Please use this page if you have any questions about building a rocket, due date, or launch date.
Two Liter Bottle Rockets
Directions for the 2014 Rocket Launch
All Rockets must be completed by Friday, May 23
Launch Day is Wednesday, May 28
For questions, please contact Mr. Tarver at:
Printed Date: 5/2/2014
Grade level: Can be adjusted for grades 3 – 8
Scientific Principles: Motion, force, mass, gravity, friction
Other subject areas: Math, writing, business, accounting, art
Materials Needed: (the list can be expanded by the students)
2 liter plastic bottles
Glue and glue guns
Sandwich bags – plastic
General Objectives: Students will:
- Work as a team to accomplish their objectives
- Create and work within a budget
- Use scientific method to test their ideas about rocketry
- Find the center of mass and center of pressure on a rocket
- Create a scale drawing of their rocket
- Design and launch a stable rocket
How to Build a Bottle Rocket
Bottle rockets are very easy to build. Make sure to test your rocket for the Center of Mass and Center of Pressure after you build your rocket.
1. Wrap and glue or tape a tube of poster board around the bottle.
2. Cut out several fins of any shape and glue them to the tube.
3. Form a nosecone and hold it together with tape or glue.
4. Press a ball of modeling clay into the top of the nosecone or you may want to try to press and type it to the top of the bottle or a little of both.
5. Glue or tape nosecone to upper end of bottle.
6. Decorate your rocket.
Center of pressure
Center of mass
A rocket that tracks true (flies straight) through the sky is said to be a stable rocket. A rocket that veers off course or tumbles is said to be unstable. The difference between the flight of the stable and unstable depends upon its design. All rockets have two distinct “centers”. The first is the center of mass. This is the point about which the rocket balances. If you could set your rocket on the edge of a ruler, the center of mass is the point that the rocket would balance horizontally like a seesaw. (It is also the place in which half the mass is in front of the rocket and the other half is behind.) If a rocket is unstable, the rocket will spend around this center.
The other center in a rocket is the center of pressure. This is a point where half of the surface area of a rocket is on one side and the half is on the other. The center of pressure differs from center of mass in that its location is not affected by the placement of payloads in the rocket. This is just a point based on the surface of the rocket, not what is inside. During flight, the pressure of air rushing past the rocket will balance half on one side of this point and half on the other. You can determine the center of pressure by cutting out an exact silhouette of the rocket from cardboard and balancing it on a ruler edge.
The positioning of the center of mass and the center of pressure on a rocket is critical to its stability. The center of mass should be towards the rocket’s nose and the center of pressure should be towards the rocket’s tail for the rocket to fly straight. That is due to the lower end of the rocket (starting with the center of mass and going downward) has more surface area that the upper end. When the rocket flies, more air pressure exists on the lower end of the rocket than on the upper end. Air pressure will keep the lower end down and the upper end up. If the center of mass and the center of pressure are the same place, neither end of the rocket will point upward. The rocket will be unstable and tumble.
So, to have a stable rocket, the center of mass must be in front of the center of pressure.
How to Determine the Stability of your Rocket
1. Tie a string around the middle of your rocket. Tie a second string to the first string or leave a very long “tail” (3 – 4 feet) on the string you used to tie around the rocket. Slide the string loop to a position where the rocket balances. Note: You might have to type the nose cone in place to keep to from falling off.
2. Mark on your rocket where the loop is. This represents your Center of Mass.
3. Draw a straight line across the scale diagram of the rocket you made earlier to show where the ruler’s position is. Label this line as the Center of Mass.
4. Lay your rocket on a piece of cardboard. Carefully trace the rocket on the cardboard and cut it out.
5. Lay the cardboard silhouette you just cut out on the ruler and balance it.
6. Draw a straight line across the diagram of the rocket. This line represents the Center of Pressure.
If your Center of Mass is in front of the Center of Pressure, your rocket should be stable. Proceed to the swing test. If the two centers are on top of each other or very close, add more clay to the nosecone. This will move the mass forward. Repeat steps 1 and 2. Check the new location of Center of Mass against the Center of Pressure.
1. Tape the string loop you tied around your rocket in the previous set of instructions so that it does not slip.
2. While standing in an open place, slowly begin swinging your rocket in a circle. If the rocket points in the direction you are swinging it, the rocket is stable. If not, add more clay to the rocket nose cone or add larger fins. Repeat the test until the rocket flies true in the direction you swing it.