Non-deployable payloads are retained by passive retention devices, and deployable payloads are secured by motor-driven, active retention devices. Payloads are secured in the orbiter payload bay with the payload retention system or are equipped with their own unique retention systems. The orbiter payload retention system provides three-axis support for up to five payloads per flight. The payload retention mechanisms secure the payloads during all mission phases and allow installation and removal of the payloads when the orbiter is either horizontal or vertical.
Attachment points in the payload bay are in 3.933-inch increments along the left- and right-side longerons and along the bottom centerline of the bay. Of the potential 172 attach points on the longerons, 48 are unavailable because of the proximity of spacecraft hardware. The remaining 124 may be used for deployable payloads. Along the centerline keel, 89 attach points are available, 75 of which may be used for deployable payloads. There are 13 longeron bridges per side and 12 keel bridges available per flight. Only the bridges required for a particular flight are flown. The bridges are not interchangeable because of main frame spacing, varying load capability and subframe attachments.
The longeron bridge fittings are attached to the payload bay frame at the longeron level and at the side of the bay. Keel bridge fittings are attached to the payload bay frame at the bottom of the payload bay.
The payload trunnions are the portion of the payload that interface with the orbiter retention system. The trunnions that interface with the longeron are 3.25 inches in diameter and 7 or 8.75 inches long, depending on their position in the payload bay. The keel trunnions are 3 inches in diameter and vary in length from 4 to 11.5 inches, depending on where they fit in the payload bay.
The orbiter and payload attachments are the trunnion/bearing/journal type. The longeron and keel attach fittings have a split, self-aligning bearing for non-release-type payloads in which the hinged half is bolted closed. For on-orbit deployment and retrieval payloads, the hinged half fitting releases or secures the payload by latches that are driven by dual-redundant electric motors.
Payload guides and scuff plates assist in deploying and berthing payloads in the payload bay. The payload is constrained in the X direction by guides and in the Y direction by scuff plates and guides. The guides are mounted to the inboard side of the payload latches and interface with the payload trunnions and scuff plates. The scuff plates are attached to the payload trunnions and interface with the payload guides.
The guides are V-shaped with one part of the V being 2 inches taller than the other part. Parts are available to make either the forward or aft guide taller.
This difference enables the operator monitoring the berthing or deployment operations through the aft bulkhead CCTV cameras to better determine when the payload trunnion has entered the guide. The top of the taller portion of the guide is 24 inches above the centerline of the payload trunnion when it is all the way down in the guide. The top of the guide has a 9-inch opening. These guides are mounted to the 8-inch guides that are a part of the longeron payload retention latches.
A set of payload active retention latches may consist of as many as five latches per payload. Three payloads can be accommodated with active latches. Each of the active latches is controlled by dual-redundant ac electric motors that release or latch the active retention latch. The active retention latches are controlled from panel A6U.
When the payload retention logic power system 1 switch on panel A6U is positioned to on, it provides main bus power to the rotary payload select switch on panel A6U. The system 2 switch, when positioned to on, provides MNB bus power to the rotary payload select switch for payloads 1, 2 and 3.
Positioning the payload select rotary switch on panel A6U to 1 provides power-on logic for the dual actuator motors of up to five latches for one payload and the talkback indications associated with up to five latches for the payload. Position 2 of the payload select switch provides power-on logic for the dual actuator motors of up to five latches for the second payload and the talkback indications associated with up to five latches for the payload. Position 3 provides power-on logic for the dual actuator motors of up to five latches for the third payload and the talkback indications associated with up to five latches.
The payload retention latches 1, 2, 3, 4 and 5 switches on panel A6U are enabled by the payload select rotary switch. Positioning the payload select switch to 1 enables up to five retention latches for payload 1, and each of the five retention latches for payload 1 would be controlled by the individual 1, 2, 3, 4 and 5 release, off, latch switches. Positioning the payload select switch to 2 or 3 has the same effect for payloads 2 and 3.
Positioning a payload retention latches switch to release provides ac power to the dual electric motors associated with the retention latch of the selected payload, driving the retention latch open. The operating time of the latch with both motors operating is 30 seconds; with only one motor operating it is 60 seconds. The talkback indicator immediately above a retention latches switch indicates rel when the latch is fully open. There are two microswitches for the rel talkback indication; however, only one is required to control the talkback indicator. The payload retention latches ready for latch talkback indicator for a retention latches switch is barberpole when the payload latch is set in the release position. There are two microswitches for the ready-for-latch talkback indication; however, only one is required to control the talkback indicator.
Positioning a payload retention latches switch to latch provides ac power to the dual electric motor associated with the latch of the payload selected, driving the retention latch closed. The operating time of one or both motors is the same as for releasing a payload. A barberpole talkback indicator immediately above each retention latches switch indicates that latch is ready to latch. The indicator shows lat when the latch is closed. There are two microswitches for the lat indication; however, only one is required to control the talkback indicator. The payload retention latches ready for latch talkback indicator for a retention latches switch is gray when the payload latch is ready to latch.
Positioning the payload select rotary switch to monitor inhibits the logic circuits of all payload actuator latch sets and inhibits the talkback indicators but provides power for payload latch telemetry.
The keel active latch centers the payload in the yaw direction in the payload bay; therefore, the keel latch must be closed before the longeron latches are closed. The keel latch can float plus or minus 2.75 inches in the X direction.