“What a DRAG, man!”

More accurately, drag is a force exerted on a moving object by the viscous fluid surrounding it which tends to slow the objects motion. The object of our interest is a Bottle Rocket traveling through air! The amount of drag the rocket “feels” is determined by two things, it’s shape (does it have a rough, or textured surface that the air can stick to?) and the “thickness” of the air itself (this property is called viscosity). Air has a relatively low viscosity when compared to water, which has a low viscosity when compared to corn syrup (imagine trying to swim through a lake of corn syrup - talk about drag!). Practically speaking, we have no control over the viscosity of the air, but we can alter the shape of our rocket to dramatically effect drag.

Bad streamlining

The first type of drag that we should consider is called Form Drag. This is related to the shape of the rocket and is directly proportional to the speed of the rocket through the air. We can minimize this type of drag by making smooth and gradual changes in the rocket’s shape so the air doesn’t encounter blunt obstructions as it passes over the rocket’s surface. This is also known as streamlining. For example, the rocket design on the left has lots of form drag and is an example of bad streamlining. The rocket design on the right has lower form drag and better streamlining, and as a result, will have better flight performance. The second type of drag is called Parasitic Drag which is related to the roughness of the rocket’s surface. This drag can be reduced by making sure the surface of the rocket (it’s skin) is as smooth as possible. To experiment with form drag, try launching rockets with different body shapes while keeping all other parameters such as pressure, water volume, and weight constant and see how this affects the rocket’s altitude.

Good streamlining

To experiment with parasitic drag, maintain the same rocket body, pressure, water volume, and weight, but change the surface texture of it’s skin. It can be a little tricky to reduce the parasitic drag , but it’s very easy to make it worse! So for experimental purposes, try roughening the rocket’s surface by gluing wrinkled tin foil over the rocket’s body to increase the parasitic drag then compare maximum altitudes of clean rockets with rough rockets.