The service propulsion system (SPS) engine was originally designed to lift the CSM off the surface of the Moon in the direct ascent mission mode, The engine selected was the AJ10-137, which used Aerozine 50 as fuel and nitrogen tetroxide (N2O4) as oxidizer to produce of thrust. A contract was signed in April 1962 for the Aerojet-General company to start developing the engine, resulting in a thrust level twice what was needed to accomplish the lunar orbit rendezvous (LOR) mission mode officially chosen in July of that year. The engine was actually used for mid-course corrections between the Earth and Moon, and to place the spacecraft into and out of lunar orbit. It also served as a retrorocket to perform the deorbit burn for Earth orbital flights. The propellants were pressure-fed to the engine by of gaseous helium at , carried in two diameter spherical tanks. The exhaust nozzle measured long and wide at the base. It was mounted on two gimbals to keep the thrust vector aligned with the spacecraft's center of mass during SPS firings. The combustion chamber and pressurant tanks were housed in the central tunnel. Reaction control system Four clusters of four reaction control system (RCS) thrusters (known as "quads") were installed around the upper section of the SM every 90°. The sixteen-thruster arrangement provided rotation and translation control in all three spacecraft axes. Each R-4D thruster measured long by diameter, generated of thrust, and used helium-fed monomethylhydrazine (MMH) as fuel and nitrogen tetroxide (NTO) as oxidizer. Each quad assembly measured and had its own fuel, oxidizer, and helium tanks mounted on the inside of an skin panel. The primary fuel (MMH) tank contained ; the secondary fuel tank contained ; the primary oxidizer tank contained , and the secondary oxidizer tank contained . The propellant tanks were pressurized from a single tank containing of liquid helium. Back flow was prevented by a series of check valves, and back flow and ullage requirements were resolved by containing the fuel and oxidizer in Teflon bladders which separated the propellants from the helium pressurant. The four completely independent RCS clusters provided redundancy; only two adjacent functioning units were needed to allow complete attitude control. The lunar module used a similar four-quad arrangement of R-4D thruster engines for its RCS. Electrical power system Electrical power was produced by three fuel cells, each measuring tall by in diameter and weighing