Fins are the guidance system for your rocket. Without them a rocket would tumble end over end. Fins can give your rocket life and beauty. Fins can portray aggressive power or aerodynamic grace. However, fins tend to be the single greatest downfall of many young rocket builders. With the incredible speeds and frightful acceleration generated at launch, many fins get ripped off the rocket body within a fraction of a second. I will discuss the reasons for this later, but for now let's list some common building materials.

Materials: (remember lightweight but sturdy)
Index cards
Clear packing tape
Cardboard (cheap, plentiful, soggy when wet)
Chipboard (cereal boxes)
Foam core (a little tougher but more $, some water damage)
Sturdi-board (like plastic cardboard, great stuff, $$, no water damage)
Balsa wood (might be a little heavy, fragile on impact)
Styrofoam sheets* (cheap, low mass, fragile on impact)

*Requires PL Premium Construction adhesive to attach

How many fins do I need?
To ensure stability and safety, the minimum number of fins on a rocket is three (3). Many people choose a 3 or 4 fin design. There is no maximum number of fins you may have but keep in mind that the more fins you have the more drag you will create and drag slows a rocket down.

Constructing fins
1. Be creative and cut out 3 or 4 identical fins. You can use any shape except "forward swept" fins.
2. Lay the fin on a flat surface.
3. Glue and/or tape an index card onto the side of the fin. Be sure to leave a one-inch tab on the index card. You will later bend this tab out 90 degrees to make an attachable area for the rocket.
4. Repeat the same for the other side of the fin.
5. Repeat with other fins.

You should now have 3 or 4 fins each with two-index card tabs on the backside. (I like to laminate the entire fin surface with clear packing tape to reduce the amount of water damage to my cardboard fins.)

Fin placement
The fins of your rocket can't be placed above the halfway point of your pressure cylinder. You want to place your fins at the base of the rocket to lower or maintain the center of gravity. If you were to place the fins above the center of gravity, the rocket would tumble and spin out of control once it left the launch pad.

Attaching the fins
This is the tricky part. Most glues including hot glue and "Liquid Nails" type adhesives are not flexible enough when it comes to the launch. The 2-liter bottle pressure chamber might expand a millimeter or more in circumference when it is pressurized. That is enough to break the bonding seal of most glues. When the rocket is launched, the fins usually rip off. I have found that the clear packing tape or strapping tape works the best at holding the fins on the rocket. (duct tape works too)

1. Apply a piece of tape to the index card tabs and them carefully tape them to your rocket.
2. Look at your fin. Make sure it doesn't curve or it isn't crooked. It should be in a direct line with the body of your rocket. If it isn't perfect, take it off and try again.
3. Attach the other fins. If you are using 4 fins they should be at 90-degree angles. 3 fins should be at 120-degree angles.
4. Test the wiggle of the fins. Your fins shouldn't wiggle more than a few centimeters from side to side. Adding more tape to the top and bottom areas of the fin might fix this problem.