Overview: The orbiter's orbital maneuvering system and reaction control system burn monomethyl hydrazine as a fuel and nitrogen tetroxide as an oxidizer. These hypergolic fluids are stored in well-separated areas on the southwest and southeast corners of the pads, respectively. Transfer lines convey the fluids through the fixed service structure to the hypergolic umbilical system located on the rotating service structure, with its three pairs of umbilicals attaching to the orbiter.
Image above: In the early morning at Launch Complex 39B, Space Shuttle Discovery stands ready for launch on mission STS-103 as rain falls all around it. Credit: NASA/KSC
NASA's John F. Kennedy Space Center 
