What is a flight path angle?

The flight-path angle is the complement of the zenith angle used to calculate the launch of a space vehicle. Complementary angles add up to 90 degrees, so the flight path angle and the zenith angle together make a right angle.

Solving a vehicle's position and velocity are described by the variables r, the vehicle's distance from the center of the Earth, and v, the vehicle's velocity. The Greek letter gamma represents the angle between the position and the velocity vectors, and is called the zenith angle. Subtract that angle from 90 degrees to get the flight-path angle, which is denoted by phi.