What happens to the coefficient of drag (Cd) as the rocket approaches the speed of sound?

As the rocket approaches the speed of sound, the coefficient of drag (Cd) tends to increase due to the effects of compressibility and shock waves that develop as the flow transitions from subsonic to transonic speeds. At lower speeds, the airflow around the rocket is relatively smooth, and the drag is primarily due to skin friction and form drag. As the rocket nears the speed of sound, shock waves begin to form, significantly altering the flow characteristics and increasing the drag experienced by the rocket. This transition also leads to a more complex flow separation that contributes to an increase in the coefficient of drag. The combination of these factors results in an overall increase in Cd as the rocket approaches supersonic speeds.