PERSHING MISSILE

CONSTRUCTION

Materials List

• [2] 7.5" QuikTube (One is for the airframe and one for the coupler and fin flanges. At less than $5 each, you can't go wrong.)
• [1] 3" LOC Kraft tube
• [1] 3" LOC Kraft coupler
• [2] 54mm LOC Kraft tube
• [3] 54mm LOC Kraft coupler
• 1/4" birch plywood for:
• - Fins
• - Bulkheads
• [2] 54mm
• - Centering & Reinforcing rings
• [2] 54mm-3"
• [2] 54mm-7.5"
• [1] 6.75"-7.5"
• [1] 54mm-7.65"
• [1] 38mm-54mm
• Poster board
• Casting resin
• Two-part foam
• Rail buttons or launch lugs
• Recovery hardware & harness
• Parachute(s)

Most of the above construction materials were purchased through Space Modeling Solutions. The recovery components (tubular nylon, heavy swivels, and Rocketman chutes) were purchased through Rocket Silo (no longer doing business) as were my Dr. Rocket motor casings.I also seem to drop my share of money at Home Depot for the sundries, paint, and epoxy.

Assembly

I'll start at the top and work down:

Nose Cone

After making the paper cone described above, I inverted the cone and filled it with casting resin. Once hardened I epoxied on a 2.5" length of 3" coupler as a shoulder. This was later filled with resin as well. A 1/4" hole was drilled about 6" deep and tapped out to take a 5/16" screw eye. Total weight of the nose cone assembly is about 2 pounds without the addition of balancing weight. I added about 20oz. of weight when I flew it on a K550 to compensate for all the propellant. This design has to be nose heavy or it won't be stable.

Transition Section

Parachute Tube subassembly

Cut a 4.25" piece off the 54mm tube. Pick an end of this piece to be the forward end and mount a 54mm-3" ring flush with the end. Mount the second 54mm-3" ring 1/4" from the aft end.

Mount the 38mm-54mm centering ring inside the tube exactly 2 " from the aft edge. Epoxy it in really well, keeping all your epoxy fillets on the forward side of the ring. This ring will hold the removable shock cord mount in place, so you measurements need to be exact and your construction tidy.

Cut the 3" tube to 30". Mount the previously described subassembly inside the aft end of the 30" x 3" tube so that the aft ring is flush and 1/4" of the 54mm tube extends past the end. (This extension will eventually be affixed to the 54mm-7.65" ring, but don't do it yet.)

Bulkhead/coupler subassembly

Select the QuikTube you want to use for the aft airframe and cut off 4". Remove the waxed liner. Take ~" vertical slice from this piece, compress the ends together and slide it inside the spare tube. Epoxy the ~" vertical slice over this joint, being careful not to glue the unfinished coupler to the spare tube. When hardened, slide it back out and fill any gaps with epoxy. Sand the outside of the joint smooth.

Center this coupler on the 54mm-7.65" ring and epoxy in place with a butt joint. (Hint: fit the aft section rings, tubes, and couplers together to help center the 54mm-7.65" ring on the coupler.)

I also bolted a 1" pipe clamp to the 54mm-7.65" centering ring extending aft inside the coupler. This is the forward shock mount used when the rocket separates in the middle.

Transition Frustum

I made a makeshift arc scriber out of plywood and coat hangers to draw the arcs as reckoned by the Transition Calculator. Unable to find a poster board of adequate size, I was forced to make it in two pieces.

Draw the arcs, cut them out and join them together. This is rolled into a frustum (that's a cone with the pointy end nipped off, for those of you who slept through geometry class). When the parachute tube is inserted it should extend about 1/8" past the end of the cone. This gap will be filled with putty and smoothed during the finishing stage. This whole transition will also be fiberglassed when I make version 3 as a two-stage missile.

Fit the transition cone, tube, and bulkhead subassembly together and glue the cone to the tube with thick CA, then seal that joint temporarily with masking tape. Balance the assembly upright on the narrow end and remove the aft bulkhead. Fill the forward 3" of the transition with 20 minute epoxy and replace the bulkhead subassembly to assure that it hardens in the proper shape. This epoxy strengthens the forward opening against zippering.

Hints for foaming the cone - mix many small batches of two-part foam and not one big one. Why? The foam expands straight up, away from the slanted sides of the cone creating gaps or dents in the cone. (I learned this lesson the hard way of v.1.) This is bad. Pour each small batch into the gaps that resulted from the previous pour/expansion/cure, and rotate the cone at a 45 angle to coat the sides of the cone. Be sure to replace the aft bulkhead subassembly after each pour to assure that the cone hardens in the proper shape.

Once the cone is filled and the foam hardens, remove any extra foam and epoxy the bulkhead subassembly in place

Aft Section

Airframe

Sand and prep the QuikTube. Mark it full length for three fins and for the rail buttons or launch lugs.

Fin Flanges

This is the other use for the extra QuikTube. Cut off a 5" section and peel off the waxed liner. Then cut it into three 2.5" x 5" rectangles (they will be arched, of course.) These are the aft flanges. Cut the remaining piece into three 2.5" x 4.5" rectangles. These are the forward flanges. Mark a line down the center of each piece.

The aft fin flanges are glued flush with the aft edge of the main airframe, centered on the fin lines. The forward flanges are 22" from the aft edge. Cut the fin slots through the flanges and airframe. I used a Dremel tool with a cutoff wheel.

Motor Tube

Cut a length of 54mm tube 6 1/4" long from the tube you cut the piece off of earlier. Attach this to the 54mm tube that is still 36" using one of the 54mm couplers. The end with the coupler mounts flush with the forward edges of the forward 54mm-7.5" centering ring. The aft end extends " past the aft ring but flush with the fin tenons.

Aft Reinforcement ring

The 6.75"-7.5" ring mounts flush inside the aft edge of the main airframe. This is added for strength. Leave it off if you want, but to me it is worth the extra weight (which isn't much).

Rail Buttons

I launched both versions off of a rail, so I planned ahead a bit to better accommodate the rail buttons. I cut three [3] 1" cubes out of pine, glued one each to the fore and aft centering rings (to the enclosed side), and one was attached directly to the inside of the tube midway between the forward fins. I drilled a " holes through the airframe into the blocks and installed threaded inserts for the rail buttons.

Aft Centering ring

There is nothing special about this ring, except that it interlocks with the aft fins. See the picture to the left taken while the rocket was being primed. This picture also shows the aft fin flanges and the aft reinforcement ring quite clearly. Also note the threaded inserts installed for the motor retainer.

Forward centering ring

I bolted a 1" pipe clamp to this centering ring, and it extends forward. This is the aft shock mount used when the rocket separates in the middle.

Forward fins

I glued the pieces of the wedge sections together first, then epoxied in the 'thru-the-wall and thru-the-fin' tabs. Then I filled the gaps with resin to make each a solid piece. They became quite sturdy.

Foam

The fin tenons were encased in two-part foam after being epoxied in place.

Shock Cord Mount and Interstage Baffle

I went to great lengths to test the operation of the my designs before I fly them. Click here for a video of a test of the ejection system. (0.7meg)

Parts list

• [3] 1/4" plywood x 54mm bulkheads
• [2] 5/16" x 4" screw eyes (or shorter if you can find them)
• [1] 5/16" extension nut
• [2] 54mm couplers
• [1] 54mm x 4" tube
• [4] 1/4" plywood x 2" bulkheads
• [1] 1/4" x 5" hardwood dowel
• [1] 1/4" plywood 54mm/38mm centering ring
• 12" 1500 lbs. Kevlar

Shock Cord Mount Bulkhead

Glue the [3] 54mm x 1/4" bulkheads together. Drill holes through this now 54mm x 3/4" bulkhead in the pattern shown in the diagram. The center hole is for the screw eye and is 1/4". The other holes are 3/8".

The two 5/16" screw eyes are mounted through the center hole. The aft side screw eye is mounted with the eye against the bulkhead. An extension nut is installed against the opposite side and the other screw eye also installed into the extension nut. You'll need to trim some of the length off the threaded section of each screw to make this fit.

The aft side screw eye needs to fit inside the forward opening of the Interstage Baffle Section. Measure the aft side screw eye so you'll know how much room you'll need.

Interstage Baffle Section

Cut a 4" piece of the 54mm tube. Mount a 54mm coupler in each end. The couplers should meet inside the tube. Make sure that the forward side coupler extends EXACTLY 2" from the 54mm tube.

Take the four [4] 2" bulkheads and drill a 1/4" hole in the center of each (if there isn't one already). Trim 5/8" off one side of each (as shown).

Insert the 1/4" dowel into the center holes, spacing the disks 1 1/2" apart. Glue disks in place.

Situations like this are great applications for Polyethylene Wood glue. Coat the inside of the tube with the glue and slide the assembly inside - the Polyethylene glue expands as it sets and forms a strong bond. When you glue the baffle assembly inside the coupler/tube assembly, leave room in the forward opening for the aft side screw eye on the Shock Cord Mount Bulkhead.

The baffle should trap all the heated particles blown forward by the ejection charge, but I still use a Kevlar/Nomex chute protector.

You needn't attach the Shock Cord Mount Bulkhead to the Interstage Baffle, and you absolutely SHOULD NOT glue it into the forward transition assembly. If you do you will only be able to fly your rocket in one configuration.

Drill a 1/8" hole at the position indicated in the baffle wall. Thread about a 12" length of 1500 lbs. Kevlar through this hole and secure to the aft side screw eye of the Interstage Baffle Section. Tie the other end into a loop. (As insurance, the Kevlar loop will be secured to the pipe clamp mounted on the aft bulkhead of the transition section by a QuikLink just in case the shock mount fails.)

Forward Assembly

Thread the recovery harness through the parachute tube and out the aft end of the forward transition section (I used 35 feet of tubular nylon). Fasten the recovery harness to the forward screw eye (I use QuikLinks as connectors). Insert the Shock Mount Bulkhead into the aft end of the forward transition section, then the Interstage Baffle. Connect the Kevlar cord to the pipe clamp mounted on the aft bulkhead of the transition section with a QuikLink. The forward section is now complete.

Flight

I've never brought this rocket out to the flight line without several onlookers peering at me with the hairy eyeball and asking, "You really gonna fly that thang? Ain't got no fins!" But fly it does. The Pershing has flown on I211, J450, and K550 motors. By far the best performance of this rocket has come during the K550 flights. Several of those flights were at meetings of the Arizona High Powered Rocketry Association (AHPRA), one of which was witnessed by the good folks at Rocket Silo. They liked the K550 flight enough to post a picture of the Pershing and my smiling mug on their web site on the Silo Cadet Project page

Above is a photo of the Pershing flying on a K550. Click here to view video of the Pershing on flying on a K550.

Global Deployment

It is said that imitation is the sincerest form of flattery. We found out how true this is when we received an email from Burl Finkelstein, a fellow we had never before met, informing us that he had created a Pershing Missile of his own based on this article. Click [HERE!] to see photos!