The RD-120 engine was designed during the early 1980's to power the second stage of the Zenit launch vehicle.  The Zenit launch vehicle was Russia's answer to Ronald Reagan's SDI (Star Wars) initiative.  Fearful that the US could wipe out their on-orbit assets, the Russian's developed the capability to rapidly (re)deploy replacement satellites on short notice.  They combined the powerful Energia liquid booster with an equally robust second stage, named the vehicle the Zenit, and designed it to hide in railway tunnels (hence the 12 foot diameter) and launch automatically in four hours.

Recently, the RD-120 has been marketed to western vehicle manufacturers by Pratt & Whitney as part of their relationship with NPO Energomash.  The standard RD-120 is an altitude engine which boasts 350 seconds of Isp.  Pioneer plans to use the RD-120 to power their Rocketplane RLV.  A booster variant of the RD-120 (the RD-120K) has also been proposed which uses a smaller area ratio nozzle.  Although some design work has been done, the booster engine is still considered a development stage engine.