A vernier thruster is a thruster used on a spacecraft for attitude control. It may be a smaller thrust motor than main attitude control motors, or a small control thruster relative to the main propulsion motors, or may be simply a small thruster of the reaction control system. Vernier thrusters are used for fine adjustments to the attitude or velocity of a spacecraft.
Vernier thrusters are used when spacecraft control requirements, such as maneuvering during docking with other spacecraft, dictate a wide range of different thrust levels for attitude or velocity control.
On space vehicles with two sizes of attitude control thrusters, the main ACS (Attitude Control System) thrusters are used for larger movements, while the verniers are reserved for smaller adjustments. As an alternative, larger thrusters which can fire for very short durations or pulses may be fired instead. Thus, by having a large pulse for a short amount of time, the same vernier end result is reached as when pulsing a much smaller jet for a longer period of time.
Vernier thruster implementations 
One type of vernier thruster is a smaller rocket engine used for attitude control in conjunction with a larger spacecraft propulsion engine. These were used on early Atlas missiles. Due to their weight and the extra plumbing required for their operation, vernier rockets are seldom used for new designs. However, R-7 rocket family, with over nineteen hundred successful launches to date, is based on Vernier rocket design solution.[not in citation given]
The Space Shuttle's reaction control system had six vernier engines or thrusters. During STS-130 commander Zamka and pilot Terry Virts used the space shuttle Endeavour's vernier thrusters to reboost the International Space Station to its correct altitude.
See also 
- "Reaction Control Systems". NASA Kennedy Spaceflight Center. unknown. Retrieved 2011-10-03. "The flight crew can select primary or vernier RCS thrusters for attitude control in orbit. Normally, the vernier thrusters are selected for on-orbit attitude hold. ... The forward RCS has 14 primary and two vernier engines. The aft RCS has 12 primary and two vernier engines in each pod. The primary RCS engines provide 870 pounds of vacuum thrust each, and the vernier RCS engines provide 24 pounds of vacuum thrust each. The oxidizer-to-fuel ratio for each engine is 1.6-to-1. The nominal chamber pressure of the primary engines is 152 psia. For each vernier engine, it is 110 psia."
- "Rocket Control: Examples of Controls". NASA's Glenn Research Center. Retrieved December 30, 2011.
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