An active flight section ( active part of the trajectory ) is the flight section of an aircraft on which the main engine of the device  is operating, as a rule a rocket one .
The concept arose in connection with the use of unguided missiles and ballistic missiles . But this concept extends to other aircraft , propelled by rocket engines that operate for only part of the flight time of the device: carrier rockets of spacecraft , spacecraft , marching engines which are switched on for a short time to correct the trajectory, rocket gliders , etc.
A passive section of a flight ( trajectory ), as opposed to an active one, is a section in which the mid- flight engine of the aircraft does not work, and the device moves by inertia, under the influence of only gravitational forces and (in dense layers of the atmosphere) air resistance (see Ballistic trajectory ).
- Active site // Cosmonautics. Encyclopedia / Ch. ed. V.P. Glushko. - Moscow: Soviet Encyclopedia, 1985.- S. 19.
- Feodosiev V., Sinyarev G. Introduction to rocketry. 2 - ed., Revised. and add. M. Oborongiz 1961