In this level-2 EZI-65 lesson, we will build the main aiframe and install the motor retainer.

• Prepare Body Tubes and Fins
• Install the Motor Mount Tube
• Install the Fins
• Install the Motor Retainer

Prepare Body Tubes and Fins

Once the fiberglass has cured (a day or so), you can cut off the excess cloth at the ends of the body tubes and around the perimeter of the fins. Don't worry about cutting the cloth off exactly flush; leave at least a 1/16" lip around the edges of the parts. Also cut off any dangling fibers. Now you can see how well you did while fiberglassing. Ideally you'd have no dry spots or drops. Dry spots (cloth not bonded to the body tube) are particularly bad. The cloth which isn't bonded to the tube must be cut out carefully and replaced with epoxy. Drops just need to be sanded off.

Before sanding, wipe down your tubes with laquer thinner to remove any "amine blush" (waxy coating produced by the curing epoxy). West System recommends using hot water and abrasive soap, but I didn't want to get water on these tubes because of the paper insides. Sand your tubes with 100 grit sandpaper to take off all high spots and drops. Don't sand through the epoxy getting a smooth surface. (Small depressions or ridges can be filled later.) You should also sand the fiberglass overlap at the ends down to the edge of the cardboard tubes. Use the cardboard tubes as a guide and don't sand down into them or your ends will not be square anymore.

Once you have sanded down the high spots, check for low spots and fill them. If you have places you had to remove dry spots, fill them with epoxy (to maintain the strength). You can use epoxy or auto body filler to fill in any low spots. I like to use West System 407 "low-density" filler to get a nice thick mixture. That way you make it slightly proud of the surface without worrying about being able to sand it flat. It also feathers out nicely. Once the filler has dried, sand the entire tube again with 150 grit sandpaper. You should now have a smooth to the touch and very hard set of body tubes. Truly things of strength and beauty!

The fins should require very little sanding. First, sand the edges flat, removing the fiberglass overlap. Then put a slight round-over on the forward, top and aft edges of the fins (leave the notch and root edges square). The surfaces of the fins should need very little or no sanding. If you have puddles of extra resin, those will need sanding smooth. After you get your fins sanded to the right shape, fill the exterior edges (not the notch and root edges) with unthickened epoxy. Paint a little on to seal the edges and wipe the excess off with a paper towel.

We've now prepared all parts for assembly. If you haven't already, read the original EZI-65 instructions all the way through so that you know what's coming up.

Install the Motor Mount Tube

Now take your motor mount tube with the two centering rings and install it into the body tube. Before mixing any epoxy, sand the inside aft end of the body tube to remove any epoxy residue and smooth the edge. Test fit the motor mount tube, it should slide in with only slight pressure. Sand the edges of the centering rings if necessary, but you don't want the assembly to be loose in the body tube.

We want to get as much epoxy on this joint as possible, even though we have only partial access to the inside. We'll concentrate on getting a good epoxy fillet on the forward edge of both centering rings. Push the motor mount tube in from the rear of the rocket until the forward centering ring is just past the end of the fin slots. Get a good thick ring of epoxy around the inside of the body tube at the top of the fin slots. Now, push the motor mount tube into the rocket the rest of the way. Rotate the assembly as you go and stop when the aft end of the aft centering ring is at the top of the fin slots. (If your aft centering ring is not exactly perpendicular to the motor mount tube, make sure the centering ring doesn't block any fin slots.)

You can stop now and let the epoxy cure. Or, for the intrepid, continue with the second fillet on the forward centering ring. Put the epoxy fillet in from the forward end of the body tube around the outside edge of the forward centering ring. Be careful not to get any epoxy inside the motor mount tube. Make sure to get a thick fillet of epoxy around the perimeter of the centering ring. I had a bit of a problem getting it behind the eyebolt, but a long thin stick helps here. Let this cure completely.

When letting this joint cure, stand the body tube upright so that the epoxy doesn't run away from the joint. Don't set the assembly on the motor mount tube though as it may get pushed up into the body. You can stand the body tube on a roll of tape with the motor mount sticking through the center.

Install the Fins

Before mounting the fins, you will need to re-open the slots in the body tube. You should still be able to see the slots through the fiberglass. My fins ended up about 1mm wider because of the fiberglass so I actually had to widen the slots a tiny bit. Draw a pencil line along the same edge of all three slots (say the left edge). Make another mark at the bottom and top of each slot. Draw a pencil line just outside the other edge of each slot (to widen them). Be careful to keep the lines parallel to the original slot, otherwise your fins will be crooked. Now cut out the three narrow rectangles. You can use a sharp hobby knife or a cutting wheel in a motor tool. If you use a motor tool, be careful not to cut outside the slot. If you use a hobby knife, be sure to use a straight edge so you cuts don't wander.

Set the body tube on its side and make sure it won't roll while you're attaching the fins. Turn the body tube so that one fin slot is at the top of the body tube. Pick an fin and test-fit it in the slot. It should bottom out on the motor mount tube and the notch should touch the outside of the body tube. (Remember that the notches on the fins go forward.) Sand the fin so that it has good contact along the base and on the shoulder of the notch.

Place a generous bead of epoxy on the base of the fin and on the notch shoulder, but not on the vertical part of the notch. Install the fin through the slot and make sure epoxy squeezes out along the length of the base and the shoulder. Make sure the fin is straight up and down. Reach into the body tube from the aft end and add more epoxy to build up a nice fillet along both sides of the base. Apply the outside fillets as well. Dab epoxy along both edges of the fins outside the body tube and smooth them out with a finger. Make sure these fillets are smooth and even as the epoxy will be very hard to sand later. Place a long pieces of tape over the top of the fin and down to both sides of the body tube so that the fin stays aligned during while curing.

Make sure you fins are as straight as possible, both in-line with the body tube and perpendicular to the motor mount tube. The straighter your fins are, the better your flight will look. Extremely misaligned fins may cause your rocket to be unstable.

Let the first fin cure at least for the recommended "handling time" and repeat with the other two fins. You now have the fins firmly attached to the motor mount tube and to the outside of the body tube with your fin fillets. You need more. You want fillets on the inside of the body tube along the fins and we still have to bond the aft end of the aft centering ring to the motor mount tube, body tube and fins. Add fillets to the inside of the body tube along both edges of each fin. When you're adding these fillets, turn the body tube so the fin your're working on is at the bottom. Space is pretty tight inside the tube, but luckily these fillets don't need to be pretty, just strong.

Once the fin fillets have set, we want to finish bonding the aft centering ring to everything it touches. Stand the body tube on the forward end and add epoxy fillets around the edges of all three compartments formed by the fins. We've now completed bonding the motor mount tube and fins to the body tube.

Install the Motor Retainer

The rear centering ring adds strength to the motor mount tube and closes the aft end of the rocket. If you're using an Aero Pack motor retainer, you can put on the last centering ring now. Put a good bead of epoxy around the inside of the body tube and outside of the motor mount tube 1/2" from the end. Slide the aft centering ring onto the motor mount tube and push it into the body tube until the aft edge is flush with the end of the body tube. Wipe of any excess epoxy and let the epoxy cure with the body tube standing upright.

Mask off the motor mount tube at the aft edge of the centering ring. Paint the centering ring with unthickened epoxy to seal the surface. Damp of any excess resin but make sure the centering ring surface is saturated. Use the epoxy to make a smooth surface along the entire aft end of the rocket, including the end of the body tube. Let cure. Wipe off the amine blush with laquer thinner and sand with 150 grit sandpaper to smooth the surface.

Now we can install the motor retainer using "J. B. Weld," which can be purchased from automotive and hardware stores. It comes as two small tubes of the two-part adhesive (mixed 1:1). One part is black and one white. You can tell when they are mixed sufficiently by the color (a uniform gray). Paint a thin layer around the outside of the motor mount tube that sticks out of the aft centering ring. Press on the Aero Pack retainer (it will go on with some difficulty) and push it down until the retainer bottoms out on the aft centering ring. Use your finger to make a nice fillet out of the squeezed-out J. B. Weld around the retainer and centering ring.

If you're not using an Aero Pack retainer, you need to drill holes and install the T-nuts in the aft centering ring for your own retainer system before bonding it. See the INFOcentral Motor Retention article for examples.