Original Design / Scratch Built
Contributed by Greg Lane(Contributed - by Greg Lane - 09/17/05)
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.
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.
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.
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.
What You Can Do