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

As a rocket approaches the speed of sound, the coefficient of drag (Cd) typically increases due to the aerodynamic changes that occur in this transonic regime. As the rocket accelerates towards the speed of sound, shock waves begin to form around it, causing a significant increase in drag.

In the subsonic regime, the airflow around the rocket is smooth, and the drag is primarily due to skin friction and form drag. However, as the rocket nears Mach 1, the flow becomes increasingly disturbed, resulting in the formation of shock waves. These shock waves can greatly increase pressure differences across the outer surface of the rocket, which in turn increases the overall drag experienced by the rocket.

This phenomenon occurs because the rocket experiences a transition from a subsonic to a transonic flow regime, where the characteristics of the airflow change dramatically. This is a crucial consideration in rocket design and aerodynamics, especially when designing vehicles intended to operate at high speeds.