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Active Thermal Control System (ATCS) Overview 

EEATCS (5A-12A.1)/EATCS (12A.1+) 

Active Thermal Control System Architecture 

Most of the Station's many systems produce waste heat, which needs to be transferred from the 

ISS to space to achieve thermal control and maintain components at acceptable temperatures. 

An Active Thermal Control System (ATCS) is required to achieve this heat rejection function 

when the combination of the ISS external environment and the generated heat loads exceeds the 

capabilities of the Passive Thermal Control System to maintain temperatures. An ATCS uses a 

mechanically pumped fluid in closed-loop circuits to perform three functions: heat collection, 

heat transportation, and heat rejection. Waste heat is removed in two ways, through cold plates 

and heat exchangers, both of which are cooled by a circulating ammonia loops on the outside of 

the station. The heated ammonia circulates through large radiators located on the exterior of the 

Space Station, releasing the heat by radiation to space that cools the ammonia as it flows through 

the radiators. . 

The ATCS consists of the Internal Active Thermal Control System (IATCS), External Active 

Thermal Control System (EATCS), the Photovoltaic Thermal Control System (PVTCS) and the 

Early External Active Thermal Control System (EEATCS). The IATCS consists of loops that 

circulate water through the interior of the U.S. Destiny Laboratory module to collect the excess 

heat from electronic and experiment equipment and distributes this heat to the Interface Heat 

Exchangers for transfer to the EATCS. At assembly complete, there will be nine separate ITCS 

water loops in the U.S. and International Partner pressurized modules. 

The Photovoltaic Thermal Control System (PVTCS) consists of ammonia loops that collect 

excess heat from the Electrical Power System (EPS) components in the Integrated Equipment 

Assembly (IEA) on P4 and eventually S4 and transport this heat to the PV radiators (located on 

P4, P6, S4 and S6) where it is rejected to space. The PVTCS consist of ammonia coolant, eleven 

coldplates, two Pump Flow Control Subassemblies (PFCS) and one Photovoltaic Radiator 

(PVR). 

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The External Active Thermal Control System (EATCS), activated for the first time on this 

mission, consists of ammonia loops to collect heat from the Interface Heat Exchangers and 

external electronic equipment mounted on coldplates and transports it to the S1 and P1 radiators 

where it is rejected to space. In lieu of using the EATCS initially, the station hardware has been 

cooled by the Early External Active Thermal Control System (EEATCS). The EEATCS has 

provided heat rejection capability rejection capability for the U.S. Laboratory Interface Heat 

Exchangers(IFHX) since assembly flight 5A through 12a.1. 

The EEATCS is the temporary system used to collect, transport, and reject waste heat from 

habitable volumes on the International Space Station (ISS). The EEATCS collects heat from the 

Interface Heat Exchangers (IFHX) located on the U.S. Laboratory module, circulates the 

working fluid, anhydrous ammonia, via the Pump and Flow Control Subassembly (PFCS), and 

rejects heat to space via two orthogonally oriented stationary radiators. 

Internal Active Thermal Control System (IATCS) 

The purpose of the U.S. Destiny Laboratory ITCS is to maintain equipment within an allowable 

temperature range by collecting, transporting, and rejecting waste heat. The ITCS uses water 

because it is an efficient thermal transport fluid and is safe inside a habitable module. The 

IATCS is a closed loop system that provides a constant coolant supply to equipment, payloads 

and avionics to maintain proper temperature. 

The U.S. Laboratory contains two independent loops, a Low Temperature Loop (LTL) and a 

Moderate Temperature Loop (MTL). This approach allows for segregation of the heat loads, 

simplifies heat load management, and provides redundancy in case of equipment failure. The 

LTL is designed to operate at 40° F (4° C) and service systems equipment requiring low 

temperatures, such as the Environmental Control and Life Support System (ECLSS) Common 

Cabin Air Assembly (CCAA) and some payload experiments. The LTL contains approximately 

16.64 gallons (63 liters) of fluid. The MTL nominally operates at 63° F (17° C) and provides 

most of the cooling for systems equipment (i.e. avionics) and payload experiments. The MTL 

contains approximately 52.83 gallons (200 liters) of fluid. The IATCS loops can be configured 

and operated as a single loop. This capability is used for a variety of purposes, including the 

reduction of wear on the pumps, reduction of pump power usage, or to compensate for a pump 

failure. 

Photovoltaic Thermal Control System (PVTCS) 

The PVTCS consist of ammonia coolant, eleven coldplates, two Pump Flow Control 

Subassemblies (PFCS) and one Photovoltaic Radiator (PVR). The coldplate subassemblies are 

an integral part of IEA structural framework. Heat is transferred from the IEA orbital 

replacement unit (ORU) electronic boxes to the coldplates via fine interweaving fins located on 

both the coldplate and the electronic boxes. The fins add lateral structural stiffness to the 

coldplates in addition to increasing the available heat transfer area. The PFCS is the heart of the 

thermal system. It consists of all the pumping capacity, valves and controls required to pump the 

heat transfer fluid to the heat exchanges and radiator, and regulate the temperature of the thermal 

control system ammonia coolant. The PVTCS can dissipate 6,000 Watts of heat per orbit on 

average and is commanded by the IEA computer. Each PFCS consumes 275 Watts during 
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normal operations and measures approximately 40 inches (101.6 cm) by 29 inches (73.7 cm) by 

19 inches (48.3 cm), weighing 235 pounds (106.7 kilograms). 

The PVR – the radiator – is deployable on orbit and comprised of two separate flow paths 

through seven panels. Each flow path is independent and is connected to one of the two PFCSs 

on the IEA. In total, the PVR can reject up to 14 kW of heat into deep space. The PVR weighs 

1,633 pounds (740.7 kilograms) and when deployed measures 10.24 feet (3.12 meters) by 44.62 

feet (13.6 meters). When the ISS assembly is complete, there will be a total of four PVRs, one 

for each PV module (S4, P4, P6, S6). 

Early External Active Thermal Control System (EEATCS) 

Function 

Since the U.S. Laboratory became operational before the permanent External Active Thermal 

Control System (EATCS) was assembled, a temporary external cooling system was needed. 

External cooling from the Russian segment is not possible because there are no operational 

interfaces between the U.S. On-orbit Segment (USOS) and the Russian On-orbit Segment (ROS) 

thermal systems. Instead, a modified version of the Photovoltaic Thermal Control System 

(PVTCS) called the Early External Active Thermal Control System (EEATCS) acts as a 

temporary thermal system. The EEATCS consists of two independent, simultaneously operating 

ammonia cooling loops (ACL). These loops transport heat loads from the Interface Heat 

Exchanger (IFHX) located on the Laboratory module's aft endcone to the radiators located on 

truss segment P6. The EEATCS is needed until the permanent EATCS is activated. Once the 

permanent EATCS becomes operational on mission, the EEATCS will be deactivated. After 

deactivation, portions of the EEATCS will be used as spare components for the PVTCS loops. 

Hardware 

Pump & Flow Control System (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 and a Flow Control Valve (FCV) which 

mixes cold radiator flow and warm IFHX return flow to regulate the temperature of the ammonia 

in the loop. The PFCS also contains the primary ammonia accumulator, which provides limited 

ammonia leakage makeup, protection against thermal expansion of the ammonia, and a net 

positive suction head greater than the minimum required to prevent pump cavitations. 

Additionally, all manner of pressure, temperature, flow, and quantity sensors used by the 

EEATCS are part of the PFCS. 

Radiators 
The EEATCS radiator ORU is a direct flow, deployable and retractable radiator system with two 

independent cooling loops. The EEATCS radiator consists of seven radiator panels, the 

deploy/retract mechanism, support structure, and the necessary plumbing. The EEATCS radiator 

has two channels (A & B) that acquire heat from the Lab Low Temperature (LT) and Moderate 

Temperature (MT) Loop Interface Heat Exchanger (IFHX) via liquid anhydrous ammonia. The 

ammonia flows from the PFCS to the associated IFHX, to the EEATCS radiator manifold tubes, 

across the radiator panels and back to the PFCS. The radiator panels reject the excess heat to 

space via two non-articulating EEATCS radiator ORUs: one AFT (Trailing) and one Starboard 

(Normal). The two radiator ORUs are located on the P6 Long Spacer Truss Segment. The 

radiator measures 10.24 feet (3.12 meters) by 44.62 feet (13.6 meters).

Interface Heat Exchanger (IFHX) 

The Interface Heat Exchanger (IFHX) units accomplish heat transfer from the IATCS water 

coolant loops to the external ammonia coolant loops. At assembly complete configuration of the 

ISS, 10 interface heat exchangers will be in operation to provide heat transfer from the IATCS 

loops of the various habitable modules to the two external ammonia coolant loops. The IFHX 

units are located on the U.S. Laboratory, Node 2, and Node 3. 

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External Active Thermal Control System (EATCS) Overview 

The EATCS provides heat rejection capabilities for all U.S. pressurized modules and the main 

power distribution electronics on S0, S1 and P1. The system uses a single-phase anhydrous 

ammonia as its working fluid for its high thermal capacity and wide range of operating 

temperatures. Ammonia has an extremely low freezing point of -107 degrees ° F (-77 °C) at 

standard atmospheric pressure. The EATCS is comprised of two independent loops labeled loop 

A on S1 (Starboard) and Loop B on P1 (Port). The independent loops were designed so that a 

failure in one would not take down the entire EATCS system. Both loops are physically 

separated to prevent orbital debris from taking out the lines and the fluid transport lines are 

buried within the truss structure. If a loop does go down, the EATCS operates at a reduced 

capacity. Each loop collects heat from up to five Interface Heat Exchangers (IFHXs) mounted 

on the Node 2, U.S. Destiny Laboratory, and Node 3 as well as externally mounted coldplates. 

Most of the cold plates and plumbing to the pressurized modules are located on the S0 center 

truss. The EATCS is designed to provide 35 kW of heat rejection per loop for a total capability 

of 70 kW. The EATCS also provides ammonia re-supply capability to the Photovoltaic Thermal 

Control Systems (PVTCS) located on P4, P6, S4 and S6. All EATCS components are located 

outside the pressurized volumes to prevent crew contact with ammonia. 

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At assembly complete, each ammonia loop will supply coolant to five Interface Heat Exchangers 

(IFHX) and five cold plates (three Direct Current-to-Direct Current Units (DDCUs) and two 

Main Bus Switch Units (MBSUs)). Two MBSU cold plates, each designed to remove 495 watts 

at 80 lbs/hr. Three DDCU cold plates are each designed to remove 694 watts at 125 lbs/hr. The 

cold plate interfaces with the component base-plate via radiant fins. IFHXs transfer thermal 
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energy from the Internal Thermal Control System’s (ITCS) water based coolant to the ETCS 

anhydrous ammonia coolant. Ammonia supply temperature is currently set at 37 °F (2.8 °C) 

The ITCS supply temperature varies as a function of the modules’ thermal load. IFHX can 

isolate and bypass the IFHX core on the ammonia side in the event a cold slug is detected at the 

pump outlet to prevent ITCS coolant from freezing. 

Key Components: 

The External Active Thermal Control System (EATCS) is the primary permanent active heat 

rejection system on ISS. It acquires, transports, and rejects excess heat from all U.S. and 

International Partner modules except the Russian modules. The EATCS contains two ammonia 

coolant loops, which cool equipment on the S0, S1, and P1 truss segments. Capable of rejecting 

up to 70kW, the EATCS provides a substantial upgrade in heat rejection capacity from the 14kW 

capability of the Early External Active Thermal Control System (EEATCS). 

Heat Acquisition Subsystem (HAS) 

The HAS consists of the Interface Heat Exchanger (IFHX) Orbital Replacement Units (ORU), 

Main Bus Switch Unit (MBSU) and DC-to-DC Converter Unit (DDCU) cold plates ORU. 

Heat Rejection Subsystem (HRS) 

The HRS consists of the radiator ORU, which is a deployable, eight-panel system that rejects 

thermal energy via radiation. The HRS also consists of the Radiator Beam Valve Module 

(RBVM) that provides radiator isolating or venting, radiator beam which carries three radiators 

and connects to the Thermal Radiator Rotary Joint (TRRJ), which rotates to the radiator beam to 

provide radiator articulation. The EATCS allows the flow of ammonia through heat rejection 

radiators that constantly rotate to optimize cooling for the station. 
Interface Heat Exchanger (IFHX) 

Interface Heat Exchanger (IFHX) provide the interface between the ITCS and the EATCS 

The Interface Heat Exchanger (IFHXs) provide the interface between a module’s internal TCS 

and the EATCS. The IFHXs transfer heat from the internal loops of the USOS modules to the 

EATCS ammonia loops. IFHXs are used to collect heat from USOS modules. There are five 

IFHXs for each EATCS loop. Some IFHXs are plumbed in series such that the cool ammonia 

flows through a module’s Low Temperature Loop (LTL) IFHX prior to flowing though another 

module’s Moderate Temperature Loop (MTL) IFHX. 

The IFHX units accomplish heat transfer from the IATCS water coolant loops to the external 

ammonia coolant loops. Each IFHX core utilizes a counterflow design with 45 alternating 

layers. IATCS water flows through 23 of the layers, while EATCS ammonia flows through the 

22 alternate layers in the opposite direction. These alternating layers of relatively warm water 

and relatively cold ammonia help to maximize the heat transfer between the two fluids via 

conduction and convection. The heat exchanger core is a simple flow through device with no 

command or telemetry capability. IFHXs are mounted on the Node 2, U.S. Laboratory, and 

Node 3. The U.S. Laboratory IFHXs have been connected to the EEATCS, until this flight, when 

the EEATCS ammonia fluid line quick-disconnect will be disconnected and reconnected to the 

EATCS. When Node 2 arrives on Assembly Flight 10A, it is equipped with six IFHXs designed 

to provide cooling for itself, the Columbus and Japanese Experiment Module. Node 3 also 

contains a set of IFHXs, which are connected to the EATCS when it arrives on Assembly Flight 

20A.

At the assembly complete configuration of the ISS, 10 interface heat exchangers will be in 

operation to provide heat transfer from the IATCS loops of the various habitable modules to the 

two external ammonia coolant loops. The IFHX units will be located on the U.S. Laboratory, 

Node 2, and Node 3. Because of the highly toxic nature of ammonia, IFHX ORUs are mounted 

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external to the pressurized modules as a safety precaution. Each IFHX measures 25 inches 

(63.50 cm) by 21 inches (53.34 cm) by 8 inches (20.32 cm) and weighs about 91 pounds (41.28 

kilograms). 

Heaters 

Each EATCS loop has electrically powered heaters wrapped around the supply and return fluid 

lines on the S0 Truss to maintain the minimum operating temperature. These heaters are used 

during low heat load conditions and are turned on and off by software in the 

Multiplexer/Demultiplexer (MDMs). These heaters can be operated in closed-loop mode 

(temperature based) or open-loop mode (time based). Numerous heaters are located on the 

EATCS plumbing on the S1 (Loop A) and P1 (Loop B) truss segments to prevent ammonia 

freezing and flexible hose damage during nonoperational periods. These heaters are 

thermostatically controlled and have no software interface.

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Cold Plate 

Each ETCS loop provides cooling 

to externally mounted coldplates 

located on the S0, S1 (Loop A), 

and P1 (Loop B) truss segment. 

These coldplates contain Electrical 

Power System (EPS) equipment 

used to convert and distribute 

power to downstream ISS loads. 

Each ammonia loop contains four 

coldplates, two attached to Direct 

Current-to-Direct Current 

Converter Units (DDCUs) and two 

attached to Main Bus Switching 

Units (MBSUs). Each MBSU 

coldplate measures 37 inches 

(93.98 cm) by 33 inches (83.8 cm) 

by 20 inches (50.8 cm) and weighs about Main Bus Switching Unit (MBSU) Coldplate 109 pounds (49.4 kilograms). 
Electrical Connectors (2)

Ammonia QDs (2)

Direct Current-to-Direct Current Converter Units (DDCU) Cold Plate 

Each coldplate ORU is connected to the EATCS ammonia loop by self-sealing quick disconnect 

(QD) couplings and contains a finned coldplate, two or three strip heaters and temperature 

sensor. The coldplates are installed such that the fins of the coldplate are positioned adjacent to 

corresponding fins on either the DDCU or the MBSU to facilitate heat transfer by radiation 

between the cooled equipment and the coldplate. Each DDCU coldplate measures 35 inches 

(88.9 cm) by 28 inches (71.12 cm) by 31 inches (78.74 cm) inches and weighs about 96 pounds 

(43.54 kilograms). 

Pump Module (PM) 

Circulation, loop pressurization, and temperature control of the ammonia is provided by the 

Pump Module (PM). Each ammonia loop contains a Pump Module Assembly (PM) ORU to 

provide flow and accumulator functions and maintains proper temperature control at the pump 

outlet. Each PM consists of a single pump, a fixed charge accumulator, a Pump & Control Valve 

Package (PCVP) containing a firmware controller, startup heaters, isolation valves, and various 

sensors for monitoring performance. The accumulator within the PM works in concert with the 

Ammonia Tank Assembly (ATA) tanks to compensate for expansion and contraction of 

9
 The Pump Module ORU circulates liquid ammonia at a constant flowrate to a network of 

coldplates and heat exchangers located on the external trusses and U.S. modules, 

respectively. 

ammonia caused by the temperature changes and keeps the ammonia in the liquid phase via a 

fixed charge of pressurized nitrogen gas on the backside of its bellows. 

The Pump Module (PM) provides fluid pumping, fluid temperature control and system pressure 

control. The PCVP provides flow control. A single pump in the PCVP provides circulation of 

the ammonia. The Flow Control Valve (FCV) located within the PCVP regulates the temperature 

of the ammonia. The FCV mixes “cool” ammonia exiting the radiators with “warm” ammonia 

that has bypassed the radiators.

Nominally, loop A will operate at 8,200 lb/hr and loop B at 8,900 lb/hr at 14,000 and 14,700 

revolutions per minute, respectively. For STS-116, initial activation with U.S. Laboratory IFHX 

where Loop A pump will run at 11,500 rpm, equivalent to 5,000 lb/hr while Loop B pump will 

run at 11,500 rpm which is equivalent to 5,200 lb/hr. 

The accumulator located in the PM provides auxiliary pressure control. The accumulator resides 

upstream of the PCVP in each PM ORU. The accumulator keeps the ammonia in the liquid phase 

by maintaining the pressure above the vapor pressure of ammonia and provides makeup 

ammonia in case of a leak. The accumulator works in conjunction with the ATA to absorb 

fluctuations in the fluid volume due to varying heat loads through the expansion and contraction 

of its internal bellows.

Nominal operating pressure for the loops is 300 psia at the pump inlet; the pressure will be 

brought up to 390 psia for start up. The maximum system design pressure is 500 psia. 

Each PM measures 69 inches (175.26 cm) by 50 inches (127 cm) by 36 inches (.91 cm) inches 

and weighs about 780 pounds (353.8 kilograms).

Low and High Pressure flow Control Monitoring 

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Failure Detection, Isolation and Recovery (FDIR) for high and low pressure conditions are 

monitored and issued by the S1/P1 Multiplexer/Demultiplexers (MDMs). For an over pressure, 

gaseous nitrogen pressure is relieved down to 360 psia when pump inlet pressure reaches 415 

psia (active control). The PVCP Inlet pressure, Radiator return pressure, and Bypass return 

pressure sensors are part of this system and two of three pressure readings are used to determine 

if an overpressure condition exists. The pump will shut down issued when the pump outlet 

pressure reaches 480 psia (active control). Various relief valves and burst disks at the IFHX, 

PM, and RBVM will relieve at approximately 70 psia (passive control) 

Low pressure (current limit set at 170 psia) is monitored by two methods to determine a low 

pressure condition (chooses higher of the two values to determine the limit). Low pressure 

conditions are monitored using the PVCP inlet pressure, radiator return pressure, and bypass 

return pressure sensors. 

Temperature Control 

The PCVP also maintains temperature set point control of the ammonia supplied to the HAS. 

The PCVP has a temperature control capability of 36 °F (2.2 °C) to 43 °F (6.1 °C) and it will be 

set at 37 °F ± 2 °F (2.8 °C). The temperature control method is by three way mixing valve that 

mixes flow from the radiators and the HRS Bypass. Heaters on the HRS Bypass leg provide an 

additional level of control. Heaters are used to provide fluid conditioning in the event the 

thermal load on the loop is not sufficient to maintain set point control and to support temporary 

transient events. Total heater power of 1.8 kW is split across two heater strips mounted on the 

HRS bypass lines (900 watts each). 

Pump outlet over temperature protection is provided by a Firmware Controller (FWC) in the 

PCVP that uses three PCVP outlet sensors to determine an over temp condition and issues zero 

pump speed. The S1/P1 MDMs use the PM outlet sensor to determine an over temp condition 

and pull power from the Solenoid Driver Output (SDO) card providing power to the PM. 

Current limit is set at 65 °F (18.33 °C). Freeze Protection in the IFHX is detected by the PCVP 

firmware which shuts down the pump (first leg). When an under temperature condition is 

detected by the S1/P1 MDMs, it will pull power from the SDO card providing power to the PM 

(second leg). Under temperature detected by the S0 1,2 MDMs pulls power from the utility rail 

(third leg, leaves many things without power). The current limit is set at 35 °F (1.67 °C). 

 
Fluid Supply 

Ammonia re-supply capability for the EATCS and the eight PVTCS located on P6, P4, S4 and 

S6 is provided by the Ammonia Tank Assembly (ATA). 

Each ammonia loop contains an ATA ORU to contain the heat transfer fluid (liquid ammonia) 

used by the EATCS loops. There is one ATA per loop located on the zenith side of the S1 (Loop 

A) and P1 (Loop B) truss segments. The ATA ORU will be used to fill the EATCS loop on 

startup, to supply makeup fluid to the system, to act as an accumulator in concert with the PM 

accumulator and provide the 

capability to vent the 

ammonia loops by way of a 

connection to an external 

non-propulsive vent. Each 

ATA primarily consists of 

two bellows ammonia tanks 

pressurized by an external 

nitrogen source, two internal 

survival heaters and two sets 

of quantity, differential 

pressure, absolute pressure 

and temperature sensors. The 

ATAs are isolatable and 

replaceable on orbit.

Multilayer Insulation (MLI) 

applied to the exterior 

surfaces of the ORU is 

provided to guard against 

excessive heat loss. The ATA 

ORU is protected against 

Micro-Meteoroid/Orbital 

Debris (MM/OD) by 

shielding on the exterior of each tank and the ORU itself. Each ATA measures 79 inches 

12
(200.66 cm) by 46 inches(116.84 cm) by 55 inches (139.7 cm) inches and weighs about 1,120 

pounds (508.02 kilograms). 

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fluid lines. 

o 

rimarily 

The ATA in combination with the Nitrogen Tank Assembly (NTA) provide fluid supply and 

primary system pressure control. A single ATA was launched on Flights 9A and 11A (ITS S1 

and P1) with approximately 640 lbm ammonia in each ATA, 320 lbm per tank. ATA provides 

necessary plumbing connection to the ammonia vent system via the vent panel. Supply to 

outboard trusses is provided through the vent panel. The ATA acts as the primary accumulator 

for the EATCS in concert with the NTA. If required, it can also be used to replenish the PVTCS 

Each ammonia loop 

contains a NTA ORU to 

provide storage for the 

high pressure nitrogen 

used for controlled 

pressurization of the 

ATA. The NTA mounts t

the S1 (Loop A) and P1 

(Loop B) truss segments 

and is connected to the 

ATA by self-sealing QDs. 

Each NTA ORU p

consists of a nitrogen 

tank, a gas pressure 

regulating valve (GPRV), 

isolation valves and 

survival heaters. The 

nitrogen tank provides a 

storage volume for the 

high-pressure gaseous nitrogen, while the GPRV provides a pressure control function as well as 

nitrogen isolation and over pressure protection of downstream components. The NTA provides 

the necessary pressure to move the ammonia out of the ATA. The single high-pressure tank 

containing nitrogen at 2,500 psia (@70 °F, ground fill) and uses the GPRV to supply continuous 

pressure up to 390 psia in one psia increments. A back-up mechanical valve limits the maximum 

nitrogen pressure to 416 psia. The GPRV provides pressure control as well as high-pressure 

nitrogen isolation and overpressure protection of downstream components. The NTA has venting 

capabilities and over pressure controls. Each NTA measures 64 inches (162.56 cm) by 36 

inches (91.44) by 30 inches (76.2 cm) inches and weighs about 460 pounds (208.65 kilograms). 

Fluid Lines and Quick Disconnects (QD): 

Fluid lines and external QDs provide the transportation path from the truss segments to the 

IFHXs. Connections between segments are made with flex hoses and QDs. There are flex hoses 

and QDs between each truss, and between the S0 truss and the various IFHXs.
14

ATCS. 

RU 

yment 

 and eight 

ls. The 

d on 

Heat Rejection System 

Heat Rejection System 

(HRS) Radiator during 

deployment testing at 

Lockheed Martin Missiles 

and Fire Control.

Heat collected by the EATCS ammonia loops is radiated to space by two sets of rotating radiator 

wings—each composed of three separate radiator ORUs. Each radiator ORU is composed of 

eight panels, squib units, squib unit firmware controller, Integrated Motor Controller Assemblies 

(IMCAs), instrumentation, and QDs. Each Radiator ORU measures 76.4 feet (23.3 meters) by 

11.2 feet (3.4 meters) and weighs 2,475 pounds (1,122.64 kilograms) 

Each ammonia loop 

contains one radiator 

wing comprised of 

three Radiator ORUs 

mounted on the 

Radiator Beam and 

six Radiator Beam 

Valve Modules 

(RBVM)and one 

Thermal Radiator 

Rotary Joint (TRRJ). 

The Radiator ORUs 

utilize anhydrous 

ammonia to reject 

heat from the E

Each Radiator O

contains a deplo

mechanism

radiator pane

deployment 

mechanism allows the Radiator ORU to be launched in a stowed configuration and deploye

orbit. Each radiator ORU can be remotely deployed and retracted. 
Each individual radiator has two 

separate coolant flow paths. Each 

flow path flows through all eight 

radiator panels. Each panel’s 

flow path has eleven flow tubes 

for a total of 22 Inconel flow 

tubes or passages (11 passages 

per flow path) per radiator panel; 

flow tubes are freeze tolerant. 

Flow tubes are connected along 

the edge of each panel by 

manifolds. Flex hoses connect 

the manifold tubes between 

panels. Each panel has a white 

(Z-93) coating which provides 

optimum thermo-optical 

properties to maximize heat 

rejection. Flow tube 

arrangement is designed to minimize ammonia freezing in the radiator. 

Shown here is 

the flow path in 

one of the panels 

of the Radiator 

15

anels. 

ach radiator path contains one Radiator Beam Valve Module (RBVM) as a part of the 

ents. 

r 

of the 

low 

There are two 

RBVMs (one 

per flow path) 

that allow or 

prevent the 

transfer of 

ammonia to 

and from the 

radiator p

E

radiator wing. Six RBVMs are mounted on the radiator beams on the S1 and P1 truss segm

Two RBVMs service each radiator ORU. Each RBVM consists of an isolation relief valve, an 

isolation valve, an Integrated Motor Controller Assembly (IMCA), QDs, and pressure and 

temperature sensors. The RBVM controls the transfer of ammonia between the Radiator 

Assembly ORU and the rest of the EATCS loop. Each RBVM contains sensors to monito

absolute pressure, temperature and valve position within the ORU. Remote control venting 

radiator fluid loop is also available through the RBVM to facilitate radiator replacement and 

prevent freezing of the ATCS coolant during contingency operations. The RBVM provides f

path isolation in the event that a panel suffers micro-meteoroid damage. Leak isolation FDIR 

functions are controlled by the S1/P1 MDMs monitoring large leaks via the STR/PTR MDMs. 

Additionally, the RBVM provides automatic pressure relief when the EATCS is over 
pressurized. Each RBVM weighs about 50 pounds (22.68 kilograms) and measures 24 inches 

(60.96 cm) by 20 inches (50.8 cm) x 5.4 inches (13.72 cm). 

 

The rotation capability for each radiator assembly is provided through a Thermal Radiator 

Rotary Joint (TRRJ). The TRRJ provides power, data, and liquid ammonia transfer to the 

rotating radiator beam while providing structural support for the radiator panels. Each TRRJ is 

composed of the following: a bearing assembly, two Rotary Joint Motor Controllers (RJMCs), 

two Drive Lock Assemblies (DLAs), a Flex Hose Rotary Coupler (FHRC), and a Power and 

Data Transfer Assembly (PDTA). The bearing assembly is the rotary interface and primary 

structural component of the TRRJ. The RJMCs provide control for the DLA system, which 

provides joint rotation and joint locking capability. The FHRC consists of four flex hoses, two 

supply and two return. The PDTA provides the data and power paths for transfer to and from the 

radiator beam. 

Thermal Radiator Rotary Joint (TRRJ) provides controlled rotation of the EATCS radiators, 

allows the transfer of power, data, and ammonia across the rotating interface, and provides the 

structural support between the S1/P1 truss segments and the associated radiator wing assembly.

TRRJ ORU provides rotation capability to the Radiator Beam to optimize the thermal 

environment of the radiators and to maximize heat rejection capability and prevent freezing in 

the radiator manifolds. Rotation angles are determined via the Radiator Goal Angle Calculation 

(RGAC) algorithm which commands the Radiator Beam to put the radiators either “edge to the 

sun” during isolation phase of the orbit or “face to the Earth” during the eclipse phase. The 

RGAC ensures the radiators stay cold enough so the heat can be rejected but warm enough so 

that the ammonia does not freeze. There is a temperature goal of –40 °F at the radiator outlet. 

The FHRC provides the transfer of liquid ammonia across the rotary joint and is capable of 

rotating 230 degrees, at ±115 degrees from its neutral position. (software command limit is 

±105°); with a variable rotation speed of 0 to 45 degrees-per-minute. Each TRRJ measures 

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approximately 5.6 feet (1.7 meters) by 4.6 feet (1.4 meters) by 4.3 feet (1.3 meters) and weighs 

927 pounds (420.5 kilograms), 

System Performance Overview 

Loop A and B operate at slightly different flow rates mainly due to differing system hydraulic 

resistance layout. . Actual heat rejection will need to be planned and coordinated between all 

modules so as not to exceed the EATCS total heat rejection capability of 70 kW (at assembly 

complete), including S1/P1/S0 mounted electrical equipment. 

Software 

Thermal Control System (TCS) software is used to control and monitor the system. The TCS 

software executes actions such as system startup, loop reconfiguration, and valve positioning for 

flow and temperature control automatically or via commands from crew laptops or ground 

workstations. Telemetry from the various temperature, pressure, flow, and quantity sensors is 

monitored by TCS software and displayed on crew laptops or ground workstations. In addition, 

Fault Detection, Isolation, and Recovery (FDIR) software is used to monitor the performance of 

the TCS and, if there is a problem, alert the crew and flight controllers. In some cases, FDIR 

software initiates recovery actions. 

EATCS Activation 

With the solar arrays attached by the shuttle Atlantis' crew in September, NASA is ready with 

STS-116 to start generating enough power so that the permanent cooling system can be brought 

on line. NASA will have to bring up the permanent electrical power distribution system on line 

first before activating EATCS. The goal is to power up the MBSUs, route power through them 

to the cooling system and gets that system activated before the MBSUs can overheat. The 

EATCS has never been tested as an entire integrated system in its on-orbit configuration. 

However, NASA and Boeing engineers did conduct an entire loop test, but it did not have a 

rotating Radiator. 

During STS-116/Assembly flight 12A.1, during the second and third spacewalks, the EATCS is 

activated as part of the entire reconfiguration. There are two major power domains, 1/4 and 2/3. 

During EVA 2, the 2/3 power domain reconfiguration takes place and loop B is activated and 

then on EVA 3, the 1/4 power domain reconfiguration takes place and loop A is activated. Both 

power-on and power-off are major reconfigurations, when half the station power is turned off for 

periods of time. NASA has implemented a large number of procedures to account for the 

situation where they are zero-fault tolerate. The key to each EVA is activation of the pump 

modules and getting the system started for the first time. Assuming a nominal activation, there 

are approximately 24 procedure steps to activate the EATCS for the first time. 

Preparations to activate the EATCS start when the ground filled nitrogen pad is vented from the 

center radiators and the rest of the system (to be vented after STS-116 reaches orbit). The 

system is presently pressured with about 80 psia of nitrogen. The next step will be to introduce 

ammonia into the system with only one radiator ORU per loop plumbed, which will take several 

hours. Filling and activating the system will be one of the major challenges that mission 

controllers and engineers will face. Activation will take about an hour for each loop. If there 
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were an anomaly during filling such as pressure begins to drop like a leak, then the entire 

reconfiguration would be interrupted. Mission controllers have thoroughly rehearsed their 

procedures to identify any potential leaks in the system. Some minor leaks are possible. Early 

activation will provide cooling to the MBSUs and DDCUs (their respective cold plates) on S1, 

S0 and P1. The activation sequence also allows for a through checkout of the EATCS loops prior 

to connecting the U.S. Destiny Laboratory IFHX to the EATCS. The US Laboratory IFHXs will 

continue to be cooled via the Early External Active Thermal Control System (EEATCS) located 

on P6 Long Spacer. 

During the 12A.1 stage, EVAs will be conducted to connect the U.S. Laboratory IFHXs to the 

EATCS. After the IFHXs are connected to the EATCS, the EEATCS will enter a dormancy 

phase. 

On Assembly Mission 10A/STS-120, the two remaining radiator ORUs per wing are deployed 

and filled. Node 2 end-cones are connected to the EATCS via the starboard and port, boom trays 

located on the forward end of S0. Pump shut down will be required. All six IFHXs on Node 2 

receive cooling from the EATCS: two for Node 2, two for Attached Pressurized Module (APMColumbus), and two for Japanese Experiment Module (JEM-Kibo). The APM and JEM IFHXs 

remain in a bypassed and isolated configuration to prevent accidental freezing of the water side 

core of the IFHXs. 

On Assembly Flight 20A/STS-132, the Node 3 end-cone is connected to the EATCS via jumpers 

from the US Laboratory aft end-cone. A bypass line with an isolation valve on the US 

Laboratory Aft end-cone allows the fluid circuit to be completed prior to Node 3 arrival. Upon 

Node 3 arrival, the end-cone connection and activation the bypass-Isolation valves on US Lab 

end-cone are closed. This mission completes the EATCS loop architecture. 

Boeing engineers in Huntington Beach, Calif. designed the EATCS as well as S0, S1 and P1 

which contains most of the EATCS hardware. Major subcontractors to Boeing were Hamilton 

Sundstrand (PCVP, coldplates, core for heat exchanger), Honeywell (tanks, accumulator, 

RBVMs), Lockheed Martin (radiators) and Marotta (valves). 
Component Reference Drawings 

Component locations on S1/P1 

Component locations on S1/P1 

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