What effect does a boat tail have on a subsonic rocket's coefficient of drag (Cd)?

A boat tail is designed to streamline the rear section of a rocket, effectively reducing the turbulence created by the abrupt change in the cross-sectional area at the base of the rocket. This design feature minimizes base drag, which is a significant factor contributing to the overall coefficient of drag (Cd) in subsonic flight. By allowing the airflow to smoothly detach from the rocket's body, the boat tail contributes to a reduction in the pressure differential at the base, thereby lowering the drag experienced by the rocket.

In essence, the streamlined shape of the boat tail aids in creating a more aerodynamically efficient profile, which is particularly beneficial in subsonic conditions where base drag can be pronounced. This decrease in base drag results in an overall reduction in the coefficient of drag, leading to improved performance and efficiency during flight. Therefore, the correct assessment of the impact of a boat tail on a subsonic rocket's drag coefficient is that it decreases the Cd by significantly reducing base drag.