I have thought the Fat Boy was a neat kit for some time, but the idea to hyperize it hit me when I was looking at Yitah Wu’s rocket fleet on the ‘net.

Yitah built a Fat Boy for two motors, and he used different length tubes for each one, so he could deploy two recovery devices at different times by simply picking different delays. I liked the concept, so shamelessly stealing it, I set out to build a multi-motor Fat Boy.

This picture shows the 3/32 plywood centering ring after it was roughed out with a band saw. I sanded the edges to size using a reciprocating sander drum. Wonderful tool...it made it easy to make the centering rings right the first time.

I switched to the tiny 1/2in drum to finish out the holes for each motor mount. I drilled a hole just large enough to slip the sanding drum into the hole, then let the drum finish our each hole.

A finished centering ring, made of 3/32 inch aircraft plywood.

I decided early that the body tube and fins would require strengthening, and I debated two ideas: (1) put couplers inside the existing tube (or cut down a BT-80 so it fit inside), or; (2) fiberglass the body tube.

Now, I have not done any fiberglassing, so that added to the anxiety. I decided to make this my Introduction to Fiberglass class, and bought some HobbyPoxy Smooth-N-Easy finishing epoxy and a package of Sig 0.75 oz/yd cloth, and set out into unknown territory.

Body Tube:

‘Glassing the body tube turned out to be pretty simple. I clamped a 2 inch plastic pipe in a vise and slipped the body tube on the pipe to keep it level while I worked.

I then cut the fiberglass to the proper size to cover the tube about 5 wraps, with an inch hanging over each end. Wearing disposable gloves (Lowe’s Lumber, $1.00 for bag of 10) and using a disposable hobby paint brush (Hobby Lobby, $0.40), I mixed a small amount of the Smooth-N-Easy epoxy and sparingly brushed it on about half the body tube, covering it top to bottom as I went.

I then laid the ‘glass on the tube, squared it up, and slowly started wrapping the tube, trying to keep a small amount of tension on the ‘glass so it laid down well. When I get to the edge of the epoxy,

I added more, so that I always had epoxy under the ‘glass as I wrapped. I found that with this lightweight ‘glass, I could then wrap it another two times around the body with the epoxy I had already applied. I then sparingly brushed on a little more epoxy and finished the last two wraps.

To get the ends to look good, I brushed epoxy into all the ‘glass hanging over the edge, then I tucked it into the body tube so it wrapped around the ends. I kept the ‘glass tight by standing up the body tube on a piece of wax paper—this kept weight on the bottom edge.

On top, I covered the nose cone with a plastic sack and put it nose down into the top opening, adding pressure there. I let the setup dry 24 hours, then cut out the excess ‘glass inside the tube with a hobby knife.

I then started my usual cycle of sandable primer followed by 220 grit sandpaper, repeated until the tube was smooth. The fiberglass added a good deal of strength (squeeze the body tube to get an idea), without a lot of weight.

Fins:

Now a crusty veteran of one ‘glassed body tube, I decided to tackle the fins. I knew the fins had to be much stronger to avoid little pieces fluttering in the wind.

My decision was to make them from 3/32 inch SIG aircraft plywood (about $2.50 per 6x12 inch sheet at local hobby store), covered with ‘glass for good measure. I copied the balsa fins to the plywood, cut them out and sanded them to shape.

To ‘glass them, I added the epoxy on both sides of the fin. I cut small pieces of ‘glass which would cover both sides of the fin in one wrap starting at the leading edge and working to the bottom.

I let the excess ‘glass hang below the bottom and side of the fin. I repeated the process with a second layer of ‘glass, then laid the fins between a sandwich of wax paper with a book on top. This dried 24 hours.

I cut the excess ‘glass from the sides and bottom with a hobby knife, then did the same sandable primer/220 grit sandpaper cycle as I did with the body tube.

Centering Rings and Motor Configuration:

I had a difficult time deciding how many motors I wanted.

I finally settled on a 7 motor configuration (see Diagram 1) because of the flexibility: by using plugs in empty holes, you can fly this on anything from 1-7 18 mm motors.

This lets me start with 1-4 motors to flight test it before I cut lose with 7 C,D, (or E!) motors.

I cut 2 centering rings from a sheet of the same 3/32 plywood as the fins. The centering rings were drilled out for each , then sanded to fit each motor tube, and to fit nicely in the body tube.

Motor Mount Assembly:

For a little spice, I designed the Fast Boy to handle a limited form of dual deployment. The body tube for the center motor extends clear up into the nose cone, while the other six motor mounts are short.

This allows me to put a parachute in the main body tube area, then stuff a parachute in the central tube. I can use a long delay motor in the center tube so the chute pops a few seconds after the streamer or drogue chute. Enough of that—on to the construction.

Using my hobby knife, I cut 6 pieces of 18mm body tube (BT-20) to the same length as a standard Estes motor.

This is optional: I made my own thrust rings for the 7 engines by cutting ¼ inch sections of a discarded Estes motor using a razor saw. You can buy them if you prefer.

I decided to install the 6 outside motor mounts, then install the tall center one after the shock cord assembly was attached. For the 6 outside (short) motor tubes, I roughed up the top of the motor tube and the thrust ring w/220 grit sandpaper, and glued the thrust ring in each tube, so that a motor inserted into the tube stuck out the bottom exactly ¼ inch.

I used yellow (wood) glue for these mounts. After the glue dried, I dripped CA inside each tube, allowed it to dry, then sanded the inside of the tube with 400 grit paper until it was smooth. This also made the tubes more rigid.

Next, I installed the six outside motor mounts in the centering rings. The critical task here is aligning everything before the motor mounts are glued.

Sliding the centering rings onto the tall center motor tube, I spaced them apart so a fin tab would slide between them. I marked this distance on the tall (central) motor tube.

Next, to make sure the motors were parallel to the center motor, I slid one outside motor tube into place, then rotated the centering rings until the outside tube appeared parallel to the tall center tube.

I gently taped each centering ring in this position, then glued the 6 outside motor tubes into place with yellow glue.

The bottom of each motor tube was flush with the outside of the aft centering ring. After the outer tubes were dried, the entire ring/motor tube assembly was slid off the tall center tube. It will be inserted last, after the fins are glued and the recovery system attached.

Next, I put fillets around each of the 6 motor tubes with yellow glue.

Recovery System Attachment:

I scrapped the Estes shock cord and replaced it with my favorite elastic, flat nylon/cotton recovery strap—a shoe lace.

I bought a 72 inch shoe lace, cut off the ends, then attached it to the bottom of the center motor tube by wrapping two small strips of left-over fiberglass (about ¾ in wide) soaked in 24hr epoxy around the shoe lace and motor tube about 3 times. This was set aside to dry overnight. The shoe lace, er, recovery attachment system, will be cut to length later.

Fin Attachment:

To install the motor mount, I inserted it in the bottom of the body, and pushed until the top centering ring was just beyond the fin slots.

Next, I twisted the motor mount until all three fin slots had a visible path into the center of the mount. I slid the fins into their slots, and they just fit between the motor tubes. Because the outer body and fins are fiberglassed, epoxy is required for fin fillets on the outer body. (yellow glue will not penetrate the fiberglass to bond).

Since the center motor tube is not installed, I put 24 hr epoxy on the inside edge of each fin and its neighboring motor tubes (see diagram below right). This makes a strong joint between motor tubes and fin, each reinforcing the other. In addition, outer fin fillets were applied using epoxy.

Central Motor Tube Installation:

After glue dried for the fins and motor mounts, the central motor tube was inserted glued into place using yellow glue. The bottoms of all 7 motor tubes were flush with the aft centering ring. I inserted a motor block and glued it (yellow glue) so the motor was ¼ inch below the aft centering ring.

Baffle Installation:

I decided at the last minute to make a baffle system for the 6 outer motors. Using the two cardboard centering rings supplied with the Fat Boy kit, along with a single hole paper punch. I punch 8 holes around the outer edge of one paper ring, and 8 holes around the inner edge of the other ring.

One ring was slid onto the central (tall) motor tube, one-half inch above the motor mounts and glued there (yellow glue). The other ring was slid onto the central tube and glued one-half inch above the first ring, again with yellow glue.

Launch Lug Attachment:

I decided to use a large launch lug, so I could go off a relay launcher. It will take a lot of current to light 7 Estes igniters at once. I bought a ½ inch brass lug and cut a 3 inch piece of it at an angle. The angled side faced forward, and was glued to the body with 24 hour epoxy. I put the lug between a pair of fins, about 4 inches from the top of the body tube.

Final Recovery System:

Assembled rocket after a couple of coats of primer. Nose cone is black because I took it off the remains of my Broadsword. I'm lazy--it was already nice and smooth, so I used it.

I decided to cut the recovery strap off at 48 inches. After cutting, I folded a 3 inch piece of the recovery strap over a 1 inch keyring (from Hobby Lobby) and sewed up the strap with some heavy polyester thread.

The 18 inch Estes nylon parachute was tied to form a loop at the end, then slid onto the key ring. The nose cone attachment was also slid onto the keyring.

Finishing:

I used Krylon red primer for my finish work. Between coats I sanded the rocket with 400 grit wet/dry sandpaper.

I carefully dripped water on the body tube and nose cose and did a true wet sanding—hey, why did you fiberglass it, anyway? After the 3rd round of paint/sand, it was ready to finish.

I painted the body with Testor’s Metalflake Burgundy, and the fins with Krylon Violet (lavender). I didn’t really like the way the colors met at the root of the fins, so I put a ¼ inch white pinstripe around the base of the fins. Much better.

I applied the FatBoy decals after dipping them in water so they could be slid around on the body. After decals were applied, two light coats of clear lacquer covered it all. The finish looked nice!

Flight Preparation:

The night before the launch (NSL at Muncie, IN) I checked the final weight of the rocket. I inserted 7 C6 motors to check the center of gravity. Not good—it was a half-caliper behind the Center of Pressure (CP is 8.25 inches from nose).

It took about 100 grams of clay and fish weights to bring the CG a half-caliper ahead of the CP. Close, but enough for a stable flight. I ran a simulation and found that the best motor was a C6-7 (1600 ft flight); however, I chose a C6-5.

First Flight:

To prep the rocket, I did a friction fit of each motor, inserted each igniter along with a trickle of black powder for good luck, and stuffed the parachute in the body. I connected the pairs of igniter leads using a homemade clip whip. It was breezy the first day at NSL.

After going through the RSO review, the LCO was concerned that my CP/CG was too close, so he put me W-A-Y out there to launch. On launch, the rocket leaped off the pad—a great flight, straight all the way. Sure enough, before apogee, a premature ejection made the nose cone take off on its own path, along with the ‘chute and keyring.

I recovered the body, otherwise undamaged, and saw that all 7 motors had ignited! Fortunately I had another nose cone, and I set off to repair the rocket.

Repair:

After wet sanding the extra nose cone (for finishing), I cut off the nose cone base, so I could freely add weight. I also wanted a stronger nose cone attachment, to avoid a repeat of Friday’s mishap.

I took a 10 in piece of 1000 lb kevlar cord and tied each end to a small fishing weight (weight had hole through the center). I turned the nose cone upside down, and put these two weights in it, forming a loop with the Kevlar cord. I started adding weights until the nose cone was as heavy as the original cone. I mixed up about 2 oz of 5 minute epoxy and poured it all over the weights to seal them and bond them to the nose cone.

The Kevlar cord formed a loop, protruding from the epoxy/lead plug several inches. I used a small quick-link to connect the new 18 inch parachute, nose cone, and recovery strap. I taped the nose cone and gave it a coat of Burgundy to match the body. As good as new!

Second Flight Preparation:

Eye test. No, it is a picture of the bottom of the rocket, taken the same time as the one with the black nose cone.

I was determined to better my first flight, and I had a stash of E25-7 Aerotech single use motors. I wanted to use 7 of them, but my simulation software told me to use 4 for the correct time delay.

This time I trusted my simulation, and plugged three holes with spent motor cases (stuffed with wadding). I did a friction fit of the 4 motors, stuffed the new 18 inch nylon parachute, inserted my Firestar igniters, and went to the flight line.

On launch, the rocket left in a hurry for a 3000ft + flight. Ejection was right on time, and rocket drifted a considerable distance downwind. Nose cone stayed with everything this time. Rocket received a damaged fin on landing (hit pavement), but will easily be repaired for another flight.

Learning Points:

There are several things I would change if doing it again. For one, I would trust my simulation software! It came very close to reality, since I knew the exact weight and diameter of the rocket at lift-off (Cd estimated at 0.6);

Second, I would mount my motors so they protrude below the body more than my design here -- The Estes motors really blackened the back of the rocket;

Third, I would definitely use the Kevlar cord/lead weight/epoxy method to weight the nose—it is a strong attachment, plus it gives more room for the recovery system;

Fourth, I would skip the longer central motor tube and make all 7 motor tubes short. The longer tube doesn’t add much, and it does remove some area for the recovery system.

Conclusion:

This was my first experience with fiberglass, and it was a fun project for sure. I liked the "sneak factor" of making the rocket look stock from the outside, but building it like high power on the inside. It definitely got attention when people saw how many motors were stuffed in there! I received lots of nice compliments from people on the flight line. Yeah, let’s go fly it again…