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Scratch - Tubular Crayon {Scratch}

Original Design / Scratch Built

Published:2005-09-17

Contributed by Greg Lane

(Contributed - by Greg Lane - 09/17/05) (Scratch) Tubular Crayon

Brief:
My Tubular Crayon is a single stage HPR rocket using parachute recovery. It is based on the RalphCo type Crayon Bank 4 inches in diameter and 35 inches long. I found the banks on sale at Toys R Us for $5.99. I decided to use tube fins instead of the usual clear fins.

Construction:
The six tube fins are made from two extra crayons. The motor tube is 54mm with a Slimline retainer. There are three centering rings for 3.9 inch tubes. I used a 3.9 inch bulkhead in the nosecone. The chute is a Rocketman R7C in a deployment bag.

The tube fins are 4.25 inches long cut from the extra crayons. They are located three inches from the bottom. The offset is to have the fins on the stiff cardboard tube instead of the plastic base. The tube fins are bolted to the body tube and bolted to each other with #8 nuts and bolts. This makes for a rigid assembly. I used no epoxy on them. I sawed off the excess length of the screws inside the tube fins.

I epoxied two centering rings in the cardboard tube. They were for a 3.9 inch tube and required some sanding to fit. The third CR is inside the plastic base. So the plastic base is wedged between the lower centering ring and the Slimline retainer.

The tubular nylon shock cord is epoxied to the motor tube between the centering rings.

I added weight to the nose by sinking some 1/4 inch all-thread into 2oz of plumbers epoxy clay at the tip. The all-thread extends through a bulkhead which has a eye hook for the shock cord. I added about fourteen ounces of additional weight. The weights were a combination of lead shot and epoxy clay molded into slices of motor tube. The weights were screwed onto the all-thread.

(Scratch) Tubular Crayon There are two PML type rail lugs attached at 9 and 16 inches from the base of the rocket. I used three layers of excess tube stacked to create an offset for the rail to clear the screw heads in the fin tube.

Through some approximation with Apogee Rocksim 8.0, I believe a conservative estimate of the CP is 24.5 in from the nose or just aft of the R on the body tube. Therefore the CG will need to be at least 20.5 or forward of the Y on the body tube. To get the CG forward I added about one pound of weight in the nose. The chute and deployment bag weigh twelve ounces, so it helps with the CG somewhat. The weight of the rocket is about 4.5 lbs without the motor.

I chose a Aerotech H242T for the first flight. I adjusted the medium delay to about 7 seconds. This was my first attempt using the Aerotech procedure to shorten a delay. I used a Slimline 54/38mm adapter with the 38mm H242T motor.

Finishing:
This rocket requires no painting since the crayon bank come finished with the colored crayon paper.

(Scratch) Tubular Crayon

Flight:
Its first flight was at the September 10, 2005, SEARS launch in Samson, AL. The flight was a success. The winds were 5 to 10 mph and the ejection was close to apogee. I was relieved that the delay adjustment worked out. This was a low flight to about 1000 feet and I got to see the rocket's entire flight.

Recovery:
The Rocketman R7C chute and deployment bag worked perfectly. I used no wadding.

Summary:
The pros for this rocket are that the body and nosecone are available from a local source for a cheap price. Tube fins are simple to make and might require less effort than rounding up the fin material and cutting it out and coming up with a through the wall mounting scheme. The tube fins were very sturdy with the screw mounting alone. There was no painting required. The rocket has a lot of visual appeal.

The cons are the weight of tube fins and a 54mm motor tube drives it into the HPR range. Most crayon rockets are launched on a G motor. The Slimline retainer also adds to the cost but certainly does the job.

(Scratch) Tubular Crayon

Most of my rockets have been kits or kits modified. So the tube finned crayon rocket was an excursion into scratch building. You have to put some thought in to make sure you are building a stable rocket. I used Rocksim 8.0 as a design aid. This version handles tube fins. My model was only an approximation as I used a conical nose instead of the true shape of the crayon's nose. I found that the Rocksim 8.0 Center of Pressure was more forward than the faked tube fin approach used with earlier versions of Rocksim. In any case, the flight was very straight. Also I am pleased to have a rocket that is unique to my knowledge having the crayon shape and tube fins. Of course the final reward is getting that "I liked that crayon rocket flight" response from my fellow fliers.

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