RS-27

The RS-27 (Rocket System-27) was a liquid-propellant rocket engine developed in 1974 by Rocketdyne to replace the aging MB-3 in the Delta. Incorporating components of the venerable MB-3 and the H-1 designs, the RS-27 was a modernized version of the basic design used for two decades. It was used to power the first stage of the Delta 2000, 3000, 5000, and the first model of the Delta II, the Delta 6000.

The RS-27 was a modified Rocketdyne H-1 built to power the first stage of the Saturn I and Saturn IB and replaced the MB-3 engine that had been used on previous versions of the Delta launcher. NASA had a large supply of surplus H-1 engines in the early 1970s, as the Apollo program was ending. In addition to its main engine, the RS-27 included two vernier engines to provide vehicle roll control during flight. RS-27 was later developed into the RS-27A and RS-56.

RS-27A
The RS-27A is a liquid-fuel rocket engine developed in 1980s by Rocketdyne for use on the first stage of the Delta II and Delta III launch vehicles. It provides 1.05 MN of thrust burning RP-1 and LOX in a gas-generator cycle. The engine is a modified version of its predecessor, the RS-27; its thrust nozzle has been extended to increase its area ratio from 8:1 to 12:1, which provides greater efficiency at altitude.

The RS-27A main engine is neither restartable nor throttleable. In addition to its main engine, it includes two vernier engines to provide vehicle roll control during flight. When used as the main booster propulsion system for the Delta II family of launch vehicles, has an operational duration of 265 seconds. The last RS-27A engine was used for the ICESat-2 launch on 15 September, 2018.