Construction Tips from and Old Fighter Pilot 3

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Joined: Thu Apr 12, 2012 9:10 am
Location: Alexandria, VA

Construction Tips from and Old Fighter Pilot 3

Post by Colonel » Wed Apr 18, 2012 10:31 am

We mounted our three fins in a triangular pattern with one in the center top and the other two equidistant on either lower corner. Draw an equilateral triangle on the tail end of the rocket to define the points where the fins will attach. This led us to taper the back end of our rocket to a triangular shape that matched the fin layout. We cut the fin sheets from corner to corner to form two fins from each sheet of plastic. We then measured and drew lines for the fin slots on the sides and top of the rocket to make sure everything stayed symmetrical.

c. We now had our symmetrical airfoil shape that tapered into a triangle at the tail. The next step was to get rid of weight. I once built a 10" high tower from 1/8" balsa wood strips for a school project that held 413 lbs before collapsing. The stress on the rocket caused by the rubber band motor is longitudinal. If 1/8" square balsa strips can withstand 413 lbs longitudinal stress, 1/8 " balsa strips can withstand the force applied by 3 rubber bands. We drew 1/8" wide lines centered on our fins (1/16" either side of each fin, 1/8" either side of the top fin to accomodate the hangar) longitudinally down the side of the rocket. We drew a line 1" aft of the nose and 1" forward of the tail. Then we carved out all of the wood between those lines and got rid of all that weight. We now had a rocket that was a solid piece of wood at the nose and tail connected by three 1/4" to 1/8" "beams". It looked cool (you can see the rubber bands work), was plenty strong longitudinally (but you can crush it your hand with minimal side-to-side force), and was very light-weight. We worked the body thickness to 1/8" all around with a dremel tool and sandpaper. Re-balanced and marked the spot for the hangar. We then carved out the hangar slot on each half (still have not glued the ship together) and glued the whole thing together at once using two-part epoxy. We then attached the fins, cut the slots in the tail for the plastic rubber band holder, put in the prop and rubber band and re-balanced, sanding to get the balance as perfect as possible.

d. Efficient power from the prop. Assemble the prop with the rounded end toward the rocket body. Use graphite on the prop shaft before assembly. Bend the end of the wire all the way over the prop to avoid prop slippage. Balance the prop by holding the wire and see if either side of the prop hangs lower than the other. Sand the heavier prop tip until both sides hang level. We did not lube our rubber bands. Dry ones worked well, but we did not test both methods.

e. Validate with a test flight. Spend the ~$4 for a hangar so you can test fly your rocket. We built a light, well-balance machine (we thought) with the goal of reducing as much friction and rocket movement as possible. Now we had to make sure the rocket preformed as designed. We tried a few fligts with 50, 75 and 100 winds on the motor before going to 125 and 150 winds. The ship flew a little left-wing down so we trimmed the left fin tip with a dremmel tool until we returned to balanced flight. One final sanding and then apply the finish.

f. Finish. The paint on a fighter jet can easily weigh over 1000lbs. That weight you feel when holding a gallon of paint goes on the thing you paint. So, be careful when finishing your rocket. I pushed for a stained finish but the boy wanted something fancier. We rumaged around in the shop and found some silver leaf I had left over from refinishing an old picture frame. The silver leaf goes on easy (paint the rocket with a thin coat of leaf adhesive, let it get tacky, the just lay the sheet of leaf on the rocket and rub on it with your finger to transfer). The leaf is nearly weightless, provides a smooth surface (better airflow) and looks like the polished aluminum jets from the 50s and 60s. Draw a few seam lines and rivet points with a fine tip Sharpie and you are finished. One more test flight to make sure nothing changed during finishing and off to the races!

PS: We strenghtened the hangar-to-body bond by heating two straight pins until red hot and "drilling" them through the rocket body and plastic hangar tab inside the body using only heat. We then cut off the ends and sanded the pins smooth with a dremel tool.

I'll post a photo whe I get it. good luck!

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Pine Head
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Re: Construction Tips from and Old Fighter Pilot 3

Post by having_wandered » Wed Feb 13, 2013 11:31 pm

This was great information. Thank you. I've heard discussions in our pack about doing one of these this year. Sure hope I get to put this info to use!

~Not all who wander are lost.

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