Build It - 6

This is a series where we build a model rocket step-by-step. You can find the rest of the series here.

The main part of the post is in the extended entry so you don’t have to deal with it if you don’t want to, but I hope you follow along because when we get done you’ll have built and flown your first model rocket. Questions asked from before are answered too.

If you have questions, please leave them in the comments or email me.

This time we’re going to do the last of the glue work, and it’s quick and easy.

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 – top and bottom. 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 ‘fired’ 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. In the next day or so I’ll put up the next part talking about the parachute and some information about flight stability. We’ll also get ready to paint the rocket.

Posted by Ted at January 12, 2004 05:16 AM | TrackBack