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Have you ever played water rocket?
In this instructable, I will teach you how to make fins for your water rocket, so your rocket can fly smooth and high.
The fins are made up of 2 parts which is basic triangular fins and extended fins around it.

This is my first instructable, I must apologize here for my poor English. Please comment on my mistakes

Step 1: Materials

To make the fins you must need:
-plastic sheet ( i get some hollow plastic sheet which call corriboard, it is light but strong )
-duct tape

you also need:
-scissors or knife
-paper
-sharpie and pencil
-a pair of compasses
-cellophane tape

Step 2: Measuring

Because we will make a prism shape fins first. So we need to know how long the board we show make.

First, measure the perimeter of the middle of the bottle.
if you don't have measuring tape, do like it:
make a strip of A4 paper,
cover it around the middle of bottle,
make a mark at the site which one end of the strip is above the paper strip

and calculate like this:
Circumference of circle=2*22/7*radius of circle
radius=(circumference*7/22)/2
if i get 29.4cm as the circumference, then the radius is 4.7cm

make a circle with radius 4.7cm on A4 paper using compasses,
Make 6 circles with same radius around the circle which center of six circles is on the line of circle in center, every six circles have the line touches the center of the circle beside it. 
now make three line around the circle at the center to become an equilateral triangle with the help of the 6 circles.
measure length of the line, it will be the length of fins.
My measurement is 15.4cm. I minus about 0.5cm so the fins made can 'clamp'  the bottle, not loose, the length of three fin pieces is 15cm with width 5cm.

Step 3: Cutting

Draw the shape of fins on the corriboard and cut it.
make sure the "lines" of corriboard is horizontal for the fins, so it doesn't bended or folded after assembly.

Step 4: Tape

Tape the 3 piece side by side with duct tape and finally become a prism like shape. 
Slide it on to the bottle to check whether it is tight enough or not. If not, shorten every fin pieces and try again.

This triangular fins is already functionable but not nice enough. if you are lazy to carry on, just use some plasticine to fix the fins and use it.

Step 5: Extended Fins

You had made the triangular fins, but it look silly. So we need to make extended fins for it !
first of all, you have to design for it, you can imitate my design but you also can make better design.

Make a template for your design with white paper and draw it on corriboard with the help of template.
the extended fins must have the lines of corriboard be vertical so the air can flow through the hollow body of it, reduce air friction.

Step 6: Connect Extended Fins

Attach the extended fins on the basic triangular fins using duct tape like before. It looks awesome now. Use enough duct tape, you will not like to see the fins be aborted after launching.

Step 7: Launch It !

Slide it onto bottle, secure with plasticine and you have a water rocket with funtionable fins.
equip the rocket with nosecone and parachute. Launch it and have fun.

i will upload tutorials about making parachute of water rocket and launcher in future, thank for viewing.

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