How to Shoot a Water Bottle

Cut four fin shapes out of the foam board with the hobby knife about 3 inches tall and about 1.5 to 2 inches wide, in the shape of a triangle or truncated rectangle. Use the hobby knife to smooth out any dangling or uneven bits, stacking the four fins on top of each other to make sure that they all have the same shape.

Trace the silhouette of the side of the bottle onto the four fins so that they will be able to mate to it tightly. Trace an area a few inches above the mouth of the bottle (which will be the nozzle). Trace so that the fins will be pointing the right direction when they are glued to the bottle.

Cut along the lines you traced on the fins so they match the contour of the bottle. After you make the first cut, match the fins against the bottle to make sure they fit tightly, and cut away extra material if it is necessary to do so.

Mix up some of the 5-minute epoxy on a piece of leftover foam board, being careful to follow the instructions on the back of the tube of epoxy. Using the piece of leftover foam board as a palette and a smaller piece of foam board as a brush, apply an even bead of epoxy to the edge of the fin that will contact the bottle. Press each fin to the bottle, holding it in place for several minutes until the epoxy sets.

Reinforce the joints between the fins and the bottle with strips of clear packing tape.

Push the needle of the ball inflater through the cork until the end is showing through the end. Attach the ball inflater to the air pump.

Fill the bottle 1/4 to 1/2 with water and insert the cork with the ball inflater needle through it into the mouth of the bottle. Press the cork into the bottle mouth until it is firmly wedged in place, and set the bottle rocket on the ground standing on it's fins. Inflate the bottle with the pump until the cork pops out and air pressure propels water out of the rocket, causing it to shoot skyward.