Aerodynamic forces are generated and act on a rocket as it flies through the air. Forces are vector quantities having both a magnitude and a direction. The magnitude of the aerodynamic forces depends on the shape, size and velocity of the rocket and some properties of the air through which it flies. By convention, the single aerodynamic force is broken into two components: the drag force which is opposed to the direction of motion, and the lift force which acts perpendicular to the direction of motion. The lift and drag act through the center of pressure which is the average location of the aerodynamic forces on an object.
Aerodynamic forces are mechanical forces. They are generated by the interaction and contact of a solid body with a fluid, a liquid or a gas. Aerodynamic forces are not generated by a force field, in the sense of the gravitational field, or an electromagnetic field. For lift and drag to be generated, the rocket must be in contact with the air. So, outside the atmosphere there is no lift and no drag. Aerodynamic forces are generated by the difference in velocity between the rocket and the air. There must be motion between the rocket and the air. If there is no relative motion, there is no lift and no drag. Aerodynamic forces are more important for a model rocket than for a full-scale rocket because the entire flight path of the model rocket takes place in the atmosphere. A full-scale rocket climbs above the atmosphere very quickly.
Aerodynamic forces are used differently on a rocket than on an airplane. On an airplane, lift is used to overcome the weight of the aircraft, but on a rocket, thrust is used in opposition to weight. Because the center of pressure is not normally located at the center of gravity of the rocket, aerodynamic forces can cause the rocket to rotate in flight. The lift of a rocket is a side force used to stabilize and control the direction of flight. Lift occurs when a flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton’s third law of action and reaction. For a model rocket, the nose cone, body tube, and fins can turn the flow and become a source of lift if the rocket is inclined to the flight direction. While most aircraft have a high lift to drag ratio, the drag of a rocket is usually much greater than the lift.
We can think of drag as aerodynamic friction, and one of the sources of drag is the skin friction between the molecules of the air and the solid surface of the moving rocket. Because the skin friction is an interaction between a solid and a gas, the magnitude of the skin friction depends on properties of both the solid and the gas. For the solid, a smooth, waxed surface produces less skin friction than a roughened surface. For the gas, the magnitude depends on the viscosity of the air and the relative magnitude of the viscous forces to the motion of the flow, expressed as the Reynolds number. Along the surface, a boundary layer of low energy flow is generated, and the magnitude of the skin friction depends on the state of this flow. We can also think of drag as aerodynamic resistance to the motion of the object through the fluid. This source of drag depends on the shape of the rocket and is called form drag. As air flows around a body, the local velocity and pressure are changed. Since pressure is a measure of the momentum of the gas molecules and a change in momentum produces a force, a varying pressure distribution will produce a force on the body. We can determine the magnitude of the force by integrating, or adding up the local pressure times the surface area around the entire body. The base area of a model rocket produces form drag.
Accurately determining the size of the drag force is very difficult in practice. The size of the drag changes depending on the thrust setting and the flow of gases at the base of the rocket. When the engine is operating, the drag is usually low. But when the engine is off during the coast phase of a model rocket, the entire base of the rocket produces a large drag. Drag is most often determined by wind tunnel testing a model of the rocket.