Flying model rockets is a relatively safe and inexpensive way for students to learn the basics of forces and the response of a vehicle to external forces. A model rocket is subjected to four forces in flight; weight, thrust, and the aerodynamic forces, lift and drag. There are many different types of model rockets. One of the first and simplest type of rocket that a student encounters is the bottle, or water rocket. The water rocket system consists of two main parts, the launcher and the rocket.

On the figure we show a generic launcher, although launchers come in a wide variety of shapes and sizes. The launcher has a base to support the rocket during launch. A hollow launch tube is mounted perpendicular to the base and is inserted into the base of the rocket before launch. The launch tube is connected to an air pump by a hollow feeder line. The pump is used to pressurize the inside of the body tube to provide thrust for the rocket. We have attached a pressure gage to the feeder line to display the change in pressure in the system. This part of the system is very similar to the simple compressed air rocket.

The other part of the water rocket system is the rocket itself. Usually the rocket is made from a 2-liter soda pop bottle. Before launch, the bottle is filled with some amount of water, which acts as the "propellant" for the launch. Since water is about 100 times heavier than air, the expelled water produces more thrust than compressed air alone. The base of the bottle is only slightly larger than the launch tube. When the rocket is placed on the launch tube, the body tube becomes a closed pressure vessel. The pressure inside the body tube equals the pressure produced by the air pump. Fins are attached to the bottom of the body tube to provide stability during the flight.

Forces acting on a rocket

Thrust - the force that acts on rockets or airplanes to make them move forward. This force is created by the rocket or airplane engine, also known as the propulsion system.

Drag - the force that acts on the surface of an object moving through a gas or liquid that slows it down. It acts in the line of motion but in the opposite direction. This force is caused by the fluid resisting the movement of the object. Drag is one of the aerodynamic forces.

Lift - the force that acts on the surface of an object moving through a gas or liquid that acts perpendicular to the direction of motion. This is the force that acts on an airplane's wings to push it upward. Lift is one of the aerodynamic forces.

Velocity - the speed of an object. When you talk about the speed of your car in miles per hour you are talking about the car's velocity.

Mass - an object's quantity of matter. The mass of an object is multiplied by the gravitational acceleration to determine its weight.

Acceleration - the change of velocity. If you are speeding up your velocity is changing and we say you are accelerating. Deceleration is when you are slowing down. If you are not moving or are traveling at a constant speed your velocity is not changing and we say you are not accelerating, or your acceleration equals zero.

Gravitational force - the force that gravity exerts on a mass. When we refer to our weight we are actually talking about the force of the earth's gravity on our body mass. We would have a different "weight" on the moon since the moon's gravity is different than the earth's, but the mass of our body would be the same. In space, astronauts are far from gravitational forces of the earth and moon so the force is weaker. That's why they float and feel weightless.

Inertia - refers to the property of an object to resist change in motion.

Newton's Laws of Motion 

You've probably heard the story we like to tell about Sir Isaac Newton sitting under the apple tree. He supposedly got hit on the head by a falling apple and discovered the laws of gravity. Well, even if it didn't happen exactly that way, Newton did develop the theories about motion and gravitational forces. He developed these theories in 1666 when he was only 23 years old and later presented his three laws of motion in a book. We continue to use these laws today and they explain the motion of everything from apples to rockets. Let's take a look at Newton's three laws and learn how we represent them in math equations.

Newton's First Law of Motion - " Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it."

This means that an object won't move unless a force makes it move. If an object is moving it will continue to go the same speed unless a force makes it slow down or speed up. This is also considered to be the definition of inertia.

Newton's Second Law of Motion - "Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration"

This means that the force on a body is dependent on the body's mass and how much it is accelerating. If the body is not moving it's acceleration is zero. The force would be equal to zero times the mass which is zero. If the body is moving and you know the acceleration you could find out how much force is acting on it by multiplying the acceleration by the mass. If the body is traveling at a constant velocity then the acceleration is equal to zero. Just as the body that is not moving, the force would be equal to zero.

Newton's Third Law of Motion - "For every action, there is an equal and opposite re-action."

This is the law of action and reaction. Think of blowing a balloon up and then letting it go? You know what happens, it flies all over the place! The air inside the balloon rushes out of the small opening (the action). This causes the balloon to move in the opposite direction (the reaction). This is similar to what happens in a rocket engine.

Newton's Third Law can be used to show how the thrust of a rocket engine makes the rocket go forward.