Here is the easiest way to make fins for a water bottle rocket. They're just cut from another bottle and taped onto your rocket. These aren't meant for optimum performance, just some easy to make fins that can be reused, and only take a few minutes to make. No glue required. NO BOTTLE PROFILE REQUIRED.
Step 1. Cut the bottom end off of a bottle (1 liter or 2 liter, whatever matches your rocket). I'm making a set for the regular 20oz bottles. I have 2 identical bottles, one for the rocket, one for the fins. These little bottles are shaped different than 2-liter bottles, and they have strange bulges, so I have to be sure I've got one that will fit on the other. Begin by cutting this as square as possible, you'll need it to set on a flat surface in a minute.
Step 2. Stand your bottle on a piece of paper, and draw a line around it. Using a protractor, compass or best guesstimate, mark lines from the center of the circle at 0, 120, and 240 degrees. It's not even all that critical unless you're really shooting for optimum performance.
Stand your cut-off bottle in the center of the circle. Use a sharpie and a triangle to transfer the points on paper to your plastic bottle. An old trick I use from when I was building Estes rockets is to lay the bottle on the inside edge of a drawer with a lip, and use that for a guide.
Step 3. Once you know the maximum available size, design your fins on a piece of paper and cut it out. The side that attaches to the rocket should be straight. Place the straight edge of the fin template along the line you drew on your cut bottle. Trace the outline on the bottle. Repeat two more times.
Step 4. Now you have a bottle with 3 fins drawn on it. Cut that bottle off square a second time, about 3/4" above the highest point of your fins. Again, as square as possible.
Step 5. Now... the original vertical line you drew on the fin template is the FOLD LINE. DO NOT CUT THE FOLD LINE. Cut out the remaining lines of your fin design.
Step 6. FOLD the fins back along the original line. An easy way to do this is to just use a ruler to get a nice straight edge for the fold. Just fold them out perpindicular to the body. Yes, they'll be curved, but they'll all curve the same way, so it's irrelevant.
Step 7. Push the fin tube onto your bottle, and just secure it with plastic packing tape using the 3/4" overhang. Or yellow electrical tape if I can't find the packing tape. DOH.
Anyway, the finished product looks like this. I messed up drawing my lines, so I just improvised some scribbly design on the fins.
Launch!
