OMS-RCS Interconnect

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.