Modification - Super Big Bertha (MPR2)
Contributed by Tim Burger Brief Construction The balsa fins that come with this kit are perfectly fine and I would have no trouble using them and flying it that way on even an F motor. But since I'm already cutting a set for a scratch built Super Ranger project it's easy to just go ahead and cut a set for this rocket while I'm at it. The new fins are of 1/8 basswood, and go through the airframe and attach to the motor mount. Through the wall fins are hard to break off; I like basswood because it isn't a whole lot heavier than balsa, but it is a lot harder and the grain won't require so much sanding and filling. So a set was cut, complete with a tab to pass through the wall and with a notch in the root edge to accommodate the motor hook retaining tube. The fins were sanded round on all edges except for the root and tab.
The rocket also uses a pair of centering rings at the front of the stuffer tube to add strength to the two piece airframe. This arrangement can be used to make an ejection baffle with just two additional parts and very little weight gain. First, the third centering ring was glued onto the tube 5" from the top. Then a plug was glued into the top of the motor tube; this plug is simply a stack of balsa disks glued together. A BT-50 size balsa block would also have worked. The block was soaked with CA to help protect it from the hot ejection charge. Next, a group of holes was cut in the tube between the plug and the centering ring. I'll normally cut the holes in three rows evenly spaced around the tube, each row having three or four 1/4 inch holes. Next, a 5" piece of BT-60 was cut from stock and a similar set of holes was cut in it. The edges of the holes were touched with thin CA on both parts. The BT-60 was glued to the last of the rings. Three equally sized and spaced notches were cut out of the edge of the ring - the remaining tabs are centered over the rows of holes. I wrapped one end the Kevlar shock cord around the top of the BT-60 and glued it there where the two parts meet. A loop was tied in the Kevlar a couple of inches above the baffle and one end of the elastic that came with the kit was tied off there. The other ends of both the elastic and the Kevlar will be tied to the nose cone. A set of fin slots was cut in the body tube the width of the fins. The motor tube was then glued into the bottom of the rocket, the joiner ring added to that, and the top part of the baffle was glued in and oriented such that the rows of holes in the stuffer tube are centered between the rows of the BT-60. After all that dried the top part of the airframe was glued to the joiner. I had a bit of trouble with the two halves not matching perfectly at the joint. The top half is a tiny bit larger in circumference than the lower half producing a sort of hump in one spot. I spent a good deal of time working on this to minimize the potential hump back appearance it might cause. Finally, the fins were glued in. I used yellow glue for everything on this kit, except for the fin fillets wherein I used epoxy. Epoxy produces perfectly smooth fillets but it adds a lot of weight so it's important to avoid getting carried away. Rating: 4/5 This was an easy but fun to build kit.
Overall, it came out OK. It looks great if you are a couple of feet away, but if you get close you can see the trouble I had with the masking tape. Rating: 3/5 An easy model to finish - the decals are OK, my masks didn't produce a nice crisp edge. Flight It was flown the first time at the club polar bear launch on January 12, 2002. The flight was OK, despite high winds. The motor was a D12-3 and it left the rod with an impressive amount of speed and smoke. It did weathervane some, and ejection was just after apogee with the nose cone down. Because of the wind, I used an 18" 'chute rather than the supplied 24" one. I misjudged the angle of the wind and it drifted over the hard top parking lot. With a smallish 'chute the rate of fall was pretty fast and it hit hard. One of the fins shattered along the grain and it lost a half inch square chip out of the bottom, but the fin did not break off. The wind also dragged it across the lot so it has a fair amount of hanger rash under the nose cone. The fin was easily repaired by running CA down the cracks and gluing in a plug to replace the missing chip. After sanding and touching up the paint one can hardly tell. Overall Rating: 4/5 Brief:
So it’s obvious that it will never fly again, and not wanting to waste fins and nose cone a rebuild was undertaken (undertaker, please drive slow).. Since the motor mount is sort of the core of the model, and since it is locked in with the fins, and also since it is integral to the baffle, it would be nearly impossible to get it out of the airframe and replace it with another. Also, the top section of the airframe is severely damaged anyway. So a complete overhaul is the best and worst I can do. The first order of business was to untie the shock-cord from both the nose cone and at the mount end (the replaceable Kevlar® cord was anchored at the motor mount). Now the airframe was cut away to reveal the badly twisted and charred motor tube. Aha! The motor tube was damaged during construction; it had probably seized when being inserted and was damaged by too much force. OK, now that we have that out of the way, the fins were removed from the tattered remains of the motor tube. So now I have a nose cone, shock-cord, and a set of basswood fins. So I need to add some centering rings, a motor tube, an airframe tube, and launch lugs! The airframe came from Totally Tubular as did the motor tube. Since I’m rebuilding, I might as well make some “improvements.” How about a 29mm mount so it can be flown with Econojets and some reloads? Sounds good! Rail buttons sound like a good idea, too. Construction:
The new body tube was measured and cut to size, then cut again at the 11-inch line. The motor tube was test fit into the airframe with a coupler to verify the size. The new version will eject at the center to form a (hopefully) zipperless design. The lower half was marked up for fins, and the slots were cut. The parts were again test fit, and finding them satisfactory the motor tube and coupler were epoxied into the lower airframe. The fins followed in pairs: the opposing fins were epoxied at the same time and a straight edge placed across the pair ensured they were square with the airframe (assuming my slot cutting is done well.) Fin fillets followed, and I noted that the airframe tube grabbed one of the fins a little and was indented at the root edge. Nuts! Wish I’d seen that while the epoxy was still wet! Oh well, it turns out that it isn’t too noticeable once the fillets cured. The rail buttons were placed carefully such that the screw is driven into the wood rings. The holes were carefully predrilled. I also opted to add a set of 1/4" lugs since a rail isn’t always readily available. I had some problems with the coupler. It was truly snug on the upper section which would cause deployment problems later. It was also pretty “fuzzy”. To correct this, I coated the whole thing down to the lower airframe with thin CA. It was sanded as soon as the CA cured until smooth. I continued working on it until the upper section fit snuggly, but not tightly and would slide smoothly. The coupler is now very hard and smooth. Finishing: Flying: The first flight was with an Estes D12-3 using an Estes plastic ’chute.. I figured it wouldn’t be too spectacular of a flight since the maximum lifting weight of the D12-3 is 13oz. It’s actually a tad over. To my surprise, it was an excellent flight and went a good deal higher than I expected. Cool! It was flown again at the club launch using an E28-4 reload and a hemispherical nylon ’chute (hand made by my young lovely). A very nice flight indeed, but the shock cord roughed up the edge of the upper section. This was smoothed out, and the inside of the tube coated with thin CA for about an inch or so. That was sanded shortly after it stopped smoking until smooth and until it fit on the coupler smoothly. My nephew and I flew it at LDRS XXII in Argonia on an F22-5 reload. It was a bit slow off the rail and arced into the wind a bit. The delay could have been shorter, but it came out OK. More motor, is the cry! We had plans to slap an F20 in it, but were so busy with all of the things to be seen at LDRS that we simply didn’t get to it. When the wind is cooperating we’ll give an F20-7 or an F23-7 a whirl. It should fly well on F40-7 reloads, and 24mm F21-6 Econojets, too, not to mention a variety of G’s. This is a pretty fun model to fly. It’s good for relatively low power motors, but can accommodate some mid-power motors, too. It could handle H motors, too, ’cept it would probably never be seen again! Flight Log
What You Can Do
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The two centering rings at the motor end were glued on per the instructions except that the forward motor ring was a little bit farther forward to allow for the fin tab. Since the new E9 motors are a bit longer than the D's I located the motor block 90mm in and set the hook so that it hangs over the end 5mm. A 95mm motor will now fit perfectly. The hook was not passed through the wall of the motor tube as shown in the instructions because it would get in the way of the longer motor. It was bent back on itself instead, and hooks on the retainer tube. A spacer will be required when using normal D size motors. This spacer is cut from an expended D12 and goes into the motor mount ahead of the new motor, and is removed when using the longer E9.
Finishing
Forward: 
The centering rings were epoxied to the motor tube, with the many-holed one at the very top, and the others spaced properly to match the fin tabs. Eight 1/4-inch holes were cut between the top two rings to form a baffle. Next, a hard wood plug — one-inch long and 1 1/8-inches in diameter — drilled and fitted with an eye-bolt and matching nylon insert nut was epoxied into the top. Finally, the aft part of the motor tube was roughed up with coarse sandpaper and the motor retainer fixed in place using the recommended JB Weld.
Several layers of Rustoleum white primer was laid on and allowed to cure for a couple of days. That was then sanded heavily. The blemishes that were revealed were filled and another layer of primer was put on. That was sanded with 400 grit paper and the dust tacked off. A layer of Rustoleum Satin White was added and allowed to cure for several days. The highlight was then taped off with blue masking tape and sprayed. I must have been standing on my head when I masked off the rings, since the pattern is upside down! A logo was created using the computer, cut into masking tape by hand, then transferred to the model and sprayed using Rustoleum Flat Black. Since this “new” model is sporting a 29mm mount I decided to call it Magnum Big Bertha; that’s reflected in the new logo. A gloss coat of clear was planned, but never done.