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 (named after Pierre 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. Instead, as modern rocket 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 (prior to the Atlas 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.
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 had 14 primaries and two vernier engines. The aft RCS had 12 primary and two vernier engines in each pod. The primary RCS engines provided 870 pounds of vacuum thrust each, and the verniers provided 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 thrusters was 152 psia. For each vernier, it was 110 psi.