The roll axis is one of three primary axes used to define and control motion in a three-dimensional space. Used primarily in the piloting of aircraft, submarines and spacecraft, the roll axis is defined by an imaginary line that runs through the center of a craft or vehicle from the front to the rear. Measured in degrees of rotation from zero to 359, 0° on the roll axis is referenced to as a fixed point in space, usually upright and level with the Earth in the case of aircraft and submarines. In spacecraft, the fixed zero point can be any point that appears to be stationary in relation to the craft’s direction of travel, such as the Earth, the sun or the orbital plane traveled by the planets as they revolve around the sun.
When an aircraft rotates on its roll axis, its nose will continue to point in the direction of travel as the wings revolve around the axis. The roll axis increases from 0° to 359° in a clockwise direction relative to the position of the pilot. At 0° roll axis, an aircraft would be flying straight and level; at 90°, the aircraft would still be pointed in the direction of travel, but the left wing would be pointed straight up and the right wing straight down. At 180°, the aircraft would be flying straight but upside-down in relation to the Earth.
There are circumstances when the fixed point of a roll axis will be changed temporarily, particularly when maneuvering or performing a task requiring movement relative to another object. A spacecraft will change the definition of its roll axis when docking with another spacecraft or an orbital station. In this case, the center of the docking apparatus will become the fixed point and all of the spacecraft’s movements are measured relative to it until the docking procedure is complete. Once complete, the fixed reference for the roll axis will either return to its former definitions or change again to the orbital station itself until the spacecraft leaves its proximity.
The roll axis functions in combination with two other axes, the pitch axis and the yaw axis, to define any relative movement or direction in a three-dimensional space. The pitch axis is an imaginary line that runs through the center of a vehicle from its left side to its right and movement is measured in relation to the vehicle’s direction of travel. Rotating on the pitch axis changes the up and down angle of the vehicle’s nose with 0° being level and 90° pointing the nose straight up. The yaw axis runs from the top of a vehicle to its bottom. When moving in the yaw axis, the nose of the vehicle rotates horizontally to the left or right in relation to the vehicle’s direction of travel.