First Stage

First-stage ascent extends from SRB ignition through SRB separation, or SRB staging. The sequence of major guidance, navigation and control events proceeds as folllows: The vehicle lifts off the pad 0.3 second after SRB ignition, rising vertically in attitude hold until the SRBs' nozzles clear the lightning rod tower by approximately 41 feet. The vehicle begins a combined roll, pitch and yaw maneuver that positions the orbiter head down, with wings level and aligned with the launch pad. The orbiter flies upside down during the ascent phase. This orientation, together with trajectory shaping, establishes a trim angle of attack that is favorable for aerodynamic loads during the region of high dynamic pressure, resulting in a net positive load factor, as well as providing the flight crew with use of the ground as a visual reference. By about 20 seconds after lift-off, the vehicle is at 180 degrees roll and 78 degrees pitch.

During the first 90 seconds of flight, the flight control system provides load relief by making adjustments to reduce vehicle loads at the expense of maintaining a precise trajectory profile. A special schedule of elevon position with respect to velocity is followed to protect the wings from excessive loads and to hold the body flap and rudder/speed brake in place. The surface position indicator displays the position of the aerosurfaces. To keep the dynamic pressure on the vehicle below a specified level, on the order of 580 pounds per square foot (max q), the main engines are throttled down at approximately 26 seconds and throttled back up at approximately 60 seconds. This also reduces heating on the vehicle. Because of the throttling at this time, the term ''thrust bucket'' evolved. Maximum dynamic pressure occurs shortly after throttle up.

Thrust vector control is the hub of flight control. In the ascent phase, the four ascent thrust vector control drivers respond to commands from the guidance system. Thus the TVC commands from guidance are transmitted to the ATVC drivers, which transmit electrical signals proportional to the commands to the servoactuators on each main engine and solid rocket booster.

Thrust vector control closes the acceleration and rate loops within the outer attitude loops to generate body axis attitude error rates, which eventually are nulled out by the main engines and SRBs. The main propulsion system processor of the digital autopilot converts body axis and attitude error signals generated in TVC into pitch and yaw nozzle deflection commands for the main engines. The SRB processor of the DAP accomplishes the same functions as the MPS, except that it is referred to as rock and tilt instead of pitch and yaw.

The SRB pitch and yaw rate gyros are used exclusively during first-stage ascent, and control is switched to the orbiter rate gyros when the SRBs are commanded to null in preparation for separation. Pitch and yaw axes and a combination of rate, attitude and acceleration signals are blended to effect a common signal to the main engines and the TVC for both SRBs. In the roll axis, rate and attitude are summed to provide a common signal to both the main engines' and SRBs' TVC.

First-stage guidance is active from SRB ignition through SRB separation plus four seconds. In this stage, guidance uses a preflight-planned (canned) table of roll, pitch and yaw attitudes referenced to relative velocity. There are canned tables defined for the cases of three good main engines, as well as for left, center or right main engine failed cases. In addition to sending commands to flight control to obtain proper vehicle attitude, guidance also sends commands to the MPS throttle in accordance with a preflight-defined throttle schedule, which is a function of relative velocity unless the flight crew has taken manual control of the throttle.

Navigation during first stage propagates the vehicle state vector through use of inertial measurement unit data and a gravity model. This function can be used to aid in driving the cathode ray tube's predictor.

Boost guidance is responsible for performing the ''table lookup'' of the appropriate roll, pitch and yaw command, depending on relative velocity. The table used is determined by the number of main engines thrusting. If one engine fails, guidance automatically recognizes the failure and lofts the trajectory, commanding the remaining two engines to a higher thrust percent, in the case of a contingency abort, for the remainder of first-stage ascent. For intact aborts, the throttle remains at the nominal maximum setting. Each main engine controller is used during the ascent phase to monitor the operation of an engine, issue inhibit commands to prevent a second engine from automatically shutting down when one has shut down, monitor the state of flight deck crew displays and switches from the switch processor and issue appropriate commands. The controllers also monitor guidance, navigation and control software for the proper time to check the main propulsion system liquid oxygen and liquid hydrogen low-level sensors, monitor GN&C; software for proper time of main engine cutoff, and issue the command to close the main propulsion system liquid oxygen and liquid hydrogen prevalves after engine shutdown.

No GN&C-related; crew actions are planned for first-stage ascent unless a failure occurs. To ensure that the auto flight control system is maintaining the expected ascent profile, the flight crew can verify that the vehicle is at the correct pitch attitude (via the attitude director indicator) and altitude rate (via the altitude/vertical velocity indicator) at each of five designated times during first-stage ascent. The flight crew can monitor that the main engines correctly throttle down and up. They can also ensure that the Pc - 50 message (chamber pressure greater than 50 psi) correctly appears on the major mode 102 (first stage) ascent trajectory CRT display before SRB separation and that SRB separation occurs on time. Manual intervention by the crew is required if these events are not automatically accomplished. The crew is also responsible for monitoring main engine performance. During first-stage ascent, only limited information is available to the crew on the PASS and backup flight system major mode 102 displays.

In the automatic mode, flight control during first-stage ascent uses commands sent to it from guidance. If the flight crew has selected control stick steering, commands are input through the rotational hand controller. Feedback from external sensors (rate gyro assemblies and accelerometers) is used to generate commands to reposition the SRB nozzles and main engine gimbals. Flight control sends the current vehicle attitude rates and the errors between current and desired attitudes to the ADI for display to the crew. As mentioned earlier, flight control also performs load relief by orienting the vehicle to reduce normal and lateral accelerations and by commanding the elevons to alleviate hinge moment loads in accordance with the premission-defined elevon schedule. If the crew does select CSS, the resultant control mode is discrete rate/attitude hold. In this mode, when the RHC is moved, a specified attitude rate is commanded as long as the stick is out of its neutral, detent position. When the stick is in detent, attitude hold commands are generated by flight control. Moreover, load relief is not performed by the software when the crew selects CSS.

The next major event is SRB separation, which occurs six seconds after the SRB separation sequence software detects both SRB chamber pressures below 50 psi within 4.3 seconds of each other and detects vehicle rates within specified limits. These checks ensure that neither SRB is burning at separation and that the SRBs will not recontact the ET after separation. SRB separation occurs at about two minutes after launch. At separation, the first stage is complete, and the software automatically shifts to major mode 103 (second stage).

SRB separation is normally performed automatically by the onboard GPCs; however, the flight crew can command separation through use of the SRB separation switches on panel C3. The SRB separation auto/man (manual) switch is positioned to man and the SRB sep push button depressed.

This manual function for SRB separation provides a backup for the automatic function; however, the manual function uses the same separation logic as the automatic. The automatic sequence is initiated by the software in the GPCs when the SRB chamber pressure is below 50 psi.

At SRB separation command, a three-axis attitude hold is commanded by the reaction control system for four seconds. When the SRB separation command is received, the SRB nozzles are positioned to null, and the flight control system is switched to the orbiter rate gyro assemblies. Four seconds after SRB separation, second-stage main engine guidance takes over. In addition, if a main engine shutdown is detected, the failed engine is positioned to null, and trim changes are commanded on the remaining engines.