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