Friday, January 14, 2011

Reviews: Estes GBU-24 Paveway III and BLU-97B Cluster Bomb

Here are my reviews of the GBU-24 Paveway III and BLU-97B Cluster Bomb kits.

GBU-24 Paveway III

This is a semi-scale model of the GBU-24 Paveway III laser guided bomb. It flies on 18mm motors and recovers under a 12" chute. You can find more info on the actual bomb [here] .


* One BT-55, 10 7/8" long
* Plastic tail cone/adapter (separated during construction)
* Two BT-20 tubes, 5" long
* Seamless elliptical plastic nose cone, BT-20
* Cardstock centering rings
* Engine hook
* Engine block
* Mylar retainer ring for the engine hook
* Launch lug, 1/8"
* Elastic shock cord, 1/8" x 18"
* Assembled 12" plastic chute
* Die cut balsa fins, 3/32"
* Die cut balsa fin protectors, 1/16"
* Peel and stick decals
* Enduring Freedom sticker (not used on model)

The first step is to assemble the motor mount. This is a typical build, and consists of the BT-20 motor mount, two centering rings, an engine hook, a Mylar retainer for the hook, and a motor block. I used carpenter's glue as recommended. Before installation in the main body tube, I epoxied a two-foot section of Kevlar® twine onto the mount, trough the top ring.

Next, I removed and sanded the fins, and filled them with Fill 'n Finish. The main fins were fine, but the protectors, which attach on either side of the rear fins, were very thin and warped badly when exposed to the moisture in the Fill 'n Finish. I pressed these as they dried and they straightened out.

The tail cone and transition come as one unit and must be separated. A razor saw is suggested, but I used an Exacto knife. The bottom of the tail cone must also be trimmed and sanded, as do the fin slots. The latter start as rectangular projections, which are cut off with a razor saw, forming the slots. The final trimming is supposed to be the top of the transition. The instructions have you cut this off, leaving a hollow unit with both the top and bottom open. The shock cord then attaches to the inside of the upper body tube using a paper mount. Instead, I poked a small hole in the recessed ridge just below the top of the transition, fed a loop of Kevlar® twine through the hole and around the transition (see photo). This formed a loop, which the elastic cord will later attach to. A small dab of 5-minute Epoxy on the hole and knot holds the twine in.

The tail cone is used to adjust the depth of the motor mount as it is glued into the main body tube. Once positioned, the tail cone is attached, making sure the engine hook is centered between the fin slots. I used black rubberized CA in place of the recommended plastic cement.

The lower fins are mounted through the slots in the tail cone. I found these slots to be excessively loose, so they didn't support the fins well. I applied yellow glue to the fin tab and the front root section. To hold the fins while this glue set, I used Liquid Nails Perfect Glue, Type 1 on the rear root section. This sets fast and the fins were good to go. There is a marking guide for the top fins, and these were attached with carpenter's glue. After the rear fins set, you position the fin deployment guides and mark their position. A thin layer of yellow glue is applied to fins up to the mark, and the guides are attached. These immediately warped like crazy, so a couple of small clamps were required on each guide/fin/guide 'sandwich' as they set. Afterwards, these still seemed flimsy, so I added a small strip of scrap balsa between each pair at their roots.

The final steps prior to finishing was the launch lug and Fill ' Finish fillets. I didn't' glue the top tube to the transition or the cone until finishing was complete. When they were connected, I again used black CA.

After finishing, all that was left was tying the elastic cord to the Kevlar® leader in the body tube and the Kevlar® loop on the transition. I stashed the cute that was provided and will use a 12" nylon chute.

I started with fill-n-finish on the fins and tubes before assembly. I found the spirals on the BT-55 to be shallow, but the ones on the top BT-20 were deeper, and required some filler. After assembly, I shot several coats of white primer, sanding and filling in between.

The instructions say to use olive drab and red spray paint, a silver paint pen, and to follow the paint scheme on the cover. The cover picture shows the fin section, top fins and nose cone to be olive drab, and top BT-20 to be more of a tan then red. I used Testors' spray for the olive drab sections. In the photos on the link I presented in the intro, the top tube seems to be somewhat of a rust colored red. I decided to use red paint, which did get toned down as it was overlaid over the olive drab. I tested some Krylon red on a piece of wood, and it seemed to be OK. However, some bubbles formed when I used it over the Testor's paint on the model. Maybe the base coat was thicker on the model. After the first red coat dried, I sanded the bubbles and re-coated it. This time it covered just fine.

There are silver bands shown at the base of the BT-55 and BT-20 tubes. Instead of silver paint, I used some chrome Trim Monokote for these stripes. Finally, I applied the peel and stick decals.

I have flown this rocket six times on C6-5's and one time each on a C6-3 and a B6-4.  The first flight was on a very windy day.  The rocket was unstable and it stripped a couple of fins.  All parts were recovered, the rocket was rebuilt and the following 7 flights have been successful.  The base of the transition is open and on one flight the 'chute was blown into the opening. Afterward, I plugged the hole with a piece of soft foam.

The model was not a difficult build, and I think a skill level 2 rating is about right. I liked the plastic parts, especially the nose cone, and think the thru-the-wall fin attachment is great. However, the fit of the fins was poor and the balsa used for the fin protectors was flimsy. As with the Cluster Bomb BLU-97B that I just built, it appears that Estes needs to pay more attention to quality control. Still, the rocket was fun to build and looks *very* cool.

BLU-97B Cluster Bomb

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.

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.

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.

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.

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 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.

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.

The first couple of flights were on C11-3's and the next five have been on D12-5's.  These delays are about right.  I started by installing a 'pom-pom' made from Teflon plumbing tape.  This didn't hold up as well as expected so I began using a 'chute protector. The rocket generally recovers well with an 18" Nylon 'chute.  On a couple of flights on windier days, the high lateral velocity has resulted in cracked fillets.