Aerodynamic Drag Force.

Fd = 0,5(de)(Cd)A(V2+Vair) ²   N

Where (de) is density of air or atmosphere through which the rocket flies.kgs/m3

Cd Aerodynamic form drag coeficient

A is the cross sectional area of the rocket m²

V2 is the actual velocity of the rocket m/s

Looking at the formula for determining the drag force restricting the rocket.

We can say that Fd is directly proportional too: (de)(Cd)A

But more importantly Fd increases with (V2)²

The faster it travels the more significant the value of Fd becomes

Conclusions:

The main restraining aerodynamic ‘Drag’ force can be reduced by

• Reducing the cross sectional area exposed to the direction of movement A

So reducing the diameter of the bottle used but maintaining the enclosed volume will help dramatically. Bigger is not always better.

• Improving the aerodynamic shape and form . Reducing the value of Cd.

If the nose cone resembles the shape of a brick then there is a considerable resistance created as the rocket punches its way through the atmosphere.

• de Reducing the density of the atmosphere in which the projectile (rocket) travels.

This is only appropriate to high altitude flights. 

This is important once the rocket has reached the outer limits of the atmosphere where the air is very thin (of reduced density).

Imagine trying to move a spoon quickly through treacle and then water.

Note: The density is also relevant when attempting a maximum apogee record.

• Vair is the vertical component of cross wind turbulence that could resist upward motion of the rocket.

Usually you avoid launching rockets when the weather conditions are poor.

Minimum aerodynamic drag for an optimised projectile volume of minimum exposed surface area.

Cd values of 0.1 and less

Parachutes

For a parachute what we want is to increase aerodynamic drag to a maximum to decelerate the descent of a valuable payload or rocket. So we need maximum drag produced by a system of low mass.  Typical Cd = 0.75 to 1.5

 Cd values

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25/04/05

This site was created on the 15th April 2003