Friday, April 13, 2012

Building A Model Rocket - part 4

Cardboard tubes, balsa, paper and plywood all fall into the category of wood products, and the best adhesive for these materials is wood glue. A well-fit joint will be stronger with wood glue than even epoxy or industrial grade cyano-acrylate (CA) - aka 'superglue'. The technique to make these indestructable glue joins is called the 'double-glue method'.

That 'indestructable' claim isn't exageration either, because the materials being joined will break before the glue bond fails. It's not unheard of to have a fin break just beyond the glue.

So what's this secret way to glue rockets together? It's simple.

1. Lay a light bead of glue along the root of the fin.
2. Put the fin into place so the glue gets onto both parts to be joined. In this case, the fin root and the body tube.
3. Pull the pieces apart. There should be a light coat of glue on both pieces. Let it dry almost completely.
4. Put another bead of glue on the fin root, then press the two pieces together for good.

The reason this works is because the first coat of glue penetrates the materials to be joined, and the second coat chemically bonds with the first, locking eveything together. This makes for an incredibly strong joint.

For the Fat Boy, I suggest just sliding the motor mount assembly into position without glue (the metal motor hook will be sticking out the bottom), then gluing the fins into position. Use plenty of glue on the fin tabs which go into the slots of the body, and less on the parts of the fin root that don't fit into the slots.

It's fairly important to get the fins straight, but vertical alignment is more important that being perfectly spaced or perpendicular to the body tube. Since the Fat Boy has slots that the fins fit into, this is taken care of for you. When gluing the fins into place, take the nose cone off and set the rocket body upside down on your work surface so that you can look down at it and better gauge alignment. Looking from above, all three fins should point to an imaginary spot directly in the middle of the motor mount tube. Wipe excess glue away with your finger, smoothing it into the joint where the fin meets the body tube.

Tip: If you've already bought motors, put one in the motor mount because it's easier to 'aim' the fins at the small nozzle than it is at the imaginary point in the empty tube.

Leave that be and let's assemble the shock cord mount. Hopefully you've taken my suggestion and picked up a package of 1/8" sewing elastic, because the length supplied with the kit is just too short.

In the instructions is a diagram for the standard Estes shock cord mount, sometimes called a 'paper sandwich'. If you're building a different kit then follow whatever directions you've got, or use the following diagram to make one like ours (click it and it gets bigger). At the end, you should have a truncated pyramid shape folded twice with one end of the elastic embedded inside. Here, you should be generous with the glue, yet squeeze it out so that it's as flat as possible.

Let everything dry. Be patient, give it a couple of hours.

For each side of each fin, run a small bead of glue along the edge where the fin meets the body tube. Then take your finger and smooth the glue into the crease. Don't wipe too much glue away, just try to leave a smooth rounded fillet. If you're using brown carpenters glue, the gel formula will keep the glue from running and you can do all the fin fillets at one time. Otherwise, just do one or two at a time and let it dry before moving on to the next. These glue fillets add lots of strength to the fin joint and you should always do them.

We didn't glue in the motor mount before attaching the fins, so lets do that now as well. Apply the glue fillet to the seam where the centering rings meet the body tube, just like you did with the fins. Smooth it with your finger, and since we're using wood glue I recommend putting a second coat on after the first is dry. Do this for both ends of the motor mount. The top fillet is deep inside the body tube, so what you can do is take a long scrap of the balsa that the fins came from, and use that to apply the glue. Don't worry about being perfectly neat, the important thing is getting the joint glued.

The instructions tell you to mark a line between the fins to help you align the launch lug. Instead of that, I usually install the lug in the corner where the fin meets the body tube. This way the lug is automatically lined up vertically (the pre-cut fin slots help), plus it's stronger for the extra surface to glue against.

Finally, it's time to glue the shock cord mount into place. You should have a "paper sandwich" which has the elastic coming out of one end. Use a good bit of glue, and attach the mount to the inside of the top of the body tube, with the elastic pointing up towards the nose cone. Make sure you get it far enough down inside the body so that it doesn't interfere with the shoulder of the nose cone.

You can trim the elastic to a length of about 24" or so before or after gluing the mount into place.

The reason for making the shock cord longer is a phenomenon known as the 'estes dent'. What happens is that during the flight, the nose cone is propelled forward by the ejection charge. If the shock cord is too short, then the nose cone stretches the elastic until it zings right back at the rocket, crunching the top of the body tube. Using a longer shock cord prevents this from happening. A good rule of thumb is to make the shock cord 2-3 times the length of the body tube.

Once the shock cord mount is dry, use more glue to make sure it's firmly glued into place. This part is going to keep your rocket attached to the parachute and nose cone, so use some care here. You also want to make sure it's as flat as possible, so that there's nothing to snag the parachute on it's way out.

When everything is dry, tie the end of the elastic shock cord to the plastic loop of the nose cone. Use a double knot and make sure it's tight.

At this point, the rocket is ready to fly except for the parachute. Next Friday we'll put up the next part talking about the parachute and discuss flight stability a little more. We'll also get ready to paint the rocket.

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