Fat Boy - 3 x 4

Dick Stafford's Descon-14 Entry

Background

OK, this is not the most unique design, but I was going to describe this rocket on EMRR anyway and though I'd hit Descon 14 early. Best-case scenario: people spend too much time on summer vacations and on the field and I pick up a prize. Worst-case scenario: people quickly start work on their unique clustered and staged concepts and I get smoked. So, y'all get out your beach towels and suntan lotion ;)

Design Concept

One afternoon I was looking for a quick project and scrounged up a section of LOC 3" tubing and an old homemade elliptical cone. Over the last year or so, I have enjoyed clustering E9s in my Thoy Snipe, a modified Art Applewhite saucer, and my El-Cheapo Pyramid, so I decided to go with another 24mm cluster. I kicked around several configurations, and quickly found out that clusters made from Totally Tubular foil-lined 24mm tubing fit nicely inside the 3" tubing. At first, I was going to stuff it full, but decided that a four motor cluster would be sufficient. The result resembles an upscale Estes Fat Boy, although it is not perfectly to scale.

Construction

Nose Cone - The cone is made from foam and is covered in a thin layer of glass. I originally built it for a thicker tube and have had to repair it a couple of times, so it is not in perfect shape. But it is, after all, my own creation and I eventually HAD to re-use it. A 3" plywood bulkhead provides the attachment point for a small eye-bolt, and the shoulder is a plastic cap from a mailing tube. I added nose weight for this project by drilling through the bulk plate and adding lead shot and epoxy until I got the CG where I wanted it.

Motor Mounts - As I mentioned above, the four 24 motor tubes fit snugly inside the LOC tubing. I quickly realized that classical centering rings weren't required and that the inter-tube joints and TTW fin attachments should hold the motor mounts securely. So, I merely made filler sections from scrap balsa to block the ejection gasses from escaping between the motor tubes. I added these near the front of the motor tubes and left the back end open. The mounts employ motor blocks, positioned to fit E9/F24 motors, and homemade retainer hooks. Four pieces of thin brass tubing were epoxied in the joints where the tubes meet. I bent the end of the piano wire, slid it into the tubing and then bent the opposite end to fit an installed motor. At launch time, the wire tabs are moved aside, the motors are inserted, and the tabs are then repositioned over the ends of the motors. A few small pieces of tape ensure that they stay in place. This is the first time I've tried this and it seems to work fine.

Fins - The fins are upscaled from the Fat Boy design and are made from 1/8" ply. The motor mount was installed so that the TTW mounts would be equally spaced between the three outer motor tubes. The fins were epoxied to the central tube and internal fillets were applied, making sure that no glue interfered with the inner tabs on the music wire.

Recovery System - A section of Kevlar cord was epoxied to the motor mount prior to its installation. A yard of thick elastic then connects the leader to the nose cone. Not my preference, but it was the only suitable material that I had on hand. I added anti-zipper protection by adding several wraps of cloth packing tape to the Kevlar leader. This has worked out well on several recent rockets. A 'portable' Kevlar heat shield and a 36" ripstop nylon chute complete the recovery system.

Finishing - The cone was already day-glo green, and I grabbed some Rusoleum day-glo orange for the body. This thing is very visible!

Flight and Recovery

I have flown the fat Boy 3 x 4 once on three E9-8s. Since I wanted to try it first on three motors, I plugged the center mount with a spent motor. The boost was slow and majestic with a bit of weather cocking. With three motors, a 6-second delay would have been better. Although ejection was a bit late, the anti-zipper doohickey did its job and there was no damage on recovery.

Conclusion

The flight on three motors was nice, and one day soon I'll try it with all four. Hmmm, I wonder if I should have gone for a x7 cluster and surface mount fins? Hmmm....