page_title: Space Shuttle orbiter
section_title: Attitude control system
hierarchical_section_title: Space Shuttle orbiter / Description / Attitude control system
caption_attribution_description: RCS thrusters on the nose of the US Space Shuttle.
context_page_description: The Space Shuttle orbiter is the spaceplane component of the Space Shuttle, a partially reusable orbital spacecraft system that was part of the Space Shuttle program. Operated by NASA, the U.S. space agency, this vehicle could carry astronauts and payloads into low Earth orbit, perform in-space operations, then re-enter the atmosphere and land as a glider, returning its crew and any on-board payload to the Earth.
  Six orbiters were built for flight: Enterprise, Columbia, Challenger, Discovery, Atlantis, and Endeavour. All were built in Palmdale, California, by the Pittsburgh, Pennsylvania-based Rockwell International company. The first orbiter, Enterprise, made its maiden flight in 1977. An unpowered glider, it was carried by a modified Boeing 747 airliner called the Shuttle Carrier Aircraft and released for a series of atmospheric test flights and landings. Enterprise was partially disassembled and retired after completion of critical testing. The remaining orbiters were fully operational spacecraft, and were launched vertically as part of the Space Shuttle stack.
  Columbia was the first space-worthy orbiter, and made its inaugural flight in 1981.
context_section_description: The Reaction Control System (RCS) was composed of 44 small liquid-fueled rocket thrusters and their very sophisticated fly-by-wire flight control system, which utilized computationally intensive digital Kalman filtering. This control system carried out the usual attitude control along the pitch, roll, and yaw axes during all of the flight phases of launching, orbiting, and re-entry. This system also executed any needed orbital maneuvers, including all changes in the orbit's altitude, orbital plane, and eccentricity. These were all operations that required more thrust and impulse than mere attitude control.
  The forward rockets of the Reaction Control System, located near the nose of the Space Shuttle orbiter, included 14 primary and two vernier RCS rockets. The aft RCS engines were located in the two Orbital Maneuvering System (OMS) pods at the rear of the orbiter, and these included 12 primary (PRCS) and two vernier (VRCS) engines in each pod. The PRCS system provided the pointing control of the Orbiter, and the VRCS was used for fine maneuvering during the rendezvous, docking, and undocking maneuvers with the International Space Station, or formerly with the Russian Mir space station. The RCS also controlled the attitude of the orbiter during most of its re-entry into the Earth's atmosphere – until the air became dense enough that the rudder, elevons and body flap became effective.
  The orbiter’s OMS and RCS fuel is monomethyl hydrazine (CH3NHNH2), and the oxidizer is dinitrogen tetroxide (N2O4). This particular propellant combination is extremely reactive and spontaneously ignites on contact (hypergolic) with each other. This chemical reaction (4CH3NHNH2 + 5N2O4 → 9N2 + 4CO2 + 12H2O) occurs within the engine’s combustion chamber. The reaction products are then expanded and accelerated in the engine bell to provide thrust. Due to their hypergolic characteristics these two chemicals are easily started and restarted without an ignition source, which make them ideal for spacecraft maneuvering systems.. 
  During the early design process of the orbiter, the forward RCS thrusters were to be hidden underneath retractable doors, which would open once the orbiter reached space. These were omitted in favor of flush-mounted thrusters for fear that the RCS doors would remain stuck open and endanger the crew and orbiter during re-entry.
caption: Space Shuttle forward reaction control thrusters
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