Scratch K.J. Express Original Design / Scratch Built

Scratch - K.J. Express {Scratch}

Contributed by Moira Jean Whitlock

Manufacturer: Scratch
K.J. Express
(Contributed - by Moira Jean Whitlock)

[Rocket Pic]Height:  43.5"
Width:  2.14"
Weight:  18 oz
Motor:  29mm
Recovery:  Parachute

Brief: This is a single-staged, three-finned 29mm engine rocket serving as a tribute to the Phoenix Suns and fans. It's also a wish that the lockout will end soon! It is meant for the NCR F62 and the G40. The fins are delta shaped.

Construction: Since I cut my own fins with a bandsaw and sanded a double bevel on the leading edge with an orbital sander, this was more work than with kits that have precut fins. But the delta shape is one of the easiest to cut. And the plywood is 1/4 inch thick, strong and unwarped birch. This can be found at hobby stores and hardware stores. Home Depot has great selections as does Hobby Bench, if you have them in your state.

The fins' root edge is 5 1/2 inches and the span is 3 1/2. The trailing edge has no sweep, going perpendicular to the body tube. Of course, the chord edge is 0, as this is a hard delta shape.

The model is further made sturdy as the body tube is a full 34-inch LOC MMT 2.14 tube, meant as a motor tube, but strong as an airframe. The nose cone is the matching hollow plastic LOC 2.14. Two centering rings (that are for 2.14 tubes to 1.14 tubes) are used to align the 8-inch length of MMT 1.14 motor tube. So far, with the F and G engines used, no mass objects have been needed in this nose cone for stability.

Aligning the fins required using a tape measure around the tube at two points per fin for straightness, and trisecting the circumference to give where each of the three fins would go. I used generous epoxying for attachment with fillets on both sides after. I've noticed that one thing about epoxy is that it's hard to sand. With such fillets, try to make them smooth from the start.

Epoxy was also used to affix the one-inch 1/4 inch diameter launch lug 16 inches from the aft end of the body tube. Some RSO's feel that two such lugs placed a foot apart or one at 3 or 4 inches length provide more stability before stable velocity is reached, but thus far, after 10 successful flights, my one lug has been enough.

The parachute is my own creation, a 26-inch diameter hemisphere made up of eight alternating purple and orange triangular gores of poly/cotton broadcloth sewn together with a 4-thread serger, with 8 braided nylon shroud lines, 24 inches in length, sewn into the gore seams. The fabric can be found at Hancock's and other major suppliers. The gores meet at the top with no spill hole. The chute inflates nicely and colorfully. If you'd like to make such a chute, make the base of the triangle 7 1/2 inches and the height 13 inches. And the other two sides aren't truly straight, but gently curve to meet the top point.

[Rocket Pic]The shock cord is two feet of 1/4 inch bungee, which I got through American Science and Surplus and also from Atomic Rockets in Phoenix. The shock cord mount is a folded paper mount affixed 5 inches from the top of the body tube, to the inside wall. A 4-inch length of Kevlar® cording, also found at Atomic Rockets, was fashioned into a loop to tie the bungee to, and also to anchor into the folded paper mount. A knot in the loop was placed at the aft end of the mount to keep it from sliding forward after the mount was glued.

Finishing: This is unique. The nose and body are blaze orange, courtesy of enamel spray paint. The fins are purple, as is the aft 8 inches of the body tube. This is a mixture of Testors blue and red enamel paint. For some reason, I couldn't find purple spray paint. At the transition between the colors on the body tube is a 5-inch wide band of silver adhesive craft mylar, found at hobby stores. It provides a flash of visibility so I can sight in on my rocket. On every alternating fin face is a 2-inch flaming basketball with flames around it and flames snaking around the body tube. Ten inches from the forward end is a 6-inch version of the same. The basketballs are made from circles cut from orange Monokote. The lines defining the basketball are made with indelible black marker. The flames are made from pieces of Monokote. Finally, for style, I put a half-inch wide band of purple Monokote at the top of the body tube.

[Rocket Pic]Flight: So far, I've used the NCR F62-6, the aerotech G35, G40, and G80 with delays around 7. 4 would be too short. The rocket flies straight every time. Worm bed wadding has been used since flight #5, with enough to fill the diameter of the tube to an inch. Three pieces of conventional paper wadding wasn't enough, as the chute bears a few light scorch marks. The recovery is just right, with no fin breaks. The motor is retained with tape to friction fit it and also a tape ring at the aft end to prevent forward kicking. There is also a loop of piano wire which goes through the aftmost centering ring, bolted and epoxied in place. The loop bends over the motor but leaves the nozzle clear. Tape is wound around it and the engine, so no aft kicking occurs either. The biggest reward of this design is the kids shouting, "Ooh, basketballs!" when they see it, and, "Go, Suns!" when the chute opens.

Summary: It's a lot of work and I needed several tools, but the rewards are many. The kids love it, and it's durable and reliable. And I'll bet its design can be lent to other teams. How about Da Bulls?

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