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.

RS-56

RS-56 engines on an Atlas II
Country of origin	United States
First flight	1991
Manufacturer	Rocketdyne
Predecessor	RS-27A
Status	Retired
Liquid-fuel engine
Propellant	LOX / RP-1
Cycle	Gas-generator
Performance
Thrust, sea-level	RS-56-OBA: 207,000 lbf (920.8 kN) RS-56-OSA: 60,500 lbf (269.0 kN)
Chamber pressure	4.8 MPa (48 bar)
Specific impulse, vacuum	RS-56-OBA: 299 s (2.93 km/s) RS-56-OSA: 316 s (3.10 km/s)
Specific impulse, sea-level	RS-56-OBA: 263 s (2.58 km/s) RS-56-OSA: 220 s (2.2 km/s)
Burn time	RS-56-OBA: 172 RS-56-OSA: 283 sec
Dimensions
Length	RS-56-OBA: 11.3 ft (3.43 m) 8.9 ft (2.7 m)
Diameter	RS-56-OBA: 8.0 ft (2.45 m) 10.0 ft (3.05 m)
Used in
Atlas II

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]