After a series of meetings and reviews of procedures, flight controllers, engineers and managers concluded that the SpaceX-3 mission could be conducted as planned without violating any launch commit criteria despite the loss Friday of a backup computer command relay box called a multiplexer/demultiplexer (MDM) that resides in the station’s S0 truss. The problem with the box, which measures 10.5 x 14.9 x 16.4 inches and weighs 50.8 pounds, occurred during a routine health check of the device. The prime multiplexer continues to operate normally. This pair of MDMs provide commanding to the station’s external cooling system, Solar Alpha Rotary joints, Mobile Transporter rail car and insight into other truss systems.
The engineering teams reported to mission managers that the station possesses enough redundancy to allow the SpaceX mission to launch Monday at 4:58 p.m. EDT from Launch Complex 40 at the Cape Canaveral Air Force Station, Fla. In advance of launch, the station’s Mobile Transporter will be moved to the proper position on the truss later today and after Dragon’s launch, the station’s solar arrays will be oriented Monday to the correct angles for the scheduled capture of the U.S. cargo craft on Wednesday and its berthing to the Earth-facing port on the Harmony module. These are steps that would properly configure the station for the mission even if the prime MDM experiences a problem.
Additionally, preparations are underway for a contingency spacewalk by two of the Expedition 39 crewmembers no earlier than around April 22 to replace the failed MDM with a spare housed inside the station. Mission managers approved a plan for the preparation of two of the U.S. spacesuits on the station and the replacement of a fan pump separator on one of the suits prior to the spacewalk. That work will begin immediately. The move of the Mobile Transporter rail car to another worksite from its current location will clear the area on the S0 truss for the spacewalking astronauts to gain access to the failed MDM during the planned excursion. The station crew will also be readying the spare MDM for its staging in the Quest airlock prior to the spacewalk.