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.

It provides 1.05 meganewtons (240,000 lbf) of thrust burning RP-1 and LOX in a gas-generator cycle.

The engine is a modified version of its predecessor, the RS-27;[7] its thrust nozzle has been extended to increase its area ratio from 8:1 to 12:1, which provides greater efficiency at altitude.

[8] When used as the main booster propulsion system for the Delta II family of launch vehicles, has an operational duration of 265 seconds.