1.2 Making a rocket  (Page 10/2)

Technology

Grade 7

Transport

Module 3

Making a rocket

Design and realise

Background

A rocket relies on jet propulsion in order to fly. When a rocket uses its fuel, a current of warm air is exhausted at its tail causing the rocket to move forward. Planes that fly at lower than 25 km use the oxygen in the atmosphere to ignite (burn up) their fuel.

Rockets have to carry oxygen with them, because above 25 km from the earth’s surface there is not enough oxygen.

M ake a Rocket

Background

We are now going to make a rocket that will use jet propulsion in order to move forward. The speed with which a rocket moves forward depends on the amount of propellant gas that is exhausted at the rear end. Water is a much better propellant than warm gas, because it is much heavier. We are going to use compressed water and air and will make observations regarding how high/far your rocket can fly.

Assignment 1:

To find the requirements needed for the rocket

[lo 1.10]

Requirements:

• Thin cardboard.
• A pen.
• A ruler.
• Coloured cardboard.
• A pair of scissors.
• Two empty.
• 2ℓ plastic bottles.
• Strong broad adhesive tape or masking tape.
• A funnel.
• A jug of water.
• A cork.
• An air valve (from a bicycle pump).
• Plastic tube.
• A bicycle pump.
• A plastic bank coin pouch.
• A protractor.
• Glue.
• A Stanley knife.
• A stiletto or a knitting-needle.
• A pair of compasses.
• Oil-based paint.

Background

A plane or rocket operates according to Newton’s third law of movement, namely if one source exerts a force on another, then the other source exerts the same amount of force on the first source, but in the opposite direction from the first source; i.e. for each action there is an opposite reaction.

If the force you exert on something is bigger than the resistance exerted by that object, the force that you exert can cause movement. The movement in the rocket is brought about by the rocket being pushed upward by the escaping gases that were generated as a result of the chemical reaction between the fuel and the oxygen that are burnt up in the combustion chambers. The large volume of gases escapes at a high speed through the rocket’s steel exhaust pipe.

There are five types of force that act upon a rocket, namely:

Thrust : the force provided by the engine, which is exhausted at the rear end and which pushes the rocket forward

Gravity : the force that pulls the rocket back to earth

Resistance : This is exerted by the air against the rocket, causing the rocket to reduce its speed. (Outside the atmosphere, there is no resistance, because there is no air.)

Lifting force : A rocket cannot be launched effectively and rise vertically unless the lifting force of its engines is greater than the weight of the rocket.

Relative wind : The air flows rapidly around the nose and down the body. Longer nose-pieces are used to get better airflow.

Assignment 2:

To do research on the fins of a rocket

[lo 1.7]

Background

A rocket needs fins in order to fly in a straight line. Do research about what the fins of a rocket look like and draw a few examples in the space provided. Also decide how many fins you would like to use and what size the fins will have to be in relation to the body section of the rocket. Also, decide what type of material will be the most suitable for making the fins. The position of the fins on the body is also extremely important.