Vernier thruster

A vernier thruster is a rocket engine used on a spacecraft or launch vehicle for fine adjustments to the attitude or velocity.

Due to their weight and the extra plumbing required for their operation, vernier rockets are seldom used in new designs.

Vernier thrusters are used in rockets such as the R-7 for vehicle maneuvering because the main engine is fixed in place.

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.

A 1960s Mercury-Atlas vernier thruster
Vernier thrusters on the side of an Atlas missile can be seen emitting diagonal flames.
The first- and second-stage engines of a Soyuz , showing the four RD-107 modules with twin vernier nozzles each, and the central RD-108 with four steerable vernier thrusters