Jupiter C

Design

The NAR Technical Report Plastic Model Conversion by Ken Brown has an article by Scott Clement detailing the conversion steps for the Glencoe kit. It uses extra fin area instead of nose weight to achieve stability. Additional ideas on this conversion found on Ninfinger's site here suggest that this isn't necessary with sufficient nose weight, and my RockSim analyses seem to bear this out. (I've created a "styrene only" file for those of you that want to try out your own ideas.) I, too, was worried about "snapback at ejection," so along with 12 inches each of ¼ inch elastic and 300# Kevlar®, I made my design mid-separation at the joint in the first stage between the fuel and LOX tanks. I also considered putting some of the nose weight in the "tub", but thought that the pin holding it on was too weak for this. I wanted the full one ounce of lead in the big transition.

I test-fitted a T-30 as a stuffer tube, and like the BT-56 suggested elsewhere, it was too big. I went with a BT-55 stuffer as it was a better fit in my bird. A thick-walled BT-20 is indeed a perfect fit in the bottom hole (which could be considered a styrene centering ring), but the BT-55 doesn't fit in the bottom can, the indentations for the fins are in the way. I designed an extended motor mount into the bottom BT-55 with two CR-2055 centering rings to secure it there. The first stage joint is also too small to pass the BT-55, but it will pass a JT-55 tube coupler; hence the mid-separation.

Assembly

For the plug in the upper tube, I cut a length of BT-55 bulkhead I had from Rogue and drilled it to accept a ¼ inch hardwood dowel. I drilled this to accept a 1½ inch long 6-32 brass machine screw, and on the head side of this I placed a small electrical lug for the shock cord mount. On the nut side of the screw, I placed two 50 caliber lead shot I'd smacked into disks with a claw hammer and drilled. A thin washer sits between them and the plug. The whole plug massed about 30 grams when I glued it into the upper BT-55.

Then I began the styrene assembly. I am not a fine scale modeler, and I knew this was going to be the hardest part. The first job was to notch one of the lower fin can pieces to pass the motor hook, which I roughed out on my hobby band saw and trimmed to shape with a hobby knife. I can see why the FSMs like this plastic, it works very well; I had a perfectly sized notch in just a few minutes with no trouble.

I tried several glues on parts from the spare kit and settled on PolyZap for the styrene-to-styrene joints and PlastiZap for the styrene-to-paper ones, both from Pacer Technology. I attached the fin can halves to the LOX tank halves before inserting the modroc pieces, and glued the fuel tank halves around the upper BT-55. I wrapped both sections with large rubber bands to hold everything tightly while the CA took hold. While these cured, I did the satellite, upper stages can and big transition separately. Finally, I assembled the fins, leaving the jet vanes off as they would interfere with the motor. Once everything was cured, I attached the fins to the lower section.

I dry fitted everything and found that the Cg was about where I wanted it, then glued the subassemblies together. I used Squadron putty to fill the most egregious gaps and attempted to smooth the visible joints, but as this was my first styrene kit in quite awhile, I didn't go overboard. Finally, I attached the fuel tank pipes between the steering nozzles, whose placement is curiously absent from the instructions. A quick consultation of Peter Alway's Rockets of the World gave me an approximate location, though I don't expect then to survive the first flight.

Finishing

Preflight

Another consideration for this bird is its velocity at the end of the launch rod. Her fins are small and for them to be effective, she'll have to be going at a goodly clip. I've included the End Of Rod Velocity (EORV) in the motor matrix. While the C5-3 is the best black powder motor for her in this respect, it ejects pretty early (~25 ft/sec). The C10 is better for both (~20 ft/sec), but is unavailable. The C6 ejects at about the same time as the C10 and this bird is over its maximum take-off weight by ½ ounce, but has a low G loading. The C4 has the best ejection timing (~10 ft/sec) but is also unavailable and probably the worst choice, being no faster off the rod than the C6 and is also not rated for this weight. In short, no motor is a perfect fit for this bird, but I was able to pick my poison for her first shot, there are plenty of semi-marginal ones to choose from.

Flight

Postflight

Scoring

Conclusion

The Glencoe Jupiter C is a favorite of PMC modelers, and given the differences between mine and others I've seen out there, I'd say it was comparitively flexible. The fit of the model rocket parts in the plastic components and the absence of any specialty materials or devices tend to make me think of this conversion as building a model rocket in a styrene bottle. The styrene takes the place of certain things, like finish detail and aerodynamics, and you only have to provide the other basic functions a model rocket would require. I'd rate this conversion as Easy (1), and further comment that the simplicity of the model itself makes the FSM portion of the effort relatively painless.

In addition, this thing taught me some good stuff that I'll be applying to my next PMC effort:

• Buy Two Kits - FSM rockets aren't very popular and go in and out of production fairly regularly. This means that the kit you get off the store shelf or from an on-line retailer might have sat awhile gathering dust. Styrene will change its shape over time in response to internal molding stresses, and this means that your kit's parts might be warped. Getting two, preferably from different sources, will let you pick from both kits to get the best, i.e., least distorted, pieces.

• Test Fit, Dry Fit, Reassemble - Test fitting everything first and repeatedly during assembly is a process most fine scale modelers do as a matter of course, but its often overlooked by rocketeers. Occasionaly, despite your best dry-fit-up efforts, an assembly will come out distorded or misaligned. Styrene joints are fairly easy to break, don't be afraid to do so and attempt the assembly again, this time with a different technique. Having two kits on hand will help here, too.

• Use CA - Most FSMs use CA exclusively now, eschewing the plastic model cement we all know from our youth. While this cement has improved over the years (the toxic-free stuff actually works, now), it still joins pieces by melting the styrene which can cause warpage, has a tendency to squish into and onto places you don't want it, and is tough to sand. CA, especially those formulated for plastic models, is much easier to control and work after cure. I don't know if its the glue's formulation or the materials its joining, but CA/styrene joints can take a long time to set up, and certainly don't tack very well. In addition to the CA, get some "kicker" to speed tack, and some debonder to clean up with. You may not use them on every joint, but having them at hand will save you at least once.

• Bind Fuselage Pieces During Cure - Wrapping the entire length of cylindrical part halves with large rubber bands as a last step in joining will help bring them back into round and insure that the edges are mated properly. Failing to do this will allow the edges to cure slightly out of alignment, and require you to use putty and sand like mad. This will destroy all the fine detail molded into the styrene body and ruin the model's appearance. On the other hand . . .

• Don't Be Afraid of Putty - Putty will fill in voids and gaps fairly easily, and other than the wicked smell, is fairly easy to use. It sands easily, takes paint well, and is relatively inexpensive. Despite claims to the contrary, it will shrink during cure, so be prepared to retreat the affected areas. If you have to do this more than once, though, you're using putty where you shouldn't be. Use white for most models, green only on those which will be painted green. You can't build missing parts from it, or use it to cover egregious gaffs, but for cleaning up those little molding or assembly imperfections, its perfect.

• Paint in Thin Coats - Nothing will ruin the finish of a styrene model faster than thick coats of paint; it obscures details and can actually warp the plastic if too much is applied at once. While most enamel paints are styrene-safe, test on a sprue or unused part to be sure. (Most R/C fuel-proof paints are not styrene-safe!) Test your entire paint "stack" first, all the way through to the clear coat; there is nothing worse than getting half-way through the finish only to find out that the next coat of paint solvates the previous three! Thin brush-on paints with laquer thinner and use a fine brush.

• Get an FSM Buddy - One of the best bits of support you can have is a fine scale modeler friend to call in moments of crisis. It is my great fortune to have the phone number of Rob Schorry, one of the best fine scale modelers in the city. While he didn't receive any panic calls during this build, his initial advice was invaluable. On the other hand, FSMs are often too eager to suggest an advanced technique beyond the talent of the average model rocketeer, e.g., cutting off the alignment pins to cure a parts misalignment. Only attempt those things they suggest if you feel comfortable with them.

Estimated Performance

Engine(s)	AGL (ft.)	Speed (ft./sec.)	EORV (ft./sec.)	Accel (Gs)
C4-3	385	115	32.3	6.9
C5-3	395	140	46.4	13.6
C6-3	365	135	32.2	8.4
C10-4	485	180	49.1	17.0
D21-7	1,165	365	61.1	20.6