Avernier thruster is arocket engine used on aspacecraft orlaunch vehicle for fine adjustments to theattitude orvelocity. Depending on the design of a craft's maneuvering and stability systems, it may simply be a smaller thruster complementing the mainpropulsion system,^[1] or it may complement largerattitude control thrusters,^[2] or may be a part of thereaction control system.The name is derived fromvernier calipers (named afterPierre Vernier) which have a primary scale for gross measurements, and a secondary scale for fine measurements.

Vernier thrusters are used when a heavy spacecraft requires a wide range of different thrust levels for attitude or velocity control, as for maneuvering during docking with other spacecraft.

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.

Due to their weight and the extra plumbing required for their operation, vernier rockets are seldom used in new designs.^[1]Instead, as modern rocket engines gained better control, larger thrusters could also be fired for very short pulses, resulting in the same change ofmomentum as a longer thrust from a smaller thruster.

Vernier thrusters are used in rockets such as theR-7 for vehicle maneuvering because the main engine is fixed in place. For earlier versions of theAtlas rocket family (prior to theAtlas III), in addition to maneuvering, the verniers were used for roll control, although the booster engines could also perform this function. After main engine cutoff, the verniers would execute solo mode and fire for several seconds to make fine adjustments to the vehicle attitude. The Thor/Delta family also used verniers for roll control but were mounted on the base of the thrust section flanking the main engine.

• TheR-7 rocket family, with over 1700 successful launches to date, still depends on the S1.358000 vernier thrusters in its first and second stage. Most of the larger Soviet missiles and launch vehicles also used verniers. Examples include theRD-8 on theZenit rocket family, theRD-855 andRD-856 on theR-36, and theRD-0214 on theProton rocket family.
• On theSM-65 Atlas, theLR-101 verniers were used for roll control and to fine-tune the vehicle attitude after the main engine cutoff. TheDelta II andDelta III rocket also used the LR-101 for vehicle roll maneuver since one engine cannot do a roll maneuver (although the laterGEM 46 had thrust vectoring nozzle, only three were equipped with this feature and only ignited a brief moment during the ascent).^[3]
• TheSpace Shuttle had six vernier engines or thrusters in its "VernierReaction Control System" (VRCS). The VRCS could also deliver a gentle steady thrust. This was regularly used to boost theInternational Space Station while docked: for example duringSTS-130 commanderGeorge Zamka and pilotTerry Virts firedEndeavour's VRCS for a duration of 33 minutes to attain an orbit between 180.5 and 190.0 nmi (334.3 and 351.9 km).^[2][4]

• Cold gas thruster
• Space propulsion
• Vernier throttle

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