Second Stage

Second-stage ascent begins at SRB separation and extends through main engine cutoff and external tank separation. The GN&C; software is in major mode 103 (second stage) at this time, calculating the required main engine steering commands to achieve preflight-defined MECO conditions. After SRB separation, attitude hold is commanded until guidance converges.

The orbiter rate gyro assemblies are used by flight control as feedbacks to find errors that are used for stability augmentation during ascent, entry and aborts and for display on the commander's and pilot's ADIs. In second-stage ascent, the body-mounted accelerometers are not used, and the elevons are held in position.

Second-stage navigation is the same as that of first stage. Second-stage flight control continues through MECO.

Second-stage guidance uses a cyclic, closed-loop scheme to calculate the necessary commands to take the vehicle to a specified set of target MECO conditions. These conditions include cutoff velocity, radius from Earth, flight path angle, orbital inclination and longitude of the ascending node. The targeting scheme is called powered explicit guidance 1. Guidance also governs the main engine throttle command so that acceleration does not exceed 3 g's. The predicted time of MECO (TMECO) is calculated and displayed to the crew on the ascent trajectory display. Following SRB separation, it may take the PEG 1 guidance algorithm several cycles to converge and for TMECO to become stable. Forty seconds before MECO, guidance begins targeting only for the desired cutoff velocity, ignoring position constraints.

The main engines are throttled down at approximately seven minutes 40 seconds into the mission to maintain 3 g's for physiological and structural constraints. Approximately 10 seconds before MECO, the MECO sequence begins; about three seconds later the main engines are commanded to begin throttling at 10-percent thrust per second to 65-percent thrust. This is held for approximately 6.7 seconds, and the engines are shut down.

At MECO, the vehicle attitude commands (roll, pitch and yaw) are frozen, and body rate damping is maintained during the coast period by the reaction control system, which is accomplished by the transition digital autopilot. During this period, an automatic sequence is initiated by GN&C; moding, sequencing and control that confirms that all main engines have shut down, the MECO confirmed flag is set, the MPS prevalves are closed and a flag is set for the external tank separation sequence after the MPS prevalves for all main engines have been commanded closed. The ET separation software sends the necessary commands to close the 17-inch orbiter/external tank feed line liquid oxygen and hydrogen disconnect valves, dead-faces the orbiter/ET interface, tests for the 17-inch feed line disconnect valves' closure, dead-faces the orbiter/ET interface, unlatches and retracts the 17-inch disconnects within the orbiter aft fuselage, arms the two aft and one forward orbiter/ET structural separation pyro initiator controllers, and fires the ET liquid oxygen tank tumble vent valve.

ET separation is performed automatically by the onboard general-purpose computers. If automatic ET separation is inhibited due to the various orbiter/ET separation tests, separation can occur only if an out-of-tolerance condition comes back within tolerance or if the flight crew elects to continue the separation by overruling the inhibit. The manual separation would be accomplished by positioning the ET separation switch on panel C3 to man and depressing the ET separation push button. Once automatic or manual ET separation is initiated, the orbiter/ET structural separation PICs are fired, separating the ET from the orbiter.

In the automatic orbiter/ET separation sequence, the transition DAP commands separation 18 seconds after main engine cutoff.

After ET separation, the two umbilical doors (one each for the 17-inch liquid oxygen and liquid hydrogen umbilical disconnects) are closed automatically for the entry phase. If the automatic function fails to close the umbilical doors, the flight crew can manually close them by using the ET umbilical door switches on panel R2.

After initiation of the orbiter/ET separation sequence, there is approximately 11 seconds of mated coast before the orbiter and external tank separate. The ET tumble system produces a tumble rate of 10 to 50 degrees per second after separation. In Kennedy Space Center launches, the external tank is on a suborbital trajectory that normally results in an impact location in the Indian Ocean. Except for direct-insertion launches from Kennedy Space Center. the tank impacts in the Pacific Ocean. External tank breakup nominally occurs during entry into the Earth's atmosphere at an altitude of approximately 185,000 feet.

Just before orbiter/ET separation, the reaction control system is inhibited. It is re-enabled immediately after ET separation to an inertial attitude hold. The transition DAP then commands the RCS to thrust the four forward and six aft negative RCS jets for a minus Z translation to achieve a 4-foot-per-second separation vertically, ensuring orbiter clearance from the arc of the rotating tank. When the required separation is achieved, the thrusting commands to the negative RCS jets are removed. The orbiter continues to coast away from the tank in the inertial attitude hold mode, gaining additional vertical clearance.

When the orbiter has gained the necessary separation, the orbiter/ET separation is flagged complete, and Mission Control is responsible for issuing a go/no-go for the impending orbital maneuvering system thrusting sequence. The software makes an automatic transition to major mode 104 (OMS-1 insertion) when the negative Z translation is complete.

During second-stage ascent, the flight crew monitors the onboard systems to ensure that the major GN&C; events occur correctly and on time. These events include closed-loop guidance convergence, 3-g throttling, MECO, ET separation and the negative Z translation following ET separation. To monitor these events, the flight crew uses the dedicated displays-the main engine status lights on panel F7 and the PASS ascent trajectory and the BFS ascent trajectory 2 displays.

The crew can monitor guidance convergence by noting if the guidance-computed time of MECO is stabilized on the ascent trajectory display. If not, the crew takes manual control of the vehicle. They can also ensure that acceleration does not exceed 3 g's via the BFS ascent trajectory 2 display as well as the accel tape on the alpha Mach indicator. The crew can monitor MECO velocity on the BFS ascent trajectory 2 display as well as on the M/vel tape on the AMI. MECO is detected by the illumination of three red main engine status lights and by the main propulsion system chamber pressure meters on panel F7 going to zero.

Depending on mission requirements, the crew may be required to translate in the plus X direction, using the translational hand controller for 11 seconds, to allow the external tank camera to photograph the tank.

At specified points during second-stage ascent, the Mission Control Center will make voice calls to the crew indicating their status with respect to aborts. For example, the ''negative return'' call indicates that it is too late to select a return-to-launch-site abort.