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Active Thermal Control System

The ATCS removes heat from the ARS at the water coolant loop/Freon-21 coolant loop interchanger and from each of the fuel cell power plant heat exchangers and warms the PRSD cryogenic oxygen in the ECLSS oxygen supply line and the hydraulic fluid systems at the hydraulic heat exchanger. The ATCS consists of two complete and identical Freon-21 coolant loop systems; cold plate networks for cooling avionics units; liquid/liquid heat exchangers; and three heat sink systems-radiators, flash evaporator and ammonia boiler.

During ground operations (checkout, prelaunch, and post landing), orbiter heat rejection is provided by the GSE heat exchanger in the Freon-21 coolant loops through ground system cooling.

From lift-off to an altitude of less than 140,000 feet-approximately 125 seconds-thermal lag is used. Approximately 125 seconds after lift-off, the flash evaporator subsystem is activated and provides orbiter heat rejection of the Freon-21 coolant loops via water boiling. Flash evaporator operation continues until the payload bay doors are opened in orbit.

When the payload bay doors are opened, radiator panels attached to the forward payload bay doors may or may not be deployed depending on the flight. If deployed away from the payload bay doors, the forward two panels on each side of the orbiter will radiate from both sides. If not deployed, they will radiate only from one side. The aft radiator panels on the forward portion of the aft payload bay doors are affixed to the doors and radiate only from the upper surface. On-orbit heat rejection is provided by the radiator panels; however, during orbital operations when a combination of heat load and spacecraft attitude exceeds the capacity of the radiator panels, the flash evaporator subsystem is automatically activated to meet total system heat rejection requirements.

At the conclusion of orbital operations, the flash evaporator subsystem is activated, and the payload bay doors are closed with the radiator panels retracted, if they were deployed, in preparation for entry.

The flash evaporator subsystem operates during entry to an altitude of 100,000 feet, at which point boiling water can no longer provide adequate Freon-21 coolant temperatures. Through the remainder of the entry phase and after landing until ground cooling is connected, heat rejection of the Freon-21 coolant loops is provided by the evaporation of ammonia through the use of the ammonia boilers. When ground cooling is initiated after landing, the ammonia boilers are shut down, and heat rejection of the Freon coolant loops is provided by the GSE heat exchanger.

Each Freon-21 coolant loop has a pump package consisting of two pumps and an accumulator. One Freon-21 coolant pump in each coolant loop is active at all times. The metal bellows-type accumulator in each coolant loop is pressurized with gaseous nitrogen to provide a positive suction pressure on the pumps and permit thermal expansion in that coolant loop. A ball check valve downstream of the pumps in each coolant loop prevents a reverse flow through the non-operating pump in the loops. The Freon pumps in each Freon coolant loop are controlled individually by the Freon pump loop 1 and loop 2 switches on panel L1. When either switch is positioned to A or B, the corresponding Freon pump in that loop operates. The off position of each switch prohibits either Freon pump in that coolant loop from operating.

When a Freon coolant pump is operating, Freon is routed in parallel through the three fuel cell heat exchangers and the midbody cold plate network to cool electronic avionics units. The Freon coolant reunites in a series flow path before entering the hydraulics heat exchanger. It then extracts energy from the Freon-21 coolant loop to heat hydraulic system fluid during on-orbit hydraulic circulation thermal-conditioning operations. During the prelaunch and boost phase of the mission and during the atmospheric flight portion of entry through landing and rollout, the hydraulic system heat exchanger transfers excess heat from the hydraulic systems to the Freon-21 loops. The Freon flows from the hydraulic fluid heat exchanger to the radiators, which are bypassed through a bypass valve during ascent and entry because the payload bay doors are closed. The radiators are located on the underside of the payload bay doors. When the payload bay doors are opened in orbit, the radiators are used for heat rejection to space. The Freon coolant flows through the GSE heat exchanger, ammonia boilers and flash evaporator. It is then divided into two parallel paths. One path flows through the ECLSS oxygen restrictor to warm the PRSD oxygen for the ECLSS to 40 F. It then flows through a flow-proportioning valve into parallel paths to the payload heat exchanger and ARS interchanger and returns to a series flow. The other path flows through aft avionics bays 4, 5 and 6 to cool some electronic avionics equipment in each avionics bay. It also flows through cold plates to cool rate gyro assemblies 4, 3, 2 and 1 and then returns to a series flow. The parallel paths return in series to the Freon coolant pump in that Freon coolant loop.

The Freon-21 coolant pumps, ARS interchanger, three fuel cell power plant heat exchangers, payload heat exchanger, flow-proportioning valve modules and midbody cold plates are located in the lower forward portion of the midfuselage. The radiators are attached to the underside of the payload bay doors. The cold plates for the hydraulic system heat exchangers; ground support equipment heat exchanger; ammonia boilers; flash evaporator; and aft avionics bays 4, 5 and 6 are located in the aft fuselage of the orbiter. The radiator flow control assemblies and RGAs are located in the lower aft portion of the midfuselage.

The radiator system consists of three radiator panels for a baseline mission configuration under the right and left payload bay doors. During ascent and entry the radiator panels are secured to the payload bay doors. The two radiator panels attached to the forward right and left payload bay doors are deployable from the forward payload bay doors when the doors are opened on orbit. The heat rejection requirements of the orbiter for a specific mission will determine if the forward radiators are to be deployed. The third radiator panel is fixed to the forward underside of the aft right and left payload bay doors and is not deployable. The baseline radiator panels are designed for missions requiring heat rejection of 21,500 Btu per hour. A fourth radiator panel, which is deployable, may be required for a specific mission and would be fixed to the aft underside of the aft right and left payload bay doors. With the addition of the fixed fourth radiator panel, the heat rejection capability is 29,000 Btu per hour. When the payload bay doors are closed, the radiators are bypassed.

The deployable radiators are secured to the right and left payload bay doors by six motor-operated latches. When the payload bay doors are opened on orbit and the mission dictates that the deployable radiators be deployed, the six motor-driven latches unlatch the radiators from the payload bay doors, and the motor-driven torque-tube-lever arrangement deploys the forward radiators at 35.5 degrees from the payload bay doors. The forward radiators would then provide heat rejection from both sides of the radiator panels.

The aft fixed radiator panels are attached to the payload bay doors by a ball joint arrangement at a maximum of 12 locations to compensate for movement of the payload bay door and radiator panel caused by the thermal expansion and contraction of each member.

The radiator panels are constructed of an aluminum honeycomb face sheet 126 inches wide and 320 inches long. The forward deployable radiator panels are two-sided and have a core thickness of 0.9 of an inch. They have longitudinal tubes bonded to the internal side of both face sheets. Each of the forward deployable panels contains 68 tubes spaced 1.9 inches apart. Each tube has an inside diameter of 0.131 of an inch. Each side of the forward deployable radiator panels has a coating bonded by an adhesive to the face sheet consisting of silver-backed Teflon tape for proper emissivity properties. The aft fixed panels are one-sided, and their cores are 0.5 of an inch thick. They have tubes only on the exposed side of the panel and a coating bonded by an adhesive to the exposed face sheet. The aft panels contain 26 longitudinal tubes spaced 4.96 inches apart. Each tube has an inside diameter of 0.18 of an inch. The additional thickness of the forward radiator panels is required to meet deflection requirements when the orbiter is exposed to ascent acceleration.

The radiator panels on the left and right sides are configured to flow in series, while flow within each panel is parallel through the bank of tubes connected by an inlet and outlet connector manifold. The radiator panels on the left side are connected in series with Freon-21 coolant loop 1. The radiator panels on the right side are connected in series with Freon-21 coolant loop 2.

If the two deployable and two fixed radiators are installed on the payload bay doors, the radiator panels will provide an effective heat dissipation area of 1,195 square feet on orbit. Each radiator panel is 10 feet wide and 15 feet long. The Freon tubing in the radiator panels is more than 1 mile long.

A radiator flow control valve assembly in each Freon coolant loop controls the temperature of that loop through the use of the variable flow control, which mixes hot bypassed Freon coolant flow with the cold Freon coolant from the radiators. The radiator flow control valve assemblies can be controlled automatically or manually by the flight crew.

In the automatic operation the rad controller loop 1 and loop 2 auto A, off, auto B switch on panel L1 is positioned to auto A or auto B to apply electrical power to the corresponding radiator flow controller assembly. The rad controller loop 1 and loop 2 mode auto , man switch on panel L1 is positioned to auto, and the rad controller out temp switch on panel L1 is positioned to norm or hi. With the rad controller out temp switch on panel L1 in norm , the radiator outlet temperature in Freon coolant loops 1 and 2 is automatically controlled at 38 F; in hi, the temperature is automatically controlled at 57 F. It should be noted that the flash evaporator is activated automatically when the radiator outlet temperature exceeds 41 F to supplement the radiators' ability to reject excess heat.

The radiator talkback indicator next to the rad controller loop 1 and loop 2 auto A , off and auto B switches on panel L1 indicates the position of the bypass valve in that Freon coolant loop. The indicator indicates byp when the bypass valve in that Freon coolant loop is in the bypass position, barberpole when the motor-operated bypass valve is in transit and rad when the bypass valve is in the radiator flow position.

When the rad controller loop 1 and loop 2 mode auto , man switch on panel L1 is positioned to man for the Freon coolant loop selected, the automatic control of the radiator flow control valve assembly in that loop is inhibited; and the flight crew controls the flow control valve assembly manually using the rad controller loop 1 , loop 2, rad flow and bypass switches on panel L1. When the switch is positioned to bypass , the loop's motor-operated bypass valve permits that Freon coolant loop to bypass the radiators. When the switch is positioned to rad flow, the valve permits coolant to flow through the radiators. The rad controller loop 1 and 2 talkback indicator for the Freon coolant loop indicates byp when the bypass valve in that loop is in bypass and barberpole when it is in transit.

The flash evaporators reject heat loads from Freon-21 coolant loops 1 and 2 during ascent above 140,000 feet and supplement the radiators on orbit if required. They also reject heat loads during deorbit and entry to an altitude of approximately 100,000 feet.

The flash evaporators are located in the aft fuselage of the orbiter. There are two evaporators in one envelope. One is the high-load evaporator; the other is the topping evaporator. There are two major differences between the evaporators. The high-load evaporator has a higher cooling capacity than the topping evaporator, and its overboard vent is only on the left side. The topping evaporator vents steam equally to the left and right sides of the orbiter, which is non-propulsive. The evaporators are cylindrical and have a finned inner core. The hot Freon-21 from the coolant loops flows around the finned core, and water is sprayed onto the core by water nozzles from either evaporator. The water vaporizes, cooling the Freon-21 coolant loops. In the low-pressure atmosphere above 100,000 feet, water vaporizes quickly. Changing water liquid to vapor removes approximately 1,000 Btu per hour per 1 pound of water. The water for the evaporators is obtained from the potable water storage tanks through water supply systems A and B.

The flash evaporators have three controllers. The primary A controller has two separate, functionally redundant shutdown logic paths (undertemperature rate of cooling). Primary B has a single shutdown logic path; secondary has no shutdown. The flash evaporator controllers are enabled by the flash evaporator controller switches on panel L1. The flash evap controller pri A switch controls controller A, the pri B switch controls controller B, and the sec switch controls the secondary controller. When the pri A , pri B or sec switch is positioned to GPC, the corresponding controller is turned on automatically during ascent by the backup flight system computer as the orbiter ascends above 140,000 feet. During entry the BFS turns the corresponding controller off as the orbiter descends to 100,000 feet. The on position of the switch provides electrical power directly to the corresponding flash evaporator controller. The off position of the switch removes all electrical power and inhibits flash evaporator operation.

The primary A controller controls water flow to the flash evaporator from water supply system A through water feed line A. The primary B controller controls water flow to the flash evaporator from water supply system B through water feed line B. Note that when a primary controller is enabled, both evaporators can be used simultaneously.

The secondary controller controls water flow to the flash evaporator from water supply system A through feed line A if the flash evaporator controller sec A sply switch on panel L1 is in the sply A position and if the hi load evap switch on panel L1 is in the enable position. If the sec B sply switch is in the sply B position and the hi load evap switch is in the enable position, the secondary controller controls water flow to the flash evaporator from water supply system B through feed line B. When the secondary controller is used and the hi load evap switch is off , both the A and B water supply systems will feed the topping evaporator in an alternate pulsing fashion. When the secondary controller is used and the hi load evap switch is in the enable position, the topping evaporator is disabled.

The primary A and B controllers modulate the water spray in the evaporator to control the Freon-21 coolant loops' evaporator outlet temperature at 39 F. The secondary controller modulates the water spray in the evaporator to control the Freon-21 coolant loops' evaporator outlet temperature at 62 F. The temperature sensors are located at the outlets of both evaporators.

The applicable flash evaporator controller pulses water into the evaporators, cooling the Freon-21. The steam generated in the topping evaporator is ejected through two sonic nozzles at opposing sides of the orbiter aft fuselage to reduce payload water vapor pollutants on orbit and to minimize venting thrust effects on the orbiter's guidance, navigation and control system. The high-load evaporator is used in conjunction with the topping evaporator during ascent and entry when higher Freon-21 coolant loop temperatures impose a greater heat load that requires a higher heat rejection. The hi load evap switch on panel L1 must be in the enable position for high-load evaporator operation. After leaving the high-load evaporator, Freon-21 flows through the topping evaporator for additional cooling. The steam generated by the high-load evaporator is ejected through a single sonic nozzle on the left side of the orbiter aft fuselage. The high-load evaporator would not normally be used on orbit because it has a propulsive vent and might pollute a payload.

Each primary controller has an automatic shutdown capability to protect the evaporator from over- or undertemperature conditions. The evaporator's outlet temperature is monitored to determine if a thermal shutdown of the evaporator is warranted. If the evaporator's outlet temperature goes below 37 F for 20 seconds or more, an undertemperature shutdown of the evaporator occurs. If the evaporator outlet temperature is greater than 41 F for 40 seconds, an overtemperature shutdown of the evaporator occurs. If the evaporator is shut down because it is over- or undertemperature, electrical power to the affected controller must be recycled to re-enable operations. The secondary controller does not have any automatic shutdown capability.

The evaporator outlet temperature of Freon-21 coolant loops 1 and 2 is transmitted to panel O1. When the Freon loop 1 or 2 switch on panel O1 is positioned to loop 1 or 2 , the evaporator outlet temperature of Freon coolant loops 1 or 2 can be monitored on the Freon evap out temp meter on panel O1 in degrees Farenheit. If the outlet temperature drops below 32 F or rises above 60 F, the red Freon loop C/W light on panel F7 will be illuminated.

The flash evaporator topping evaporator can be used to dump excess potable water from the potable water storage tanks, if required, on orbit. The radiator flow control valve assembly has an alternate control temperature of 57 F that is used for this excess water dump into the topping evaporator.

Electrical heaters are employed on the topping and high-load flash evaporators' steam ducts to prevent freezing. The flash evap hi load duct htr rotary switch on panel L1 selects the electrical heaters. Switch positions A and B provide electrical power to the corresponding thermostatically controlled heaters on the high-load evaporator steam duct and steam duct exhaust. The A/B position provides electrical power to both thermostatically controlled heaters. The C position provides electrical power to the thermostatically controlled C heaters. The off position removes electrical power from all the heaters.

The flash evap topping evaporator duct rotary switch on panel L1 selects the thermostatically controlled electrical heaters on the topping evaporator. Positions A and B provide electrical power to the corresponding heaters, while A/B provides electrical power to both A and B heaters. The C position provides power to the C heaters. The off position removes electrical power from all the heaters.

The topping evaporator's left and right nozzle heaters are controlled by the topping evaporator heater l and r switches on panel L1. When the left and right switches are positioned to auto A or auto B, electrical power is provided to the corresponding left and right nozzle heaters, and the corresponding nozzle temperature is maintained between 40 and 70 F. The off position removes electrical power from both heater systems.

The ammonia boilers use the low boiling point of ammonia to cool the Freon-21 coolant loops when the orbiter is below 100,000 feet during entry. There are two complete, individual ammonia storage and control systems that feed one common boiler containing ammonia passages and the individual Freon-21 coolant loops 1 and 2.

Each ammonia boiler storage tank contains a total of 49 pounds of ammonia, all of which may be used for cooling. Each ammonia tank is pressurized with gaseous helium at an operating pressure between 550 psia to 83 psia. Downstream of each ammonia storage tank to the common boiler are three control valves: a normally closed isolation valve, a normally open secondary control valve and a primary control valve. A relief valve in each ammonia boiler storage system provides overpressurization protection of that ammonia storage tank.

Ammonia boiler supply systems A and B are enabled by the corresponding NH3 controller A and B switches on panel L1.

When the NH3 controller A switch is positioned to pri/GPC before entry, it enables the computer to control electrical power to the primary and secondary controller within ammonia controller A. When the orbiter descends through 100,000 feet, the backup flight system computer commands the ammonia system A controller on. The primary controller in the ammonia system A controller energizes the ammonia A system isolation valve open, permitting ammonia to flow to two motor-operated controller valves and commands the primary motor-operated valve to regulate the flow to the ammonia boiler. Three temperature sensors are located on each Freon-21 coolant loop. One sensor on each Freon-21 coolant loop is associated with the primary controller and its motor-operated valve to regulate ammonia system A flow to maintain Freon-21 coolant loop 1 and 2 temperatures at the outlet of the ammonia boiler at 34 F. One sensor on each Freon-21 coolant loop is associated with the ammonia system A controller fault detection logic. If the Freon-21 coolant loop 1 and 2 temperatures drop below 31 F for greater than 10 seconds, the fault detection logic automatically inhibits the primary controller, which removes power from the ammonia system A isolation valve and the primary controller's motor-operated valve. The fault detection logic switches to the secondary controller in the ammonia system A controller, which energizes a redundant coil in the ammonia system supply A isolation valve. It opens the valve and commands the primary motor-operated valve to full open and allows the secondary controller to control the secondary motor-operated valve to regulate the ammonia A flow to the ammonia boiler. The third sensor on each Freon coolant loop is associated with the secondary controller and secondary motor-operated valve. It regulates ammonia supply system A flow to maintain the Freon-21 coolant loop 1 and 2 temperatures at the outlet of the ammonia boiler at 34 F. This automatic switchover is only from the primary to the secondary.

The ammonia boiler is a shell-and-tube system with a single pass of ammonia on the ammonia side and two passes of each Freon-21 coolant loop through the boiler. The ammonia flows in the ammonia tubes and the Freon-21 coolant loop flows over the tubes, cooling the Freon-21 coolant loops. When the ammonia is sprayed on the Freon-21 coolant lines in the boiler, it immediately vaporizes, and the heat and boiler exhaust is vented overboard in the upper aft fuselage of the orbiter next to the bottom right side of the vertical tail. The ammonia boiler operations continue through the remainder of entry, landing and rollout until a ground cooling cart is connected to the GSE heat exchanger.

When the NH3 controller A switch is positioned to sec/on, the ammonia system A controller is electrically powered and enabled directly (no computer command is required). The primary controller in the ammonia system A controller energizes the system's isolation valve open, permitting ammonia to flow to two motor-operated controller valves. The primary controller commands the secondary controller's motor-operated valve to the open position and the primary controller's motor-operated valve to regulate the ammonia flow to the ammonia boiler. The three temperature sensors on each Freon-21 coolant loop operate and control Freon-21 coolant loop 1 and 2 temperature in the same manner as in the primary/GPC mode. The fault detection logic also operates in the same manner as in the primary/GPC mode.

The off position removes all electrical power from the ammonia system A controller, rendering ammonia system A inoperative.

The NH3 controller B switch controls the ammonia system B controller and ammonia supply system B in the same manner as the ammonia system A controller and ammonia supply system A are controlled by the A switch.


Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
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