Brief:
Cheap Dumb Rocket (CDR) is a 40% scale version of the 8 1/2" tall, 7.5" diameter, tube-fin Big Dumb Rocket (BDR) built as a club project by the SSS rocketry club of Arizona. As a fan of tube-fin rockets, the huge, twin J570-powered BDR is the biggest tube-fin design I’ve seen, so I made plans to fly one like it for my Level 2 shot.

Construction:
Details of the original are on the SSS website (www.sssrocketry.org). Before embarking on this project, I built an exact 40% scale model of BDR out of 3-inch mailing tubes to check balance and flying qualities. For fun, and in this period of AP famine, I decided to power it with a cluster of six D12 engines (“G72 the hard way”).

The result turned out to be one of the sturdiest and best flying rockets I’ve yet built. The big 16 oz. rocket(see photo -- flying buddy Brock is a 6-footer) flies well on either 3, 4 or 6 D12-5’s arranged symmetrically (triangle, square, hexagon) and is stable enough to survive a failed motor ignition on any of these combinations. CDR is as simple and low cost to build as a rocket can be. Shopping for materials (Office Depot), cutting them out, assembling and painting took only an afternoon. There are no complicated multi-hole centering rings or bulkheads -- the 6 individual 5-inch long D-sized Estes motor mounts (assembled with Estes hooks in the usual way) are simply epoxied in a hexagonal array around a central 4-inch piece of 24mm tubing, motor hooks facing inward (see close up of CDR’s business end). The entire cluster affair is then inserted into the main body tube after liberal slathering with epoxy -- fit is perfect. Shear loads from the 72 Newtons of D12 power are transferred to the hull by all the motor tube attachment points. Spaces between the motor tubes are then packed with gobs of epoxy mixed with cotton to insure hull pressurization on ejection firing (this is NOT an optional step). Tube fins are cut with a miter box with a 22..5 deg. bevel on the bottom to resist edge crushing on landing impact (and because it looks cool that way). Dimensions of the 3" tube fins are 3" on the short side, 4 1/4" on the long side. Tube fins are epoxied to the main tube in pairs in the usual self-jigging method for tube fin designs. The nylon shock cord (10' of USMC shoe lace) is attached through a hole in the hull 4" from the top of the body tube, with the knot covered with a fairing made from a piece of plastic Garcia-Vega cigar tube. A 1/4" LOC launch lug is centered 6" above the tube fin can, aligned with one of the triangular holes between two tube fins. Nose cone (3" plastic)is borrowed from a deceased Estes Big Daddy. Finish was with Flat Aircraft Gray primer and Flat white Testor sprays. No sealing of the paper surface is necessary. See components photo of CDR on the trunk of my car.

Flight:
I fly CDR exclusively on D12-5’s or (when all 6 motors are used) on D12-7’s. CG fully loaded is 9" from the aft end. Cp on tube fin designs is always safely somewhere down in the tube fin array. First flight was on 6 x D12-7’s and climb out was straight, steady and just perfect. Ejection (at apogee) was less perfect -- the 24" Top Flight chute hung halfway out of the hull, and CDR tumbled horizontally from about 800-900'. Not a bit of damage -- one strong rocket. Much as I hate baffles, I then installed a 3" x 38mm 1/8" ply centering ring 9" below the top of the body tube to insure that the ejection blast from the ring of D12’s was directed toward the center of (and not around) the recovery system. Ejection has been perfect since this fix. On the third flight, one of the six D12’s failed to ignite, which fact I didn’t become aware of until I went to retrieve it (landing photo, if you look carefully at the 2 o’clock position). I’ve found that 4 motors is actually the best combination of economy and performance, and I mostly fly with two motor tubes plugged with tape-filled spent Estes casings. On the fifth flight, with 4 motors, one of the D12-5’s CATOed like a cherry bomb 3-feet off the pad and blasted out the cone and recovery system in a ball of flame as the rocket climbed at full power. After the expected wild sky dance, Cheap Dumb Rocket settled to the ground on its chute and fired its remaining ejection charges at the sky. Thank heavens for those Kevlar® chute-protectors Andy Woerner sold me -- I won’t leave home without them. Again, not a bit of damage to the rocket, except for a little scorching of the tube interior. CDR is one sturdy rocket.

Summary:
What started out as a low-cost test bed that I intended to fly only once or twice has turned out to be one of the most enjoyable fun-fly rockets in my fleet. Its original purpose has not been forgotten, however, and I am now at the painting stage of my Level 2 project, a 4", 5ft tall, 60 oz. version of the BDR for Pro38 J330 power.

CDR Pros: quick building, cheap ($5 airframe) jumbo-sized cluster rocket, sturdy and versatile to fly (3 power settings on D12 engines). Good, safe cluster trainer.

CDR Cons: cluster complexity combined with tube-fin drag

Other:
Last but not least, I’d like to share my approach to constructing whip clips for cluster ignition. I simply purchased a package of Radio Shack® double ended alligator clip test wires (14"), and strip about an inch of insulation from the middle of each. By clipping one of these set-ups to the exposed middles of two others, you have a 4-motor launch clip. With two of these, you can launch a 4-motor cluster form a 12 volt High Power controller (don’t even think of using a dinky Electron Beam controller -- not enough juice), while retaining the ability to check continuity on each individual motor igniter (with a little off-on alligator clip gymnastics), something you can’t do with a hard-soldered whip clip.