In which operational environment is the second stage typically engineered to function?

The near-vacuum of space.

The design philosophy dictates that different stages of a launch vehicle are optimized for different environmental conditions. The first stage handles the dense lower atmosphere. In contrast, the second stage ignites only after the first stage separates, often while the vehicle is already traveling at thousands of miles per hour at high altitude. This stage is specifically engineered to operate efficiently where air resistance is negligible—the near-vacuum environment of space. Its primary function is to continue acceleration until the required orbital insertion velocity and altitude are achieved, tasks that are best performed without the drag associated with an atmosphere.