld (for the retainer base) stick to that? Scotch tape instead of blue? Or should I just slop on the glue and hope the tube is centered when it dries?

The tube supplied in the kit is heavy, not the lightweight tube found in most LPR MMT kits. And Apogee's product page states the 29L retainer will fit this tube.

 

[jimzcatz]

I would just epoxy it and go

 

[Dane Ronnow]

I just tried one full wrap with 20lb copy paper. It takes up the gap with just enough slack for a thin film of glue. I'll probably go with that—glue the paper to the tube with Elmer's white, then epoxy the rings and JB Weld the retainer.

 

[jqavins]

I'd make sure there is plenty of contact from tube to glue to component.

• First, notify Apogee of the issue. Include pictures. See what they have to say.
• Carefully trim the paper under the rings so that it barely sticks out, then apply fillets wider that the paper strip.
• Put paper at the aft end of the tube no wider than half the insertion depth into the retainer.

That way the paper assures that everything is centered around the tube, but you don't have the paper in the load path. In other words, the paper functions as a spacer, but is not structural.

 

[neil_w]

I generally do a wrap of paper, and if it's still too gappy I do another. Or if I'm in the mood I'll use cardstock instead, which usually will take care of it in one shot. If it ends up too big, that's not a problem, just takes a few seconds of sanding to get the perfect fit.

I do wish that hole in centering rings would be sized a bit too small rather than too big; it's very easy to sand it out to make it looser, much more effort to make it tighter. This is my one complaint with the light ply centering rings I get from BMS (often a bit loose), but it doesn't stop me from using them.

 

[mbeels]

I've bought two of those motor mounts from Apogee (29mm in BT-80) and both had a very loose fit. I contacted Apogee and their response was:

"
Hello Marten,


Do your centering rings say 13315 on them? That is the correct ring for the tube. I just checked them. They aren't snug on the tube because we do have to allow for tube tolerances during the manufacturing process, but they were tight enough that a nice layer of glue and fillets should have worked fine.


Michelle Mason
Customer Service"

I wrapped the motor tube with a single layer of paper, and that seemed to be enough to fill the gap. I glued it all together with white glue and it has held up fine. So far the biggest motor I used was an H128.

 

[Dane Ronnow]

In other words, the paper functions as a spacer, but is not structural.

What I have in mind is applying a thin film of Elmer's Glue All, then wrapping the paper as tightly as possible (using my fingers as a squeegee), thus making the paper 'one with the tube', so to speak. The paper strip at the aft end of the tube would be 1/4" wide, since the base of the retainer will butt up against the aft CR. The forward strip, just wide enough for the 1/8" CR.

 

[neil_w]

What I have in mind is applying a thin film of Elmer's Glue All, then wrapping the paper as tightly as possible (using my fingers as a squeegee), thus making the paper 'one with the tube', so to speak. The paper strip at the aft end of the tube would be 1/4" wide, since the base of the retainer will butt up against the aft CR. The forward strip, just wide enough for the 1/8" CR.

That is fine. With the paper glued to the tube, and the ring glued to the paper, you should be fine. I have done this many times. (Sometimes I glue the paper to the inside of the ring instead, but same result).

 

[Dane Ronnow]

I do wish that hole in centering rings would be sized a bit too small rather than too big; it's very easy to sand it out to make it looser, much more effort to make it tighter.

My sentiments exactly. I'd much rather sand a little, than figure out how to 'pack the gap'.

 

[Dane Ronnow]

Do your centering rings say 13315 on them?

Yep, that's the one. I understand the need to accommodate tolerances, but when they get wide (at least wider than I'm used to seeing), that "nice layer of glue" Michelle speaks of, becomes a nice, thick layer. Not good.

 

[mbeels]

Yep, that's the one. I understand the need to accommodate tolerances, but when they get wide (at least wider than I'm used to seeing), that "nice layer of glue" Michelle speaks of, becomes a nice, thick layer. Not good.

Agreed. It is really big. I think that gap is much bigger than normal tolerances for tubes. I think it is a small error on Apogee's part.

I just tried one full wrap with 20lb copy paper. It takes up the gap with just enough slack for a thin film of glue. I'll probably go with that—glue the paper to the tube with Elmer's white, then epoxy the rings and JB Weld the retainer.

I did basically that, but used white glue for both joints in one step (paper to tube, and ring to paper).

 

[jqavins]

I don't disagree with the current plan; even as I was reaching for the "Post reply" button I reminded myself of the fact (often stated, sometimes overlooked) that the glue is the strong point in a good joint, and the glue is going to permeate the thin paper. You'll be fine. I'd still feel more comfortable, just for my own warm fuzzy feeling, keeping the added paper out of the load path. But don't sweat it.

Many people here are better, more experienced builders that I am, and that includes both Neil and Marten. If they say you're good, you're good.

(Neither the paper nor the glue is in the load path for the retainer during the motor burn, as the end of the tube is, as you say, hard against the aluminum. But it's called a retainer because it keeps the motor in the rocket during ejection. At that moment, the load path is, indeed, through the glue and possibly the added paper. But I'm overthinking this. I do that.)

 

[neil_w]

Keep in mind that when you fillet the centering rings, the fillets will completely cover the paper spacer, and you'll have a solid motor mount tube -> centering ring joint.

I am unclear how the paper spacer could be a weak point. If anything, I would think that a loose fit (requiring the glue to fill the gap) would be worse.

 

[Dane Ronnow]

But it's called a retainer because it keeps the motor in the rocket during ejection. At that moment, the load path is, indeed, through the glue and possibly the added paper.

That was on my mind when I decided to move the aft CR back to butt up against the retainer base, hoping to add a little more strength against the rearward yank on ejection.

 

[neil_w]

I'm going to add a disclaimer here that although I have used the paper spacer technique often and it has always worked fine, I don't subject my rockets to extreme G forces. So this advice is worth what you paid for it.

 

[Dane Ronnow]

Keep in mind that when you fillet the centering rings, the fillets will completely cover the paper spacer, and you'll have a solid motor mount tube -> centering ring joint.

That's what I'm thinking too. If the paper has a good, thin film of glue underneath (no unglued spots), it becomes part of the tube, essentially a thicker-walled tube. And I agree that the wider the gap and the more glue required to fill it, the weaker the joint becomes.

 

[Dane Ronnow]

I did basically that, but used white glue for both joints in one step (paper to tube, and ring to paper).

Agreed. After I said "epoxy the rings" (which you quoted from an earlier post), I decided against that, opting for white glue instead. And if I'm understanding you correctly, doing the paper-to-tube and rings-to-paper in one step, the glue permeates the paper from both sides, making it stronger.

 

[mbeels]

That's what I'm thinking too. If the paper has a good, thin film of glue underneath (no unglued spots), it becomes part of the tube, essentially a thicker-walled tube. And I agree that the wider the gap and the more glue required to fill it, the weaker the joint becomes.

Yeah, I'd agree with that. The paper spacer is a matrix for the glue. And I think that is basically what cardboard is (paper fibers held together with glue).

 

[mbeels]

And if I'm understanding you correctly, doing the paper-to-tube and rings-to-paper in one step, the glue permeates the paper from both sides, making it stronger.

That's how I imagine it in my head! It's a bit messy to do, because the paper gets softer and tears easily once soaked with glue, but it will all harden up solid in the end.

 

[jqavins]

Keep in mind that when you fillet the centering rings, the fillets will completely cover the paper spacer, and you'll have a solid motor mount tube -> centering ring joint.

Which is what I suggested in post #4, bullet 2. And then (perhaps I was imagining it) you seemed to say that wasn't important. And I concede, it's not important, but just feels better.

I am unclear how the paper spacer could be a weak point.

So am I, and see above.

If anything, I would think that a loose fit (requiring the glue to fill the gap) would be worse.

That was never on the table, for the rings.

That's how I imagine it in my head! It's a bit messy to do, because the paper gets softer and tears easily once soaked with glue, but it will all harden up solid in the end.

And if the fit with the paper is snug then the paper will want to bunch up, pushed ahead of the ring going on. Just watch out for that.

Where I was talking about glue filling a gap is for the retainer, with paper filling part of the gap as a centering spacer. The retainer is put on with JBW. Will JBW wet the paper the way white glue does? I don't know. Will Tube → JBW → paper → JBW → aluminum be any weaker than tube → JBW → aluminum? I doubt it a lot but, as above, it just feels better. And unlike most overbuilding for better feeling, this doesn't cost anything in materials or weight.

 

[mbeels]

Where I was talking about glue filling a gap is for the retainer, with paper filling part of the gap as a centering spacer. The retainer is put on with JBW.

Ah, I see. I had forgotten about the retainer.

 

[Dane Ronnow]

And then (perhaps I was imagining it) you seemed to say that wasn't important.

If I did, it was my mistake. (I think when posts start stacking, sometimes things get a little confusing. They do for me.)

Everything you told me in your earlier post makes sense to me.

Will Tube → JBW → paper → JBW → aluminum be any weaker than tube → JBW → aluminum?

FWIW, I'm thinking tube > Elmer's All Purpose > paper > JBW > aluminum. On the forward ring, tube > Elmer's > paper > Elmer's > CR.

 

[jqavins]

If I did, it was my mistake. (I think when posts start stacking, sometimes things get a little confusing. They do for me.)

They do indeed; that statement was aimed at Neil.

 

[BABAR]

when my centering rings are going to be hidden from view (which is most of the time) I really like gorilla polyurethane glue. It’s ugly but it sure fills the gaps.

 

[Tractionengines]

DON'T USE Gorilla Glue to FILL GAPS. YES it foams up and looks nice. But the OVERLY Foamed Glue, has almost no real strength. I will do that to fill a gap for painting, but do not use it to fill a gap in a structural joint.

Here is a really good write up and test. Plus the manufacturer responded, and agreed exactly with what the testing found.
https://woodgears.ca/joint_strength/gorilla_glue.html

 

[jqavins]

It's good to see the myth dispelled. I had heard it before, had no factual information to either corroborate or dispute it, and probably wouldn't have tried it, since a tight joint is always better than a gapped one anyway.

Or is it? The real surprise from that report is that the gapped wood glue joint was stronger than the ungapped one. That came as a shock, where the Gorilla performance did not. Maybe that's why the double glue method works? A tacky layer here plus a tacky layer there makes the small submillimetre gap?

 

[mbeels]

It's good to see the myth dispelled. I had heard it before, had no factual information to either corroborate or dispute it, and probably wouldn't have tried it, since a tight joint is always better than a gapped one anyway.

Or is it? The real surprise from that report is that the gapped wood glue joint was stronger than the ungapped one. That came as a shock, where the Gorilla performance did not. Maybe that's why the double glue method works? A tacky layer here plus a tacky layer there makes the small submillimetre gap?

Yeah, interesting. It's nice to see someone test something like that carefully.

I do recall reading that it is important to avoid excess clamping pressure when using wood glue, as it would starve the glue joint of glue.

Here's my guess why a gapped wood glue joint may be slightly stronger than the un-gapped one. Because the wood glue is stronger than the fibers of the wood itself, if the joint is gapped, there is a continuous layer of glue that can uniformly spread out the load over the entire area under shear. If it was un-gapped, some of the high spots of one surface would line up with high spots on the other surface, resulting in small glue starved areas. Then the glue layer would look more like swiss-cheese, than a continuous layer.

 

[jqavins]

Makes sense. Right? Maybe. Testable? Probably difficult. (High speed X-ray camera, maybe.)

 

[Back_at_it]

Simply coat the outside of the centering ring with glue and put a healthy amount inside the body tube and push it home. You need to remember that glue needs to be able to get between the joint in order to hold it together. If the fit is too tight, all you're doing is pushing the glue up the tube.

I will typically sand a centering ring or at the very least, bevel the forward edge so that glue gets down into the join.

I'd say, glue and go.

 

[Dane Ronnow]

You need to remember that glue needs to be able to get between the joint in order to hold it together. If the fit is too tight, all you're doing is pushing the glue up the tube.

For sure. But it has to be tight enough that there's no movement from side to side between tube and CR. Otherwise, I end up with a cockeyed tube. The gap was nearly 1/16" to begin with.

I ended up gluing two layers of 24 lb. paper around the tube, 3/16" wide (allowing 1/4" for the retainer base, 1/8" for the CR). Once dried, I lightly sanded it until the retainer and CR would slide on easily, then epoxied both in place with J-B Weld, with the CR butting up against the retainer. Same process with the forward CR, which was glued with Elmer's wood glue.

Turned out great.