An interconnect between the OMS crossfeed line and the aft RCS
manifolds provides the capability to operate the aft RCS using
1,000 pounds per pod of OMS propellant for orbital maneuvers.
The aft RCS may use OMS propellant from either OMS pod in orbit.
The orbital interconnect sequence is available during orbit operations
and on-orbit checkout.
The flight crew must first configure the following switches (using
a feed from the left OMS as an example): (1) posi tion the aft
left RCS tank isolation 1/2, 3/4/5A and 3/4/5B and aft right RCS
tank isolation 1/2, 3/4/5A and 3/4/5B switches on panel O7 to
close; (2) check that the talkback indicator above these switches
indicates cl, and position the aft left RCS crossfeed 1/2, 3/4/5
and aft right RCS crossfeed 1/2, 3/4/5 switches to open; (3) check
that the indicators show op and open the left OMS tank isolation
A and B valves (panel O8) and verify the talkback indicators show
op ; (4) open the left OMS crossfeed A and B valves and verify
the indicators show op ; (5) close the right OMS crossfeed A and
B valves and verify the indicators show cl; and (6) position the
left OMS He press/vapor isol valve A switch in the GPC position.
The left OMS-to-aft-RCS interconnect sequence can then be initiated
by item entry on the RCS SPEC display.
The left OMS helium pressure vapor isolation valve A will be
commanded open when the left OMS tank (ullage) pressure decays
to 236 psig, and the open commands will be terminated 30 seconds
later. If the left OMS tank (ullage) pressure remains below 236
psia, the sequence will set an OMS/RCS valve miscompare flag and
will set a Class 3 alarm and a CRT fault message. The sequence
also will enable the OMS-to-RCS gauging sequence at the same time.
The flight crew can terminate the sequence and inhibit the OMS-to-RCS
gauging sequence by use of the OMS press ena-off item entry on
the RCS SPEC display. The valves can then be reconfigured to their
normal position on panels O7 and O8. The OMS-to-aft-RCS interconnect
sequence is not available in the backup flight control system.