The vertical stabilizer of the orbiter had a leading edge that was swept back at a 45-degree angle. There were four elevons mounted at the trailing edges of the delta wings, and the combination rudder and speed brake was attached at the trailing edge of the vertical stabilizer. These, along with a movable body flap located underneath the main engines, controlled the orbiter during later stages of descent through the atmosphere and landing. Overall, the Space Shuttle orbiter was roughly the same size as a McDonnell Douglas DC-9 airliner. 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. 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. The orbiter crew cabin consisted of three levels: the flight deck, the mid-deck, and the utility area. The uppermost of these was the flight deck, in which sat the Space Shuttle's commander and pilot, with up to two mission specialists seated behind them.