scope:

Description of Driving Event:

The mass SPEC is used to leak check the SSME heat exchanger (HEX) coil, to evacuate the MPS fuel vacuum jacketed (VJ) lines and for external leak checks using the hand-held probe. The heat exchanger converts liquid oxygen to gaseous oxygen for pressurizing the shuttle external oxygen tank. The MPS fuel line vacuum jackets provide the insulation necessary to meet SSME pre-start pump inlet temperatures. If mass SPEC vacuum is lost for any reason (power loss or belt breakage) vacuum roughing pump oil may be sucked into the SSME heat exchanger (back-streaming) unless the hex is isolated immediately.