The first step was to epoxy a thrust ring to the end of the motor tube.  The ring came with three 10-32 threaded PEM nuts pre-installed.  These will be used to hold the motor retainer plate in place.

Motor tube with thrust ring attached.

To make sure the fins will end up straight, I marked the motor tube with some fin alignment lines.  To get these lines  to be exactly 120 degrees apart around the circumference, I used a piece of blue masking tape.  First step was to wrap the tape around the tube and put a line across the tape at the overlap joint.  Then remove the tape and lay it out straight.  If done right, part of the line on the tape will end up on each end of the tape once it is laid out straight.  Then measure the distance between the lines at each end of the tape.  Divide the distance by three and mark the tape at each third.  Then carefully re-apply the tape to the tube and get the original marks at the overlap joint to line up again.  The marks on the tape for the equal thirds are now exactly 120 degrees apart. 

The alignment lines for the fins can now be drawn down the length of the tube.  To make sure they are exactly parallel with the axis of the tube, I use my Black&Decker Workmate with the top opened-up just enough to allow the tube to sit into it without falling completely though.  In this case, about 3 inches for a 4 inch tube.  (Make sure the opening is a constant width for its entire length.)  A line can then be drawn along the length of the tube by running a pencil along the surface of the Workmate or by using a right angle piece of metal as shown in the photo.  I prefer this method over just using a standard right angle applied directly to the tube because it tends to give much more accurate results.

The two photos above show the "dry fit" check for the fins and the centering rings in the booster. Everything is held in place by the C-clamps and the fin alignment fixture.  If it all looks good, then the parts are removed and then re-assembled with epoxy applied.

The photo on the left above also shows two other types of epoxy adhesive that use this same scheme.  The E-120HP gives a longer working time (120minutes).  It is a non-sagging, aerospace grade with superior resistance to rapid temperature changes. It has high shear strength and resistance to peeling. The E-60HP is a metal and plastic bonder with high sheer strength and peel resistance. They are part numbers 6430A24 and 6430A28 respectively.  There are many other types of epoxy available as well.

Here the centering rings and the fins have been epoxied into place.  Some blue masking tape was used near the top of the fins to hold them in exact alignment with the marks on the motor tube.  The fin alignment fixture did a good job keeping them aligned at the bottom, but the top of the fins could easily be slightly out of place due to the long root cord of the fins. 

A good way to cut Kevlar is with the special shears (that look like scissors) available from CST for $17.95.  Part number E814.

The Kevlar strips were saturated with West System Epoxy and laid into the fin root joint on the motor tube.

Close-up of Kevlar reinforcing strips epoxied into the fin joint.

I did not have the tooling required to install the special threaded PEM nuts like those that came with the kit, so I decided to just use a more conventional implementation.  I used two threaded inserts into a bulkhead that is secured to the motor tube and centering rings.

First I created two curved wood blocks that were shaped to match the outside of the motor tube and the inside of the body tube.  A #10-24 threaded steel insert was screwed into the center of each block. (McMaster-Carr part number 90192A114.)  The photo at right shows the two curved wood blocks with inserts installed.  Two extra inserts are also shown in the lower right corner of the photo.

A couple of clamps were used to position the wood blocks onto one of the centering rings on the booster.

West System epoxy was used to secure the wood blocks in place.  They are epoxied to the motor tube and the adjacent centering ring.

The long aluminum alignment piece was removed after the epoxy had cured.

Next step was to slide the booster body tube section into place.  The fins slide nicely into the precut slots.  The rail guide anchor can still be seen because the tubing is translucent.   This is handy because it allows the center of the rail guide mount to be marked on the body tube.

Once the center point was marked, the tubing was removed and drilled.  The tubing was then slid back into place to check the alignment.

This photo shows a test fit of the rail button installed into the anchor.  Two nylon washers serve as standoffs to move the rail button a little further away from the body tube.  Doing so helps prevent the rail from touching the rocket and scuffing the paint at the forward end. 

Four #10 x 6-inch threaded rods were also mounted in the four holes around the perimeter of the bulkhead.   These will be used to secure the bulkhead to the motor tube.

The motor tube that comes with the Air-X "Black Rock" kit is not quite long enough to reach the bulkhead if length is allowed for an Aerotech 98/15360 motor case.  Therefore, these four threaded rods allow the bulkhead to be mounted a few inches beyond the end of the motor tube. 

The four threaded rods are tacked into place on the end of the motor tube using Hysol epoxy and then the bulkhead was removed.

The Kevlar cloth used here is the same as was used for reinforcing the fin joints described above.  It is 8 oz cloth 0.026" thick and available from Thermostatic Industries, Inc.

Some small black paper clips were then used as miniature clamps to hold the Kevlar tightly formed around the threaded rods while the epoxy cured.  Small strips of wax paper between the Kevlar and the paper clips prevented the paper clips from becoming stuck in the epoxy.

The paper clips and the wax paper were removed once the epoxy had cured. 

A test fit of the bulkhead shows the rods are still properly aligned. 

The next step was to epoxy the bulkhead to the top of the booster coupler.   To help strengthen this joint, I used more Kevlar. (of course!).

A small strip of Kevlar about 16 inches long and 2 inches wide was cut with some tabs along one edge that are about 0.75 x 2 inches. 

The Kevlar strip was then rolled into a shape to fit inside the coupler tube.

The Kevlar strip is then placed in position at the end of the coupler tube and with the tabs up against the bulkhead.  Here you can also see the bottom side of the U-bolt that is installed in the bulkhead.  The photo is looking into the inside of the booster coupler tube.

The Kevlar was then saturated with West System epoxy to bond everything in place.  Once the epoxy was cured, the holes for the four threaded rods were re-drilled.   This coupler tube and bulkhead are now ready to be installed into the booster body tube.

Next the coupler was "dry fit" onto the forward end of the booster section.  The four threaded rods on the motor tube extended through the coupler bulkhead and were trimmed to precisely the right length using a hack saw.

The main body tube needed to be cut into two pieces.  I did this by installing a steel hose clamp at the location to be cut.  The hose clamp serves as a very nice guide for a hack saw blade.  This method results in a fairly accurate and a nice "square" cut without needing any specialized tools. 

The airframe section can be seen in the foreground.  It came already slotted for the fins.  The inside surface of it has been roughed-up with sandpaper to give the epoxy a better bonding surface.

These two photos show the "dry fit" test to check things out prior to applying all the epoxy.   The airframe slides down over the top of the booster fin can.

The idea is to keep the airframe body tubing spaced out and away from the edges of the centering rings as it slides down into place.  This will avoid smearing off all the epoxy on the edges of the centering rings.  Once the body tube is fully down and in place, then the wood strips can be pulled out to allow the inside of the body tube to contact the edges of the centering rings.   All this ensures the epoxy stays in place and then produces a nice solid bond between the body tube and the centering rings.

Hysol epoxy was then applied to the outside edges of all the centering rings.  The applicator gun and mixing tip made this step very quick and easy.  This epoxy is thick enough to stay in place until we slide the body tubing over it.

The blue masking tape is protecting the threaded nuts on the thrust ring to make sure that they don't get accidentally clogged with epoxy.

After epoxy was applied to all the fin joints, the wood sticks were pulled straight out the bottom.  This allowed the body tube to come down into contact with the centering rings and fins. 

The Black Rock kit included a short 1" section of coupler tubing that fits neatly into the rear end just aft of the thrust ring.  This coupler ring provided a nice stiff structural  piece for the hose clamp to tighten down against.

The Angelfire booster section was then hung on a chain from the ceiling to hold it vertical while all the epoxy cured.  It is finally starting to look like a rocket!

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