According to “A guide to building and understanding the physics of water rockets”, the drag force can be reduced by the following points:
- Design a cone-shaped nose and put most weight in the cone.
The functions of the cone are to reduce the windward area and to add stability. The reason why the center of mass should be kept in the front is because the center of pressure from the air drag will act more on the back of the water rockets, and will help it return to its original line. If the center of mass lies behind the center of pressure, the air drag will act more on the front end of the water rocket, and creating momentum around this line. This can be related to the design of a ferry at sea (mentioned in class).
2. Choose a long and thin bottle rather than a short and fat one and make the bottle smooth as possible.
This can reduce the windward area, or the perpendicular surface area that is in first contact with the air. A smooth bottle can reduce the air friction between the bottle and air.