LESSON THREE: Building the motor mount and fin canister assemblies

In our first installment, we made some awesome fiberglass fin shells, some of which were assembled onto G-10 fin cores for use on our 38mm birds, and others which were left unmounted so the motor mount could be assembled first, for our 54mm model. In this installment, we bring all of those assembled fin units and remaining fin cores and shells, and merge them with two 38mm motor mount tubes and one 54mm motor mount tube.

In all three rockets, the motor mount tube is the same length: 9 inches. Only the diameter is different, which will yield us the same amount of recovery space in the airframe for our drogue chutes to reside in during flight. In addition to the three 9 inch lengths of motor mount tube, we will use four 38mm-to-2.56" centering rings, two for each of the two 38mm rockets; two 38mm-to-2.56" coupler tube centering rings, one for each 38mm rocket; and two 54mm-to-2.56" centering rings. All of these centering rings are 3/16" birch plywood except the 54mm rings, which are 1/4" thick.

We'll start with the 38mm version first. To begin, we'll need to layout the placement on the motor mount tube where the Giant Leap Groove-Lok will reside. Our considerations are the placement of the AeroPack motor retainer, the thickness of the rear of the styrene fin canister and the aft 38mm-to-2.56" coupler tube centering rings.

In the photo above, you can see the upright motor mount with the AeroPack retainer, the rear most centering ring and the Groove-Lok in place over the motor mount tube. In the tube lying horizontally, the red, yellow and blue shaded areas show where these items will be on the tube, adding in an extra 1/16" for the thickness of the styrene fin canister.

Once you have successfully marked your tube where the Groove-Lok will be applied, you can go ahead and attach the Groove-Lok to the 38mm tube using J-B Weld. It is a good idea to scuff the inside of the Groove-Lok with some 80 grit sandpaper as well as the area on the motor mount tube. The Groove-Lok is the perfect choice for this project because it allows you to securely attach the thin G-10 fin roots in a positive 90 degree orientation without worrying that they might come off or end up out of line.

While the J-B Weld is curing, which I recommend you leaving overnight to do, you can prep the styrene fin canister for use, which will consist of cutting the four fin slots, enlarging the rear opening to fit the 38mm motor mount tube and square up the front opening. If you haven't cut your fin canister apart from the nose cone base, do so at this time, taking care to cut the two pieces apart squarely in the groove. You can use a hacksaw for this if you are careful — I used a bandsaw.

The fins slots are relative straight forward — I laid out where the slots should be with masking tape using the four raised lines Estes put on the fin canister for alignment as guides and then used a razor knife to cut them out. After they were cut out, I used a Dremel with a cut-off wheel to clean them up.

The opening in the rear of the fin canister is nearly 38mm out of the box, but the hole may be slightly off center. Take the 38mm-to-2.56" coupler tube centering ring and lay it over the rear of the fin canister: you will see immediately where you need to remove material. Mark it with a pencil and carefully trim it away. Don't over cut it, you want it snugly fitting the 38mm tube, and be centered.

Once you get the aft fin canister hole cut to size, turn your attention to the other end. First, sand the face of the forward opening smooth and flat. I find that a piece of 80 grit self-adhesive sandpaper stuck to a flat piece of cardboard allows me to use the part to sand in a circular motion and get a good, square result. As the opening squares up, you will need to sand the inside diameter of the opening uniformly around its circumference, out to the beginning of the rolled edge. Just so you know, you'll have to squeeze the opening into an "O" shape to get the rear centering ring inside, so don't over do the sanding on this end.

In the photo above, the J-B Weld is cured and we can get ready put the motor mount tube in the fin canister. Before doing anything, take a black permanent marker and mark the four fin groove sets on the Groove-Lok that will be used so you can plainly see that you have the proper fin grooves lined up with the fin slots in the fin canister. Also, draw pencil lines up and down the motor mount tube that line up with the proper grooves in the Groove-Lok, to ease lining up the unit later. Then, run a piece of sandpaper up and down the inside of those four fin grooves to rough up the inside of the slots.

Next, take the fin canister and form an "O" with the forward opening, dropping in the 38mm-to-2.56" coupler tube centering ring. Look at the aft opening, using your fingers to hold the centering ring into place at the aft opening — you should be able to see on the inside at two points along the seam where excess styrene is built up inside the fin canister, keeping the centering ring from being able to seat flatly against the rear of the fin canister — this excess styrene has to be removed. I used a fiber cut-off wheel on my Dremel to cut the excess styrene away, allowing the centering ring to now sit flat.

You can see in the photo above the two centering rings sitting at the bottom of the fin canister (bottom red arrows), while the upper red arrows point to the location were we are going to apply epoxy. We have to mount the rear centering ring to the motor mount tube with the centering ring inside the fin canister, because if it were mounted to the motor mount tube beforehand, it wouldn't fit through the forward opening. So, we are going to apply epoxy to the rear of the motor mount tube, insert the motor mount tube into the fin canister, insert it into the aft centering ring and square everything up to dry.

To square it up, once the epoxy is in place, we need to hold the motor mount tube in place inside the fin canister, properly centered. Since we already have 38mm-to-2.56" centering rings, all you have to do, once you have the epoxy on the motor mount tube and it is in place, slip one of the centering rings over the forward end of the motor mount tube, push it down to tbe bottom, and slip a short section of scrap 2.56" body tube over the centering ring onto the fin canister to hole it in place. You may want to try doing this in a dry run to see how it goes.

Once you are comfortable with the process, apply epoxy around the aft end of the motor mount tube where the centering ring will fit, drop it in place, square it up and clean up the excess epoxy. The goal here is to mount the centering ring to the motor mount tube, not slop so much excess epoxy that you fill up the Groove-Lok's grooves or cause the motor mount to adhere to the rear of the fin canister. But! Since those things can happen, be sure you look through the fin slots, and using your marks on the motor mount tube and the marks on the fin grooves themselves, align the fin canister with the slots on the Groove-Lok. You should have plenty of time to move things around, clean up and then make sure your work is properly aligned. You can still move things around afterward, but be safe anyway.

Once your rear centering ring has dried, we can start attaching fins. I like getting all the help I can get to do this easier with better results, and I like using jigs to help me. I took the two V blocks I use to fiberglass with and used them to help me align my fins. In the photo above, this is my "fin factory" which has a singular 90 degree jig on the left, and a 180 degree jig on the right. It takes a little time to get the jig patterns into the proper placement, but once its clamped down, it's not going anywhere.

Using this jig arrangement, I only attach one fin to each fin canister at a time. It's a time consuming process, but getting it right is worth the time invested. Before even mounting each fin canister in the jig, I custom-fit the fin to be used to that particular fin slot. I cut the tabs about 1/8" longer than needed, so I could sand away the root edge to get the perfect fit. Once I had that run's two fins sanded and ready to mount, then I put my piece of 1.5" conduit through the motor mount — it's a perfectly snug fit, just like a motor — and place it in my V blocks.

I started out on the left jig, attaching one fin in the upright position. I sand my entire fin root tab, both sides, and then using J-B Weld, coat both sides all the way up the root edge where the fin shells will butt up with the outside of the fin canister. I really butter these things up! Then I insert the fin, line it up, tape it to my particular jig with masking tape and clean up the fin root-to-fin canister joint. Using a craft stick, with one end cut square, I fill in any gaps with excess J-B Weld, and try to make the joint as clean as possible. Then leave it to dry overnight.

Once one fin is on the canister, you can move it to the right side of my jig arrangement, and lay out the 180 degree spread. Then move it back to the right and attach the first perpendicular fin using the 90 degree jig, then back to the right for the last 180 degree spread. I added small square wood blocks for the last two fins on the 180 degree side to offset the other two fins. Don't be afraid to experiment with various jigs, they make life a lot easier.

Once finished with attaching the fins, you should be able to remove the upper centering ring and scrap of body tube and the motor mount tube and fin assembly will retain its position in the fin canister, with the fins positively attached to the motor mount tube. For added strength, I then foamed the fin canister with two-part epoxy foam from Mr. Fiberglass. This fin canister is so rock solid it's unbelievable. Once we add our upper two centering rings and the recovery attachment points, these fin canisters will be done.

Before we finish the 38mm motor mount assemblies, let's cover the 54mm motor mount construction. It's more straight forward in that there just isn't much room inside the fin canister to put anything! For that reason, even if there were a 54mm Groove-Lok, there wouldn't be room for it. So our only option was the conventional method of attaching our G-10 fin cores directly to the motor mount tube, installing that in the fin canister, and then adding the fin shells.

The incredibly tight fit requires that we not use a rear centering ring. In fact, the 54mm AeroPack motor retainer acts as the rear centering ring, because it fits just barely inside the rear of the styrene fin canister! So, the placement of the fin cores on the 54mm version is dictated by how much needs to be left out for the AeroPack retainer — just mark the distance, and that is the rear edge for the fin cores to be mounted.

The first thing I did was create another jig, shown in the photo above how it would be used. It is two pieces of luaun door skin mounted to a 1x4. This one would hold a 54mm motor mount tube level between the two uprights that had a half circle cut-out to fit the tube. The half circle cut out was placed so that the horizontal surfaces of the jig intersected with the mid point of the tube, minus half the thickness of the .062 fin core thickness. This would allow for the fin cores to be lined up at first perfect 180 degree angles and then with a perpendicular cut in the jig, to rotate and make perfect 90 degree fin placements. It's a simple thing, but the results are great.

Using medium CA, I tacked my pre-sanded G-10 fin cores to both sides of the 54mm motor mount tube. I kicked it a little with CA activator, flipped it over and repeated the process. Kicked again, I rotated the motor mount tube 90 degrees, placing the fin cores in the vertical slots and repeated the previous process.

Once the fin cores were in the right place, I moved them to my V blocks so I could do two opposing fin fillets at the same time with 15 minute epoxy. Because of the close fit, you can't do extravagant fin fillets, so I did minimal ones using my finger to smooth them out.

As they cured, I created a template from paper that would allow me to cut tip-to-tip 6 oz. fiberglass cloth in receeding layers, pulling back from the edge 1/2" each on three layers. So each layer would be smaller and smaller, each by 1/2" on each edge, and run over the motor mount tube and up the other side. Once the three layers of fiberglass cloth were added tip-to-tip, the strength was just what this minimalistic model was looking for.

Since the 54mm version would have the fin cores already attached to the motor mount tube, this dictated that the fin canister would have to be slotted from the rear to allow the fin canister to slip over the fin cores. This was fine, since the rear of the fin canister was so hogged out stuffing the motor mount tube in there in the first place, but I wanted to reinforce the aft edge where it covered the motor retainer and to strengthen the places where it was split to fit. I mean, this is styrene.

So what I needed was a ring 1/4" wide that I could glue inside the fin canister after the motor mount tube was installed and still clear the motor retainer. I solved this by cutting a 1/4" strip from the excess 2.56" airframe tube I had fiberglassed, and then cutting a narrow strip out of it, effectively shrinking the tube ring down until I got the diameter I wanted. I did this before I cut my fin slots all the way down to the bottom on the fin canister, but after I had sized the aft opening for the 54mm tube. Once it was finished, I CAed the ends together and sanded it smooth. Now, all I had to do was slap some Testor's cement on the outer surface and slip it into the rear of the fin canister after the motor mount was installed. Done deal.

The other end of the fin canister required only slight sanding to enlarge enough to allow the 54mm motor mount tube to pass through. Just make sure you sand consistently so that the tube remains centered in the fin canister.

Once I had the fin canister ready to receive its motor mount tube, I cut the four fin slots with a razor knife all the way to end of the fin canister. I slipped the fin canister down over the motor mount/fin assembly, noting where additional material needed to be removed until the fit was dead on. Once the fit was right, I covered the AeroPack motor retainer with a couple layers of masking tape and used it to position the ring I had made, using Testor's plastic cement to glue it in place, and then secured with a wrap of masking tape to dry. Medium CA tacked the fin canister sides where they contacted the fin cores — I just needed them to hold until the fin shells were attached and secured.

So, the next day, with the rear ring in place, it was time to start attaching the fin shells to the fin cores. I removed the motor retainer and admired my nice clean fin canister now housing a 54mm motor mount tube.

In the photo above, you can see the fin shell attachment in process. The right fin shell is attached, while on the left fin core, you can see the layered approach to the tip-to-tip fin root reinforcement. The fin canister assembly is coming along nicely.

In this photo, more fin shells are being attached. This goes faster than the 38mm versions, since the cores are already on and perpendicular to each other. That's a huge motor mount for a rocket this small. It's almost ... evil!

This process is repeated until all the fin shells are attached. We've come a long way from 24 fin shells to now. Once all the fin shells were attached, I used 15 minute epoxy and colloidal silica called Cab-o-sil to make an epoxy paste, and used that to fill the gaps between the fin shells and the fin canister, then setting it aside to dry.

Side view of the 54mm model's recovery attachment system using flat Kevlar ribbon.

Now that the initial assemblies are done, it's time to finish up the motor mount units by installing the upper centering rings and the appropriate recovery attachment methods.

The 38mm version is fairly straight forward: epoxy the rear-most 38mm-to-2.56" centering ring to the upper motor mount, sliding it all the way to the top of the fin canister and filleting the joint. The upper centering ring will have two 3/16"-24 eye bolts installed, one on each side of the centering ring, 180 degrees apart. I got mine from Public Missiles, Ltd.

After the eye bolts are assembled in the centering ring, with a nut and washer on the top and bottom, and taking care to orient the eye bolts so the heads are parallel to each other, I wick some CA into the threads so the nuts can't vibrate loose. Mount the centering ring back about 3/16" from the forward end of the motor mount tube with epoxy, and then fillet the joint. This version is now ready for the next lesson.

The 54mm version, being so big with so little room, requires a different approach for the recovery attachment point. I decided to epoxy Kevlar ribbon to the motor mount tube, running it through the ultra-small centering rings on both sides, forming a long looped bridle to attach the recovery system to. I decided to use 11mm Kevlar ribbon that John Cox of Rincon Rocketry sells here in the Rocketry Planet Auctions. The ribbon is a little over 7/16" wide and is rated at 1500 pounds, which is plenty substantial for what we are doing here.

The first step for the 54mm version was to attach the rear-most 54mm-to-2.56" centering ring just like we did on the 38mm version. Unlike the 38mm versions, there isn't enough room to foam the fin canister, but I think we will be just fine without that step.

In the photo above, you can see the 54mm motor mount/fin canister assembly. Sticking out the top is the two ends of 12 feet of the Kevlar ribbon, doubled over at a length equal to the distance that the two centering rings would be apart when installed. I did this simply to have two thicknesses of Kevlar behind the upper centering ring. The rest of the length of Kevlar is stuck inside the motor tube to get it out of the way.

The bottom arrows point to notches cut into the upper-most centering ring to clear the Kevlar ribbon when installed. To assemble, I apply epoxy to the centering ring and forward end of the motor mount, slide the centering ring over the two Kevlar ribbon ends, onto the motor mount tube, aligning the centering ring about 3/16" from the forward edge.

While the epoxy is still wet, so I can move things around a little if needed, I remove the masking tape holding the doubled-over ends of the Kevlar ribbon and coat each side liberally with epoxy. To make it all lay down flat until the epoxy cures, I wrap masking tape around the motor mount tube, compressing the Kevlar ribbon tightly onto the tube, and leave it to dry. Check your ribbon length before applying epoxy, you may have to trim it to get it to fit into the area between the two centering rings when doubled over.

When the epoxy had hardened, I went back and wrapped a couple layers of 6 oz. fiberglass cloth around it and glassed it in solidly, as indicated in the graphic to the upper right. Maybe not needed, but again, more cheap insurance.

Here are our three completed motor mount/fin canister assemblies. Rock solid fin canisters, much stronger than the ones I had in the prototype, so I am very pleased with the construction at this point.

In our next installment, we'll mate these to the three airframe tubes we fiberglassed in the last class, plus address launch lug methodology and how to get the scale band with the numerous holes in it that goes around the middle of the airframe. Until then, fly 'em high!