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
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
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
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
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
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
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
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
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.
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
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.
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.
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.
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.