Saturday, December 6, 2014

N00b Mistakes #1 - The Wrong Amount of Glue

I'm starting a new, occasional series of posts on this blog - N00b Mistakes. These are mistakes I've made while building (or launching) rockets, and hopefully, things I've done to fix them. If you are new to rockets, I hope you'll find these useful, as they may help you from screwing up something in your builds. Or maybe they'll give you ideas for fixing problems; sometimes, it looks like you've "ruined" a kit, when in fact, you can fix many little flaws with a little patience and care.

This first N00b Mistake is about glue. How much do you need?

Well, for most applications, particularly attaching fins to your rocket, you don't want too much. A thin layer of wood glue or white glue will often be much stronger than a thick, globby layer.

Here, I'm building a rocket with compound fins - made of two pieces
which must be carefully joined together before attaching to the rocket...

This is where a very thin layer of glue is much better and stronger
than a thick layer, which would make more mess, and be less strong.
 This is also where a double glue joint will help you a lot - you apply a thin layer of glue to the fin root edge, stick it in place on the rocket, then remove it, and let it dry for a few minutes. Then, when you apply a new, thin layer of glue to the fin edge, and attach it to the rocket, it will be much stronger, and take less drying time. I have no idea why this is - probably something to do with polymers or chemicals or magic or something.

But sometimes, you do want a more generous amount of glue, and that's what I'm going to address here.

What Have I Done??

When building the Estes Cosmic Explorer, I needed to join two body tubes with a tube coupler. This is a short, slightly narrower section of tube which goes inside the two main sections of body tube, and it's used to make longer rockets. It's also used to join the stages of a multistage rocket, like my Janus I, although in that case, you're not gluing the tubes together, you're holding them together with friction - you want them to separate in flight!

An assortment of couplers for various sized tubes

I digress...

Anyway, I was building the Cosmic Explorer, and went to join the two body tubes. First, I glued the coupler into  the bottom tube, sticking halfway out.

The red coupler inserted into the lower body tube section

Knowing that less glue is stronger than too much glue, I applied a thin ring of glue inside the upper tube, and shoved the two together.

Then... disaster! Wood glue and white glue tend to seize up when they get spread really thin. This is what happened with my tube coupler.


I have read that this tends to happen a lot with the Estes red couplers, particularly with Elmer's Glue All white glue (they changed formulas not long ago, and I guess a lot of rocketeers have had issues with it).

As I pushed the two body tubes together, there was a gap. I could see the red coupler between the two white tubes, and could not get them together. I pushed as hard as I could, trying to get the tube ends flush with one another, but it wouldn't budge - I even crimped the body of the rocket!

Trying to force the two tubes together didn't work - and it only gave me this nasty crimp in the body tube.
 How Did I Fix This?

Well, once the tubes were glued together, they were together forever, like Romeo and Juliet.

Primer did nothing to hide the gap in the tubes - primer, in fact, only magnifies flaws you didn't see before.

I used the trick for filling in those ugly spiral grooves in the tube - CWF (carpenter's wood filler). I brushed a ring of it over the gap, let it dry, and sanded it down.

I hadn't perfected my tube filling yet, so if you look at the finished rocket really, really closely, you can see a slight bulge from the filler. But this is pretty well masked by the fact that the color of the paint job switches from white to black at that point - so it's nearly invisible.

Conclusion, or How To Avoid This In the First Place

When building rockets, there are a number of things you must glue inside other things. The motor mount, of course, but also sometimes a tube coupler, or even an ejection baffle - a little device that allows gas from the ejection charge to push the nose cone and parachute out at apogee - the top of a rocket's flight - while protecting the parachute from flaming particles of black powder. It's an alternative to using recovery wadding on every flight.

An ejection baffle is inserted halfway into the body tube.
It allows ejection gas to pass through to eject the nose cone and
parachute, but protects the parachute from burning particles.
Hence, no need for recovery wadding.

Here's a picture of an ejection baffle inside my 3D Rocketry Nautilus II rocket. I got it in there successfully, but I made another really dumb n00b mistake when I installed this. Can you guess what it was?

Hint: The little metal thing in the middle of the baffle is a screw eye. Guess
what I forgot to do... (I'll cover this in another N00b Mistakes for you Rocket Noobs).
 Anyway, whenever you need to glue a thing inside something else, you should do several things. First, you should always "dry fit" the part - basically stick it in at least partway without any glue to make sure you can get it in there in the first place. If you put glue in place and then try to slide the motor mount in without checking the fit first, you might find that you can't get it in all the way, because one of the centering rings needs a little sanding. Then, you're pretty much in trouble, because the glue will start to set as you try to get the motor mount in or out of the rocket and fix the problem.

Always test internal parts to make sure they fit
before trying to glue them into place.

See - the aft centering ring needs to be sanded just a bit so it slides into
place. If I'd already applied glue to this before testing, I'd have a problem.

Second, make sure there's enough glue so that the part will slide in easily. You don't need to overdo it, but a paperthin layer of glue is probably too little. Kit instructions will tell you to apply a ring of glue inside a body tube with a craft stick or dowel rod, but whenever possible, I find the best way to do this is to use my fingers to apply the glue in the first place.

Sometimes this isn't possible - you need to get glue too far into too tight a space to use your fingers, and you have to put a bead of glue on the end of a dowel rod or craft (popsicle) stick. In that case, just make sure you get a good amount in there. But if you can use your fingers, do. You can feel with your fingertips if the glue is thick enough to slide easily, or if it's really thin and tries to grab hold of your fingers.

Finally, insert the component in smoothly and in one motion. Once it's in place, you can give it a little twist if you're able to, to make sure the glue is evenly spread. But once it's in, leave it alone.

While you do want to be conservative with glue on things like fins and launch lugs - and then strengthen those joints later with glue fillets - when installing something on the inside of a tube, just make sure you get enough glue around the inside of the tube so that it slides into place smoothly before it grabs hold of the parts. 

It's also important to remember that when you're putting in something that receives a lot of force during rocket flight, like a motor mount, you want there to be an internal fillet to reinforce the joint. Using a little extra glue will create a fillet when you push the part in place, because it will form a thick little ring of glue wherever you stop pushing it.

In the end, though I thought I'd really screwed this rocket up, it's become one of my favorites. It flies really straight, and barely rolls at all! Here's a video of the launch - a full launch, from liftoff to landing. There's a treat at the end - a slo-mo replay of the launch.


  1. I've had trouble with motor mounts and couplers seizing up. I even took a bunch of scrap couplers and body tube pieces and attempted a controlled experiment using different glues and techniques to find what works and what doesn't. In my experience, and some but not all builders report similarly: Yellow wood glue (my preferred glue for most wood-to-wood and wood-to-paper construction) seizes up very quickly; so does Elmer's Glue-All. Some have reported no trouble with Aleene's Tacky Glue, and I've used it successfully, but the last two times it froze up immediately after (and was trying to freeze up immediately before) I got the part in place. As for the amount of glue, a thin application is definitely not the way to go... but I've had trouble when using larger amounts too. That last time with the Aleene's I used a LOT of glue, and it still seized up.

    At present, what I do depends on the tightness of the part. Occasionally, if it's a coupler or motor mount that slides in easily, and I'm feeling lucky, I'll fall back on the yellow glue or the Aleene's. But if it's at all tight, or I'm not feeling so lucky, I'll mix up a little epoxy and use that. I'm not a fan of epoxy in low- and medium-power builds for almost any other purposes, but it's great for motor mount and coupler installation, because it does not seize at all.

    1. I've been wondering about that tacky glue stuff. I've heard it's pretty easy (usually), but I was wondering how strong it is. I meant to post a question about that on TRF, but never got around to it.

      As for epoxy, I haven't used it yet, but I plan to soon, as I have a few mid power rockets to build (three Estes Pro Series II rockets, actually). At least one of these - the Leviathan - uses a tube coupler.

      Perhaps once I've used the epoxy, I'll update this post based on what I've learned.