The Science Behind Two Liter Bottle Rockets

It Ain’t Rocket Science…

Aerodynamics

• The word comes from two Greek words: aerios, concerning the air, and dynamis, which means force

• Aerodynamics is the study of forces and the resulting motion of objects through the air.

Center of Gravity

• Point where mass is evenly distributed• The balance point• Rocket would rotate around center of gravity

Center of Pressure

• Point where aerodynamic forces are equally distributed

Rocket StabilityIn a stable rocket, the Center of Gravity (CG) should be closer to the nose than the Center of Pressure (CP).

Rocket Stability

Ways to make Center of Gravity (CG) closer to the nose to increase stability1. Make the rocket longer which moves the CG

closer to nose.2. Add weight to nose which moves CG closer to

nose.

Rocket Stability

Ways to make Center of Pressure (CP) further from the nose to increase stability1. Place the fins as far to the bottom of the

bottle as possible.2. Do NOT make the fins too large.3. Select a fin shape that is larger at the bottom

than the top (triangle).

Newton’s Laws of Motion

Newton’s 1st Law of Motion

• An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Newton’s 1st Law of Motion

How does this law apply to rockets?

Newton’s 2nd Law of Motion

• Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

Newton’s 2nd Law of Motion

How does this law apply to rockets?

The heavier the rocket, the more force must be applied to get the same acceleration.

Newton’s 3rd Law of Motion

• For every action there is an equal and opposite reaction.

Newton’s 3rd Law of Motion

How does this law apply to rockets?

As the water and air shoot out of the bottomof the bottle (action), the rocket is thrust Upward (reaction).

Forces Acting on a RocketA rocket undergoes several forces during flight

Gravity

• A natural phenomenon where objects of mass attract each other

• Gravity gives weight to physical objects and causes them to fall toward the ground when dropped.

Drag

• Sometimes called air resistance• A type of friction• Forces acting opposite the relative motion of

any object

Thrust

• Thrust is a reaction force quantitatively by Newton’s 2nd and 3rd Laws of Motion

• When something expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on the mass which is the force called thrust

Rocket Launch