Overview
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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