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.
Vernier thrusters are used in rockets such as the R-7 for vehicle maneuvering because the main engine is fixed in place. For 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 mounted on the base of the thrust section flanking the main engine.
- On the SM-65 Atlas, the verniers were used for roll control and to fine-tune the vehicle attitude after main engine cutoff.
- The R-7 rocket family, with over 1700 successful launches to date, is still depending on vernier thusters in its first and second stage. Most of the larger Soviet missiles such as the R-36 also used verniers.
- The Delta II and Delta III rocket used two vernier thrusters for vehicle roll maneuver since one engine cannot do a roll maneuver (although the later GEM 46 had thrust vectoring nozzle, only three were equipped with this feature and only ignited a brief moment during the ascent).
- 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.
- "Rocket Control: Examples of Controls". NASA's Glenn Research Center. Retrieved December 30, 2011.
- "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.
- Bergin, Chris (19 February 2010). "STS-130 prepares for undocking – MMOD impact on hatch cleared". NASAspaceflight.com. Retrieved 20 February 2010.
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