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4:11 p.m. - The recovery operations convoy has arrived. When the vehicle is considered safe from all potential hazards and free of toxic gases, the purge and coolant umbilical access vehicle moves into position at the rear of the orbiter.
The first staging position of the convoy after Discovery lands is 1,250 feet from the orbiter. Safety assessment teams dressed in protective attire and breathing apparatus use detectors to obtain vapor level readings around the orbiter. They test for possible explosive or toxic gases such as hydrogen, hydrazine, monomethylhydrazine, nitrogen tetroxide or ammonia. Once the forward and aft safety assessment teams successfully
complete their toxic vapor readings around the orbiter, purge and coolant umbilical access vehicles are moved into position behind the orbiter to gain access to the umbilical areas. Checks for toxic or hazardous gases are completed in the areas of the aft fuselage. If no hydrogen gases are present, convoy operations continue. If hydrogen gases are detected, the crew is evacuated immediately, convoy personnel are cleared from the
area and an emergency power-down of the orbiter is conducted. After carrier plates for the hydrogen and oxygen umbilicals are installed, coolant and purge air begin flowing through the umbilical lines. Purge air provides cool and humidified air conditioning to the payload bay and other cavities to remove any residual explosive or toxic fumes that may be present. The purge of the vehicle normally occurs within 45 to 60 minutes after an orbiter comes to a full stop. Transfer of the air conditioning function to ground services occurs at about the same time, allowing onboard cooling to be shut down.
The Orbiter Recovery Convoy consists of a number of specially-designed vehicles and a team of specialists who safe and service the orbiter and assist in crew egress. Included in the convoy are ll special vehicles and units. A brief description of these follows.
Scape Trailer . Self-Contained Atmospheric Protection Ensemble (SCAPE), vehicle, parked at a midfield location during landing, contains the equipment necessary to support recovery including recovery crew SCAPE suits, liquid air packs, and a crew who assist recovery personnel in suiting-up in protective clothing.
Vapor Dispersal Unit. The Vapor Dispersal Unit is a mobile wind-making machine able to produce a directed wind stream of up to 45 mph. It is an adaptation of a standard 14-ft. agricultural wind machine designed to protect fragile agricultural crops from frost damage or freezing. It is used by the recovery team to blow away toxic or explosive gases that may occur in or around the orbiter after landing. The fan can move 200,000 square feet of air a minute.
Coolant Umbilical Access. This apparatus is a stair and platform unit mounted on a truck bed which permits access to the aft port side of the orbiter where ground support crews attach coolant lines from the Orbiter Coolant Transporter.