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.
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|
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).
|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.