Vernier thruster

From Wikipedia, the free encyclopedia
Jump to: navigation, search
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,[1] or it may complement larger attitude control motors,[2] 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.[1] 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.


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.

See also[edit]


  1. ^ a b "Rocket Control: Examples of Controls". NASA's Glenn Research Center. Retrieved December 30, 2011. 
  2. ^ a b "Reaction Control Systems". NASA Kennedy Spaceflight Center. Retrieved 2011-10-03. 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 has 14 primary and two vernier engines. The aft RCS has 12 primary and two vernier engines in each pod. The primary RCS engines provide 870 pounds of vacuum thrust each, and the vernier RCS engines provide 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 engines is 152 psia. For each vernier engine, it is 110 psia. 
  3. ^ Bergin, Chris (19 February 2010). "STS-130 prepares for undocking – MMOD impact on hatch cleared". Retrieved 20 February 2010.