 The orbiter propulsion system has three main engines to provide the initial thrust plus reaction control system engines which are used to provide attitude pitch, roll and yaw (rotational) maneuvers. It also has orbiter maneuvering system engines for placing the orbiter in orbit, conducting major velocity maneuvers, and to slow the orbiter for reentry to Earth.
The external tank is the largest element of the shuttle. It supplies pressurized liquid hydrogen fuel and liquid oxygen oxidizer to the main engines during launch and ascent.
Two solid rocket boosters (SRBs) provide the main thrust to lift the space shuttle off of the pad. They are reusable and have a propellant mixture which consists of ammonium perchlorate (an oxidizer at 69.6 percent), aluminum (fuel at 16 percent), iron oxide (a catalyst at 0.4 percent), a polymer (a binder that holds the mixture together at 12.04 percent), and an epoxy curing agent (at 1.96 percent).
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