Drag Force is equal to one-half the drag coefficient times density, velocity squared, and area.
There are two types of drag:
Friction Drag is the friction of an object generated by moving in a medium.
Pressure Drag is uneven pressure distribution in the direction of motion of a moving object.
The drag coefficient is a nondimensional value that describes how much drag a cross-sectional area will generate. The drag coefficient encompasses both friction drag and pressure drag. The greater the drag coefficient the higher the drag force.
Typical values for drag coefficient:
Modern automobile: 0.25 – 0.35
Flat plate: 1.28
Sphere: 0.07 to 0.5
Here is a figure that shows the drag coefficients for rockets with different nose cones
In order to reduce the pressure drag on a rocket, one would need to:
- Make the rocket as narrow as possible without hindering its structural integrity
- Optimize the shape of the nose cone so that it has the least drag coefficient
Here are some common shapes:
This is from Benson’s “Volume” from May 2017 from Nasa’s Glen Research Center.
This is from Milligan’s “Drag of Nose Cones” from 2013 published in the National Association of Rocketry
The nose shapes are ordered from generating the least drag to the most drag
This is figure 22 from Dr. Gregorek’s “AERODYNAMIC DRAG OF MODEL ROCKET” from 1970
This figure represents the pressure distribution of an ogive nose cone with a drag coefficient of 0.04
In order to reduce the friction drag on a rocket, one would need to:
- Make sure that the rocket body is smooth and that there isn’t anything impinging from the rocket body
- This will help to keep the airflow around the rocket streamlined and laminar. This will keep the boundary layer small and reduce the overall drag coefficient.
- If the rocket body is not smooth and there are protruding objects then airflow around the rocket will be turbulent. This will produce a large boundary layer leading to flow separation and increase the overall drag coefficient.
This is figure 8 from Dr. Gregorek’s “AERODYNAMIC DRAG OF MODEL ROCKET” from 1970
General tips on reducing the drag coefficient of each part of the rocket
- Nose cone
- Parabolic or ogive shape nose cone
- Body tube
- Smooth finish
- No protruding parts
- Have a tapper base of the rocket with the body to form a gentle boat tail
- Smooth out the base where the fins connect with the body
- Airfoil shaped
- Optimize thickness
- Elliptical or tapered shape
- Align the fins properly so that they are exactly perpendicular to the body
- Good Workmanship
To elaborate further on the drag equation:
The density will decrease as the rocket increases in altitude. The higher the rocket gets, the less drag that the rocket will experience due to decreasing density.
The coefficient of drag will increase a little bit after the rocket has reached the sound barrier at Mach 1. Then the coefficient of drag will decrease with increasing Mach number.
This figure from Sutton, and Biblarz’s “Rocket Propulsion Elements” from 2016 represents the coefficient of drag with Mach number for the cross-sectional area based on a V-2 missile.
Here is another figure from Braeunig’s “Saturn V Launch Simulation” from Dec-2013 that represents the simulated coefficient of drag with Mach number for the cross-sectional area Saturn 5.