Design - CG & CP - Water Bottle Rockets

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The Basics of Rocket Stability When building your own rocket, it will help to understand a little about what makes rockets stable and fly straight, and how to check for yourself.

If a rocket, hanging from a string, was balanced front to back, the wind would have a greater effect on the fins due to their larger surface area. Like the feathers on the end of an arrow, this makes it stable. A stable rocket flies straight and does not tumble through the air. Stability causes the rocket to fly farther, faster, and under control. Next, you will learn how to find the Center of Pressure and the Center of Gravity on your own rocket.

Finding the Center of PressureGet a piece of cardboard that is slightly larger than your completed rocket on all sides. Trace a line completely around your rocket staying as tightly as possible. The trace should look like the rocket's profile. Cut out the profile of your rocket.

Get a ruler to use as a pivot point. Balance the cut-out profile of your rocket on the ruler. Mark the point where it balances. Mark the same location on your actual rocket (CP). This is the Center of Pressure of your rocket.

Finding the Center of Gravity (Center of Mass)Tie a piece of string around the body of your rocket somewhere near the middle. Use a couple pieces of tape so the string does not slip. Holding the rocket up by the string, see if it balances. If it does not balance, move the string towards the nose or the fins until the rocket balances. Mark the location of the string on your rocket (CG).This is the Center of Gravity of your rocket.

Now you know the two most important points on your rocket that determine its stability. The CG is the point where your rocket balances. The CP is the point against which the wind appears to push. The Center of Pressure will always try to follow the Center of Gravity. If the CG is in front of the CP, your rocket will be stable. If the CP is in front of the CG, your rocket will try to fly backwards.

If you find that the Center of Pressure is in front of the Center of Gravity, you can do one of two things:1. You can remake the fins of your rocket so they are bigger/wider. You will have to complete both CP and CG tests again. 2. You can add some weight to the nose of the rocket to move the Center of Gravity forward. The Swing TestThere is one more test you can do to help determine how stable your rocket will be in flight; the Swing Test.

Get a piece of string that is 3 or 4 feet long. Tie it around your bottle at the CG mark you made earlier. Use some tape to ensure the string does not move. Start spinning around slowly and gradually speed up so the rocket "flies" around you on the string. It would probably be best to do this outside.

One of three things will likely happen:

If you swing your rocket and the nose points in the direction of the swing, your rocket will likely fly stable. If you swing your rocket and it starts cartwheeling, this is likely evidence that the CG and CP are too close together. Adding some weight to the nose will likely help straighten out its flight.

If you swing your rocket and it flies backwards, this means the Center of Pressure (CP) is in front of the Center of Gravity (CG). Try adding some weight to the nose and test again. If that does not help, the fins may be too small.

Here are some examples (If you have access): Successful Swing Test Unsuccessful Swing Test You are well on your way to becoming a Rocket Scientist! On the rockets below, the CG is shown with a yellow dot. The CP is shown with a red dot. How will each of the rockets below perform? Can you explain why?