RS-56

RS-56 (Rocket System-56) was an American liquid-fueled rocket engine, developed by Rocketdyne. RS-56 was derived from the RS-27 engine,[1] which itself is derived from the H-1 engine used in the Saturn I and Saturn IB.

Two variants of this engine were built, both for use on the Atlas II first stage. This was the last Atlas rocket to use the "stage-and-a-half" technique, where it ignited all three engines at liftoff and then jettisoned the two side engines and their support structure during ascent.

The two RS-56-OBA engines, with high thrust but moderate efficiency,[2][3] were integrated into a single unit called the MA-5A and shared a common gas generator. They burned for approximately 164 seconds before being jettisoned, when acceleration reached approximately 5.0–5.5 g.

The central sustainer engine on the first stage, an RS-56-OSA, would burn for an additional 125 seconds. It featured less thrust but better efficiency at high altitudes than the RS-56-OBAs.[2][3][1][4]