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.
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.
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
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.
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.
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.
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.
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
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).
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.
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.