In the launch configuration, the orbiter and two SRBs are attached
to the external tank in a vertical (nose-up) position on the launch
pad. Each SRB is attached at its aft skirt to the mobile launcher
platform by four bolts.
Emergency exit for the flight crew on the launch pad up to 30 seconds
before liftoff is by slidewire. There are seven 1,200-foot-long
slidewires, each with one basket. Each basket is designed to carry
three persons. The baskets, 5 feet in diameter and 42 inches deep,
are suspended beneath the slide mechanism by four cables. The slidewires
carry the baskets to ground level. Upon departing the basket at
ground level, the flight crew progresses to a bunker that is designed
to protect it from an explosion on the launch pad.
At launch, the three Space Shuttle main engines - fed liquid hydrogen
fuel and liquid oxygen oxidizer from the external tank - are ignited
first. When it has been verified that the engines are operating
at the proper thrust level, a signal is sent to ignite the SRB.
At the proper thrust-to-weight ratio, initiators (small explosives)
at eight hold-down bolts on the SRB are fired to release the Space
Shuttle for liftoff. All this takes only a few seconds.
Maximum dynamic pressure is reached early in the ascent, nominally
approximately 60 seconds after liftoff. Approximately 1 minute later
(2 minutes into the ascent phase), the two SRB have consumed their
propellant and are jettisoned from the external tank. This is triggered
by a separation signal from the orbiter.
The boosters briefly continue to ascend, while small motors fire
to carry them away from the Space Shuttle. The boosters then turn
and descend, and at a predetermined altitude, parachutes are deployed
to decelerate them for a safe splashdown in the ocean. Splashdown
occurs approximately 141 nautical miles (162 statute miles) from
the launch site. The boosters are recovered and reused.
Meanwhile, the orbiter and external tank continue to ascend, using
the thrust of the three Space Shuttle main engines. Approximately
8 minutes after launch and just short of orbital velocity, the three
Space Shuttle engines are shut down (main engine cutoff), and the
external tank is jettisoned on command from the orbiter.
The forward and aft reaction control system engines provide attitude
(pitch, yaw and roll) and the translation of the orbiter away from
the external tank at separation and return to attitude hold prior
to the orbital maneuvering system thrusting maneuver.
The external tank continues on a ballistic trajectory and enters
the atmosphere, where it disintegrates. Its projected impact is
in the Indian Ocean (except for 57-degree inclinations) in the case
of equatorial orbits KSC launch) and in the extreme southern Pacific
Ocean in the case of a Vandenberg launch.
Normally, two thrusting maneuvers using the two OMS engines at
the aft end of the orbiter are used in a two-step thrusting sequence:
to complete insertion into Earth orbit and to circularize the spacecraft's
orbit. The OMS engines are also used on orbit for any major velocity
In the event of a direct-insertion mission, only one OMS thrusting
sequence is used.
The orbital altitude of a mission is dependent upon that mission.
The nominal altitude can vary between 100 to 217 nautical miles
(115 to 250 statute miles).
The forward and aft RCS thrusters (engines) provide attitude control
of the orbiter as well as any minor translation maneuvers along
a given axis on orbit.
At the completion of orbital operations, the orbiter is oriented
in a tail first attitude by the reaction control system. The two
OMS engines are commanded to slow the orbiter for deorbit.
The reaction control system turns the orbiter's nose forward for
entry. The reaction control system controls the orbiter until atmospheric
density is sufficient for the pitch and roll aerodynamic control
surfaces to become effective.
Entry interface is considered to occur at 400,000 feet altitude
approximately 4,400 nautical miles (5,063 statute miles) from the
landing site and at approximately 25,000 feet per second velocity.
At 400,000 feet altitude, the orbiter is maneuvered to zero degrees
roll and yaw (wings level) and at a predetermined angle of attack
for entry. The angle of attack is 40 degrees. The flight control
system issues the commands to roll, pitch and yaw reaction control
system jets for rate damping.
The forward RCS engines are inhibited prior to entry interface,
and the aft reaction control system engines maneuver the spacecraft
until a dynamic pressure of 10 pounds per square foot is sensed,
which is when the orbiter's ailerons become effective. The aft RCS
roll engines are then deactivated. At a dynamic pressure of 20 pounds
per square foot, the orbiter's elevators become active, and the
aft RCS pitch engines are deactivated. The orbiter's speed brake
is used below Mach 10 to induce a more positive downward elevator
trim deflection. At approximately Mach 3.5, the rudder becomes activated,
and the aft reaction control system yaw engines are deactivated
at 45,000 feet.
Entry guidance must dissipate the tremendous amount of energy the
orbiter possesses when it enters the Earth's atmosphere to assure
that the orbiter does not either burn up (entry angle too steep)
or skip out of the atmosphere (entry angle too shallow) and that
the orbiter is properly positioned to reach the desired touchdown
During entry, energy is dissipated by the atmospheric drag on the
orbiter's surface. Higher atmospheric drag levels enable faster
energy dissipation with a steeper trajectory. Normally, the angle
of attack and roll angle enable the atmospheric drag of any flight
vehicle to be controlled. However, for the orbiter, angle of attack
was rejected because it creates surface temperatures above the design
specification. The angle of attack scheduled during entry is loaded
into the orbiter computers as a function of relative velocity, leaving
roll angle for energy control. Increasing the roll angle decreases
the vertical component of lift, causing a higher sink rate and energy
dissipation rate. Increasing the roll rate does raise the surface
temperature of the orbiter, but not nearly as drastically as an
equal angle of attack command.
If the orbiter is low on energy (current range-to-go much greater
than nominal at current velocity), entry guidance will command lower
than nominal drag levels. If the orbiter has too much energy (current
range-to-go much less than nominal at the current velocity), entry
guidance will command higher-than-nominal drag levels to dissipate
the extra energy.
Roll angle is used to control cross range. Azimuth error is the
angle between the plane containing the orbiter's position vector
and the heading alignment cylinder tangency point and the plane
containing the orbiter's position vector and velocity vector. When
the azimuth error exceeds a computer-loaded number, the orbiter's
roll angle is reversed.
Thus, descent rate and down ranging are controlled by bank angle.
The steeper the bank angle, the greater the descent rate and the
greater the drag. Conversely, the minimum drag attitude is wings
level. Cross range is controlled by bank reversals.
The entry thermal control phase is designed to keep the backface
temperatures within the design limits. A constant heating rate is
established until below 19,000 feet per second.
The equilibrium glide phase shifts the orbiter from the rapidly
increasing drag levels of the temperature control phase to the constant
drag level of the constant drag phase. The equilibrium glide flight
is defined as flight in which the flight path angle, the angle between
the local horizontal and the local velocity vector, remains constant.
Equilibrium glide flight provides the maximum downrange capability.
It lasts until the drag acceleration reaches 33 feet per second
The constant drag phase begins at that point. The angle of attack
is initially 40 degrees, but it begins to ramp down in this phase
to approximately 36 degrees by the end of this phase.
In the transition phase, the angle of attack continues to ramp
down, reaching the approximately 14-degree angle of attack at the
entry Terminal Area Energy Management (TAEM) interface, at approximately
83,000 feet altitude, 2,500 feet per second, Mach 2.5 and 52 nautical
miles (59 statute miles) from the landing runway. Control is then
transferred to TAEM guidance.
During the entry phases described, the orbiter's roll commands
keep the orbiter on the drag profile and control cross range.
TAEM guidance steers the orbiter to the nearest of two heading
alignment cylinders, whose radii are approximately 18,000 feet and
which are located tangent to and on either side of the runway centerline
on the approach end. In TAEM guidance, excess energy is dissipated
with an S-turn; and the speed brake can be used to modify drag,
lift-to-drag ratio and flight path angle in high-energy conditions.
This increases the ground track range as the orbiter turns away
from the nearest Heading Alignment Circle (HAC) until sufficient
energy is dissipated to allow a normal approach and landing guidance
phase capture, which begins at 10,000 feet altitude. The orbiter
also can be flown near the velocity for maximum lift over drag or
wings level for the range stretch case. The spacecraft slows to
subsonic velocity at approximately 49,000 feet altitude, about 22
nautical miles (25.3 statute miles) from the landing site.
At TAEM acquisition, the orbiter is turned until it is aimed at
a point tangent to the nearest HAC and continues until it reaches
way point 1. At WP-1, the TAEM heading alignment phase begins. The
HAC is followed until landing runway alignment, plus or minus 20
degrees, has been achieved. In the TAEM pre-final phase, the orbiter
leaves the HAC; pitches down to acquire the steep glide slope, increases
airspeed; banks to acquire the runway centerline and continues until
on the runway centerline, on the outer glide slope and on airspeed.
The approach and landing guidance phase begins with the completion
of the TAEM pre-final phase and ends when the spacecraft comes to
a complete stop on the runway.
The approach and landing trajectory capture phase begins at the
TAEM interface and continues to guidance lock-on to the steep outer
glide slope. The approach and landing phase begins at about 10,000
feet altitude at an equivalent airspeed of 290, plus or minus 12,
knots 6.9 nautical miles (7.9 statute miles) from touchdown. Autoland
guidance is initiated at this point to guide the orbiter to the
minus 19- to 17-degree glide slope (which is over seven times that
of a commercial airliner's approach) aimed at a target 0.86 nautical
mile (1 statute mile) in front of the runway. The spacecraft's speed
brake is positioned to hold the proper velocity. The descent rate
in the later portion of TAEM and approach and landing is greater
than 10,000 feet per minute (a rate of descent approximately 20
times higher than a commercial airliner's standard 3-degree instrument
At 1,750 feet above ground level, a pre-flare maneuver is started
to position the spacecraft for a 1.5-degree glide slope in preparation
for landing with the speed brake positioned as required. The flight
crew deploys the landing gear at this point.
The final phase reduces the sink rate of the spacecraft to less
than 9 feet per second. Touchdown occurs approximately 2,500 feet
past the runway threshold at a speed of 184 to 196 knots (213 to