Noodleman wrote:I also have the same problem. But, it tumbles only when the rocket is filled with water. Whenever i do an all air launch, it flies perfectly straight up without the need of any fins or nose cones. How does this happen?
In physics terms, we are dealing with the center of mass versus the center of pressure.
Center of mass is the point where the rocket balances horizontally.
Center of pressure is the point where wind blowing on the rocket in any direction exerts an equal turning force on either side of the point.
For stability, the center of mass must be
ahead of the center of pressure.
Think of an arrow with fletching (feathers) on at the tail end. The center of mass will be about the middle of the arrow. You can balance it on your finger at that point in still air. To find the center of pressure, you would hold the arrow horizontally between two fingers on a windy day, and find the point along the length to hold it where the arrow doesn't turn into the wind. Because the feathers exert a turning force, you will have to hold the arrow farther toward the back to find this balance point. The center of mass is in front of the center of pressure.
In an all-air launch, the mass of the bottle is probably about the middle of the bottle, maybe a bit toward the base if the base is a heavier material (it's usually a bit thicker there). The center of pressure may be about the same place, or even slightly behind the center of mass, so the bottle won't have much reason to turn (if the center of mass and center of pressure are in the same place, there's no reason
not to turn either, so typically we see finless bottles as unstable).
Similarly, if you remove the fletching from an arrow, it
might fly straight, or it might not. A feather-less arrow will fly reasonably straight if you add weight to the nose, because the body of the arrow provides a weak weathervane effect.
Now consider when the bottle is full. Almost all the weight is at the back. Suddenly the only "fin" available (the body of the rocket) is
ahead of this mass. So it will try to flip around, just as if you tried to launch a rocket backward, or tried to push a boat through the water rudder-first. This, by the way, is also why it's so hard to keep a canoe on a straight path when you're the only occupant and sitting toward the back: the body of the canoe is acting like a rudder and it's in front of you, causing the canoe to want to turn.
If you add fins (this site has a good tutorial on how to create "box fins" which you can do with cardboard), then you shift the center of pressure backward. Sweeping the fins back (not just sticking straight out) puts the fin area even further back for better stability.