The Raptor engine is powered by subcooled liquid methane and subcooled liquid oxygen using a more efficient Full Flow Staged Combustion cycle, a departure from the simpler "open-cycle" gas generator system and LOX/kerosene propellants that current Merlin engines use.[1] The RS-25, with hydrolox propellant also used a staged combustion process,[22] as do several Russian rocket engines, including the RD-180[21] and the 25.74 MPa (3,733 psi) chamber pressure RD-191.[3] The stated design size for the Raptor engine varied widely during 2012–2017 as detailed design continued, from a high target of 8,200 kN (1,800,000 lbf) of vacuum thrust[24] to a more recent, much lower target of 1,900 kN (430,000 lbf).[citation needed] In its 2017 iteration, the operational engine is expected to have a vacuum Isp = 382 s (3,750 m/s) and a sea-level Isp = 334 s (3,280 m/s).[25]