Physics: Newton's 3rd Law- Every action has an equal and opposite reaction
Aerodynamics: The center of gravity must be forward of the center of pressure, or it wont fly straight. (fins in the back, weight in front)
Economics: Added value. You could spend $10 on a kit, or PENNIES on a homemade rocket which takes more work.
Materials: Hot Glue, paper clips, clay, paper, glue sticks, cardboard, straws, elmers glue.
Tools: Hot glue gun, scissors, toothpicks, string
Become familiar with rockets by assembling 1 from a kit. The nosecone from this will be your form for a mold. I advise that one's 1st rocket use "A" engines, the smallest ones, especially when experimenting.
Press a factory nosecone between 2 balls of clay. This makes a mold. Leave the original nosecone in it for now. Wrap string around the clay tight enough to leave a mark in the clay. Deepen this groove with a toothpick. Groove allows you to align mold later. Take mold apart and let it dry a bit. It doesn't need to be very dry, just enough that it doesn't get squashed. Tye the 2 halves of this mold back together, and fill it with hot glue. While glue is hot, press a paper clip into it. Allow to cool, then remove your new nosecone from the mold. Dozens of nosecones can be cast from 1 mold.
THE BODY TUBE:
apply gluestick to the paper, a little at a time while wrapping it around 2 engines. Trim the ends neat and square.
ENGINE RETAINING RING:
Saw a piece off of a spent engine about 1/2 inch long. Glueing this inside the body tube gives the engine something to press against. If there is a rocketry club in the area, you can collect their discards after a launch. It's good to chat with them about appropriate launch sites too.
Cut your first fin from cardboard. Use this as a template to make the other 2. Use a thin paper pad (or a stack of tongue depressors) to draw parallel lines on the body tube. Glue fins to this.
About 1.5 inches of a straw glued to the body tube glides along the launch rod upon launch.
Curl a ribbon by scraping it against back of scissors. Glue 1 end into a folded bit of paper. Glue this inside the top of the body tube, about 1/2 inch down. Tie the other end to the nosecone. This is a streamer recovery, only advisable for small rockets.
Put an engine in the rocket. If it's loose, apply a bit of tape around engine. Hang rocket from a string such that it balances horizontally when hung from string. Swing rocket over your head and see if it goes nose forward. If not, you need larger fins or more weight in nose.
Follow all safety rules. Launch in an open space away from trees, bushes, and dry grass. This is really fun. When chasing rockets, Newtonian physics becomes aerobic exercise!