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Old 03-10-2018, 10:31 AM
luke strawwalker's Avatar
luke strawwalker luke strawwalker is offline
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Join Date: Dec 2007
Location: Needville and Shiner, TX
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Quote:
Originally Posted by olDave
clhug---- unscrew the bottle top and stick in the "handle" end of a cheapo paint brush, or also can use a bamboo shishkebab skewer (which we all have among our tools and scratch build materials, right?). Stir to your little heart's content.
Elmers glue has always separated a little if left on the shelf for long, IME. Not a big deal. I usually squirt some out of the bottle and thin it down 1:1 with distilled water before working with it. Which leads me to....

neil----position your MMT assembly into the body/airframe tube DRY until you get it where you want it, *****THEN***** apply a little glue around the outer edge of the aft centering ring. Not accusing you personally, but have seen many modrocs built by many different people that were waaaay over-glued. I have long built mine by inserting the MMT, brushing a little water around the perimeter of the aft CR (using a cheapo water-color-style brush from the dollar store), then brushing ONE or TWO brush-loads of thinned white glue (see above) around that same perimeter. If your parts are properly sized and fitted, this will be plenty of glue. This approach has worked just fine for me for decades, including mounts for D and E motors.


Good point on the "grabbing" problem, particularly with motor mounts. Yellow wood glue is more prone to that problem than white glue, though white glue CAN suffer that effect as well, PARTICULARLY with those narrow motor mounts that use a long sleeve to bridge the motor tube to the body tube (like a BT-20 in a BT-50) that has a lot more surface area. What causes it is when the glue get smeared into a super-thin layer as the centering ring slides into the body tube and essentially "squeegees" the glue super-thin as it slides into the tube. The super-thin layer can essentially "dry" almost instantly, as the moisture in the glue RAPIDLY is wicked away into the paper fibers on the inside wall of the tube and of the centering ring itself.

The solution is to use your handy-dandy bamboo skewer as a glue applicator. Mark the skewer with a magic marker with one end at the top centering ring and mark the skewer at the bottom of the centering ring PLUS the distance you want or need the bottom centering ring recessed into the bottom of the body tube (say, a half inch, or whatever is called for in the instructions. In this case, we'd put the mark 1/2 inch below the bottom centering ring since it will be recessed inside the tube 1/2 inch. Apply a dollop of glue to the skewer, and use the skewer to apply and smear the glue on the inside wall of the body tube, after sliding the skewer in to the proper depth where the mark it even with the back end of the body tube. Make take 2-3 applications or so to work the glue all the way around the body tube, and it doesn't have to be perfect. I then slide the motor mount in halfway, apply a little white glue just inside the back end of the body tube, and then slide the motor mount in the rest of the way to the proper depth in one quick, smooth motion. Then I rotate the tube very slowly for a few moments to make sure any excess glue spreads itself evenly around the circumference of the joint, and set it aside to dry. A thin fillet around the aft centering ring/body tube joint and you're all set.

I personally wouldn't rely on one aft fillet on the aft centering ring to lock the motor mount in place... under thrust this fillet will be in TENSION and prone to tearing away, which would allow the motor mount to shove itself up inside the body tube toward the front of the rocket. Maybe it works, but *I* wouldn't trust that alone. If you want to install the front ring "dry" and then apply a bead of glue inside the aft end of the tube to lock the aft centering ring from above and then fillet it from below, I'd be a lot more convinced that would hold up under thrust, but basically at that point the front centering ring is just "along the for ride" and contributes essentially *nothing* to the strength of the motor mount under thrust... and filleting the aft ring from the back to resist the ejection charge "kick back" would still be a good idea IMHO...

Personally I prefer applying *some* glue to both rings, so they share the load of thrust and ejection. It's easy enough to do without risk of "locking" the motor mount in place prematurely if you use the skewer to apply the forward ring of glue at the proper depth, and install the motor mount quickly and smoothly to the correct depth once the glue is applied.

Later! OL J R
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