Service Module Reaction Control System
The Service Module Reaction Control System (SM RCS) is utilized to control spacecraft rates and rotation in all three axis in addition to any minor translation maneuvers including SPS ullage thrust and separation maneuvers. It consists of four individual, functionally identical packages, located 90° apart around the forward (+X axis) of the SM periphery. Each package, called a "quad", has four reaction engines mounted on the outer surface. The engines use monomethylhydrazine (MMH) as fuel and inhibited nitrogen tetroxide oxidizer, each engine has about 100 lbf (441.5 N) thrust. This hypergolic fuel combination is pressurized with helium during thrusting periods, so no propellant pump is needed. The propellant flow from the tanks to the thrust chambers is controlled by electric engine injector solenoid valves.
