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Fireball XL-5 Construction |
The Fireball XL-5 construction project has the following steps:
In my first pass through scaling the XL-5, I picked BT-80 as the size for the main body section. After printing and measuring, it became clear that this was not an optimal size. The ship's tapered "neck" led to a 2 inch upper body tube and nose cone, which is not a readily available size. Wanting to maintain a solid internal tube, I set the nose cone size to BT-80 and got 3 inches for the main body tube size. This is also a non-standard size - but in this case the non-standard curse of NCR was a blessing. One of the NCR tube sizes is 3 inch, and I had some left over from my 3 inch V2. This way, I could use a solid BT-80 running the length of the ship, with the 3 inch tube around it, and some kind of reducer to make the ship's 'neck'.
I created the blueprints based on an image at the Fireball XL-5 Web Site. This is a tedious but simple process that can be done using any image editor that supports true image sizes in printing - i.e., allows you to set the image dimension in inches, or the pixel resolution, and actually prints out at the scale you specify. I used Photoshop, which makes this an easy proces when on-screen measurement units are set to inches. With this feature, all that is required is to measure the body tube diameter directly on the screen and adjust the resolution until the desired size is obtained. Then the project can be printed on as many sheets as required and assembled with tape to create a full-size blueprint.
In addition to the blueprint, it is convenient to create patterns for each fin. With this rocket, I had a problem creating a fin template for the forward fins. Since they're at a 45 degree angle, there was no good edge-on shot from which to extract a template. I started with the 45 degree shape and stretched it in Photoshop by dividing the width by the cosine of 45 degrees (about 0.707) and increasing the pixel size in that dimension accordingly. The seemed to create the correct shape.
Basic construction methods selected were:
The overall size and construction methods chosen immediately imply a 29mm motor mount, mostly likely an F motor, or perhaps a small G. Before calculations are performed to determine nose weight and thus overall liftoff weight, the final engine selection cannot be performed. Unfortunately, none of the popular simulation programs can handle a two-finned configuration - they all presume 3, 4, or more symmetric fins. Additionally, the unusual fin shapes and large side pods will result in an unusual drag profile making it more difficult to compute an exact delay.
Construction was undertaken in sections by module. These are the wings, body, nose (Fireball Jr.), and reactor/engine area.
The first step in making the wings was to carve the outer rocket pods. The pod starts as a block made by laminating two 3/8" balsa sheets together. The rough shape was cut with a Dremel tool using a large abrasive cutting wheel. The next step is to create the compound curved surface and rounded edges of the pod. This was done by roughing out the shape with a Dremel sanding cylinder, then finishing the shape with a sanding block and 150 grit paper. The shape can be done by first pointing the sides of the nose, then slanting the sides by sanding at a right angle.
Building the wings themselves turned out to be one of the more complex parts of the construction process. Assembly should be about the same as building a model airplane wing - but I've never built a model airplane wing, so that didn't help me much. The tricky part is getting the right taper from root to tip while maintaining a plane surface and keeping the leading edges aligned. When I originally looked at the blueprints, I thought the wing had a curved surface, but it is indeed composed of two plane surfaces.
A reinforced leading edge was added by shaping a wedge from square basswood stock. The taper of precut model aircraft wing parts was not right for this wing. For additional structural strength, a basswood square runs from the outer jet pod through the two walls of the body tube. This provides a framework for attaching and laying out the ribs, and should provide the structure required to hold the heavy jet pods in place during the take-off acceleration.
The wing's rib structure is visible in the fireball design - the ribs are both a structural and design element. For this reason, they need to be straight and sanded flush with the wing's aft surface. Glue joints need to be neat in this area that can't be seen in most model airplane wings.
When the wings are ready for assembly, a square hole was cut in the jet pod to hold the square rod. This sounds complex, but was done easily by visual alignment and cutting with a pointed xacto blade.
The top fin was made with a pre-cut model airplane trailing edge running along the outer edge. The walls are made from the same balsa as the wings. Once glued to the body,this is a very sturdy triangle. The ribs across the back are just large enough to give the impression of ribs - they don't serve any structural purpose.
The main challenge of the body is to create the "neck" taper. This was done by laying down a bed of "Patch and Paint" filler then covering with several coats of finishing epoxy for strength and surfacing quality. To get a smooth surface, the outer layer must be thinned with a considerable amount of water. A metal ruler was used to make the final surface. The material works much more easily if the shaping tools are ept wet.
The nose cone and forward fins are built-up from an Estes Broadsword size nose cone. his part of the ship was refered to as "Fireball Jr." in the show. It had the ability to leave the ship to land on a planet while the mother ship remained in orbit. Fireball Jr. has a more classically pointed nose cone than any of the production nose cones, so the tip must be built up with epoxy. For quick rough casting, I use play-dough for a mold. This sounds difficult, but is actually very easy. Just shove the nose cone into a big lump of play-dough, then sculpt the desired new shape into the impression - in this case, a pointed nose. Fill the depression with epoxy and shove the nose cone in. When it epoxy cures, only a small amount of play-dough will stick to it. Clean up the shape with sandpaper, and you're done. If you're worried about your sculpting skills, just be sure to make the hole in the play-dough too large rather than too small so you can fix your errors by removing material when you sand the shape smooth.
An important trick when adding epoxy to a nose cone (inside or outside) is to drill through the tip and place a screw inside so the epoxy will have an irregular shape to adhere to. The screw head sits on the outside tip of the nose cone, and the threads project inside. This will be important later when lead and epoxy are added to the inside of the nose for stability.
The "reactor" was created using 3/16" Evergreen styrene tubes. Cut them to length and stack around the tube, leaving any gap on the underside where it won't show. Fill the ends with spacking compound or some other sandable filler.
The wings have small matching streamlined bubbles on the top and bottom surfaces. There are also large motor/jet shapes on the bottom side of the wings. The teardrop wing bubbles were created by carving the basic shape from balsa freehand using a Dremel sanding cylinder. After creating the basic shape, it is attached and smoothed into the wing shape with finishing epoxy. The cylindrical jet/motor shapes were created using styrene and balsa. An unusual feature of these shapes is that they are aligned with main body tube - consequently the aft portion is almost half covered by the sloping wing.
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| Clamping pods to wing | Tail Cone | Building a layer | Wing underside |
The finish foundation was created by many coats of finishing epoxy with sanding in between. Some wing areas were filled with "Patch-N-Paint" and sanded prior to the first coat of expoxy. This helped to create a very level surface on the wings and fill any voids around the leading edges.
After repeated coats of finishing epoxy, and sanding, several coats of Rustoleum red oxide primer was used to fill any remaining voids and produce a very fine surface for the final metalic coat. At this point I actually painted the ship silver and red to see how it would look and reveal any surface finish problems. It is much easier to see problems on paint than resin-covered wood. After painting, the ship was sanded back to mostly primer and surface problems corrected.
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