Rockets and Newton’s Third Law
Rockets need energy to fly, this energy is provided by burning fuel in a rocket engine. In a rocket engine, the propellant burns rapidly, producing hot gas under high pressure. As exhaust gas streams out through the nozzle, the rocket responds by moving in the opposite direction. This is an example of Newton’s Third Law of Motion: “For every action there is an equal but opposite reaction.”
To understand Newton’s law, blow up a balloon and let it go. The action of the air escaping through the open end of the balloon causes a reaction that results in the balloon shooting forward.
The nose cone carries the payload or cargo. Common payloads include astronauts, satellites, scientific instruments, and even explosives. The nose cone may also contain the guidance system that controls the flight direction of the rocket. To minimize friction and drag, the nose cone should have a rounded or bullet shape. This gives the rocket a small frontal area.
Fins keep the rocket flying straight. Without fins, the rocket would tumble out of control. If the fins are not spaced evenly, or are different sizes or shapes, the rocket will not fly straight. When building the rocket, the designer must consider factors such as the size, shape, number and location of the fins. The fins must always be located at the rear of the rocket unless the rocket has an onboard computer controlled guidance system.
The rocket body holds the propellant and rocket engine. Fins are attached to the rocket body. The stability of the rocket depends upon the rocket’s length. You’ll find that longer rockets fly straighter than shorter rockets. Rolled tubes of paper can be used to lengthen your rocket body.
Forces Affecting Flight
Thrust is the force generated by a rocket engine that propels the rocket. Thrust must be greater than the pull of gravity for a rocket to fly upwards or forward. The more resistant to motion a rocket is, the more thrust is needed to propel it.
Gravity is the downward force applied to all objects on Earth. More force is required to a heavy object than a light rocket. The heavier the rocket, the more thrust is required to lift the rocket into space. Once the model rocket has used all of its fuel, it no longer accelerates. However, Earth's gravity continues to act on this rocket to slow it down. If the rocket's speed is slow enough, Earth's gravity eventually will pull it back to Earth.
Drag is the amount of air resistance or friction on the rocket as it flies. Drag is minimized by reducing the frontal surface area of the rocket and by having a smooth exterior finish. If two rockets of similar size and weight were launched with the same force, the rocket with the lowest drag would fly farther.