The ECLSS consists of an air revitalization system, water coolant loop systems, atmosphere revitalization pressure control system, active thermal control system, supply water and waste water system, waste collection system and airlock support system. These systems interact to provide a habitable environment for the flight crew in the crew compartment in addition to cooling or heating various orbiter systems or components.

The ARS controls relative humidity between 30 and 75 percent, maintains carbon dioxide and carbon monoxide at non-toxic levels, controls temperature and ventilation in the crew compartment, and provides cooling to various flight deck and middeck electronic avionics and the crew compartment. The ARS consists of water coolant loops, cabin air loops and pressure control. Cabin air is ducted to the crew compartment cabin heat exchanger, where the cabin air is cooled by the WCLs; therefore, cabin air cools the crew cabin, flight crew and crew compartment electronic avionics. The water coolant loop system collects heat from the crew compartment cabin heat exchanger and heat from some of the electronic units in the crew compartment and transfers it to the water coolant/Freon-21 coolant loop heat exchanger of the ATCS.

The ATCS provides orbiter heat rejection during all phases of the mission. It consists of two Freon-21 coolant loops, cold plate networks for cooling electronic avionics units, liquid/liquid heat exchangers for cooling various orbiter systems, and four heat sink systems for rejecting excess heat outside the orbiter-ground support equipment heat exchanger, flash evaporators, radiator panels and ammonia boilers. The Freon-21 coolant loops transport excess heat from the fuel cell power plant heat exchangers, payload heat exchangers and midbody and aft avionics electronic units; heat the hydraulic systems; and deliver that heat to the heat sinks. During checkout, prelaunch and postlanding ground operations, the GSE heat exchanger in the orbiter's Freon-21 coolant loops rejects excess heat from the orbiter through ground systems cooling. Approximately 125 seconds after lift-off, the flash evaporator system is activated and provides orbiter heat rejection of the Freon-21 coolant loops via water boiling. When the orbiter is on orbit and the payload bay doors are opened, radiator panels on the underside of the doors are exposed to space and provide heat rejection. If combinations of heat loads and orbiter attitude exceed the capacity of the radiator panels during on-orbit operations, the flash evaporator can be activated to meet the heat rejection requirements. At the conclusion of orbital operations, the payload bay doors are closed, rendering the radiator panels inoperative for heat rejection; and the flash evaporator is again brought into operation through deorbit and entry until atmospheric pressure buildup no longer permits the boiling water to provide adequate cooling at approximately 100,000 feet altitude. At this point the ammonia boilers reject heat from the Freon-21 coolant loops by evaporating ammonia through the remainder of entry, landing and postlanding until ground cooling is connected to the GSE heat exchanger.

The ARPCS controls crew compartment cabin pressure at 14.7 psia, plus or minus 0.2 psia, with an average of 80-percent nitrogen and 20-percent oxygen mixture. Oxygen partial pressure is maintained between 2.95 psia and 3.45 psia, with sufficient nitrogen pressure of 11.5 psia added to achieve the cabin total pressure of 14.7 psia, plus or minus 0.2 psia. The pressurization control system receives oxygen from two power reactant storage and distribution cryogenic oxygen systems in the midfuselage of the orbiter. Gaseous nitrogen is supplied from two nitrogen systems consisting of two nitrogen tanks for each system located in the midfuselage of the orbiter. An optional mission kit consists of an emergency gaseous oxygen tank, and the system can be located in the midfuselage of the orbiter. The gaseous nitrogen system is also used to pressurize the potable and waste water tanks located below the crew compartment middeck floor.

Potable water produced by the three fuel cell power plants is directed and stored in potable water tanks for flight crew consumption and personal hygiene. The potable water system is the supply to the flash evaporator system when it is used to cool the Freon-21 coolant loops. A waste water tank is also located below the crew compartment middeck floor to collect waste water from the crew cabin heat exchanger and flight crew waste water. Solid waste remains in the waste management system in the crew compartment middeck until the orbiter is serviced during ground turnaround operations.

The orbiter crew compartment provides a life-sustaining environment for a flight crew of eight. The crew cabin volume with the airlock inside the middeck is 2,325 cubic feet. For extravehicular activity requirements, only the airlock is depressurized and repressurized. If the airlock is located outside of the middeck in the payload bay, the crew cabin volume would be 2,625 cubic feet.



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