This is a quad, or cluster, of four Apollo Reaction Control System (RCS) engines. The RCS was a vital rocket engine system for maintaining the attitude of the Apollo Service Module (SM), helped it to maneuver in space, plus undertake minor midcourse velocity corrections, abort separations and emergency attitude maneuvers. Each thruster produced 100 pounds of thrust. The propellants were monomethyl hydrazine (MMH) and nitrogen tetroxide. The quad shown here was not flown but was from Apollo-Soyuz Test Project, possibly as a backup RCS. It was transferred to the Smithsonian in 1980 from the NASA Johnson Space Flight Center.