Construction Package

1) Booster Section:

Booster Tube Preparation

The booster section was constructed from a 49 inch long piece of 16 inch diameter concrete forming tube bought from Borders Construction supply in Tucson, Az. The tube was mounted to the sanding jig, sanded smooth with 60 grit sandpaper, mounted on a 3" X 18" flexible sanding board, to remove surface bumps and to proivide a good surface for the epoxy. It was then covered with a single layer of 7 oz. fiberglass cloth and epoxy.

After a two day cure, the tube was then sanded level with 60 grit sandpaper and the filled with a filler consisting of West Systems epoxy, microspheres, and silica. Again, after curing for a couple of days, the tube was sanded with progressively finer grades until a smooth surface was obtained. At that point the tube was removed from the tube turning jig and internal components were installed.

After all internal components were installed, the tube was remounted to the tube turner, and scratches created during other phases were filled with epoxy filler. After curing, the tube was sanded smooth. The tube was primed with Duplicolor High Build Primer and then a thin layer of automotive body filler was applied and sanded smooth. An additional layer of high build primer was then applied. Finally, minor imperfections were spot puttied and the whole assembly was wet sanded with 320 grit sandpaper and a final coat of Duplicolr Primer/Sealer was applied. After 30 minutes the assembly was painted with Duplicolor Metalic Red. The original Fat Boy had light blue fins and a red body. I decided to use metallic paint colors that would still appear very close to the original paint colors and yet still provide a distinctive paint job. Finally, the entire body section was painted with 2 coats of Duplicolor Clear for durability and allowed to dry. The decals were created by scaling up the original decal set that came with the Estes kit and cutting them into plastic by a local sign shop. The decals were then applied to the side of the rocket in the same locations as the original.

Booster Section Internals

The plan for the booster included removable fins to make transportation of the rocket manageble, one 98mm motor tube for the Hypertech hybrid motor, and 3 - 54mm motor tubes to allow for airstarted boosters or launching with 3 - 54mm motors. For the certification flight only the single hybrid configuration will be used of course, I just wanted the flexibility to try other motor configurations. Additionally, there needs to be room for the redundant recovery systems and the parachute. I decided to attach the fins with 1/4 inch threaded rods running from the aft centering ring, through the fin root, and fastening into the middle centering ring.

Each fin box was created from 2 pieces of 1/4 inch 7-ply Birch plywood measuring 6 inches X 12 inches and a Douglas fir spacer with a thickness of 3/4 of an inch. The centering rings were cut from 3/4 inch 7-ply birch plywood on a router table.

For assembly, one set of spacers were first glued to the middle centering ring and the second set attached with machine screws and nuts to the aft centering ring. I wanted the aft ring removable after fin can assembly so I could foam the fin can area. As a safety measure I put strips of polypropylene plastic between the aft centering ring and the spacers to prevent glue from seeping into the joint. The middle and aft centering rings were then aligned with the engine tube and then the sides of the fin boxes were attached to the spacers with small wood screws. When all the alignment was checked the sides were removed one at a time and bonded to the spacers. After all of the sides for the fin boxes were attached to the spacers then whole assembly was bonded to the body tube with West Systems epoxy and 3 inch fiberglass tape. The middle centering ring was bonded to the main motot tube on both the bottom and top side of centering.

I needed a way to get wiring down to the 54mm motors in the event of an airstart so I bonded 3 - 34 inch long 3/16 inch diameter aluminum tubes to the sides of the 54mm motor tubes. I then fit the modified tubes to the aft and middle centering rings. When I was ssatified with the alignment, I bonded the tubes to the middle centering ring and 3 inch fiberglass cloth was again applied to both the top and bottom at the middle centering ring.

Next, the top centering ring was fitted to the body tube. 18 inch long 3/8 inch diameter threaded steel rods were fitted between the middle and top centering ring with nuts and washers installed both sides of the middle centering ring and a nut and washer set to the correct height for installation of the top centering ring. All fasterners were cleaned with acetone and the coated with epoxy before assembly. Additionally, the inside of the body tube that was to be between the middle and top centering rings as well as the surface of the centering rings were coated with epoxy before assembly (I generally seal all surfaces with epoxy for strength and stability). Finally, the top centering ring was installed, nuts and washers installed and tightened on the threaded rods and the top centering ring bonded to the body tube and motor tubes with epoxy and 3 inch fiberglass cloth.

After the epoxy had set overnight, the body tube was turned over and the cavity between the motor tubes and fin boxes was filled with 2-part expandable foam. A big issue that I failed to realize here was the pressure the expanding foam put on other parts. I should have put a spacer in the fin slots to keep the foam from pushing them in; I didn't and later a fair amount of time was used to sand the middle of the fins slots enough that the fins would fit properly again. Finally, the aft centering ring was attached to the body tube and motor tubes with epoxy and 3 inch fiberglass cloth. After everything was cured I used a laminate trimming bit in my router to open up the fin slots. So sanding was needed in the corners to get a good fit for the fins.

The last components to go into the booster section was the instrument section for the two Missile Works altimeters. I cut 2 - 6 inch long sections of 3 inch body tube and 2 wedge shaped pieces of 1/4 inch 7-ply Birch plywood. In the plywood I cut a hole for the tubing, as well as, a hole for a 1 inch PVC threaded coupler. The powder charge for nose cone separation will be in a 1 inch threaded end cap with a hole in the bottom for the igniter. I cut the threaded coupler in half and epoxied it to the plywood wedge. The plywood was contoured to fit the inside edge of the booster tube and the outside edge of the main 98mm motor tube. I then bonded the whole assembly into the top of the booster section adding fillets as required for strength. The altimeters will be mounted to 1/4 inch plywood and placed into the instrument bays for flight. When everything was installed and finished the booster section was remounted to the tube turner and the outside was finished.

2) Fins:

The fins presented a major challenge. Attaching the fins in a normal through the tube configuration was ruled out due to the size of the finished rocket. In order to make transport easier, it was determined that it would be easier to have removable fins. Several options were considered. I finally decided to build fin boxes in the body tube and attach the fins with threaded rods running through the fin root and attaching into a threaded insert in the middle centering ring.

The fins were built with 3 layers. The middle layer is 1/4 inch 7-ply birch plywood and follows the full outline of the fin. 3 - 4 inch holes were drilled in the base of the fin in an effort to lighten the weight. The outer layers were made in two parts, first the bottom 1/3rd was made with 1/4 inch 7-ply birch plywood. It was cut at an angle to ensure that the piece of plwood would protect the bottom of the fin as well as strengthen the fin root where it was attached in the fin box. The top 2/3rds of the fin was made with 1/4 inch foamboard. 2 - 9/32 inch aluminum tubes were embedded in the fin root to allow for attachment of the fin to the body tube in the fin box (9/32 aluminum tubing has an internal diamter of 1/4 inch). In final assembly only a single bolt was used to attach the fin because it was apparent that two would be overkill. A layer of 7 oz. fiberglass cloth was applied to the foamboard and then 3 inch wide strip of fiberglass was applied to the edges. Finally, another layer of 7 oz. fiberglass cloth was applied to the entire fin. West Systems epoxy was used throughout. After curing, the fins were sanded level with 60 grit sandpaper and the filled with a filler consisting of West Systems epoxy, microspheres, and silica.

After cure the fins were sanded smooth and then primed with Duplicolor High Build Primer. A thin layer of automotive body filler was applied and the process of prime and sand continued until I was satified with the surface. I painted a final layer of Duplicolor Primer/Sealer and then painted the fins with Duplicolor Light Blue Metallic. After sufficient time I painted the fins with two coats of Duplicolor Clear.

In retrospect, it would have been easier to just build the fins out of 2 layers of 1/4 inch plywood and cover in a single layer of fiberglass. I suspect it would have been just as strong and probably lighter. As it is the fins weight 17 lbs fot the set and on a short fat design weight at the bottome is not a good thing.

3) Nosecone:

The nose cone was built in 4 parts - First, an 8 inch section of the 16 inch concrete forming tube would provide a shoulder for the nosecone to fit in the main body tube. Second, 2 centering rings would hold a 32 inch long 8 inch diameter tube centered in the shoulder tube. Third, Styrofoam rings would be attached to the 8 inch diameter tube and carved with a hot wire to match the profile of the Fat Boy's nose cone. The styrofoam would be covered with fiberglass cloth. Finally, a plywood plug would be constructed to make the top of the nose cone.

The shoulder tube was built using an 8 inch section of 16 inch diameter concrete forming tube, the same tube as used in the body. The tube was cut lengthwise and a section was removed so it would fit inside the main body tube. To ensure adequate clearance and to protect the body tube from epoxy the body tube was lined with polypropylene plastic. The shoulder tube was then inserted into the body tube and glued at the cut with epoxy. The inside of the shoulder tube was fiberglassed with fiberglass cloth for strength.

Next, 2 centering rings were cut on the router table from 3/4 inch 7-ply birch plywood. One ring was cut with an outside dimension equal to the finished diameter of the nose cone and an internal diameter set to accept the outside of the 8 inch concrete forming tube. The second ring was similar to the first except the outside dimension was cut to fit inside the rocket body tube minus the thickness of the shoulder tube. A third centering was cut from 1/4 inch 7-ply Birch plywood with the same dimension as the second ring to provide an attachment point for the top of the shoulder tube.

For assembly, the 1/4 inch ring was glued to the larger 3/4 inch thick ring. Then the shoulder tube was glued to the 1/4inch ring and fiberglass cloth was used to reinforce the joint. Finally, the last centering ring was glued to the bottom of the shoulder tube with a slight inset so I could strengthen the joint with a fillet.After the glue had set, the 8 inch diameter tube was glued into the shoulder assembly with the bottom of the tube even with the bottom ring. This assembly was set aside to cure. After curing the Styrofoam rings were stacked on the 8inch tube and then placed in the hot wire cutting jig.

A template was made of the original Fat Boy's nose cone and then scaled up to the correct size. 2 pieces of masonite were cut to match the template and attached to the hot wire cutting jig. The nose cone was cut with the hot wire by following along the template then the nose cone was rotated slightly and cut again. This procedure was repeated many times until the final nose cone shape was created. More information on this process is available on Brad Vatsaas's web page. When the shaping with the hot wire was completed the nose cone was mounted to the tube turning jig and sanded smooth with 120 grit sandpaper. 2 layers of 6 inch wide 7oz. fiberglass tape was applied to the nosecone in overlapping strips and allowed to semicure. A filler made of epoxy resin and silica was then applied and smooth over the entire shape and the whole assembly was given a couple of days to cure to cure.

The nosecone was mounted to the tube turning jig and sanded smooth with 60 grit sandpaper. Once again the pattern of fill and sand, prime and sand carried on until I was satified with the result. I then applied a coat of Duplicolor Primer/Sealer and after 30 minutes painted the nose cone Duplicolor Metallic Red.

I then removed the nose cone from the tube turner and installed the nose plug, which is the attachment point for the main deployment bag from below and the drogue chute from above, into the center tube of the nose cone. 3 inch fiberglass tape was applied top and bottom for srength.

The top cap was made from 3 layers of 3/4 inch plywood and a 4 inch section of the 8 inch concrete forming tube. The 3/4 inch plywood was cut in rings according to the nose cone template. A ring of 1/4 inch plywood was cut to provide and attachment point for the nose cap shoulder and a fourth ring of 3/4 inch plywood was cut to form the base of the nose cap.

For assembly, the top 3 - 3/4 inch rings and the 1 - 1/4 inch ring were glued together. Then the shoulder tube was split in the same manor as the nose cone shoulder and glued. Then the shoulder was attached to the base of the nose cap and strengthened with fiberglass tape. A 3/8 inch eyebolt was attached to the bottom ring and the bottom ring was attached to the nose cap shoulder. A layer of fiberglass was attched to the outside of the shoulder for strength. The assembled nose cap was placed in the unfinished nose cone and rough shaped with a belt sander. Finally shaping was done by hand and then the nose cap was sealed with epoxy and finished in the same manner as the rest of the parts.

4) Launch pad interface:

The rocket will be launched from a 10 foot long Blacksky launch rail which uses the oversize rail buttons. I bought two 1500 Series Rail Buttons from Railbuttons.com and mounted them in threaded inserts in the top and bottom centering rings.

5) Avionics:

Avionics consists of two Missile Works RRC2 Altimeters installed in separate electronics bays. I mounted each altimeter to a piece of 1/4 inch plywood that slides into the electronics bay. The plywood is attached to a 4 inch circle of plywood that forms the seal to the electronics bay. Three normally closed phone plugs will be used for the arming and safing mechanisms. One plug will connect to the battery connection on the altimeter, and one each the the apogee and delayed ejection circuits. Since the nose cone contains and ejection charge and since the nose cone separates from the booster at 800 feet AGL it was necessary to build the apogee circuit in a manner that allows pulling the wires going to the apogee charge free from the citcuit without damaging the electronics bay. I used banana jacks for this connection. I mounted the phono jacks through the skin of the booster tube next to the electronics bay. Wires run from the phono jacks to the appropriate terminal on a terminal strip mounted to the top of the altimeter section.