Modification - BLU-97B Cluster Bomb (24mm) {Modification} Modification

Contributed by Dick Stafford

Manufacturer: Modification

(MOD) Estes Cluster BombBrief:
This kit is a semi-scale version of an actual Combined Effects Munition (CEM). As I was applying the decals, I noticed that they identify the model as a GBU-87, whereas the package calls it a BLU-97B. If you look at this website, it looks like the overall bomb unit is the CBU-87, and the individual bomblets are BLU-97s. This is a brief description of how I upgraded my Cluster Bomb with a 24mm motor mount. I need to point out that I generally followed the modifications that Carl Tulanko made. If you read his review, however, you will see that I did use slightly different techniques here and there.

Modifications:
Fin can/motor mount

I used several glues for the fin can, including: plastic cement to hold the tail cone together; epoxy for the motor hook, motor block, and Kevlar® cord; carpenter's glue for the centering ring/motor tube joints; Liquid Nails Perfect Glue for the fins, and Gorilla Glue (polyurethane) to attach the fin can to the body tube.

I started by cutting a piece of Totally Tubular aluminum-lined 24mm motor tubing, making it the same length as the stock tube. Using this tube as a guide, I got a rough idea of how much the tail cone had to be trimmed, and cut that much off with a razor saw. When I test fit the tube, I found there were internal tabs that had to be trimmed. These were easily removed with a Dremel tool. From there, I slowly sanded the tail until the tube fit. I also trimmed the inside of the tail cone with an X-acto knife, so that the tube transitioned smoothly to the tail cone. This way, no filling of this joint was required.

(MOD) Estes Cluster Bomb I decided to keep the motor hook, which precluded me using an E9 motor, but I thought that the resulting rocket might be too heavy for the E9 anyway. The motor hook was installed with masking tape and a dab of 5-minute epoxy. A small notch was made in the tail cone to allow the hook to flex. I also added a motor block above the hook as in the stock kit. Epoxy was used here since the surface of the motor tube is metal.

The centering rings were trimmed to fit the 24mm tube. The bottom ring was installed flush with the top of the tail cone. The fins were added, a 28" piece of heavy Kevlar twine was attached through the motor mount and the assembly was mounted in the body tube.

Nose cone assembly

I followed Carl's philosophy for the nose cone: make sure all the nose weight is on the parachute attachment and keep the 2-piece cone as just an outer shell. Since more nose weight was required anyway, I thought that his idea of using an eyebolt affixed to a plywood bulkhead was a great idea. Where I differed was that I decided to use only the eyebolt hardware for weight.

I started by assembling the two-piece cone with plastic cement and breaking off the nose cone's eyelet. I then cut a bulkhead using a hole saw. I positioned this over the end of the cone and used it as a guide to drill a center hole and four smaller holes for mounting screws. For added internal support, I partially filled the small tip of the cone with Gorilla Glue. Once it expanded, the tip overflowed slightly.

I inserted the eyebolt into the plate and ran one nut to the end. A small dab of epoxy affixed this to the back side of the plate. Two other nuts were installed at the end of the eyebolt and were held in place with LocTite. This was dry fit in the nose and the nose in the body tube. I also loaded an 18" nylon chute and an E30 to get a good look at the CG. Mine came out about 8 inches from the top of the nose. This gives a static margin of almost 1 for the Barrowman CP and 1.6 for the Rocksim CP.

(MOD) Estes Cluster Bomb Final assembly

Because of the added weight in the cone, I used the stock elastic cord and a second piece of similar size (length and width). These elastic cords are tied on one end to the Kevlar leader, and on the other end to the eyebolt. I used the stock lugs, one just above the tail cone and one around the CG. In addition, I added buttons from railbuttons.com. I mounted one just above the transition so it bolted through the shoulder, and one just above the top centering ring. The top bolt actually goes through the polyurethane glue that expanded above the top ring.

Finishing

I started with Fill 'n' Finish on the fins before assembly. After assembly, I shot several coats of white primer, sanding and filling in between. This was lots of work due to the tube imperfections. I then painted it with Testor's flat olive drab. I then added the decals and overlaid a layer of Testor's Dull Coat. These peel and stick decals went on nicely, but the next morning I saw places where the stickers weren't adhered perfectly.

I hand painted the tip with Testor's silver, and added red, white, and blue rail buttons to go with the Enduring Freedom patriotic theme.

I didn't apply the Enduring Freedom decal, but may add it on later. I also didn't finish the bombs, at I am pondering using them on the exterior of another project.

Construction:
First a few comments about the quality of the parts provided with the kit. The first thing I noticed that the balsa was a lot harder than the typical Estes kit. I found this to be a good thing, especially since I was upgrading to a bigger motor and the weight at landing was going to be greater. The nose and tail cones are both two-piece and fit together well. I do have two beefs with them however. The nose itself had a couple of dimples, which keeps it from having a perfectly uniform surface. I guess I could have filled them, but I decided this was not worth the effort. Later in construction, I found that the fins would not fit in the tail cone slots. This took a lot more sanding and trimming than I would have expected. Not a big deal, but annoying. Next, the BT-70 tube was the worst I've ever seen in an Estes kit. The spiral line wasn't bad, but there was an intermediate spiral a little under 1/8" wide that had a washboard pattern (for lack of a better description) all the way along the tube. There also was a wide band near one end that was very rough. If I had to rate the kit solely on the parts, I'd have to give it a 2.5 out of 5.

Flight:
I decided to submit this partial review since I missed the November launches and may not be on the field until spring. I'll provide an update at that time.

Summary:
I immediately fell in love with this rocket. Stubby rockets are among my favs and this one has a unique look. The parts were sub-par but with just a little work the rocket turned out fine. Having read Carl's article, I am really itching for good weather!

And, I too am planning an upscale. Mine will be 5" in diameter and will fly on G80s. The nose cone is built and is currently being primed/filled.

Related Products

Flights

comment Post a Comment