The International Space Station: Operating an Outpost in the New Frontier Page 203 (237 of 438)

203 SYSTEMS: THERMAL CONTROL—THE “CIRCULATORY SYSTEM” OF THE INTERNATIONAL SPACE STATION CHAPTER 11 each of the two loops sharing the radiator—to carry ammonia across the radiator panel. Ammonia flows down one side of a PVR/TCR radiator through the tubes running through the panels and then back up tubes on the opposite end of the panels. The tubes along the ends of the panels are connected from one panel to the next with flexible hoses where the panels hinge together so that ammonia can get all the way to the last panel. Heat from the warm ammonia radiates into the coldness of space as the fluid moves along the large radiator panels. Figure 7. Deployment of the PhotoVoltaic Radiator on the P3 truss during STS-115/ISS- 12A in September 2006. The sequence from top to bottom shows the radiator unfolding from the compact launch configuration to its fully extended length, approximately 12.53 m (~493.4 in.). Each of the seven radiator panels is approximately 3.12 x 1.79 x 0.02 m (~124 x 70.6 x 0.69 in.), or about the size of two queen-sized mattresses placed next to one another. In the EETCS, ammonia flowed from the PFCS through the P6 lines to Z1, through the Z1 lines to the aft end of the LAB where it joined the fluid lines on the LAB Aft Endcone, and through the heat exchanger to collect heat. The warmed ammonia then flowed back through Z1 and P6, this time all the way up to the radiators. The flow from each EETCS loop split, some flowing through the TTCR and some flowing through the Starboard Thermal Control Radiator. Some of the flow bypassed both of the radiators to provide warm ammonia to the FCV in the PFCS, which performed temperature regulation. Most flow from the heat exchanger flowed through the radiators to provide cold ammonia to the FCV, which then merged the warm and cold ammonia to achieve the desired temperature for ammonia to return to the pump, where it started the journey again. The PVTCS follows the same basic path, except that all heat loads are contained on one truss so the loops never leave that truss segment. Ammonia flows from the PFCS to the batteries and other control equipment that require cooling, then both PVTCS loops flow through the single PVR to the PFCS. If the electrical equipment is cool enough to not require cooling at that moment, the FCV flow will bypass the radiators and go directly to the pump where it starts the loop again. The ETCS radiators are functionally the same as the EETCS/PVTCS radiators, though far larger and somewhat more complex. Each Heat Rejection System radiator has eight panels, each measuring 3.4 x 2.7 m (131.25 x 107.00 in.), or a bit bigger than two king-size mattresses placed side by side. Together, the eight panels that make up one HRS radiator can reject up to 11.67 kW. The HRS radiator panels are built in much the same way as PVR types. Ammonia flows through the radiators the same way, though the HRS panels have only 22 stainless steel tubes to carry ammonia across the radiator with each tube—about double the size of those in the PVR radiators at 0.32 cm (0.13 in.) in diameter. Each HRS radiator is used by only one ETCS loop, but each has two independent flow paths, thus allowing some flexibility of cooling capability. The 22 tubes that run across each radiator panel are distributed between the two independent flow paths in an alternating pattern. Each ETCS loop has three HRS radiators installed side by side on a large rotating plate called a Radiator Beam Truss Structure, as shown in Figure 8. Three radiators provide a total of six radiator flow paths for a combined heat rejection capability of up to 35 kW for each ETCS loop. The radiator flow paths were filled with nitrogen for launch and installation, and the cooling capability has proven

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203 SYSTEMS: THERMAL CONTROL—THE “CIRCULATORY SYSTEM” OF THE INTERNATIONAL SPACE STATION CHAPTER 11 each of the two loops sharing the radiator—to carry ammonia across the radiator panel. Ammonia flows down one side of a PVR/TCR radiator through the tubes running through the panels and then back up tubes on the opposite end of the panels. The tubes along the ends of the panels are connected from one panel to the next with flexible hoses where the panels hinge together so that ammonia can get all the way to the last panel. Heat from the warm ammonia radiates into the coldness of space as the fluid moves along the large radiator panels. Figure 7. Deployment of the PhotoVoltaic Radiator on the P3 truss during STS-115/ISS- 12A in September 2006. The sequence from top to bottom shows the radiator unfolding from the compact launch configuration to its fully extended length, approximately 12.53 m (~493.4 in.). Each of the seven radiator panels is approximately 3.12 x 1.79 x 0.02 m (~124 x 70.6 x 0.69 in.), or about the size of two queen-sized mattresses placed next to one another. In the EETCS, ammonia flowed from the PFCS through the P6 lines to Z1, through the Z1 lines to the aft end of the LAB where it joined the fluid lines on the LAB Aft Endcone, and through the heat exchanger to collect heat. The warmed ammonia then flowed back through Z1 and P6, this time all the way up to the radiators. The flow from each EETCS loop split, some flowing through the TTCR and some flowing through the Starboard Thermal Control Radiator. Some of the flow bypassed both of the radiators to provide warm ammonia to the FCV in the PFCS, which performed temperature regulation. Most flow from the heat exchanger flowed through the radiators to provide cold ammonia to the FCV, which then merged the warm and cold ammonia to achieve the desired temperature for ammonia to return to the pump, where it started the journey again. The PVTCS follows the same basic path, except that all heat loads are contained on one truss so the loops never leave that truss segment. Ammonia flows from the PFCS to the batteries and other control equipment that require cooling, then both PVTCS loops flow through the single PVR to the PFCS. If the electrical equipment is cool enough to not require cooling at that moment, the FCV flow will bypass the radiators and go directly to the pump where it starts the loop again. The ETCS radiators are functionally the same as the EETCS/PVTCS radiators, though far larger and somewhat more complex. Each Heat Rejection System radiator has eight panels, each measuring 3.4 x 2.7 m (131.25 x 107.00 in.), or a bit bigger than two king-size mattresses placed side by side. Together, the eight panels that make up one HRS radiator can reject up to 11.67 kW. The HRS radiator panels are built in much the same way as PVR types. Ammonia flows through the radiators the same way, though the HRS panels have only 22 stainless steel tubes to carry ammonia across the radiator with each tube—about double the size of those in the PVR radiators at 0.32 cm (0.13 in.) in diameter. Each HRS radiator is used by only one ETCS loop, but each has two independent flow paths, thus allowing some flexibility of cooling capability. The 22 tubes that run across each radiator panel are distributed between the two independent flow paths in an alternating pattern. Each ETCS loop has three HRS radiators installed side by side on a large rotating plate called a Radiator Beam Truss Structure, as shown in Figure 8. Three radiators provide a total of six radiator flow paths for a combined heat rejection capability of up to 35 kW for each ETCS loop. The radiator flow paths were filled with nitrogen for launch and installation, and the cooling capability has proven

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CHAPTER 11 SYSTEMS: THERMAL CONTROL—THE “CIRCULATORY SYSTEM” OF THE INTERNATIONAL SPACE STATION 202 The three-way valve in the heat exchanger assembly provides the ability to divert ammonia flow such that it bypasses the heat exchanger core. This valve is called a bypass valve (Figure 2). When the ETCS loop is off, this valve is adjusted to allow the ammonia flow to bypass the heat exchanger so that when the loop starts up, the super-cold ammonia will not be able to remove too much heat from the internal system. Although it sounds desirable to reject as much heat as possible, there is a point where things can get too cold. Because water expands as it freezes, the heat exchanger is not allowed to reach the freezing point of water. Otherwise, the frozen water will expand such that it ruptures the stainless steel parting sheet and then contracts as it thaws, leaving a hole for ammonia to leak through and into the cabin. An Isolation Valve—a two-way (i.e., two-position) valve that can be closed to isolate the heat exchanger core from ammonia flow— helps prevent the water from freezing in the IFHX. The ETCS ammonia is kept at a temperature of about 3.3°C (~37.9°F). If the temperature drops below that level, software in the system will automatically stop ammonia flow and configure these valves to bypass and isolate the ammonia side of the IFHX. When a heat exchanger is bypassed and isolated, it is hydraulically locked such that any sudden temperature increases within the ammonia will put the heat exchanger at risk of overpressurization. As another check to prevent overpressurization, the heat exchanger has a small ammonia fluid line, called a bleed line, which connects the ammonia inlet close to the heat exchanger core with the ammonia outlet outside the isolation valve. Some ammonia can escape through this line if pressure gets too high inside the isolated IFHX core. Heaters attached to the heat exchanger core and the water inlet and outlet lines are designed to prevent freezing of the water side of the heat exchanger. Additional safety checks in the ETCS software help detect issues and prevent the heat exchanger from freezing. The first response was detailed above: if the ammonia temperature drops to about 1.1°C (34°F), the ETCS pump will automatically shut down and the heat exchanger will be bypassed and isolated to prevent cold ammonia from entering the heat exchanger. The second and third checks monitor the quantity of fluid in the internal system pump accumulator. If the quantity of water measured in the accumulator begins to rise, the only source of fluid would be ammonia leaking into the water. Therefore, if the accumulator quantities in the ITCS go above a threshold, the pumps will be shut down to help prevent pushing ammonia into the cabin. As discussed in Chapter 19, the case of toxic ammonia leaking into the crew cabin is one of the three critical emergency events for which the crew and ground regularly train (see also Chapter 10). External Thermal Control Systems— Loads and Radiators The final component included in all the external thermal systems is the heat rejection point: the radiators. Although there are two types of radiators, they work the same way by providing radiative heat transfer from ammonia loops on the ISS. One type is known as a PhotoVoltaic Radiator (PVR), which is installed in the eight PVTCSs. The second type is known as the Heat Rejection Subsystem (HRS) or, more commonly, the ETCS radiators. Both types of radiators consist of a base panel connected to a series of radiator panels that are hinged together, accordion-style, such that they can be deployed (i.e., straightened out, end to end) or retracted (folded back together, face to face) (Figure 7). Each radiator was fully retracted at launch and has an automated deploy/ retract capability with a manual override that can be controlled by a spacewalking astronaut. The P6 truss segment houses a total of three radiators. One radiator, known simply as the 2B/4B PVR, faced forward to be shared between the two PVTCS loops. The remaining two radiators were shared between the two EETCS loops. One radiator faced aft and therefore is still known as the Trailing Thermal Control Radiator (TTCR). The other radiator faced starboard in the original P6 location, and is known as the Starboard Thermal Control Radiator. Note: Now that the outboard truss segments rotate 360° to support solar array pointing, neither name is accurate. See also Figure 4. The PVR radiator type is always shared between two independent loops and has seven radiator panels. The seven panels together can reject from 9.5 to 14 kW of heat, depending on the thermal environment. This is equivalent to the capacity of the air conditioner in an average house in the southern United States. The inside of each radiator panel is a honeycomb made from aluminum sheeting, with that honeycomb sandwiched between two 0.254 mm (0.010 in.) sheets of aluminum that are then coated with either silver or white coating to ensure optimal reflectivity. Each panel has 24 stainless steel tubes that measure 0.17 cm (0.067 in.)—12 tubes for

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CHAPTER 11 SYSTEMS: THERMAL CONTROL—THE “CIRCULATORY SYSTEM” OF THE INTERNATIONAL SPACE STATION 204 greater than the need. Therefore, some of the flow paths still have their original nitrogen fill and have not yet been used. Figure 8. A major component of the ETCS as seen from the Space Shuttle. The left image shows the port and starboard radiator beams (three radiators per beam, with each radiator consisting of eight panels). On the right is a close-up of the Radiator Beam Truss Structure that is rotated by the Thermal Radiator Rotary Joint to maximize heat rejection. Though the EETCS radiators on the P6 truss were fixed, each set of ETCS radiators can be rotated to improve heat rejection by positioning the radiator beam so that its three radiators are in an edge-to-sun position. This rotation is accomplished via the Thermal Radiator Rotary Joint (TRRJ) (Figure 8), which uses many of the same components as the Solar Array Rotary Joint (SARJ) described in Chapter 9. As with the SARJ, the TRRJ consists of a pair of drive/lock assemblies and a pair of rotary joint motor controllers (see Chapter 9). Unlike the SARJ, the TRRJ has a limited rotational capability because it is impractical to use fluid connectors that have a full 360 degrees of rotation. Instead, the fluid hose rotary coupler provides fluid connections from one end of the TRRJ to the other, and the hoses within the fluid hose rotary coupler limit rotation to 210 total degrees of rotation, 105 degrees in each direction. The TRRJ interface is also much smaller than the SARJ interface. Whereas the SARJ rings are as large as the truss segment that houses it (i.e., large enough for a person to stand inside), the TRRJ is a fairly compact cylinder installed in the center of the S1 and P1 truss segments. Also, unlike the SARJ, the TRRJ generally does not need to rotate very much and can be left in the same position, changing only for specific attitude changes or certain solar beta angles (see Chapter 7). The primary purpose of all three external systems is to cool the heat exchangers that provide heat transfer from pressurized modules. However, the ETCS also supports several EPS components that are located on the truss segments. These electrical components are mounted on an interface known as a coldplate, which both connects the component to the truss and provides an interface for ETCS cooling (Figure 9). Each coldplate is a broad plate with narrow- set fins extending from it. Those fins interleave with fins extending from the electrical component such that heat from the electrical component fins radiates to the coldplate fins. This finned plate is bonded to a stainless steel flow plate, which is sealed to allow ammonia flow through the unit to pick up heat transferred through the aluminum fins. Each ETCS loop provides cooling to five electrical components in addition to five heat exchangers. Though the heat load from the electrical components is a small fraction of the heat load from the heat exchanger, the electrical components are fully dependent on ETCS cooling to function at full capacity. Water in Water out Heat Coldplate Hardware Figure 9. A schematic showing how a coldplate works. Cold liquid flows in from the left side and picks up heat generated by the attached hardware. The warmed liquid then flows out toward the radiators.

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