Vernier Thrusters are a special type of rocket engines used primarily in spacecraft. They are a component of the Attitude control systems or the Reaction Control systems employed in spacecraft. Vernier Thrusters are so called because they are used to provide fine adjustments to the direction and orientation of the spacecraft much like the vernier scale is used to provide fine measurements in the vernier caliper.
The Reaction Control System in a spacecraft is responsible for providing small levels of thrust in any direction desired to adjust its position and provide translation. It is also used to generate torque for the spacecraft allowing it to rotate and control its roll, pitch and yaw.
Vernier thrusters work in combination with large primary thrusters which are responsible for large movements while the vernier thrusters provide small adjustments. This system is used to control the spacecraft’s position during re-entry, adjust alignment during docking procedures and for stationkeeping during orbit.
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Examples of Vernier thrusters used in spacecraft are the R-7 rocket which was the worlds first intercontinental Ballistic missile and was later used in the Soviet Union’s space missions. The R-7‘s main engine was fixed so the vernier thrusters were used to provide directional thrust to the rockets.
Another example is the Atlas rockets where vernier thrusters were not only used for positioning but also for rolling maneuvers once the booster engines are detached.
Vernier thrusters add a lot of weight to the rockets due to additional plumbing requirement and are being replaced by more controllable rocket motors capable of controlling the pulse of larger thrusters as well. However they still remain widely used in many spacecraft and rockets currently in operation.