The Soviet RD-0120 (also designated 11D122) was the Energia Core rocket engine, fueled by LH2/LOX, roughly equivalent to the Space Shuttle Main Engine (SSME). These were attached to the Energia Core rather than the orbiter, so were not recoverable after a flight, but created a more modular design (the Energia Core could be used for a variety of missions besides launching the shuttle). Baselined from the more mature American hydrogen–oxygen engine technology, but considerably modified with Russian innovations and methods, the RD-0120 and the SSME have both similarities and differences. The RD-0120 achieved nearly identical specific impulse and combustion chamber pressure with reduced complexity and cost, as compared to the SSME, primarily at the expense of lower thrust-to-weight ratio. It uses fuel-rich staged combustion cycle and a single shaft to drive both the fuel and oxidizer turbopumps. Some of the Russian design features, such as the simpler and cheaper channel wall nozzles, were evaluated by Rocketdyne for possible upgrades to the SSME. It achieved combustion stability without the acoustic resonance chambers that the SSME required.