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).