A vernier thruster is a rocket engine used on a spacecraft for fine adjustments to the attitude or velocity of a spacecraft. Depending on the design of a craft's maneuvering and stability systems, it may simply be a smaller thruster complementing the main propulsion system, or it may complement larger attitude control thrusters, or may be a part of the reaction control system. The name is derived from vernier calipers 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 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. Instead, as modernrocket engines gained better control, larger thrusters could also be fired for very short pulses, resulting in the same change of momentum as a longer thrust from a smaller thruster. Vernier thrusters are used in rockets such as the R-7 for vehicle maneuvering because the main engine is fixed in place. For earlier versions of the Atlas rocket family, in addition to maneuvering, the verniers were used for roll control, although the booster engines could also perform this function. After sustainer 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.
On the SM-65 Atlas, the LR-101 verniers were used for roll control and to fine-tune the vehicle attitude after main engine cutoff. The Delta II and Delta III rocket also used the LR-101 for vehicle roll maneuver since one engine cannot do a roll maneuver.
The Space Shuttle had six vernier engines or thrusters in its "Vernier Reaction Control System". The VRCS could also deliver a gentle steady thrust. This was regularly used to boost the International Space Station while docked: for example during STS-130 commander George Zamka and pilot Terry Virts fired Endeavour's VRCS for a duration of 33 minutes to attain an orbit between 180.5 and 190.0 nmi.
