the possible replacement of the Russian-supplied RD-180 engine used on the first-stage booster of the ULA Atlas V. Formal study contracts were issued in June 2014 to a number of US rocket-engine suppliers. In September 2014, ULA announced that it has entered into a partnership with Blue Origin to develop the BE-4 LOX/methane engine to replace the RD-180 on a new first-stage booster that would succeed the Atlas V. At the time, the engine was already in its third year of development by Blue Origin, and ULA expected the new stage and engine to start flying no earlier than 2019. Two of the -thrust BE-4 engines will be used on the new launch vehicle booster. Dynetics and Aerojet Rocketdyne (AJR) have also offered their AR1 hydrocarbon-fueled rocket engine as replacement of the RD-180. ULA CEO Tory Bruno has said in early 2015 that both the AR-1 option and the US manufacture of the RD-180 by ULA under license are backup options to the primary option ULA is pursuing with the Blue Origin BE-4 engine. By March 2016, the US Air Force had signed development contracts with AJR and Blue Origin to provide funding toward engine development for both engines. , the first Vulcan flight with new engines is expected to fly in 2022. As of May 25, 2020 (20 years since the first launch of the Atlas LV with RD-180), 116 engines have been delivered to the USA, 90 launches have taken place, all of them are recognized as successful. Design and specifications The combustion chambers of the RD-180 share a single turbopump unit, much like in its predecessor, the four-chambered RD-170. The RD-180 is fueled by an RP-1/LOX mixture and uses an extremely efficient, high-pressure staged combustion cycle. The engine runs with an oxidizer-to-fuel ratio of 2.72 and employs an oxygen-rich preburner, unlike typical fuel-rich US designs. The thermodynamics of the cycle allow an oxygen-rich preburner to give a greater power-to-weight ratio, but with the drawback that high-pressure, high-temperature gaseous oxygen must be transported throughout the engine. The movements of the engine nozzles are controlled by four hydraulic actuators. The engine can be throttled from 47% to 100% of nominal thrust. Applications During the early 1990s, General Dynamics Space Systems Division (later purchased by Lockheed Martin) acquired the rights to use the RD-180 in the Evolved Expendable Launch Vehicle (EELV) and the Atlas program. As these programs were conceived