The port and starboard doors are 60 feet long with a combined area
of approximately 1,600 square feet. Each door is made up of five
segments that are interconnected by circumferential expansion joints.
Each door hinges on 13 Inconel 718 external hinges (five shear and
eight idlers). The lower half of each hinge attaches to the midfuselage
sill longeron. The hinges rotate on bearings with dual rotational
surfaces. There are five shear hinges and eight floating hinges.
The floating hinges allow fore and aft movement of the door panels
for thermal expansion.
Each door actuation system provides the mechanism to drive each
door side to the open or closed position. Each mechanism consists
of an electromechanical power drive unit and six rotary gear actuators,
which are connected by torque tubes to each other and to the power
drive unit. Linkages transmit torque from the rotary actuators to
the doors.
The forward 30-foot sections of both doors incorporate radiators
that can be deployed; they are hinged and latched to the door inner
surface in order to reject the excess heat of the Freon-21 coolant
loops from both sides of the radiator panels when the doors are
open. An electromechanical actuation system on the door unlatches
and deploys the radiators when open and latches and stows the radiators
when closed. The radiators may be left in the stowed position for
a given flight and will only radiate the excess heat from the one
side. Fixed radiator panels are installed on the forward end of
the aft payload bay doors and radiate from one side only. Kitted
fixed radiator panels may be installed on the aft end of the aft
payload bay doors when required by a specific mission; they also
will radiate from only one side.
During payload bay door closure, the aft flight deck payload bay
door crewman optical alignment sight is used to check door alignment.
When the payload bay doors are closed, they are fixed at the aft
fuselage bulkhead and allowed to move longitudinally at the forward
fuselage. The doors also accommodate vehicle torsional loads (a
force that causes a body, such as a shaft, to twist about its longitudinal
axis), aerodynamic pressure loads and payload bay vent lag pressures.
The payload bay is not a pressurized area.
Thermal and pressure seals are used to close the gaps at the forward
and aft fuselage interface, door centerline and circumferential
expansion joints.
The doors are 60 feet long. Each consists of five segments interconnected
by expansion joints. The chord of each half of these curved doors
is approximately 10 feet, and the doors are 15 feet in diameter.
The doors are constructed of graphite epoxy composite material,
which reduces the weight by 23 percent over that of aluminum honeycomb
sandwich. This is a reduction of approximately 900 pounds, which
brings the weight of the doors down to approximately 3,264 pounds.
The payload bay doors are the largest aerospace structure to be
constructed from composite material.
The composite doors will withstand 163-decibel acoustic noise and
a temperature range of minus 170 to plus 135 F.
The doors are made up of subassemblies consisting of graphite epoxy
honeycomb sandwich panels, solid graphite epoxy laminate frames,
expansion joint frames, torque box, seal depressor, centerline beam
intercostals, gussets, end fittings and clips. There are also aluminum
2024 shear pins, titanium fittings, and Inconel 718 floating and
shear hinges. The assembly is joined by mechanical fasteners. Lightning
strike protection is provided by aluminum mesh wire bonded to the
outer skin.
Extravehicular activity handholds are attached in the torque box
areas.
The payload bay doors are covered with reusable surface insulation.
The left door with attached systems weighs approximately 2,375
pounds and the right weighs about 2,535 pounds. The right door contains
the centerline latch active mechanisms, which accounts for the weight
difference. These weights do not include the radiator panel system,
which adds 833 pounds per door.
The PL bay door open/stop/close switch on panel R13 initiates the
payload bay door power and control system through the aft flight
deck data processing system, general-purpose computer and associated
cathode ray tube display and keyboard. The normal operational mode
for opening and closing the payload bay door bulkhead latches, centerline
latches and payload bay doors is through keyboard entries in an
automatic mode in which the latches are cycled and the doors controlled
in a predetermined sequence. If a problem occurs in the predetermined
automatic sequence, a manual keyboard capability permits selection
of automatic sequence groupings that can be commanded individually.
The open position of the switch on panel R13 provides the signals
to a GPC to initiate and sustain the automatic or keyboard manual
bulkhead latches and door opening sequence. The close position accomplishes
the same as the open position except for the closing sequence. The
stop position removes the open and close signals, stopping the sequence
in progress.
The PL bay door talkback indicator on panel R13 is functional only
in the automatic sequence and would remain in its initial state
in the manual keyboard mode. The signal source for the talkback
indicator is a combination of ready-to-latch and door-open limit
switch inputs that are processed by software to establish the talkback
indicator state. The talkback indicator indicates op when the bulkhead
latches, centerline latches and doors are open; cl when the doors
are closed and the centerline and bulkhead latches are closed; and
barberpole when the bulkhead and centerline latches and doors are
in transit or are stopped between open and closed.
When closed, the doors are latched to the forward and aft bulkheads
and along the upper centerline of the doors. The latching system
consists of 16 bulkhead latches (eight aft and eight forward) and
16 payload bay door centerline latches. The forward and aft bulkhead
latches are in groups of four ganged latch hooks. The centerline
latches are also in groups of four ganged latches. Each centerline
latch gang incorporates four latches, bellcranks, push rods, levers,
rollers and an electromechanical actuator.
The forward and aft bulkhead latches are arranged in groups of
four ganged latches. Each group is opened or closed by an electromechanical
actuator with two redundant, three-phase ac reversible motors that
receive ac power from mid motor controller assemblies when commanded
in the automatic predetermined sequence or by manual keyboard entries.
In the automatic mode, the forward and aft bulkhead latches operate
simultaneously.
The forward and aft bulkhead latch groups consist of two ac reversible
motors. These groups also control an actuator output arm, which
positions active latch mechanisms and disengages or engages four
latch hooks on four corresponding passive rollers on the bulkhead.
The two ac motors of a bulkhead latch group are commanded through
limit switches to open or close that group of latches. When the
ac motor is in operation, the brake associated with that motor is
released and is applied when power is removed from the motor. The
limit switches apply or remove ac power from the motor when that
latch group reaches its open or closed position. When both motors
are operating, the latch group operating time is 30 seconds; it
is 60 seconds when only one motor is operating. In addition, each
MMCA has its own timer set to twice the normal operating time to
allow for single-motor operation of a bulkhead latch group without
causing a sequence fail signal PLB doors CRT message and SM alert.
During latching operations for a bulkhead group, the payload bay
door comes in contact with a bulkhead switch module striker when
the door is nearly closed. A two-out-of-three voting logic of the
ready-to-latch switches precludes premature start signals to the
bulkhead latch motors. The ready-to-latch switch then activates
the bulkhead latch ac motors, which latches the door closed. The
door-closed limit switches turn the ac motors off. The limit switch
contact closures are sent to the CRT display under micro-sw stat
(switch status), which permits the flight crew to observe the change
in the status of the microswitches. Telemetry can also monitor the
microswitch status. Torque limiters in each bulkhead latch group
permit slippage if a limit switch fails to turn off the ac motors
or the mechanism jams during latching operations in order to prevent
damage to the motors or mechanisms. Extravehicular activity disconnects
are provided to permit an EVA flight crew member to close the door
latch manually from inside the payload bay if the mechanism jams
when the doors close.
The payload bay door centerline latch groups are controlled automatically
in a predetermined sequence or manually by individual latch groups
through keyboard entries in a manner similar to the bulkhead latch
groups. The 16 centerline latches are arranged into groups of four,
similar to the bulkhead latches.
Each centerline latch group consists of two ac reversible electric
motors that drive a rotary shaft and bellcrank and four hooks to
engage a corresponding passive roller to latch the door closed or
disengage the passive roller to unlatch the door. All 16 centerline
hook assemblies contain alignment rollers to eliminate payload bay
door overlap due to thermal distortion. Passive shear fittings in
each centerline latch group align door closure and cause the fore
and aft shear loads to react once the doors are closed.
The centerline latch group ac reversible motors are automatically
turned off by limit switches when the latches are opened or closed.
Each motor has a brake that operates similarly to the brakes in
the bulkhead motors. When both motors are operating, the nominal
operating time is 20 seconds. If only one motor is operating, the
time is 40 seconds. Each mid motor controller assembly has its own
timer set to twice the normal operating time to allow single-motor
operation of the centerline latch group without causing a sequence
fail signal PLB doors CRT message and SM alert.
Torque limiters in the centerline latch groups allow slippage if
limit switches fail to turn off an electrical drive motor or the
mechanisms jam to prevent damage to the motors or mechanism.
EVA disconnects in a centerline latch group can be used to isolate
a jammed latch from the group.
The payload bay doors are driven by a rotary actuator consisting
of two electrical three-phase reversible ac motors per power drive
unit. There is one power drive unit for right doors and one for
the left doors.
The power drive unit drives a 55-foot-long torque shaft. The shaft
turns the rotary actuators, which causes the push rod, bell crank
and link to push the doors open. The same arrangement pulls the
doors closed.
The payload bay door opening and closing sequence is controlled
automatically through in a predetermined sequence or manually through
keyboard entries. The starboard doors must be opened first and closed
last due to the arrangement of the centerline latching mechanism
and the structural and seal overlap. Limit switches on each power
drive unit turn the ac motors off when the doors are open or closed.
Each ac motor has an associated brake that operates similarly to
the bulkhead and centerline latch motors. When both motors are operating,
the nominal time for payload bay door opening or closing is 63 seconds.
If only one motor is operating, the time is 126 seconds. Each MMCA
has its own timer set to twice the normal operating time to allow
single-motor operation of the payload bay doors without causing
a sequence fail signal PLB doors CRT message and an SM alert .
Torque limiters are incorporated into the rotary actuators to avoid
damaging the drive motors or mechanisms if limit switches fail to
turn off an electrical drive motor or the mechanisms jam.
Two bolts on the bellcrank and the bolt connecting the link to
the rotary actuator can be EVA disconnect points if the linkage
fails when the doors close. The power drive unit can be disengaged
manually on the ground or on orbit.
The payload bay doors open through an angle of 175.5 degrees.
Two radiator panels on each forward payload bay door can be deployed
when the doors are opened on orbit and stowed when the doors are
closed before entry, or they can be left in the stowed position
for a given flight. Freon-21 coolant loop 1 flows through the left-hand
radiator panels, and the No. 2 loop flows through the right-hand
panels. On orbit, the panels radiate excess heat collected by the
Freon-21 coolant loops from heat exchangers and cold plates throughout
the orbiter. Coolant flows through the radiators from aft to forward.
The radiator panels mounted on the forward end of the aft payload
bay doors are fixed to the bay doors.
The radiator deploy and stow operation is controlled manually from
the aft flight deck panel R13. The PL bay mech (payload bay mechanisms)
pwr, radiator latch and radiator control sys switches control the
panels. Four indicators show the radiator latch and deploy status.
When the payload bay doors are fully open, the PL bay mech sys
1 and sys 2 switches are positioned to on . The sys 1 and sys 2
switches positioned to on provide ac bus power to both right- and
left-side radiator latch control actuators.
The radiator latch control sys A switch positioned to release applies
ac power to one ac reversible drive motor on each starboard and
port panel. When each motor is in operation, the brake is removed.
Each ac drive motor rotates a torque shaft, which operates push
rods that unlatch six latches on each of the two right and two left
radiator panels. The linkages and latches are attached to the payload
bay doors, and passive rollers are attached to the radiator panels.
The operating time for releasing the latches with one motor is approximately
52 seconds. Limit switches remove power from the ac motors. The
brake is applied for each motor. The radiator stdb (starboard) and
port talkback indicators above the latch control sys A and B switches
indicate rel when the corresponding latches are released and barberpole
when in transit. When the radiator latch control sys B switch is
positioned to release, ac power is applied to the remaining ac reversible
drive motor on each right panel and each left panel. This remaining
ac drive motor will operate the same rotating shaft and unlatch
the same six latches on each of the two right and two left radiator
panels. Separate limit switches remove power from these ac motors.
The radiator stdb and port talkback indicators above the latch control
sys A and B switches indicate rel when the corresponding latches
are released and barberpole when in transit and have the same operating
time as in system A. If both switches were positioned to rel simultaneously,
the operating time would be approximately 26 seconds.
Positioning the radiator latch control sys A and/or B switch to
latch reverses the action and latches the radiator panels to the
payload bay doors. The talkback indicators indicate lat when the
panels are latched and barberpole when in transit.
The off position of the radiator latch control sys A and/or B switch
removes power from the corresponding control system, which stops
the motors and latch system movement.
Torque limiters in the power drive system prevent damage to the
system in the event of jamming or binding during operation.
The radiator control sys A switch positioned to deploy applies
ac power to one ac reversible drive motor on the right panel and
one ac reversible drive motor on the left panel. The motors are
not operable until the MMCAs have received two signals from the
radiator panel unlatch drives, which prevents inadvertent deployment
of the radiators while still latched. When power is applied to the
left and right motors, the brake is removed and the rotary actuator
shaft rotates and pushes the respective radiator panels away from
the payload bay doors to the deployed position. Separate limit switches
turn the ac motors off and apply the brake for each motor. The operating
time for deployment with one motor is 86 seconds. The radiator stdb
and port talkback indicators above the radiator control sys A and
B switches indicate dpy when the corresponding panels are deployed
and barberpole when in transit. When the radiator control sys B
switch is positioned to deploy, ac power is applied to the remaining
ac reversible drive motor on the right radiator panel and the remaining
ac reversible drive motor on the left panel. This remaining ac drive
motor operates the same rotary actuator shaft and pushes the respective
radiator panel away from the payload bay doors to the deployed position.
Separate limit switches turn the ac motors off. The radiator stdb
and port talkback indicators above the radiator control sys A and
B switches indicate dpy when the corresponding panels are deployed
and barberpole when in transit and have the same operating time
as in system A. If both switches are positioned to deploy simultaneously,
the operating time is 43 seconds.
Positioning the radiator control sys A and/or B switch to stow
reverses the action, stowing the radiators to the payload bay doors.
The talkback indicators indicate sto when the panels are stowed
and barberpole in transit.
The off position of the radiator control sys A and/or B switch
removes power from the corresponding control system, stopping the
motors and radiator panel movement.
When the radiators are deployed, they are 35.5 degrees from the
payload bay doors.
Torque limiters on the power drive system prevent damage to the
system in the event of jamming or binding during operation.
Each rotary crank can be disengaged from the rotary actuator (via
EVA operations) by retracting a shear pin. Retraction allows the
crank to rotate around an alternate pivot and permits the crew to
stow the panels if the system fails. If the power drive unit fails,
all four shear pins must be removed to allow manual stowing of the
radiators. The pins are accessible when the radiators are fully
deployed. No disengagement is planned if the radiators fail to deploy.
The contractors are Rockwell's Tulsa Division, Tulsa, Okla. (payload
bay doors); Curtiss Wright, Caldwell, N.J. (payload bay door power
drive unit, rotary actuators, drive shafts, torque tubes and couplings,
radiator deploy/latch actuator and latch mechanism); Hoover Electric,
Los Angeles, Calif. (payload bay door electromechanical rotary actuators);
Vought Corp., Dallas, Texas (radiators); Rockwell's Space Transportation
Systems Division, Downey, Calif. (latches, linkages and actuators).