The Happy Birthday Party Napkin
Rocket of the Apocalypse

Background

Our nephew, Johnny Schubert, had a rocket-theme birthday party when he turned six, and our sister, Jan, saved a napkin to give to Brad when he came to town. Such thoughtfullness should not go without a corresponding action; in this case, the build of a cool rocket!

Design

Brad started noodling with a flightworthy design almost right away, and using an enlarged photocopy and a set of calipers, created a RocSim file. The upscale of the napkin drawing produced a design that stood 45 inches high and with a 15 inch diameter (at the widest point) and a 31 inch fin span.

The initial puzzle revolved around materials and construction techniques. The aerodynamics of a short, squat, high-drag design required that everything possible be done to reduce weight, particularly aft. The bulbuous front of the fuselage will move the center of pressure forward, as will the tapered tail. The large fins will compensate for this impact to CP, but conventional materials would just be too heavy, moving the Center of Gravity too far aft. Each and all of these factors will reduce stability. CP is a function of the design, weight distribution can be controlled. But it will take care during construction and selection of materials or the result will not be airworthy.

Full-Size Templates

I printed the RocSim two-dimensional drawing onto a transparency. I taped a huge piece of paper on the wall, and using an overhead projector I traced a full-size outline of the drawing. (Those of you with CAD and plotter access will no doubt find this hilarious -- but you make do with what you have.) This drawing was transferred onto 1/8" masonite and two full-sized templates were cut from it. These were used in the construction of the Styrofoam Shaping Fixture.

Nose Cone

Knowing from experience that the nose will have to be weighted heavily to stabilize the design, this is the one area where I didn't select lightweight materials. Using a hole saw, I cut round plugs of various diameters from 1" pine (the actual thickness of 1" pine is 3/4"). These were stacked to get the unusual profile of the nose cone and glued.

For final shaping I ran a bolt through the center holes, which was then secured in the chuck of my drill press. I shaved off the corners with a rasp as the assembly rotated. Gaps were filled with spackle and sanded smooth. I countersunk a bolt (and washers for weight) in the tip of the nose and attached a screw eye to it at the bottom (extra weight can be added here at final balancing). The countersink hole was filled with spackle and sanded smooth.

Fuselage Construction

Styrofoam Block

The fuselage is constructed of two layers of 6 ounce fiberglass laid over shaped Styrofoam. A 4" tube runs through the center length of the foam. 1/2" launch lugs are attached to the center tube.

Home Depot sold me one 4'x8'x2" and one 2'x4'x1" sheet of white Styrofoam. The 2" sheet was cut into eighteen 16"x16"x2" squares. A 4" hole was bored in the center of 17 of these, and in one square cut from the 1" Styrofoam. All of these were stacked around the central 4" tube to form a 16"x16"x35" block. (I did not glue the pieces of the block together until AFTER I was done shaping the styrofoam with the Hot Wire Foam Cutter. I didn't want the hardened glue to interfere with the hot wire as it cut through the block.)

I cut two 4" disks with 1/4" center holes and friction-fit these in opposite ends of the center tube. A 1/4" aluminum rod mounted the block in the Styrofoam Shaping Fixture.

Excess Styrofoam (the part of the block that extended beyond the edges of the fuselage outline templates) was removed using the Hot Wire Foam Cutter. The block was then turned 90 in the fixture and the trimming process repeated. The block was then turned 45 and trimmed, turned 22.5 and trimmed, etc., until the block was almost completely shaped and rounded.

The fuselage, (at this point about 95% completely shaped) was moved to the motorized Airframe Tube Fiber Glassing Rotisserie Stand for final smoothing. The fuselage stayed on the spit through the application of two layers of 6 ounce fiberglass.

Nozzle

These are the step I took in the construction of the :

• Cut forward and aft rings from 1/8" Masonite
• The the forward ring is 5" diameter x 1/8" Masonite.
• The aft ring is 8" diameter x 1/8" Masonite.
• 8" Squares of foam were stacked 4.25" high
• 4" wide hole was bored through the center
• Rings and foam were all stacked on a 4" tube prior to trimming.
• The forward ring, aft ring, and Styrofoam are all fiberglassed together to be 4.5" high.

Fin Construction

I bought two sheets of 1/4" foam core art board and glued them together using 3M spray adhesive. This created a 1/2" foamboard with a double-thickness of paper running through the center. The fins were cut out from this using the full-size drawing.

I drew lines at 3/4" parallel to the trailing and leading edges. By angling a knife from this line to the double-thickness paper at the edge center. These cuts made perfect bevels at each edge.

Each fin was reinforced with two layers of 6 ounce fiberglass.

Fin Installation

Marking the fin slots was a real adventure, because there are no straight lines to use as a reference. I'm really glad I remembered my Dad's repetitive advice; "Measure twice, cut once". (Dad was an Industrial Arts teacher, so he's always passing on these bits of wisdom. His other favorite is, "Don't fall off the roof." God only knows how many time that saved me from a thoughtless tumble....)

I ultimately stood the rocket on its nozzle on a flat countertop and used a builder's square to mark all four fin slots perpendicular to the flat surface. Then I painstakingly measured the distances between fin slots (several times) to make sure they were straight and evenly spaced.

Fin slots were cut through the fiberglass 'skin' with a Dremel tool. That was intense. Then the foam was cut down to the center tube with a sharp knife and removed using a combination of violent gouging motions and a shop vac.

I created a fixture to hold each fin perpendicular to the fuselage as the epoxy cured. The fin tenons were epoxied to the 4" center tube. After the epoxy cured on the center joints, 2" fiberglass tape was applied at all the joints between the fin and the 'skin'.

Raised Viewports

The raised viewports are made of one layer of 6-ounce fiberglass laid over Styrofoam. Before I cut the foam, I used a hole saw to cut two 1/8" Masonite disks, one 4" diameter and one 2.6" diameter. I sandwiched a piece of 1.5" Styrofoam between the two discs and used a 1/4" bolt to hold them together. Finally I used the Hot Wire Foam Cutter to trim the foam extending past the discs to create a 1.5" thick disc with a trapezoidal cross section. This was fiberglassed and the bottom was shaped to fit the curvature of the fuselage.

I roughed the fuselage contour on the backside of the raised viewports by using a drum sander attached to my drill press. Then I refined the contour by sanding it against a sheet of 100 grit sandpaper I had taped to side of the brine tank of my water softener (20" diameter).

I took a lot of care marking the positions for the veiwports. Like the fin slots, it was difficult to get an accurate measurement with no straight lines to use as a guide. A cloth measureing tape really came in handy.

Last, I cut four 2" discs from the 1/8" Masonite, this time without a center hole. I sanded the edges smooth and centered them on the outward face of the raised viewports. These were later painted purple to match the napkin drawing.

Computer Flight Simulation

If you believe the computer simulations, this rocket will fly like a wiffle ball.

Avionics Bay/ Motor Mount

The computer simulations convinced me that it would be necessary to rely on electronic ejection. I had been concerned with achieving a vertical trajectory all along, and now that I was also concerned about altitude it seemed prudent to avoid a dependence on a delay charge only.

I had intentionally waited to install the Motor Tube until the rocket was finished because I had suspected that I would need to use the space between the two centering rings for an Avionics bay. I chose a 54mm tube. I cut a slot in the aft centering ring that would fit my G-Wiz flight computer. I mounted the G-Wiz to the Avionics Bay cover, which is another ring that fits over the fixed aft centering ring, and it all removes between flights. The screws used for motor retention also hold on the cover of the avionics bay.

The forward centering ring holds a 1.5" pipe clamp for the shock cord mount. A small piece of 1/2" copper pipe with an end cap was installed to hold the ejection charge. An aluminum tube connects the charge holder to the avionics bay -- this will be used to feed the squib leads into the av bay. I added another centering ring at the midpoint of the tube to compensate for any strength lost in the aft ring when I created the opening for the avionics.

Stabilization and Testing

The RocSim calculations of CP (center of pressure) are simply not reliable for the data I loaded in it for this rocket. I doubt it was designed to accomodate so many transitions. The Barrowman, RocSim, and Cutout calculations each produced radically different answers for the CP location. So I decided to do a swing test just to assure myself that the design will be stabile.

Imagine tying a rope to a 16 pound frozen turkey and swinging it around your head.

First, I fully loaded the rocket with recovery gear and a K550 motor. Then, I located the Center of gravity with these items installed. Then I unloaded the rocket, found the new center of gravity, and attached a rope at that point. (It was two inches ahead of the fully-loaded CG, and and two inches further back from the point where RocSim said it needed to be to be stabile).

Then I took it in the back yard and tried it out. (Someday I'll post the video so you can hear me grunt as I swing that albatross around in a circle.) I started swinging with the rocket facing the wrong way -- it quickly turned and assumed a stabile flight path. To test the stability of the design further, I removed the nose cone. This moved the CG back about six inches, 26" from the tip of the rocket -- right where RocSim calculated the CP based on the Cutout method. Once again, I started swinging the rocket tail first, and it flipped around and assumed a stabile flight path. I feel assured there is plenty of tolerance in stability of the design.

After the swing test I added about three pounds of weight to the nose cone by attaching a stack of large fender washers to the eye bolt. This moved the Center of Gravity ahead another 5 inches, well ahead of the points I already proved were stabile using the swing test.

About three pounds of nose weight was added when balancing the rocket for flight. Fully loaded, the rocket weighted 22.5 pounds.

Click HERE for photos of the maiden flight!

In May I was asked to write a magazine article for Extreme Rocketry magazine covering the NAR National Sport Launch sponsored by the Superstition Spacemodeling Society (my club in Phoenix). This gave me the chance to write about this rocket as part of the event article, and my picture was prominently displayed in the magazine.