Vernier thruster

File:Mercury Atlas vernier motor, Truax variant.jpg

A 1960s Mercury-Atlas vernier motor

Vernier thrusters on the side of an Atlas missile can be seen emitting diagonal flames.

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 next to the main propulsion motors, or it may complement larger attitude control motors, 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 motors became better controllable, larger thrusters could also be fired for very short pulses, resulting in the same change of momentum as a longer thrust from a smaller motor.

Examples

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.

• In the early Atlas missiles vernier thrusters were used for attitude control.
• The R-7 rocket family, with over seventeen hundred successful launches to date, is still depending on vernier thusters in its first and second stage.
• The Space Shuttle reaction control system had six vernier engines or thrusters in its VRCS or "Vernier Reaction Control System". The system could also deliver a gentle steady thrust which was regularly used to reboost 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.

See also

• Reaction control system
• Space propulsion
• Attitude control
• Cold gas thruster
• Vernier throttle