Thursday, December 23, 2010

Review: Upscale BLU-97B Cluster Bomb

This is a description of my G-powered upscale of the Estes BLU-97B Cluster Bomb. The body consists of two 5" Quaker Oats bins. Here it is next to it's Estes cousin.

The nose cone tip was turned from 2" pink Styrofoam using a hand drill clamped in a vise. I first cut circles top and bottom with my Rotozip, and because the cuts didn't reach through the foam, finished up with a hand saw. A metal bolt epoxied into the core provided a sturdy attachment point for the drill. This worked better than a wooded dowel and I knew I'd need to add nose weight anyway. To extend the cone slightly, I cut a 3/4" circle from the bottom an oat bin. I glued this to the bottom of the cone. At this point, I coated the cone (without the plastic insert) with a layer of West epoxy. The 'nipple' portion of a cone is a plastic cap from something-or-another.

The shoulder is another foam disc encircled with a piece of an oat bin. I did not glue these sections together until I completed the rest of the rocket and determined how much nose weight was required. The required weight was determined by adjusting the CG of the completed rocket, including the motor and chute, based on the Rocksim model. The weight was provided by a long eye-bolt and several nuts. This eye-bolt is slightly off-center, and extends through the shoulder section and into the top of the nose. The two halves were epoxied together and the center hole was filled with some epoxy, some lead shot (just for insurance), and more epoxy.

The motor mount is a piece of Giant leap 29mm phenolic that extends from the top of the oat bin to 4" below the bottom. There are two thin plywood bulkheads in the bin, one near the top and one resting against the bottom lip of the bin (inset about 3/8" from the bottom). An 8' length of 1/4" Kevlar® is attached through the top ring.

I built the through-the-wall fins from foam board and framed them with wood dowels. I wanted to keep the bottom light, and there will be plenty of room for a BIG chute. Having the fins extend below the end of the motor tube allowed me to clamp them together in pairs with a plastic ruler, which helped with their alignment.

The rear is a 29mm x 2.5" centering ring and has a T-Nut to accommodate a motor clip. I added some pieces of foam board to fill the void in the rear of the body tube (remember the lower ring is offset into the tube a bit) and to extend past the end by 1/4". These pieces of foam board were beveled to provide support for the tail cone sections. I also ran strips of foam board along the line where the tail cone sections will meet the fins, forming a narrow shelf. The tail cone sections themselves are 1/64' plywood attached with Perfect Glue, Type 2 (similar to CA).

To connect the two sections of 'body tube', I made a tube coupler from another oat bin. This was split length-wise, a section removed, and the ends were glued back together to from a coupler tube. Since the oat bins are not very strong, I treated the inside of the top edge with CA, and built a makeshift anti-zipper device from small-cell bubble wrap and cloth tape. This was attached to the Kevlar® cord where it would hit the tube at deployment.

One fin buckled slightly after the first launch (see flight section) so I reinforced the fin joints with carbon fiber tape. A yard of this stuff cost more than the rest of the rocket, but on a whim, I decided to try it. On good thing about this tape is that the lengthwise edges are woven closed so they will not unravel.

The rocket was painted with Testor's flat olive drab and the tip was painted silver. I made upscale decals (actually stickers) from Avery full-page label stock. This material is not as nice a real decals, but I over-ran my budget with the carbon tape.

The 5" body tube provides plenty of room for a big 'chute so the foam board fins have survived reasonably well. On the third flight, the BLU-97B went off target and terrorized a flock of ducks. The unit was removed from the pond and had to be re-conditioned. I was amazed that it survived.  It had flown well on the following motors:

AT RMS G64-4, AT RMS G64-6, AT RMS G71-5, AT SU G80-4, AT SU G80-7, RoadRunner SU F60-4

I like the unique look of the cluster bomb, I love short fat rockets, and I enjoy making rockets out of junk, so what's not to like about this upscale? The main downside is the foam board fins. Plywood would have been stronger but it would also have been heavier, although maybe not after I had to add reinforcement.

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