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There was an old upholstery fabric shop in Atlanta that had moved out of its previous workspace, and left behind were hundreds of cardboard tubes. Needless to say I told a rocket buddy, Joe Lacina, and the idea of free tubes was all we both needed to start a road trip. After we visited the location, the tubes turned out to be 12 and 16 inch inside diameter concrete form tubes. Large was right! Nonetheless, we decided to go and load up a dozen for the ride home and figure out what we needed them for latter. When I told another rocket friend, Jorge Blanco, of the free tube offer, he drove to Atlanta to get his own dozen.
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Once at home the tubes took up half of one stall in the garage and looked twice as big. Now that several members of the Atlanta rocket community had big tubes we needed big nose cones to fit them. What project was good to use 15 pound, 5 foot lengths of sonotube?
In a flash of blinding inspiration it hit me:
FATBOYS!
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They would not get to heavy from using a short piece of the thick walled tube and they would not get to large to get out of the house, or to transport in our van. As an added benefit the big diameter nosecone would not be as long as an similar diameter ogive style and therefore easier to mold.
The first consideration when using non-standard tubes as a rocket airframes is, where do you get a nosecone? Now that I had so many free tubes, a custom nosecone(s) made to fit them was a must. The first step was to layout a template for the profile of the nosecone. I scaled up a Fat Boy nosecone from 2.6 diameters to 12.5 diameter. The resulting pattern was transferred to a piece of foam core by rolling a serrated wheel around a carbon paper backed pattern, (borrowed from sewing pattern transfer). The cut pattern was put in a tube to judge the appearence.
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PLUG
Now I was starting to realize that making one nose was just not going to be enough. Between Jorge and me we had almost two-dozen of the 12.5 inch tubes. I had to make a mold to mass produce nose cones and equip an army of FATBOYS. (Hinting at another world domination conspiracy, but we wont discuss that right now). A mold for the nose cones was in order. That changed things a little as the cone I was about to make was to be a plug and need not be a flight capable cone.
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To make the plug I decided to pattern it after the construction of the Hindenburg. This method entails a series of rings (frame), covered with glued on paper (skin).
The board that the pattern was cut from made a great template to figure the sizes of the rings. To make the tip easier to get a true shape I turned 6-inch diameter a foam plug. The foam plug and all the forming rings were hot glued to a piece of broomstick that acted as the spine.
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The broomstick spine made it easy to get all the rings centered on the same axis and gave a convent handle for the plug. After the frame was complete, a coupler was made by cutting a strip out of a piece of tubing, slipping it inside the larger tube and glassing the seam. A large wood disk was hot glued to the top of the coupler and one inside the bottom of the coupler. The first formed the shoulder the second helped the coupler stay round. Next, the coupler section was slid on the broomstick and joined to the frame. Finally the frame was covered with vertical strips of paper bonded with Elmers glue.
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To make the paper Hindenburg structural, the entire papered area was covered with strips of 6oz fiberglass cloth and polyester resin. Next, a tedious step that is familiar to most rocket builders Putty, Sand, Repeat.
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Using Bondo and a spreader the whole cone was given a final shaping and smoothing before priming and painting.
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Now it starts to get interesting, using a process similar to the one Brad illustrated in making a normal size nose cone, I embarked on making a mold for this Big Boy. A piece of melamine clad Masonite large enough to form a flange on three sides of the plug was cut to match the template. This parting line board was then positioned at the mid point of the plug and secured to the plug by packing a filet of non-hardening modeling clay on the underside juncture.
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Next, I affixed four bottle caps to the corners of board. These will form interlocking locators in the completed mold. The plug and board face was waxed and buffed 5 times, then sprayed with PVA release film.
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The mold half was constructed by wet lay-up of one layer of 10 oz cloth and a minimum of three layers of 1.5 mat over a brushed on coat of black tooling gelcote. After completion of the mold half, the Masonite board and clay were stripped off. Here I realized that I made a mistake. The underside of the plug was scuffed where it contacted the table, being scrubbed with errant glass fibers. I had to repair the damage before I could continue. The plug was not yet removed from the mold half.
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Next the exposed plug and newly made mold flange were waxed let dry and buffed 5 times. It was then sprayed with a mist of PVA release film and allowed to dry overnight. A layer of black tooling gelcote was brushed over the whole mess and then the lay-up process repeated. The moment of truth came the next day when I tried to separate the mold. Would it be glued together?
Lots of prying, sweating, thin putty knives and water wetting to dissolve the PVA were used. It finally yielded, and a new mold was brought into the world. So far the free tubes accounted for several weeks of work and a couple hundred dollars in cost. But the big black mold was pretty impressive!
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MOLDING:
I was somewhat afraid to use the mold after all the work I invested in it. After all, it could get glued together with the molded part and be a useless but large chunk of fiberglass. The free tubes already had created several weeks of work and I was still pretty far away from making a rocket out of one. Ready to continue, I cleaned up, trimmed and washed the mold halves. Following Brads advice I waxed them 5 or 6 times, letting each coat dry and then buffing it hard.
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Spraying the mold with PVA became the next problem. It looked good after application but the slow drying water base material ran or puddled an hour or more afterward. After three failed attempts of spraying drying all night and then pealing off the film, I learned to do three fine mist coats of PVA and walk away. After drying the good coat of PVA overnight the mold was clamped together.
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I decided to try to lay-up the monster nosecone in one piece instead of in halves. I was easily able to reach inside the assembled mold, but I could just reach the tip at full arms length. To minimize gluing my under arm to the wet polyester resin I had a plan. Lay-up all of the dome part of the nose to from tip to shoulder while holding the mold vertical. Then, lay-up the coupler with the mold horizontal and requiring only a short reach. The lay-up took a couple hours but went well. I discovered that I could not flex the cloth and matt from the coupler over the shoulder. Instead I built up the shoulder with about 5 strips of mat and ran the coupler plies over the built up area.
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The nosecone was allowed to cure several hours and the mold was striped uneventfully. It looked like a real store bought nosecone. I finally had my first nose cone to make the free tubes into rockets!
The saga continues. Now on to build a rocket that uses the nosecone and one of the free tubes.
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Airframe:
As you recall, the decision was to make a Fat Boy scale up. I chose a 98mm motor mount so I could fly it on anything from a high impulse 54mm J to a M. With a motor mount long enough for the 7680 case there was little room to make a conventional dual deploy system. I also wanted to keep the recovery system weight forward to minimize the need for nose weight. Rocsim was run using the base cone simulation for short fat flat-bottomed rockets. This simulation adds a base drag factor and increases the calculated stability to better match reality with out adding excessive ballast. The final design was to have an altimeter bay behind a hatch below the top centering ring.
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Since the piece of tube for the Boy already was about 12 pounds I wanted to minimize weight of the other components, particularly those on the aft end. The first item on the build menu was the fins. After laying out a pattern for a thru tab scale up fins, I chose to make them from foam core and fiberglass. Since the Boy was likely to come in like a bag of rocks, the fins had to be strong. My plan was to rip strips of oak to the thickness of the fins and miter them into a perimeter frame for the fins. A piece of foam core filled the center and a layer of 1.5 mat was laminated over them. Laminating was done in a stack, the fins, a layer of rip stop nylon peal ply, waxed paper, Masonite and more of the same. Photo of before and after laminating fins.
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Assembly was by gluing the forward centering ring and motor tube in place in the pre-slotted airframe. The massively thick airframe was slotted with a up cutting saber saw as that frayed the cardboard the least.
The fins were filleted to the airframe with glass tape on the inside of the airframe thru the open aft end.
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Now the fun began, how to make a reliable dual deploy system in a space about a foot round and foot high? I decided to try something really different. A partition in the chute bay would separate the main and apogee compartments. The main would be retained by a pin secured strap and ring (like a real parachute). The main would be a USAF bail out chute I cut down to 17 feet from its original 28-foot diameter. To go completely military I used a torpedo chute I as a drogue.
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Deployment is as follows: At apogee a charge holder mounted above the chutes blows (removes), the nose. A large nomex made from a firemans hood protects the chutes under the charge holder. The nose and drogue shock cord are attached to a Defy Gravity Tether. The rocket rides down from apogee on its drogue with the hose hanging near the drogue. After the tether releases the drogue shock cord via an altimeter command it pulls the pin retaining the main. The now freed drogue shock cord then pulls the main to full extension.
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Electronics are mounted inside a hatch to a slide out (rack) for easy access. Pull pins that hold micro switches in the off position accomplish effortless arming of the electronics. The arming switches are mounted on their side to eliminate the effect of acceleration on their moving parts.
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The Big Fat Boy flew at Orangeburg S.C., Dec 05 in gray primer on a AT L1300 and everything worked.
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Today it is painted as a Big Boy Fat Boy (a parody on the Big Boy resturant mascot) and ready to fly on a M.
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Two more nosecones have been molded using the mold, one for me and one for Jorge with his assistance. It got easier each time. Looking back, I cant (or don't want to), add up the cost of the glass, resin, parts and months of time it took to get one rocket and a spare nosecone to be able use on the free tubes. Each cone took over a half-gallon of resin, the mold well over a gallon of resin and many yards of glass cloth and mat. Several friends are inspired to start work on the Team fat boy project. Let the army of Boys begin!
Dont be surprised if a fleet of 12.5 diameter Big Fat Boys is seen taking over your range some day in the future.
According to Burl, the Bigboy blew the nozzle on its sorbitol M motor. This is the result of vectored thrust at reduced power. What the photo doesn't show you is the land shark it did when it hit the ground.