Improving Aerodynamics: 5 Types of Drag

What Makes an Aerodynamic Rocket?

Well, for one thing: patience. If you try to slap a rocket together because you're under a deadline, or just can't wait to finish it, the rocket will turn out sloppy and unfinished.

There are 5 types of aerodynamic drag:

-Friction drag

-Interference drag

-Parasite drag

-Pressure drag

-Induced drag

Now, I know what you're thinking. "Uh! What a drag!" Well, it is, and that is why it is certainly worth doing something about! I will go over the effects of each of these types individually.

Friction Drag

There is always a layer of turbulent air surrounding a speeding model. The velocity of the air relative to the rocket increases with distance from it. This layer is only a few thousandths of and inch thick, and it only takes .0005 inches to transition the laminar boundary layer to turbulent. That’s why it is crucial to keep the model as smooth and glossy as humanly possible. While it is impossible to keep the boundry layer from turning turbulent, a good idea is to trip the boundry layer manually near the nose of the rocket so that the boundry layer will be tripped in exactly the same position all the way around the rocket. Some claim this helps reduce drag.

Interference Drag

Interference drag is the result of an interruption of the boundary layer. The abrupt disturbance causes much drag. Such causes of interference drag are as follows: a protrusion, cavity/gap, joint, bump, or wavy surface. The most prominent source of interference drag is from the fins. Obviously, a rocket with triform (3 fins) fin configuration will have 25% less interference drag than a rocket with cruciform (4 fins) configuration. Also, interference drag will be reduced with a fillet in the fin root connection to make the interruption less sharp.

Pressure Drag

In order for a model rocket to fly through the air it has to force the air aside and let it slip back into place with as little disturbance as possible. To minimize positive pressure drag, you can either fly on very hot days or high altitudes, or select nose shapes with low drag coefficient (Cd). The nose shapes with the lowest pressure Cd’s are parabola and hemispherical.

Also, the air has to close behind the speeding model with as little disruption as possible. If the rocket is going faster than the air can close behind it there will be a negative pressure drag due to the partial vacuum tugging at the back side of the rocket. So sometimes a boat tail or transition to narrower diameter will reduce these effects.

Parasite Drag

This form of drag is actually just another facet of pressure drag. It is caused primarly by the launch lug, or any other protrusion sticking out the side of the rocket's body. Parasite drag can be as much as 35% of the entire drag on a model rocket. It can be reduced by ~25% when the launch lug is positioned in a fin root connection. Or, you can build a tower launcher, and remove the launch lug entirely!

Induced Drag

Induced drag is the drag that occurs when the fins generate lift. More specifically it is called the lift-induced drag. This type of drag is the hardest to picture.

The induced drag occurs at the tip of the fin. If the fin had no finite length, there would be no induced drag. The way this happens is when the airflow around the fins “leaks” around the fin tip to the opposite side creating vortices of swirling air in its wake:

The drag due to lift can be expressed with the following diagram:

The induced drag becomes greater if:

-The fins are swept back

-The fin span is shorter

-The fins have a rounded tip (leave the very tip square, but give the leading and trailing edges a good airfoil)

Summing It All Up

When studying drag, one realizes that there is no "correct" or "ideal" way to make a rocket. When you include a feature that is supposed to help in one area in aerodynamics, you're going to have to make a compromise in another area. But with insight as to what causes drag, you should be able to come up with a happy medium.