RD-253 engine - Credit: Khrunichev. 10,372 bytes. 162 x 178 pixels.

Version of RD-253 with thrust increased by 7%. Includes an additional gas generator for tank pressurization. This enhancement is accomplished with only a minor modification to the propellant flow control valves. This modification first flew as a mission-unique enhancement on the Proton K that delivered the MIR space station core module in 1986. Engines incorporating this change have undergone extensive additional qualification firings since then, in order to approve them for use in standard production vehicles. Shipsets have already been produced and incorporated onto production Proton K vehicles. To 1999, 65 Proton K's had been launched with the modified engines operating at 102% of rated thrust. Eight Proton K vehicles had flown with the 107% engines. There had been no flight anomalies attributed to the increased thrust engines.