The Rocketdyne LR-101 is a fixed thrust, single start vernier thruster developed by Rocketdyne in the mid-to-late fifties and used in the Atlas, Thor and Delta launch vehicles until 1990.
Each of these rockets used two LR-101 secondary engines to provide yaw, pitch and roll control during their ascent to space. The pair of LR-101 vernier thrusters would receive their propellant flow from the turbopumps of the sustainer engine until it was shut off. After sustainer burnout, this pair of thrusters would switch to feed off the remaining propellant, effectively becoming a Pressure-fed engine. The remaining fuel would then be spent for the last trajectory corrections and stabilization of the rocket before separating the payload.
Variants
Due to several modifications the LR-101 underwent over the years, there are many different variants of this engine (LR101-NA-3,[1] LR101-NA-7,[2]