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Wednesday, 28 July 2010 21:36

Bruce Bell's Doberman Featured

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Bruce Bell's Doberman Bruce Bell's Doberman Bru

I have always built short fat rockets because I liked seeing them take off slow. So for Hellfire this year I decided to go in the opposite direction and build long and lean. My project started with a Liberty Launch Systems Competitor 4. This is a 10 foot long all fiberglass kit with G-10 fiberglass fins, coupler and a 5:1 nose cone, and comes equipped with a 24" long 75mm motor mount tube and centering rings. The booster section is only 48" long, which could present some problems with a few motors that I might want to use and not provide enough room for a proper installation of a drogue system. So I also purchased an additional coupler and 24" airframe section. With the additional section added on, the booster is now 72" long, plenty long enough for even the longest 75mm motors.  

I started with the motor mount by gluing the forward CR on to the motor tube about 2" from the end. I drill a ¼" hole in the forward CR and feed the Kevlar shock cord through the hole and tie it around the motor tube. I fill in with generous amounts of epoxy to hold in place. I drilled 2 small holes in the other centering ring (CR) and mounted (temporarily) a pair of small screw eyes.
This way I can slide that CR on and off while gluing the fins in place and gluing the motor mount tube in place. Once the 3 fins are glued in place, I pull the rear CR off and now apply the fillets to the fin roots. Once that is done, I glue the rear CR on, leaving enough MMT tube to glue on the SlimLine motor retainer. I cut about ½" off the base of the nose cone and use this ring to reinforce the tail of the rocket by gluing it in behind the rear CR. This helps with those hard landings out on the Salt Flats. I now apply some nice fillets to the fins on the outside using some thinned JB-Weld.
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The Liberty kits do not come with an electronics bay, but they do come with a coupler and end caps. I purchase the hardware including ¼" threaded rods, nuts, nylon locking nuts, washers, wire, terminal strips and a 9v battery holder. The altimeter of choice for me is the PerfectFlite ALT-WD. I start by gluing the end plates together after dry fitting them in the coupler tube. I marked where the threaded rod holes and the U-bolt holes need to go and drill those.
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I then cut the 2 threaded rods to about 3" longer than the coupler. I mounted the hardware in the mains side first including the threaded rods and the U-bolts. Note, I used nylon locking nuts to make sure nothing falls off at a time when you most need things to stay together, like just before your chute deploys.
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I mounted the drogue side end plate to the threaded rods as shown below. I cut out a piece of 1/8" plywood so that it fits comfortably inside the bay and marked along both sides of the threaded rods with a pencil on the plywood. Then I glued on some heavy ¼" I.D. tubing for the sled to slide on the rods.
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I then very securely mounted the 9v battery holder and the altimeter on to the sled, on opposite sides. If there is a crash, the battery would normally be a projectile, so this keeps the battery away from the altimeter.
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Now with the electronics mounted, I glued on the charge cups and the terminal strips. I used #20 gauge strip wire, and went ahead and wired everything up, leaving a generous loop of wire inside the bay to allow accessing the bay for battery replacement. I used a on/off switch from Aerocon to turn on the electronics from the outside. I notched the bottom of the mains compartment to accommodate this switch.
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I painted the whole thing with a coat of "glass-friendly primer" and a coat of black enamel. I then added a pair of PML rail guides. I plan on flying the Doberman on a Aerotech M-1297 at Hellfire, will weigh 25 pounds and fly to about 8,000 feet.
doberman






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