1.3 Finding arocket's centre of gravity

Technology

Grade 7

Transport

Module 4

Finding the rocket’s centre of gravity

Assignment 1:

How to find your rocket’s centre of gravity

[lo 1.14]

Make a loop at the end of a 1 m piece of string.

• Tie the loop around the body section of your rocket.
• Let the rocket hang free in your hand and shift the loop until the rocket hangslevel/parallel to the ground.
• Indicate the centre of gravity on the rocket by drawing a line right around the body with a marking-pen. Indicate the symbol for centre of gravity.

Background

The aerodynamic forces at a rocket’s centre of pressure are centred. The centre of pressure is usually also close to the tail end of the rocket. The large surface of the fins and the smooth surface of the rest of the rocket cause it to be the position where the pressure is the same on both sides. To get the centre of pressure closer to the tail of the rocket, the front section (nose) of the rocket has a more pointed shape or the body section of the rocket could be made longer, because in that way it becomes more aerodynamic. By putting smaller fins on the front section of the rocket, the centre of pressure is moved closer to the front end.

R ules : The centre of gravity should be closer to the front end of the rocket than the centre of pressure. The centre of pressure should be the same distance or longer than the diameter of the rocket from the centre of gravity to the tail end. This will make the rocket stable and will help it to fly in a straight line.

Assignment 2:

How to find a rocket’s centre of pressure

[lo 1.14]

Make a sketch of the side elevation of your rocket. Sketch the fins as well and draw as accurately as possible.

• The centre of pressure is determined by drawing a line that spreads the surface of the rocket equally on both sides of the line.
• Indicate the centre of pressure in the sketch by drawing a line through it.
• Indicate the centre of pressure on the rocket by drawing a line right around the body with a marking-pen. Indicate the symbol for the centre of pressure.

Assignment 3:

Testing whether your rocket is stable during flight

[lo 1.13]

• Make a loop at the end of a 2 m piece of string.
• Tie the loop around your rocket at its centre of gravity. Affix the loop with adhesive tape if necessary.
• The rocket should hang parallel to the ground surface (Sketch 1). Swing it around with a circular movement (Sketch 2).
• Change the centre of gravity or the centre of pressure until your rocket swings forward in a straight line while hanging horizontally.

If you want to make any changes to your rocket to make it more stable, you can do it now. Decorate the bottles and fins so that it looks more like a rocket, for instance by painting it with an oil-based paint.

Now you are ready to launch your rocket.

Assessment

Memorandum

Assignment 1 - 3

Learners can help each other to find the centre of pressure and centre of gravity of every rocket. They will differ seeing that the appearance of each rocket should differ.