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One of the signs accompanying the LR-105 engine. It reads

LR-105 Sustainer Engine

Attached to the base of the Atlas was a gimbal-mounted Rocketdyne LR-105 sustainer engine of 57,000 lb thrust, which operated together with two 1,000 lb swiveling motors used for roll control.

Two Rocketdyne LR-89 booster engines burned at takeoff with the central sustainer. Each of these 165,000 lb-thrust engines operated for about 145 seconds before separating (half-stage). The central Atlas engine continued to burn for a total of about 270 seconds. All the engines drew their liquid oxygen-kerosene (RP-1) propellants from common tanks in the sustainer.