External Active Thermal Control System

External Active Thermal Control System (EATCS) ^[1]

The External Active Thermal Control System (ATCS) is required when the environment or heat loads exceed the capabilities of the Passive Thermal Control System (PTCS) such as external surface materials, insulation or heat pipes. The ATCS uses a mechanically pumped fluid in closed-loop circuits to perform three functions: heat collection, heat transportation, and heat rejection.

Heat Rejection System Radiator (HRSR). The heat rejection subsystem (HRS) consists of a base, eight panels, torque panel, torque arm, an interconnected fluid system, a scissors-type deployment mechanism and a computer controlled motor/cable deployment system. Part of the station’s external active thermal control system (EATCS), the HRS radiator rejects thermal energy via radiation.
Plasma Contactor Unit (PCU). The Plasma Contactor Unit (PCU) is used to disperse the electrical charge that builds up by providing an electrically conductive “ground path” to the plasma environment surrounding the ISS. This prevents the electrical discharges and provides a means of controlling crew shock hazard during EVA. There are two PCUs located on the ISS Zenith 1 Truss, both of which are operated during EVA.
Pump Flow Control Subassembly (PFCS). Each external loop contains a Pump & Flow Control System (PFCS) which contains most of the controls and mechanical systems that drive the EEATCS. There are 2 pumps per PFCS which circulate ammonia throughout the external coolant loops
Utility Transfer Assembly (UTA). The UTA is a processor that allows power, signals and data to flow across the SARJ by roll rings incorporated within.
Flex Hose Rotary Coupler (FHRC). The FHRC provides the transfer of liquid ammonia across the Thermal Radiator Rotary Joint (TRRJ) between the P1 & S1 truss segments and the Heat Rejection System Radiators (HRSRs).
Pump Module (PM). Truss. The PM is part of the station’s complex External Active Thermal Control System (ETCS), which provides vital cooling to internal and external avionics, crew members, and payloads. The station has two independent cooling loops. The external loops use an ammonia-based coolant and the internal loops use water cooling.
Ammonia Tank Assembly (ATA). The primary function of the ATA is to store the ammonia used by the External Thermal Control System (ETCS). The major components in the ATA include two ammonia storage tanks, isolation valves, heaters, and various temperature, pressure, and quantity sensors. There is one ATA per loop located on the zenith side of the Starboard 1 (Loop A) and Port 1 (Loop B) truss segments.
Nitrogen Tank Assembly (NTA). The NTA provides a high-pressure gaseous nitrogen supply to control the flow of ammonia out of the ATA. The ATA contains two flexible, chambers incorporated into its ammonia tanks that expand as pressurized nitrogen expels liquid ammonia out of them.
Direct Current Switching Unit (DCSU). The DCSU routes battery power to the MBSU distribution bus to satisfy power demands. In addition to primary power distribution, the DCSU has the additional responsibilities of routing secondary power to components on the PV modules.
Battery Charge/Discharge Unit (BCDU). The Battery Charge Discharge Unit (BCDU) is a bidirectional power converter that serves a dual function of charging the batteries during solar collection periods (isolation) and providing conditioned battery power to the primary power buses during eclipse periods.
Main Bus Switching Unit (MBSU). The MBSUs act as the distribution hub for the EPS system. The four MBSUs onboard the ISS are all located on the Starboard Zero (S0) truss. Each of MBSU receives primary power from two power channels and distributes it downstream to the DDCUs.