Physics of the Rocket

Propulsion:
This bottle rocket is propelled by a mixture of air and water. How this propels the rocket is that the water provides the mass that the rocket pushes against in order to take off and the compressed air provides the force that pushes the water out.  According to Newton's 3rd Law the equal and opposite reaction is the water and rocket being forced apart in opposite directions making the rocket fly.

Body of the Rocket:
I didn't need to reinforce the fuselage of the rocket because we are limited to 80 psi.  I made sure the body of the rocket was smooth so that it was aerodynamic. I also made sure that no unnecessary weight was added so that my rocket would have a greater range capability.  A light weight rocket can get a greater range using less water than heavier rockets because you can add more water before the rocket starts loosing range due to the excess.  Light rockets can get the same range as a heavy rocket using less water.

Fins:
Number of fins: four fins is an optimal number to have on this rocket because it provides the needed stability to keep the rocket on course and because it fits on the launch pad more easily without risk of bending the fins.
Placement: I place the fins as close to the back where the nozzle is because the fins will keep the back from rotating and the rocket from flying off course. It's all about the stability.
Shape: I chose a generic shape because I know this trapezoidal shape cuts through the air nicely and does a better job of directing the rocket of smaller more streamline fin shapes.  The way i designed the fins to hug the contour of the bottle makes the fins less likely to bend and warp in flight keeping the rocket stable.
Angle Orientation:  I chose to angle the fins straight up and down as opposed to angled to one side for a number reasons.  One reason is that when building rockets the choice is either fins or no fins; when you have no fins, you want the rocket to spin for stability like a bullet but if you have fins, you do not want the rocket to spin because of the fins because it will not keep the rocket stable since the whole point of fins is to keep the rocket from spinning out of control and to steer the rocket.  The straight up and down orientation is also much easier to accurately place because there is less risk of angling the fins at different angles.

The Nose:
When making a rocket the best nose shapes are either round or pointed.  Typically pointed tips are used on high speed rockets and round tips are used on low speed rockets but when it comes to bottle rockets, the difference between pointed and round tips won't have any significant effect on the rocket. I chose pointed because I wanted the nose to be destructible to lessen the force on the egg when it crashes into the ground.

Weight Distribution:
The weight distribution of a rocket should have the majority of the weight in the front above the center of pressure.  When building rockets, having the center of pressure closer to the bottom of the rocket and the center of mass closer to the top of the rocket gives it more stability.  The weight in the front leads the rocket and keeps the rocket flying straight.  If the mass was in the back, the rocket would be easily taken off course because the tip would easily be blown around and the rocket would go out of control.

In the diagram on the left, cg is center of gravity (mass) and cp is center of pressure.

 *I hand drew all the pictures and added text in photoshop*