The instructions for the Big Dog suggest using wood glue for installing the motor mount. I decided against this, because I wanted to be able to position the mount exactly where I want it. Sometimes wood glue will seize up at the wrong moment when you install an item inside a tube, and then it's stuck there - sometimes halfway in place.
So I mixed up some 30-minute epoxy.
If you're new to rockets, epoxy is a type of adhesive which comes in two parts - a resin and a hardener. It stays in liquid form until you mix the two parts, then it slowly sets, or "cures." The advantages are added strength, and the ability to reposition the part for a little while - because it stays gooey until it cures. It doesn't simply dry out; the two parts mixed together create a chemical reaction in which long polymer molecules are formed, creating a strong bond.
Usually with epoxy, longer cure time means a stronger joint, and 30 minutes is pretty good. 5 minute epoxy might be too brittle, and with a high thrust motor, might crack.
Now, the shock cord is mounted directly to the motor tube. When I slide the motor mount into the back of the rocket, I need to avoid getting the shock cord stuck in the epoxy inside the tube, and potentially cemented to the inner wall of the rocket. The solution for that is easy - I run the shock cord down through the motor tube and pull it out the back.
I also covered the threads of the screw-on motor mount with tape, to avoid getting stray epoxy on them.
Epoxy is bad for your skin, so when using it, you really need to wear gloves - the blue, nitrile gloves are the ones I always see recommended, so that's what I use. You can buy them at drug stores and hardware stores.
I mixed up a small batch of epoxy and ran a ring of it around the inside of the aft end of the airframe, then inserted the motor mount.
Then I realized I'd made a wise choice using epoxy. It turned out the centering rings were a little loose on this rocket. Very loose, in fact. I wonder if this was because of the same problem that led to a fin marking guide that was too short - that the body tube diameter is slightly larger than stated on the kit.
Anyway, epoxy is quite strong, and I have more confidence that it will hold such a loose motor mount in place than I would have with wood glue. And it oozes a bit, so it was more likely to fill in the gap between the centering ring and the airframe.
The centering rings were so loose, in fact, that I couldn't tell by feel if I'd gotten any epoxy on them at all - I couldn't feel much resistance by moving the mount in and out of the rocket. I decided to let things set and see what the result was.
Because it was so loose, and would take up to thirty minutes to set, I taped the body upright on a launch rod, then loosely taped the mount in place for support. If I'd simply left it in place, it would have fallen out, and then I'd have epoxy on the carpet!
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| See the shock cord hanging out the back? |
After a few hours, I checked the bond, and it was set and very strong. But I didn't have any epoxy on the aft centering ring - and it was a little loose. I had to fix that.
First, I needed to get the shock cord out of the way, and back inside the rocket where it belonged. So I clipped a clothes pin onto the end, and dropped it back through the motor tube.
As you can see, there's not much space between the motor mount and the airframe. Epoxy is messy, and I needed a way to get a small amount in there around the centering ring, and not anywhere else.
I taped the rocket to the launch rod again, this time upside down. I grabbed a spare medicinal dosage syringe and mixed up some epoxy.
This isn't the same syringe I use for filling tube spirals. You can clean up uncured epoxy with rubbing alcohol (you can see some in the little cup in the photo), but I wasn't sure I wouldn't ruin the syringe.
In any case, I mixed up some more epoxy and drew it up into the syringe. I cleaned off the tip with a paper towel and a little alcohol, then injected the epoxy all the way around the centering ring. A coffee stir stick helped me spread it around so I got it fully around the edges of the ring, creating a good bond with the body tube all the way around its circumference.
When using epoxy, you need to examine everything carefully to make sure you don't have any unwanted drips anywhere, and immediately clean them up with alcohol. I removed the tape from the motor retainer to avoid any drips permanently cementing the tape in place, cleaned up the edges of the body tube, and then flushed the syringe with alcohol. The syringe cleaned up just fine, so I can use it again.
After letting the epoxy set up overnight, the whole thing was really strong.
After this, I moved on to shaping the fins into airfoils.
This is as far as I've gotten with this rocket so far. We'll talk about the fins, and maybe even attach them to the rocket, next time I post on this series.
Click here for Part 4.
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