Motor mount glue issues.

[Senior Space Cadet]

I decided to start a separate thread. Maybe I shouldn't have, but here it is.
Last Saturday I had one engine block fail, on launch, and a second that would have failed, on the next launch, if the whole motor mount hadn't come out the back on the ejection charge.
Feedback has been along the lines of: it can't be the glue, and I must be doing something wrong.
It is unlikely that it's the glue, but it isn't out of the realm of possibilities. I would venture a guess that I'm one of the few using Gorilla wood glue. It is possible that Gorilla wood glue is crap for building motor mounts.
Obviously something is wrong, but it isn't a procedural problem on my part.
I glue in the engine blocks by smearing some glue on the inside of the motor tube and pushing the engine block ring forward with a motor.
I glue the whole mount into the body tube by soaking the end of a Q-tip, with glue, and smearing it about where I think the forward centering rings will be and then smearing some glue on the aft centering ring, then pushing the unit into the body tube until in place.
What hasn't failed, so far, is the screw type motor retainers, which I super glue on, and the centering rings on the motor tube.
So, the question is, how do I fix the problem.
Despite advice to the contrary, I think one first step is to change glues.
In most gluing situations I'd apply glue to both surfaces being glued. This isn't really practical with motor mounts. I tried it and ended up having to toss the whole thing when the paper swelled to the point that the parts no longer fit. Besides changing glues, about the only thing I can do is just use more of it and make sure I don't put it too far forward in the body tube. Too much glue can end up being a problem because a bunch of dried glue in the wrong place can prevent the motor from going in all the way, or the mount from sliding smoothly into the body tube.
I'm trying some new glues. Hopefully that, and more glue, will fix the problem.
Sorry the photo isn't sharper. I didn't want to bother with a tripod.

 

[Antares JS]

-Your problem is either the placement of your glue or not using enough glue.
-Your glue is absolutely not the problem. I use Aleene's Tacky Glue for low power rockets and have done so for twenty years and never had a problem, despite it probably not being as strong as actual wood glue. Your technique matters far more than what glue you use.
-Test fit your parts before applying any glue. Sand them if they don't slide together easily. This will prevent seizing.
-Apply glue inside the body tube well aft of your forward centering ring.I usually put it about half the length of the motor mount into the tube. This absolutely assures that the forward centering ring will pick up a bunch of glue as it slides into place.
-If you're that worried about seizing, practice pushing your motor mount into place and doing it in one quick, smooth motion.
-Apply enough glue to create a good bond. Finding that it was really annoying to apply a decent amount of glue with an applicator, I started just using my fingers to put glue into a body tube. Put a nice dollop on your finger and smear it around evenly until there's a generous film covering the entire inner surface. (This is fine to do with white glue but you'll probably want to make sure it's okay to do with whatever glue you use. Obviously this is a no-no with epoxy)
-Consider not stopping to apply glue to your aft centering ring and just applying a fillet of glue around it after it is in place.

 

[lakeroadster]

Are those home made centering rings made out of painter's tape?

If the answer is yes then the motor mount is only as strong as the bond between the layers of tape.

Use regular masking tape... not a painters tape. Painters tape is specifically made not to adhere well so it doesn't damage the substrate when it's removed.

Or make your own centering rings from lite-ply or home made balsa ply.

 

[Jay Rairigh]

The centering rings are Estes rings and are fine. Generally I would think I'd see some trace of glue at those rings after they are glued. Possibly you are not using enough glue in general with your glueing. Also, more glue gives the parts more lubrication when assembling and less chance the parts will 'grab' and lock up while glueing your assemblies.

Type/choice of glue is minimal here. Any wood glue should be plenty good.

 

[afadeev]

I glue the whole mount into the body tube by soaking the end of a Q-tip, with glue, and smearing it about where I think the forward centering rings will be and then smearing some glue on the aft centering ring, then pushing the unit into the body tube until in place.

-Your problem is either the placement of your glue or not using enough glue.

+1.
When in doubt that enough glue soaked into the centering rings, you can always use a long dowel to deposit more glue around the forward centering ring (through the airframe), and the aft centering ring (helps if you do not glue the motor retainer until after the MMT is in the airframe).

Also, throw away the Q-tip. Instead:
1). Use a toothpick to deposit a solid and drip-worthy ring of glue to an area 1/4" BELOW where the forward centering ring will settle.
2). Insert the MMT half way into the airframe (from the aft end).
3). Now apply another drip-worthy ring of glue to the aft end of the airframe, all the way around.
4). Now push the MMT into place with one firm movement, and let it rest on the aft end of the rocket (support it sitting vertically, if necessary).

The centering rings will displace the beads of glue and coat themselves, and the airframe walls, with glue to spare. The excess glue will be pushed forward, and form a glue barrier, preventing the MMT from moving forward even if you didn't soak enough glue into the centering rings.
Shine a flashlight into the airframe. If you can see the glue barrier/dam, congrats - you are done.
If not, deposit some more glue to the FWD centering ring area with a dowel.

5). After everything has dried, apply another ring of glue to the aft end of the MMT. This builds another glue dam that will prevent the MMT from ejecting out the tail end, even if you didn't soak enough glue into the aft centering ring. Let it dry sitting tail up.
6). Now glue the motor retainer.

Now the MMT will outlive the rest of the rocket!

HTH,
a

 

[Senior Space Cadet]

Are those home made centering rings made out of painter's tape?

If the answer is yes then the motor mount is only as strong as the bond between the layers of tape.

Use regular masking tape... not a painters tape. Painters tape is specifically made not to adhere well so it doesn't damage the substrate when it's removed.

Or make your own centering rings from lite-ply or home made balsa ply.

No, they are from Apogee.

 

[Senior Space Cadet]

I think I've found the problem.
I live in Colorado. Humidity is very low. The glue dries or skins over very quickly.
I think what is happening is I'm putting down a thin layer of glue and before I get the parts together, it is drying out and skinning over, resulting in a poor bond.
I'm not sure what to do about it except work quickly and use tons of glue.
I did buy a bottle of glue that might have a longer working time. I'm going to do some rocket building today, so I'll try the new glue and use more of it.
I won't know if I'm successful till I launch some more rockets.
I asked Apogee about it and I'll see what they have to say. They are in Colorado too, of course, and a great source of information as well as parts.

 

[Senior Space Cadet]

Yeah, Tim at Apogee says work faster and use more glue. I guess it's not rocket science.

 

[ChicagoDave]

+1.
When in doubt that enough glue soaked into the centering rings, you can always use a long dowel to deposit more glue around the forward centering ring (though the airframe), and the aft centering ring (helps if you do not glue the motor retainer until after the MMT is in the airframe).

Also, throw away the Q-tip. Instead:
1). Use a toothpick to deposit a solid and drip-worthy ring of glue to an area 1/4" BELOW where the forward centering ring will settle.
2). Insert the MMT half way into the airframe (from the aft end)
3). Now apply another drip-worthy ring of glue to the aft end of the airframe, all the way around.
4). Now push the MMT into place with one firm movement, and let it rest on the aft end of the rocket (support it sitting vertically, if necessary).

The centering rings will displace the beads of glue and coat themselves, and the airframe walls, with glue to spare. The excess glue will be pushed forward, and form a glue barrier, preventing the MMT from moving forward even if you didn't soak enough glue into the centering rings.
Shine a flashlight into the airframe. If you can see the glue barrier/dam, congrats - you are done.
If not, deposit some more glue to the FWD centering ring area with a dowel.

5). After everything has dried, apply another ring of glue to the aft end of the MMT. This builds another glue dam that will prevent the MMT from ejecting out the tail end, even if you didn't soak enough glue into the aft centering ring. Let it dry sitting tail up.
6). Now glue the motor retainer.

Now the MMT will outlive the rest of the rocket!

HTH,
a

Note: bolding is mine and the key to how I glue in the MMT.

This is .EXACTLY. what I do for all my rockets - Low, Mid and High Power. For Mid & High Power I usually leave the aft centering ring off (I don't glue it in) and I glue the forward centering ring again from the aft end as well.

Using this method the maximum acceleration my rockets have taken is 31g actual peak. And I've flow that rocket multiple times with the same motor so several times it's taken 30g+ with no issue. Oh, and that is using just regular Titebond II or III wood glue on plywood and cardboard tubing.

 

[jrap330]

Yeah, Tim at Apogee says work faster and use more glue. I guess it's not rocket science.

Yes more glue, as stated act as a lub and take longer to dry and as stated forget the Q-Tip...use a dowel and your scrap balsa wood...I bet too much glue remains on the cotton tip.... Built One and fly it and see if your new construction procedure is a success.

 

[jqavins]

As I noted in the other thread talking about this, a pen mark on the applicator stick can help you place the forward glue ring in the right place. To deal with the skinning thing, as others have said, more glue. But it doesn't have to be a ton more. Think of how thin that skin is, and how much depth of glue it will take to have a decent amount of liquid underneath. It doesn't take much, but it does take more than a tiny bit. So, in the range of "some"; more than a little and less than a lot. And the same goes for how far short you put the glue rings of the final centering rings' locations: more than a tiny bit, but less than a lot.

Wood glues are not all the same, but for this purpose they might as well be. Elmers White, Elmers Wood, TB, TBII, TBIII, and Gorilla Wood glues are all PVA, and any differences between them are in the details that are of little or no importance in LPR construction. (I've read somewhere that TBIII is not a PVA glue, but I just did some Googling to find out what it is, and came up with the answer that it's PVA after all.) The only somewhat relevant differences are in their working times, for which TBIII is better than most at 10 minutes. Elmers White glue is probably better yet. I don't know what you've got there.

Some more glue, right placement of glue. Work faster doesn't mean rush anything; don't rush and don't dawdle.

 

[Jay Rairigh]

+1
...don't rush and don't dawdle.

And yet glueing tubing couplers still frightens me. Yikes!

 

[jqavins]

Then don't use wood glue. White glue and epoxy both don't grab.

 

[o1d_dude]

“Some is good, more is better, too much is just enough.” -Old Mopar drag racing saying.

Therefore:

”MOAR GLOO!”

 

[Ronz Rocketz]

I add epoxy to the end without gluing the motor hook. Maybe unnecessary and adds weight yet I like the reassurance. The fins may look crooked but they’re not.

 

[Gramparocket]

If your using gorilla glue its a poly type glue and required water or humidity in the air to cure

 

[Ronz Rocketz]

If your using gorilla glue its a poly type glue and required water or humidity in the air to cure

Perhaps for the foaming type but maybe not for the wood glue he's using.

 

[Gramparocket]

For all my model rockets i use Tightbond II for glueing duty and epoxy for fillets. When you put a good bread of glue up inside for the forward cr, as you push in the MMT it should make a nice filet above the mount check it in the light if you don't have a nice filet in there you didn't use enough glue. Then fillet the bottom as shown in the photo above

 

[Gramparocket]

For all my model rockets i use Tightbond II for glueing duty and epoxy for fillets. When you put a good bread of glue up inside for the forward cr, as you push in the MMT it should make a nice filet above the mount check it in the light if you don't have a nice filet in there you didn't use enough glue. Then fillet the bottom as shown in the photo

 

[Gramparocket]

Ok I haven't used that blend before. Well when all else fails
Read the directions

 

[jqavins]

Yeah, the Gorilla wood glue is like any other PVA wood glue, as close to the same as all the rest are to each other. It capitalizes on the Gorilla name, which many have learned to like thanks to the polyurethane glue.

 

[RocketTree]

I have used that same Gorilla wood glue on over 40 rockets this year alone, including a couple large G power with no failures. Wait 24 hours after glue before launching. Make sure the lower motor mount ring is glued to the body. Install the screw-on motor retainer after. This glue takes a while to dry, and multiplied if confined to a sealed space (inside the body). All the best!