Rocket Design

Flight Direction

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Center of Mass

Center of Pressure

Drag

Stable

Spacecrafts or Rockets are designed in such a way, to reduce drag ( air resistance ) and provide stability and control. Rocket aerodynamics deals with how air flows over the rocket body and how the design affects rocket stability and drag.

- Nose cone and Rocket diameter

The amount of air that opposes rocket motion depends mainly on the following three factors :

  1. Nose cone

  2. Diameter of Rocket

  3. Speed of Rocket


​The first point of the rocket body that meets air is nose cone at the front end of the rocket. If the speed of the rocket is less than the speed of sound ( 340 m/s ) the best shape of the nose cone is a rounded curve. At supersonic speeds, i.e; speed greater than 340 m/s, the best shape is narrower or a sharper point.

Rockets with a larger diameter have more drag because there is more air being pushed out of the way. Drag depends on the cross-sectional area of the object pushing through the air. Making a rocket as narrow as possible is the best way to reduce drag.

The air drag increases as the rocket speeds up. Speed is directly proportional to the square of drag

 

Rocket Speed ∝ Drag^2

- Rocket Fins

The stability of a rocket is its ability to keep flying in the right direction of air without wobbling or tumbling

Fins are used on smaller rockets to provide this stability and control direction. It works in the same way as placing feathers at the tail of an arrow. The greater drag on the feathers keeps the tail of the arrow at the back so that the point of the arrow travels straight into the wind.

- Center of Mass

The Center of Mass of an object is that point on the body where the entire mass of the body is assumed to be concentrated. It is the point of stability of the body. The centre of mass can be moved closer to the nose cone end of a rocket by adding some mass near the nose cone. This will increase stability.

- Center of Pressure

The Center of Pressure is that point at which all of the aerodynamic forces are assumed to be concentrated upon.

To find the approximate position of the center of pressure, outline the rocket on a piece of paper. The center of the area of the outline shape is approximately the center of pressure.

For a rocket to have greater stability, the center of pressure must be closer to the tail than the center of mass. If the center of pressure is the same as the center of mass, the rocket loses its stability and it would tumble down. Stability of the rocket increases as the distance between the center of mass and the center of pressure increases. 

Placing fins at the tail end of a rocket moves the center of pressure closer towards the tail end and increases the rocket's stability. However, this also increases the air drag, so there is an optimal size for fins so that the rocket has enough stability without having too much air drag.