What is the primary factor affecting the specific range of a turbojet airplane during cruise?

The specific range of a turbojet airplane during cruise is primarily influenced by altitude. As an aircraft ascends to higher altitudes, the air density decreases, leading to a reduction in drag. This allows the airplane to achieve better fuel efficiency because the engines can operate more efficiently in the thinner air. The optimal altitude for a turbojet typically maximizes fuel economy and minimizes drag, which is essential for achieving a greater specific range—defined as the distance traveled per unit of fuel consumed.

While engine performance, gross weight, and aircraft design certainly play significant roles in overall performance and efficiency, it is the effects of altitude on drag and engine performance that primarily dictate the specific range during cruise. Therefore, understanding how altitude impacts aerodynamic efficiency is crucial in optimizing the specific range of turbojet aircraft.