Four Forces of Flight

Rocket Project Day 1

Aerodynamic Forces

Act on a rocket as it flies through the air

Lift & Drag

Lift Force – Acts perpendicular to the direction of motion

Drag Force – Opposes the direction of motion

Generated by the interaction and contact of a solid body with a fluid, a liquid or gas

Lift

In a rocket, lift is used to stabilize and control the direction of flight

In a plane, lift is used to overcome the force of weight; in a rocket, thrust is used to overcome weight

In a rocket, lift is generated by the nose cone, the rocket body and the fins

Drag

Think of drag as aerodynamic FRICTION

Sources of Drag

Skin Friction – Friction between air and the solid surface of the rocket

Form Drag – Depends on the shape of the rocket, a resistance to upward motion

WeightThe force generated by the gravitational attraction of one object on another object

This force is always directed towards the center of the Earth

The magnitude of this force depends on the mass of all of the parts of the rocket itself, plus the amount of fuel, plus any payload on board

The weight is distributed throughout the rocket, but we can often think of it as collected and acting through a single point called the CENTER OF GRAVITY

Center of GravityAverage location of the weight of an object

Mass and weight are distributed throughout an object

How can we find the center of Gravity?

For simple geometries we just balance the component or the entire rocket using a string or an edge. The point at which the component or rocket is balanced is the center of gravity. This is just like balancing a pencil on your finger! Obviously, we could not use this procedure for a large rocket like the Space Shuttle, but it works quite well for a model.

Weight Formula

Weight = mass x gravitational acceleration

On Earth

W = m x 9.8 m/s2

Rocket Thrust

The force which moves the rocket through the air, and through space.

Thrust is generated by the propulsion system of the rocket through the application of Newton's third law of motion

The direction of the thrust is normally along the longitudinal axis of the rocket through the rocket center of gravity

Bottle Rocket Thrust

All rocket engines produce thrust by accelerating a working fluid

A bottle rocket uses water as the working fluid and pressurized air to accelerate the working fluid

Thrust (Continued)

In rockets, the hot gases in the combustion chamber press against all sides equally. Water bottle rockets work the same way. The water bottle acts as the combustion chamber of the rocket.

When the bottle is opened, the pressure on the opposite side of the combustion chamber is now unbalanced and pushes the rocket.