LESSON FOUR: Assembling the airframe/fin canister assemblies

Our Honest John project is moving along rather nicely, and with this installment, we'll have the airframe finished up and will be moving in the next installment on to the electronics bay/main recovery compartment, all encased in the unique two-part signature nose cone profile.

When we finished the last installment, we had our three fin canister assemblies ready for the fiberglass-reinforced body tubes. In this installment, we are going to join both of those together to form a completed air frame assembly, then add a decorative band around the mid-section and attach two launch lug standoffs. At that point, we will be ready to prime the air frame.

To begin our assembly, we need to mount the fiberglass-reinforced air frame tube onto the fin canisters. All three rocket airframes will assemble exactly the same — there is no differentiation between any of the three with regard to the rest of the lessons. They only differed in motor mount size.

To assemble them, you need to apply a band of epoxy 2" wide 1/2" up inside the airframe tube. This will allow us to use Testors plastic cement on the forward 1/2" of the fin canister joint. I used a regular wood craft stick to spread a thick coat of epoxy in the tube. Work quickly, because we want to get this assembled before the epoxy starts gelling.

In the photo above, the red area marks the band of Testors plastic cement around the forward band of the fin canister. By applying the cement and epoxy separately in this manner, we can keep them from mixing in with each other and not doing their job. I apologize for the picture quality, but sometimes you don't know these things until its too late.

Once you get the epoxy and cement in place, slide the two parts together, twisting the airframe in a circular motion as you slide it down on the fin canister. The idea is to spread the cement and epoxy around on the parts, getting an even distribution of the adhesives. If you have a 54mm version, make sure you check your Kevlar recovery harness to make sure it is inside the motor mount tube and out of the way. Clean up any excess glue on the fin canister/body tube joint with acetone.

Here are our airframes with the body tubes in place. Let's set the airframes aside and talk about the decorative band and the launch lugs while the epoxy and styrene glue sets. In the original Estes kit, the decorative band around the middle of the airframe was just a series of printed bands on card stock, and getting the holes into them were your problem. That would hardly work for me, so I approached it a little differently.

Using an extra 3" phenolic coupler tube, I cut off bands the same width as the printed bands in the kit using my combination cutoff saw. Then I built a little jig that would positively hold the band in place so I could drill the series of holes in the band.

The photo above shows my jig holding the bands in place on my drill press. I drill a hole, rotate the band and drill another. Repeat. It's a rather simplistic approach, but it does the job, puts the holes the same distance from the edge and gives the resemblance of the original version.

Let me offer this suggestion if you do this: build an extra. These are flimsy enough with the holes drilled in them that they are easy to break. Another suggestion is to go slow. If you break the band while sanding it, put thin CA on the break and let it dry. It will take some work, but the results are really good.

To install the bands, we are going to have to take a slice out of its circumference. I made my cuts inside of an opening, and overlapped the ends, and cut off the excess, remembering that it is easier to cut more off, but really hard to add some back. So take your time and don't get in too big of a hurry.

The bands mount half-way the length of the airframe tube. When you have yours fitted, hold the joint firmly and secure it with thin CA. You want to place your joint on the back side of the rocket, where the launch lug standoffs will go. Work your way around the band, filling each hole with a drip of thin CA. When you get all the way around, the band will be firmly joined to the airframe. Some touch up work with sandpaper and you will be ready for primer, which will also help seal the edge of the bands and secure the band's bond.

While the bands were drying, I attacked the launch lug attachment issue. I had settled on using the new Public Missiles Ltd's Linear Rail Lugs, which are made of durable yet light-weight urethane. They are advertised as gliding smoothly and securely along both the Blacksky ProRail and the standard 1" (#1010) 80/20 brand rail. According to PML's website, they have the same frontal area as rail buttons but with much greater surface area so that they can be mounted using epoxy alone in those situations where a screw would interfere with internal components. In this case, because I would need a standoff to clear the nosecone, we could use a screw with them to hold them on.

I thought the full-length launch rail lug was too long, so I cut each one in half, as shown in the picture above, which was similar to the standard forward rail guide shown in the original kit. I then set out to build a standoff that carried on the lines the rail guides displayed.

The results are shown above, and consist of gluing two layers of 1/4" birch plywood strips together and then sanding it down to roughly a 3/8" square stick of plywood that was then cut on taper to match the rail guide. The results look pretty sweet. The lugs have an inset on the back, which is 3/8" wide, so they slip over the 3/8" wide standoffs. In the previous photo, you can see sandpaper on a craft stick — I stuck some adhesive backed sandpaper on the stick to true up the back mounting surface of the guides, as it looked like it had a little off-square surface going on and this allowed me to square things up.

After carefully marking my airframe for their locations, I epoxied the plywood standoffs in place, one at the very rear of the airframe tube, the other about 2" back from the forward edge. I would attach the rail guides after paint, but the standoffs were filleted so I could prime everything.

Before I got the primer out, I filleted the fins to the fin canister using 15 minute epoxy and using my finger to form the shape, smoothed out each one and waited until it dried, then rotated and continued on to the next. Remember, I am having three times the fun.

Once the fillets were done, I used 150 grit sandpaper on the airframe and fin canister to carefully sand the fins, fillets, fin canister and airframe before coating each rocket with a couple of good layers of Duplicolor automotive grey filling primer in the spray can. This is a good filling primer that goes on light and more important, is friendly to styrene plastic.

Hint on sanding the fin canister: the better you prep the underlying surfaces, the better the end result will look. Take your time and do it well.

In the photo above, you can see how my decorative band turned out. I think it works well.

In this photo, you can see the rear of my decorative band and where it was joined on the back side. By joining the decorative band on the inside of an opening, it doesn't look so unusual, yet is another reason to put the seam on the back of the rocket.

And finally, here are all three airframes sitting in primer, just awaiting a nose cone and some olive drab paint.