2.8 ELECTRICAL POWER SYSTEM (EPS)

CONTENTS

Description ........................................... 2.8-1Power Reactants Storage and

Distribution System ..................... 2.8-1Fuel Cell System .................................. 2.8-10Electrical Power Distribution and

Control ........................................... 2.8-20Operations ............................................ 2.8-29EPS Caution and Warning

Summary ....................................... 2.8-31EPS Summary Data ............................. 2.8-37EPS Rules of Thumb............................ 2.8-37

Description

The electrical power system (EPS) consists of theequipment and reactants that produce electricalpower for distribution throughout the orbitervehicle, and fulfill all the orbiter external tank,solid rocket booster, and payload powerrequirements, when not connected to groundsupport equipment. The EPS operates duringall flight phases. For nominal operations, verylittle flight crew interaction is required bythe EPS.

The EPS is functionally divided into threesubsystems: power reactants storage and distri-bution (PRSD), three fuel cell power plants (fuelcells), and electrical power distribution andcontrol (EPDC).

Through a chemical reaction, the three fuel cellsgenerate all 28-volt direct-current electricalpower for the vehicle from launch minus 3minutes and 30 seconds through landing roll-out. Prior to that, electrical power is providedby ground power supplies and the onboardfuel cells.

Power is controlled and distributed by assem-blies located in the forward, mid, and aftsections of the orbiter. Each assembly is a housingfor electrical components such as remote switching

devices, buses, resistors, diodes, and fuses. Eachassembly usually contains a power bus or busesand remote switching devices for distributing buspower to subsystems located in its area.

Power Reactants Storage and DistributionSystem

The power reactants storage and distributionsystem stores the reactants (cryogenic hydrogenand oxygen) and supplies them to the three fuelcells that generate all the electrical power for thevehicle during all mission phases. In addition,the subsystem supplies cryogenic oxygen to theenvironmental control and life support system(ECLSS) for crew cabin pressurization. Thehydrogen and oxygen are stored in tanks atcryogenic temperatures (-285°F for liquidoxygen and -420° F for liquid hydrogen) andsupercritical pressures (above 731 psia foroxygen and above 188 psia for hydrogen).

The PRSD system components are located in theorbiter midbody underneath the payload bay oron a payload bay pallet for 10+ day missions inextended duration orbiter (EDO) vehicles. Thesystem stores the reactants hydrogen andoxygen in double-walled, thermally insulatedspherical tanks with a vacuum annulus betweenthe inner pressure vessel and outer tank shell.Each tank has heaters to add energy to thereactants during depletion to control pressure.Each tank is capable of measuring quantityremaining.

The tanks are grouped in sets of one hydrogenand one oxygen tank. The number of tank setsinstalled depends on the specific missionrequirement and vehicle. Up to five tank setscan be installed in the midfuselage under thepayload bay liner of OVs-102, -103, -104, and-105. Up to four additional tank sets can beflown on the EDO pallet in the payload bay ofOV-102 and -105. OV-105 may be configured ata later date to fly up to four additional tank sets(total of eight additional sets). Switches tocontrol these tanks are already installed.

Powerreactan tssto rage &

distr ibu tionsystem

Fue l ce llsystem

(3)

DC busdis tr ibu tion

system(m a in , essen tia l,

con trol, andpayload buses)

Environm enta lcon tro l and

life support system

Loads

AC genera tion& d is tr ibution

H2

O2

H2O

D Cpower

307.cv s

The Electrical Power System

Switch inputs and dedicated displays

Dataprocessing

system

To annunciator

Displaysand

controls

To CRTs

Matrix

Environmentalcontroland life

support system

Fuelcells

Switch inputs and dedicated displays

Oxygen

Heat and water

Caution/warning

Instrumentation

Telemetry

308.cvs

Powerreactantstorage

anddistribution

Electrical Power System Interfaces

Cryogenic Storage Tanks

All oxygen tanks are identical and consist ofinner pressure vessels of Inconel 718 and outershells of aluminum 2219. The inner vessel is33.435 inches in diameter and the outer shell is36.8 inches in diameter. Each tank has a volumeof 11.2 cubic feet and stores up to 781 pounds ofoxygen. The dry weight of each tank is 215pounds. Maximum fill time is 45 minutes.

All hydrogen tanks also are identical. Both theinner pressure vessel and the outer shell areconstructed of aluminum 2219. The innervessel’s diameter is 41.51 inches and the outershell’s is 45.5 inches. The volume of each tank is21.39 cubic feet, and each stores up to 92 poundsof hydrogen. Each tank weighs 227 pounds dry.Maximum fill time is 45 minutes.

The inner pressure vessels are kept supercold byminimizing conductive, convective, and radiantheat transfer. Twelve low-conductive supportssuspend the inner vessel within the outer shell.Radiant heat transfer is reduced by a shieldbetween the inner vessel and outer shell

(hydrogen tanks only), and convective heattransfer is minimized by maintaining a vacuumbetween the vessel and shell. During groundoperations, a vacuum ion pump maintains therequired vacuum level and is also used as avacuum gauge to determine the vacuum’sintegrity. The pump consists of an electricalpower supply and an anode, which absorbs freeions between the tank walls.

Cryogenic Tank Heaters

Each hydrogen tank has one heater probe withtwo elements; each oxygen tank has two heaterprobes with two elements on each probe. As thereactants are depleted, the heaters add heatenergy to maintain a constant pressure in thetanks.

The heaters operate in automatic and manualmodes. In the automatic mode, the heater iscontrolled by a tank heater controller. Eachheater controller receives a signal from a tankpressure sensor. If pressure in a tank is lessthan or equal to a specific pressure, thecontroller turns the heater on. When the

SPLYVLVTO ECS

SYS 1

MANFVLV

TK9

TK8

TK7

TK6

TK3

TK5

TK4

MANFVLV

SPLYVLVTO ECS

SYS 2

FC1

O2

TK1 P

MANF P

REACVLV

H2

TK1 P

MANF P

REACVLV

MANFVLV

TK9

TK8

TK7

TK6

TK3

FC2 TK2 P

MANF P

REACVLV

TK2 P

MANF P

REACVLV

MANFVLV

FC3

REACVLV

REACVLV

TK5

TK4

OV103,OV104,OV105

OV103,OV104,OV105

OV102ONLY

CRYO

309.cvs

The PDRS System

pressure in the tanks goes above the upper limit,the respective controller turns the heater off.The O2 TK1, 2, 3, and H2 TK1, 2, 3 heaterswitches are located on panel R1; switches forthe O2 and H2 TK4 heaters (tanks 4/5 on OV-102) are on panel A11. (On OV-102, the oxygenand hydrogen tank 4 B heaters are disconnected,and the logic and power are used for the tank 5A heaters.) OV-103, -104, and -105 heaterswitches for tank set 5 are on panel A15. Theheater switches for EDO pallet tanks 6-9 arelocated on panel A15 in both EDO configuredvehicles. OV-105 also has heater switches fortank sets 10-13 on panel A11 for the future useof a second EDO pallet.

Heater controller pasia

Pressure limits(psia)

Low High

H2 tank 1, 2 200-206 220-226H2 tank 3-9 217-223 237-243O2 tank 1,2 803-819 838-854O2 tank 3-9 832-848 866-882

O2 and H2 TK HEATERS Switcheson Panel R1

Dual-mode heater operation is available forpairs of oxygen and hydrogen tanks. If theheaters of both tanks 1 and 2, or tanks 3 and 4,are placed in the automatic mode, the tankheater logic is interconnected. In this case, theheater controllers of both tanks must sense a

low pressure before the heaters will turn on.Once the heaters are on, a high pressure signalfrom either tank will turn off the heaters in bothtanks.

In the manual mode, the flight crew controls theheaters by using the ON OFF positions for eachheaters switch on panel R1, A11, or A15.Pressure in each tank is shown as O2 TK P andH2 TK P on the DISP 68 CRYO SYSTEM or DISP168 CRYO PALLET displays. Pressure is alsodisplayed on the CRYO O2 and H2 PRESS meterson panel O2. The specific tank (1, 2, 3, or 4) isselected by the rotary switch below the meters.Tank 5 TK P is also displayed on OV-105.

CRYO TANK 4 HEATER Switcheson Panel A11

Cryogenic System Meters and Switches onPanel O2

Each tank set has a hydrogen/oxygen controlbox that contains the electrical logic for thehydrogen and oxygen heaters and controllers.The control box is located on cold plates in themidbody under the payload bay envelope. OnOV-102, a single control box controls tanks 4and 5.

2011/ /068 CRYO SYSTEM 4 000/02:34:40 000/00:00:00 TK O2 1 2 3 4 5P 827 826 871 826 826TK P 828 828 871 828 828QTY 99 99 100 100 100T -249 -249 -249 -249 -249HTR T 1 -249 -249 -182 -249 -249 2 -249 -249 -182 -249 -249HTR 1ACUR 1B 4/5SNSR 2A 2B 4/5MANF P 871 871 VLV OP OP H2P 215 213 224 213 213TK P 215 214 224 214 214QTY 97 97 99 99 99T -417L -417L -417L -417L -417LHTR T -417L -417L -417L -417L -417LMANF P 222 222 VLV OP OP

313

DISP 68 CRYO SYSTEM Display (on orbit)

The CRYO O2 HTR ASSY TEMP meter on panelO2, in conjunction with the rotary switch belowit, selects one of the two heaters in each tankand permits the temperature of the heaterelement to be displayed. The range of thedisplay is from -425° F to +475° F. Thetemperature sensor in each heater also is hard-wired directly to the caution and warningsystem, which drives the yellow O2 HEATERTEMP light on panel F7. This light isilluminated if the temperature is at or above344° F. A signal also is sent to the GPCs wheresoftware checks the limit; if the temperature is ator above 349° F, the BACKUP C/W ALARM lighton panel F7 is illuminated. This signal also isdisplayed on DISP 68 CRYO SYSTEM andtransmitted to the ground.

2011/ /168 CRYO PALLET 4 000/02:38:52 000/00:00:00 TK O2 6 7 8 9P 515L 515L 515L 515L PALLETTK P 0L 0L 0L 0L STBQTY - 5L - 5L - 5L - 5L STBT -417L -417L -417L -417L POPHTR T 1 -417L -417L -417L -417L POP 2 -417L -417L -417L -417LHTR 1A PALLETCUR 1B PPOSNSR 2A 2B H2P 145L 145L 145L 145LTK P 0L 0L 0L 0LQTY - 6L - 6L - 6L - 6LT -417L -417L -417L -417LHTR T -417L -417L -417L -417L

314

DISP 168 (CRYO PALLET)

Two current level detectors are built into thecircuit of each oxygen tank heater to interruptpower in case of electrical shorts. The seconddetector is redundant. Each detector is dividedinto A and B detectors, which monitor theheater A current and the heater B currentrespectively. The detectors are powered bycircuit breakers on panels O14, O15, O16, andML86B. The detectors monitor the current inand out of a heater. If the current difference is0.9 amp or greater for 1.5 milliseconds, a tripsignal is sent to the heater logic to removepower from the heaters regardless of the heaterswitch position. If one element of a heatercauses a “trip-out,” power to both elements isremoved. The O2 TK 1, 2, 3 HEATERS RESET/TEST switches on panel R1 and the O2 RESET/TEST switches on panels A11 and A15 can beused to reapply power to that heater bypositioning them to RESET. The TEST positionwill cause a 1.5-amp delta current to flowthrough all four detectors of a specified oxygentank, causing them to trip out. During on-orbitoperations, the flight crew will be alerted to acurrent level detector trip-out by an SM ALERT,S68 CRY0 02 message, or S168 CRYO message.

Cryogenic System Heater Sensor CircuitBreaker on Panel O14

Each oxygen and hydrogen tank has a quantitysensor powered by a circuit breaker. These areidentified on panel O13 as CRYO QTY O2 (or H2)TK 1 and TK 2 and on panel ML86B as CRYOQTY O2 (or H2) TK 3, TK 4, and TK 5. Data fromthe quantity sensors are sent to panel O2, wherethe rotary switch below the meters is used toselect the tank for display on the CRYO O2 (orH2) QTY meters. The range of the meters is zeroto 100 percent. The data are also displayed onDISP 68 or DISP 168.

Cryogenic System Heater Sensor CircuitBreaker on Panel O15

Cryogenic System Heater Sensor CircuitBreaker on Panel O16

There are two pressure sensors for each oxygenand hydrogen tank. One sensor transmits itsdata to the tank heater controllers, the cautionand warning matrix on panel F7, and is dis-played as "P" on DISPs 68 and 168. The yellowO2 PRESS light is illuminated if oxygen tankpressure is below 540 psia or above 985 psia.The yellow H2 PRESS light illuminates ifhydrogen tank pressure is below 153 psia orabove 293.8 psia. Data from the other sensor isdisplayed as TK P on DISPs 68 and 168 and thePRESS meters on panel O2. The SM ALERT andBACKUP C/W ALARM lights on panel F7 arealso driven by the data. The range of the O2

meter is 0 to 1,200 psia. The H2 meter’s range is0 to 400 psia. The range of "P" displayed on thePASS and BFS SM SYS SUMM 1 and DISPs 68and 168 is 515 to 1,215 psia for oxygen and 145to 305 psia for hydrogen.

The data from oxygen and hydrogen fluidtemperature sensors is displayed on DISP 68 orDISP 168 and transmitted to the ground.

O2 TK1, TK 2 HEATERS RESET/TESTSwitches on Panel R1

O2 TK 4/5 RESEST/TEST SWITCH ON PanelA11 (OV-102)

Cryogenic Tank Quantity Sensor CircuitBreakers on Panel O13

Cryogenic System Heater Sensor Circuit Breaker on Panel ML86B

Reactant Distribution

The cryogenic reactants flow from the tanksthrough a relief valve/filter package moduleand a valve module. They then flow to the fuelcells through a common manifold. Hydrogen issupplied to the manifold from the tank at apressure of 200 to 243 psia and oxygen issupplied at 803 to 883 psia. The pressure of thereactants will be essentially the same at the fuelcell interface as it is in the tanks, since only asmall decrease in pressure occurs in thedistribution system.

Each tank has a relief valve/filter packagemodule that contains the tank relief valve and a12-micron filter. The filter removes contaminantsthat could affect the performance of componentswithin the PRSD and the fuel cells. The valverelieves excessive pressure overboard that buildsup in the tank. The oxygen tank relief valverelieves at 1,005 psia and the hydrogen tank reliefvalve relieves at 302 psia. Tanks one and twoalso have a manifold relief valve that relievespressure in the manifold lines. The manifoldrelief valves are a built-in safety device in theevent a manifold valve and fuel cell reactantvalve are closed because of a malfunction. Thereactants trapped in the manifold lines would bewarmed up by the internal heat of the orbiter andoverpressurize. The manifold relief valve willopen at 290 psi for hydrogen and 975 psi foroxygen to relieve pressure and allow the trappedreactants to flow back to their tanks.

The reactants flow from the relief valve/filterpackages through four reactant valve modules:two hydrogen (hydrogen valve modules 1 and2) and two oxygen (oxygen valve modules 1 and2). The valve modules contain a check valve foreach cryogenic tank line to prevent the reactantsfrom flowing from one tank to another in theevent of a leak. (This prevents a total loss ofreactants. However, hydrogen tank 5 on OV-102 shares a common check valve with tank 4. Aleak in either tank will drain the other as well.For this reason, unless the EDO pallet is flown onOV-102, hydrogen tanks 4 and 5 are used first.)The oxygen valve modules also contain theenvironmental control and life support systematmosphere pressure control system 1 and 2oxygen supply valves. Each module also containsa manifold valve and fuel cell reactant valves.

Each fuel cell has two reactant valves—one forhydrogen and one for oxygen. The valves arecontrolled by the FUEL CELL 1, 3, 2 REACswitches on panel R1. When the switch ispositioned to OPEN, the hydrogen and oxygenreactant valves for that fuel cell are opened andreactants are allowed to flow from the manifoldinto the fuel cell. When the switch is positionedto CLOSE, the hydrogen and oxygen reactantvalves for that fuel cell are closed, isolating thereactants from the fuel cell and rendering thatfuel cell inoperative. Each FUEL CELL REACswitch on panel R1 also has two talkbackindicators, one on each side of the switch. The

Cryogenic Tank Quantity Sensor Circuit Breakers on Panel ML86B

corresponding talkback indicates OP when thevalve is OPEN and CL when the valve is closed.There is redundant capability to close the REACVALVES via the FUEL CELL REAC VLV circuitbreakers and switches on panel C3.

FUEL CELL 1, 3, 2 REAC Switches andTalkbacks on Panel R1

WARNINGThe FUEL CELL REAC switches on panelR1 are in a vertical column with FUEL CELL1 REAC on top, FUEL CELL 3 REAC in themiddle, and FUEL CELL 2 REAC on thebottom. This was done to allow theschematic to be placed on the panel.Because the switches are not in numericalorder, it is possible to inadvertently closethe wrong fuel cell reactant valve whenshutting down a fuel cell.

It is critical to have reactants available to thefuel cells. When any fuel cell reactant valve isclosed, the red FUEL CELL REAC light on panelF7 is illuminated and a caution/warning tone issounded. The computers sense the closed valve,which causes the BACKUP C/W ALARM light onpanel F7 to be illuminated, an SM ALERT tooccur, and a closed indication to be displayedon DISP 69 and SM SYS SUMM 1. This alertsthe flight crew that the fuel cell will beinoperative within approximately 20 to 30

seconds for a hydrogen valve closure and 130seconds for an oxygen valve closure. The timevaries with power loading.

The O2 and H2 MANIFOLD VLV switches onpanel R1 control the respective hydrogen andoxygen manifold valves. When the two hydro-gen and two oxygen manifold valves are in theCLOSE position, fuel cell 1 receives reactantsfrom cryogenic tank set 1, fuel cell 2 receivesreactants from cryogenic tank set 2, and fuel cell3 receives reactants from cryogenic tank sets 3and above. ECLSS atmosphere pressure controlsystem 1 receives oxygen from oxygen tank 1,and system 2 receives oxygen from oxygen tank2. When the switches are positioned to CLOSE,the talkback indicator associated with eachswitch indicates CL.

With both H2 and O2 MANIFOLD VLV 1switches positioned to OPEN and theMANIFOLD VLV 2 switches positioned toCLOSE, cryogenic tanks 1, 3 and above supplyhydrogen to fuel cells 1 and 3, and oxygencryogenic tanks 1, 3 and above supply oxygen tofuel cells 1 and 3, as well as to ECLSSatmosphere pressure control system 1. Thetalkback indicator associated with each MANFVLV 1, 2 switch indicates OP, CL, respectively.O2 and H2 TK 2 continue to supply fuel cell 2.

O2 and H2 MANIFOLD VLV Switches andTalkbacks on Panel R1

CAUTION

There are no relief valves for O2 and H2

manifold 3's with manifold valves 1 and 2closed. This configuration should beavoided except for leak isolation andtroubleshooting

When the H2 and O2 MANIFOLD VLV 2switches are positioned to OPEN, and theMANIFOLD VLV 1 switches are positioned toCLOSE, hydrogen cryogenic tanks 2 and 3 andabove supply hydrogen to fuel cells 2, 3, andoxygen cryogenic tanks 2 and 3 and abovesupply oxygen to fuel cells 2 and 3, as well as toECLSS atmosphere pressure control system 2.The talkback indicator associated with eachMANF VLV 1, 2 switch indicates OP, CL,respectively. H2 and O2 tank 1 continue to sup-ply fuel cell 1.

With the H2 and O2 MANIFOLD VLV 1 and 2switches positioned to OPEN, all hydrogencryogenic tanks are available to supply hydro-gen to all three fuel cells, and all oxygencryogenic tanks are available to supply oxygento all three fuel cells, as well as to ECLSSatmosphere pressure control systems 1 and 2.Because tank sets 1 and 2 are regulated at aslightly lower pressure, tank sets 3 and abovewill feed before tank set 1 and 2. Reactants willbe supplied to the fuel cells by the tank(s) withthe highest pressure.

Manifold pressure data from two pressuresensors located in the respective hydrogen andoxygen valve modules is displayed on DISP 68CRYO SYSTEM. This information is also sent tothe systems management computer where itslower limit is checked. If the respective hydro-gen and oxygen manifold pressures are below150 psia and 200 psia respectively, an SMALERT will occur.

Data for installed EDO pallet cryo tank sets canbe monitored during ascent and entry on CRTdisplay BFS SPEC 168 CRYO PALLET. On orbitthe data is available on SM SPEC 168 CRYOPALLET.

Fuel Cell System

The three fuel cells are located under the payloadbay area in the forward portion of the orbiter’smidfuselage. Each fuel cell is 14 inches high, 15inches wide, 40 inches long, and weighs 255pounds. Each fuel cell is reusable and restartable.

The three fuel cells are individually coupled tothe PRSD subsystem, the active thermal controlsystem (ATCS), the supply water storagesubsystem, and the electrical power distributionand control (EPDC) subsystem. The fuel cellsgenerate heat and water as by-products ofelectrical power generation. The excess heat isdirected to fuel cell heat exchanger, where it isrejected to the Freon coolant loops. The water isdirected to the supply water storage subsystem forthe environmental control and life support system.

ACCESSORIES POWER

� Reactant flow monitor 96 cells� Heat rejection Converts H2 & O2 into:� Water rejection� Thermal control � Power� Cell performance monitor � Potable water� Electrical control unit � Heat

Fuel Cell Block Diagram

The nominal voltage and current range of eachfuel cell is 2 kW at 32.5 volts dc, 61.5 amps to 12kW at 27.5 volts dc, 436 amps. Each is capableof supplying up to 10 kW maximum continuouspower in nominal situations, 12 kWcontinuously in off-nominal situations (with oneor more fuel cells failed), and up to 16 kW for 10minutes. The average on-orbit power consump-tion of the orbiter is approximately 14 kW,leaving additional capability available forpayloads. Each fuel cell is serviced betweenflights and reused until it has accumulated 2,000hours of on-line service.

The orbiter’s three fuel cells operate asindependent electrical power sources, eachsupplying its own isolated, simultaneouslyoperating dc bus. Each fuel cell consists of apower section and an accessory section. Thepower section, where hydrogen and oxygen aretransformed into electrical power, water, and

heat, consists of 96 cells contained in threesubstacks. Manifolds run the length of thesesubstacks and distribute hydrogen, oxygen, andcoolant to the cells. The cells contain an oxygenelectrode (cathode), and a hydrogen electrode(anode) that are separated by a porous matrixsaturated with potassium hydroxide electrolyte.

The accessory section monitors the reactantflow, removes waste heat and water from thechemical reaction, and controls the temperatureof the stack. The accessory section consists ofthe hydrogen and oxygen flow system, thecoolant loop, and the electrical control unit.

The fuel cell generates power through anelectrochemical reaction of hydrogen and oxygen.At the hydrogen electrode (anode), hydrogen isoxidized according to the following reaction:

2H2 + 4OH 4H2O + 4e

forming water and releasing electrons. At theoxygen electrode (cathode), oxygen is reducedin the presence of water. It forms hydroxyl ionsaccording to the following relationship:

O2 + 2H2O + 4e 4OH

The net reaction consumes one oxygen moleculeand two hydrogen atoms in the production oftwo water molecules, with electricity and heatformed as by-products of the reaction.

The fuel cell coolant system circulates a liquidfluorinated hydrocarbon and transfers the wasteheat from the cell stack through the fuel cell heatexchanger to the Freon coolant loop system inthe midfuselage. Internal control of thecirculating fluid maintains the cell stack at aload-dependent operating temperature ofapproximately 200° F.

When the reactants enter the fuel cells, they flowthrough a preheater (where they are warmedfrom a cryogenic temperature to 40° F orgreater), a 6-micron filter, and a two-stage,integrated dual gas regulator module. The firststage of the regulator reduces the pressure ofthe hydrogen and oxygen to 100 to 120 psia.The second stage reduces the oxygen pressureto a range of 60 to 62 psia and maintains thehydrogen pressure at 4.5 to 6 psia differentialbelow the oxygen pressure. The regulated

oxygen lines are connected to the accumulator,which maintains an equalized pressure betweenthe oxygen and the fuel cell coolant. If oxygenand hydrogen pressure decrease, the coolant’spressure is also decreased to prevent a largedifferential pressure inside the stack that coulddeform the cell stack structural elements.

Upon leaving the dual gas regulator module,the incoming hydrogen mixes with the hydro-gen-water vapor exhaust from the fuel cellstack. This saturated gas mixture is routedthrough a condenser, where the temperature ofthe mixture is reduced, condensing a portion ofthe water vapor to form liquid water droplets.The liquid water is then separated from thehydrogen-water mixture by the hydrogen pump/water separator. A centrifugal water separatorextracts the liquid water and pressure-feeds it topotable tanks in the lower deck of the pressur-ized crew cabin. Water from the potable waterstorage tanks can be used for crew consumptionand for cooling the Freon loops by feeding theflash evaporator system.

The hydrogen pump uses three-phase ac powerto circulate the hydrogen gas back to the fuelcell stack, where some of the hydrogen isconsumed in the reaction. The remainder flowsthrough the fuel cell stack, removing theproduct water vapor formed at the hydrogenelectrode. The hydrogen-water vapor mixturethen combines with the regulated hydrogenfrom the dual gas generator module and theloop begins again. The performance at thepump is monitored via the H2 pump statusvoltage display on DISP 69.

The oxygen from the dual gas regulator moduleflows directly through two ports into a closed-end manifold in the fuel cell stack, achievingoptimum oxygen distribution in the cells. Alloxygen that flows into the stack is consumed,except during purge operations.

Reactant consumption is directly related to theelectrical current produced: if there are nointernal or external loads on the fuel cell, noreactants will be used. Because of this directproportion, leaks may be detected by comparingreactant consumption with current produced.An appreciable amount of excess reactants usedindicates a probable leak.

Fuel Cell 1 Typical Display and Controls

Fuel Cell - Typical

Water Removal

Water and electricity are the products of theoxidation/reduction reaction of oxygen andhydrogen that takes place in the fuel cells. Thewater must be removed or the cells will becomesaturated with water, decreasing reactionefficiency. With an operating load of about 7kW, it takes only 110 amp-hr or approximately20 minutes to flood the fuel cell with producedwater, thus effectively halting power generation.Hydrogen is pumped through the stack and theportion that is not consumed in the reaction actsas a carrier gas, picking up and removing watervapor on the way. After being condensed, theliquid water is separated from the hydrogen bythe hydrogen pump/water separator anddischarged from the fuel cell to be stored in theECLSS potable water storage tanks.

Product water is routed to tank A; when tank Ais full, it is routed to tank B, then tanks C and D.An alternate water delivery path is alsoavailable to deliver water to the ECLSS tanks ifthe primary path is lost. If water tanks are full,or there is line blockage, the water relief valvesopen at 45 psia to allow the water to ventoverboard through the water relief line andnozzle. Check valves prevent water tanks fromdischarging through an open relief valve.

For redundancy, two thermostatically activatedheaters are wrapped around the discharge andrelief lines to prevent blockage caused by theformation of ice in the lines. Two switches onpanel R11U, H2O LINE HTR and H2O RELIEFHTR, provide the flight crew with the capabilityto select either A AUTO or B AUTO for the fuelcell water discharge line heaters and the waterrelief line and vent heaters respectively.

Thermostatically controlled heaters willmaintain the water line temperature above 53° Fwhen required. The normal temperature ofproduct water is approximately 140° to 150° F.Thermostatically controlled heaters also main-tain the water relief valve’s temperature whenin use between 70° to 100° F.

Fuel cell 1, 2, or 3 dc voltage and current can bemonitored on the DC VOLTS and DCAMPS/SIG STR meters on panel F9, using therotary switch below the meters to select aspecific fuel cell, and on DISP 69. Temperaturesensors located on the fuel cell water discharge

line, relief valve, relief line, and vent nozzle aredisplayed on the DISP 69 FUEL CELLS .

If the potassium hydroxide electrolyte in anyfuel cell migrates into the product water, a pHsensor located downstream of the hydrogenpump/water separator will sense the presenceof the electrolyte and alert the crew via an SMALERT and display on DISP 69. A common pHsensor is located downstream of the interfacebetween each fuel cell’s primary water path,prior to entering the ECLSS supply H2O tanks.

H2O LINE and H2O RELIEF HTR SwitchesOn Panel R11U

2011/ /069 FUEL CELLS 4 000/02:35:11 000/00:00:00 FC O2 1 2 3VOLTS 31.3 31.2 31.2 H2O RLF LINE T 72AMPS 161 164 163 NOZ T A 244 B 244FLOW O2 2.7 2.7 2.6 HTR SW A H2 0.5 0.5 0.5 PURGE LN O2 T 32REAC O2 OP OP OP H2 T1 40 H2 OP OP OP T2 40 H2O LINE PHSTACK T +204 +204 +204EXIT T 151 151 151COOL T 73 73 73 FC P 61 61 61 1 2 3 PUMP PHH2 PUMP 0.4 0.4 0.4 ∆V SS1 15 18 18 READY RDY RDY RDY SS2 16 20 11 SS3 22 26 26H2O ∆AMPS - 3 + 1 + 2PRI LN T 144 144 144 VLV T 93 93 93ALT LN T 79 79 79

328

FUEL CELLS CRT Display (DISP 69)

DC VOLTS and DC AMPS Meters and RotarySwitch on Panel F9

Fuel Cell Purge

During normal fuel cell operation, the reactantsare present in a closed-loop system and eitherconsumed in the production of electricity orrecirculated through the stack. Any inert gasesor other contaminants will accumulate in andaround the porous electrodes in the cells andreduce the reaction efficiency and electrical loadsupport capability. Purging is performedperiodically to cleanse the cells. The purgesequence can be controlled manually by thecrew, automatically by flight software, or viacommands sent by Mission Control. When apurge is initiated by opening the purge valves,the oxygen and hydrogen systems becomeopen-loop systems; increased flows allow thereactants to circulate through the stack, pick upthe contaminants, and blow them overboardthrough the purge lines and vents. Electricalpower is produced throughout the purgesequence. No more than 10 kilowatts (350amps) should be required from a fuel cell beingpurged because of the increased reactant flowand preheater limitations.

Fuel cell purge can be activated automatically ormanually by the use of switches on panel R11U.

At the initiation of either an auto or manualpurge, the purge line heaters are turned on toheat the purge lines and ensure that thereactants will not freeze in the lines. Thehydrogen reactant is the more likely to freezebecause it is saturated with water vapor.Depending on the orbit trajectory and vehicleorientation, the heaters may require 27 minutesto heat the lines to the required temperatures.Due to limitations on the hydrogen and oxygenpreheaters in the fuel cells, the crew checks thefuel cell current to ensure each fuel cell has aload of less than 350 amps. As the currentoutput of the fuel cell increases, the reactantflow rates increase and the preheaters raise thetemperature of the reactants to a minimum of40° F in order to prevent the seals in the dualgas regulator from freezing prior to purging. Atloads greater than 350 amps, additional flowduring a purge would exceed the maximumflow the preheaters are capable of warming.The crew also checks that the fuel cell is not shutdown, that time to deorbit is more than 3 hours,and, for an automatic purge, that an SM OPStransition is not scheduled within the next hour.An SM OPS transition will halt the automaticpurge sequence.

FUEL CELL Purge Switches on Panel R11U

The purge lines from all three fuel cells aremanifolded together downstream of their purgevalves and associated check valves. The lineleading to the purge outlet is sized to permitunrestricted flow from only one fuel cell at a time.

NOTE

If more than one fuel cell is purgedsimultaneously, back pressure may build inthe purge line. Due to the sizing of thehydrogen and oxygen purge lines, the backpressure in the H2 line can cause thehydrogen pressure to rise above the oxygenpressure. Although the dual gas regulator isdesigned to prevent this from happening, itwould not be advisable to expose all threefuel cells to this condition at the same time.If there is a generic problem with the dualgas regulator, this could cause all three fuelcells to fail simultaneously.

For an automatic purge, the FUEL CELLPURGE heater switch is positioned to GPC andthe FUEL CELL PURGE VALVES 1, 2, and 3switches on panel R11U are positioned to GPC.The FUEL CELL GPC PURGE SEQ switch isthen positioned to START and must be helduntil 3 seconds after the talkback indicatorbeside the switch indicates gray. The automaticpurge sequence will not begin if the indicatorindicates barberpole. First, the GPC turns thepurge line heaters on and monitors thetemperature of the lines. The one oxygen linetemperature sensor must register at least 69° Fand the two hydrogen line temperature sensors79° F and 40° F respectively, before the purgevalves will open. When the proper tempera-tures have been attained, the GPC will open thehydrogen and oxygen purge valves for fuel cell1 for 2 minutes and then close them, and repeatthe process for fuel cells 2 and 3. If the tempera-tures are not up to the minimum limit after 27minutes, the GPC will issue an SM ALERT,display the data on DISP 69, and stop the autosequence. Thirty minutes after the fuel cell 3purge valves have been closed (to ensure thatthe purge lines have been totally evacuated ofwater vapor by the heaters), the GPC will turnoff the purge line heaters. This providessufficient time and heat to bake out anyremaining water vapor. If the heaters areturned off before 30 minutes have elapsed,water vapor left in the lines may freeze. The

GPC purge sequence start can also be initiatedusing the SPEC 60 SM TABLE MAINT display.The sequence is normally performed by MissionControl uplink on orbit.

A manual fuel cell purge would also be initiatedby the flight crew using the switches on panelR11U. In the manual mode, the three fuel cellsmust be purged separately. The FUEL CELLPURGE HEATER switch is positioned to ON forthe same purpose as in the automatic mode, andthe flight crew verifies that the temperatures ofthe oxygen line and two hydrogen lines are atthe same minimum temperatures as in theautomatic mode before the purge sequence isinitiated. The FUEL CELL PURGE VALVE 1switch is then positioned to OPEN for 2 minutesand the flight crew observes that the oxygenand hydrogen flow rates increase on DISP 69.The switch is then positioned to CLOSE, and adecrease in the oxygen and hydrogen flow ratesis observed on DISP 69, confirming the purgevalves are closed. Fuel cell 2 is purged in thesame manner using the FUEL CELL PURGEVALVES 2 switch. Fuel cell 3 is then purged inthe same manner using the FUEL CELL PURGEVALVES 3 switch. After the 30-minute linebakeout period, the FUEL CELL PURGE HEATERswitch is positioned to OFF. Manual purges usedifferent heaters than automatic purges. At leastone manual purge is scheduled per flight

Fuel Cell Cooling/Temperature Control

In order to cool the fuel cell stack during itsoperations, distribute heat during fuel cell start-up,and warm the cryogenic reactants entering thestack, the fuel cell circulates fluorinatedhydrocarbon throughout the fuel cell. The fuel cellcoolant loop and its interface with the ECLSS Freoncoolant loops are identical in fuel cells 1, 2, and 3.

The temperature of the coolant returning from theFreon coolant loops is sensed before it enters thefuel cell. This is displayed as COOL T on DISP 69.It then enters the fuel cell and passes through a 75-micron filter. After the filter, two temperature-controlled mixing valves allow some of the hotcoolant to mix with the cool returning coolant toprevent the condenser exit control valve fromoscillating. The control valve adjusts the flow ofthe coolant through the condenser to maintain thehydrogen-water vapor exiting the condenser at atemperature between 148° and 153° F.

The stack inlet control valve maintains thetemperature of the coolant entering the stackbetween 177° and 187° F. The accumulatorinterfaces with the oxygen cryogenic reactant tomaintain an equalized pressure between theoxygen and the coolant (the oxygen andhydrogen pressures are controlled at the dualgas regulator) to preclude a high pressuredifferential in the stack. The pressure in thecoolant loop is sensed before the coolant entersthe stack and is displayed as COOL P on DISP 69.

The coolant is circulated through the fuel cellstack to absorb the waste heat from thehydrogen/oxygen reaction occurring in theindividual cells. After the coolant leaves thestack, its temperature is sensed and the data isdisplayed on the FUEL CELL STACK TEMPmeter through the three-position switch locatedbelow the meter on panel O2 and on the DISP 69display as STACK T. The yellow FUEL CELLSTACK TEMP, the red BACKUP C/W ALARMlight, and the SM ALERT light on panel F7 willbe illuminated if fuel cell and stack temperaturesexceed upper or lower limits. The hot coolantfrom the stack flows through the oxygen andhydrogen preheaters where it warms thecryogenic reactants before they enter the stack.

FUEL CELL STACK TEMP Meter and FUELCELL 1, 2, 3 Switch on Panel O2

The coolant pump utilizes three-phase ac powerto circulate the coolant through the loop. Thedifferential pressure sensor senses a pressuredifferential across the pump to determine thestatus of the pump. The FUEL CELL PUMPlight on panel F7 will be illuminated if fuel cell1, 2, or 3 coolant pump delta pressure is lost anda fault message will be sent to the CRT. If thecoolant pump for fuel cell 1, 2, or 3 is off, theBACKUP C/W ALARM light will be illuminatedand a fault message and a �P� will bedisplayed.

The temperature-actuated flow control valvedownstream from the pump adjusts the coolantflow to maintain the fuel cell coolant exittemperature between 190° and 210° F. The stackinlet control valve and flow control valve havebypass orifices to allow coolant flow throughthe coolant pump and to maintain some coolantflow through the condenser for watercondensation, even when the valves are fullyclosed due to the requirements of thermalconditioning. The hot coolant that is not usedat the mixing valves exits the fuel cells to thefuel cell heat exchanger where it transfers itsexcess heat to be dissipated through the ECLSSFreon coolant loop systems in the midfuselage.

Electrical Control Unit

The electrical control unit located in each fuelcell power plant contains the startup logic,heater thermostats, 30-second timer, andinterfaces with the controls and displays for fuelcell startup, operation, and shutdown. The unitcontrols the supply of ac power to the coolantpump, hydrogen pump/water separator andthe pH sensor. It also controls the dc powersupplied to the flow control bypass valve (openonly during startup) and the internal startupand sustaining heaters. It also controls thestatus of the FUEL CELL 1, 2, 3 READY FORLOAD and COOLANT PUMP ÆP talkbackindicators on panel R1.

The nine fuel cell circuit breakers that connectthree-phase ac power to the three fuel cells arelocated on panel L4. Each fuel cell electricalcontrol unit receives its power from an essentialbus through the FC CNTLR switch on panelsO14, O15 and O16.

FUEL CELL Circuit Breakers on Panel L4

FC1 CNTRL Switch on Panel O14

Cell Performance Monitor

As fuel cell life increased with the introductionof the third substack, the likelihood ofindividual cell failures caused by “wearout” orage increased. Consequently, the cellperformance monitor (CPM) was designed todetect individual cell performance problems orimminent failures such as cell crossover.

Crossover is defined as an uncontrolled mixingof the reactants that results in the generation ofexcessive product heat. The combination ofhydrogen and oxygen requires little heat toexplode. During normal fuel cell operation,hydrogen and oxygen are diffusely mixed toproduce electricity. The matrix, a fibrousasbestos blotter device within each cell, holds

the KOH electrolyte and limits the H2/O2

mixing. A crossover occurs when the matrixfails to do its job. A manufacturing flaw in thematrix or an impurity in the matrix fibers maycause a pin hole to develop after hours andhours of exposure to the caustic KOH in thepresence of the waste heat within the cell.

As the direct combination of the reactantmolecules occurs at the pin hole, increasedwaste heat may cause the pin hole to burn andenlarge allowing more reactants to combine. Ifthis reaction continues, it might uncontrollablypropagate and possibly result in a violentexplosion. A crossover may proceed rapidly orgradually. Before the introduction of the cellperformance monitor prior to STS-9, theelectrical performance indicators of a crossover-distressed cell were not detectable, since the lostelectrical performance of the substack with thedegraded cell was picked up by the healthysubstacks.

The cell performance monitor compares eachhalf substack voltage and calculates a delta voltsmeasurement. By comparing both halves of asubstack, the cell performance monitor candetect cell changes. Since all the cells in asubstack are producing the same current, bothhalves of the substack should be atapproximately identical voltages. Ideally �V = 0but small fluctuations are common. Asignificant voltage loss in any one cell willproduce a change in the �V with 300 mV beinga critical performance loss. �V can be eitherpositive or negative, so the cell performancemonitor converts the positive/negative �Vbetween the upper and lower substack halves toan absolute value and then outputs that value (0to 500 mV). To provide margin andaccommodate the absolute value, the C/W limitis set at 150 mV. The output is then convertedto a 0 to 5 V dc signal for telemetry and crewdisplay systems by a “times 10” amplifier.Calibration curves rescale this to electronic units(EU) (0 to 50 mV).

Every 7.5 minutes the cell performance monitorperforms a self-test to ensure data accuracy andto prevent fuel cell shutdown due to faulty data.During the self-test period:

� The voltage measurement of the substackhalves ceases.

� A test signal of 50 mV is input to the voltagemeasurement logic.

� The output of the “times 10” amplifier isexamined for 425 to 575 mV.

� If the test fails, the output voltage is driven to5 volts until the next self-test is performed 7 ±1 minute later. A test voltage of 50 mV isintroduced and the sequence repeats itself.

� If the self-test is passed, the unit resumessubstack half voltage measurements.

Fuel Cell Start

The FUEL CELL START/STOP switches on panelR1 are used to initiate the start sequence or stopthe fuel cell operation. When a switch is held inits momentary START position, the control unitconnects the three-phase ac power to the coolantpump and hydrogen pump/water separator(allowing the coolant and the hydrogen-watervapor to circulate through these loops) andconnects the dc power to the internal startupand sustaining heaters and the flow controlbypass valve. The switch must be held in theSTART position until the COOLANT PUMP ÆPtalkback shows gray (approximately three tofour seconds), which indicates that the coolantpump is functioning properly by creating adifferential pressure across the pump. Whenthe COOLANT PUMP ÆP talkback indicatesbarberpole, the coolant pump is not running.

The READY FOR LOAD talkback for each fuelcell will show gray after the 30-second timertimes out or when the stack-out temperature is

above 187° F, whichever occurs last. Stack-outtemperature can be monitored on the SM SYSSUMM 1 (STACK T) and DISP 69 FUEL CELLSdisplays (STACK T). It can also be monitoredon the FUEL CELL STACK OUT TEMP meteron panel O2, using the switch beneath the meterto select the fuel cell. When the READY FORLOAD talkback shows gray, the fuel cell is up tothe proper operating temperature and is readyfor loads to be attached to it. It should not takelonger than 25 minutes for the fuel cell to warmup and become fully operational; the actual timedepends on the fuel cell's initial temperature.The READY FOR LOAD indicator remains grayuntil the FUEL CELL START/STOP switch foreach fuel cell is placed to STOP, the FC CNTLRswitch on the overhead panel is placed to OFFor the essential bus power is lost to the electricalcontrol unit.

2011/ /078 SM SYS SUMM 1 4 000/14:44:12 000/00:00:00SMOKE 1/A 2/B DC VOLTS 1/A 2/B 3/C CABIN 0.0 FC 30.6 30.1 31.0 L/R FD 0.0 0.0 MAIN 30.6 30.1 31.0 AV BAY 1 0.3 0.3 ESS 29.6 29.6 29.3 2 0.3 0.4 A B C A 3 0.3 0.3 CNTL 1 29.4 29.4 29.6 CABIN 2 29.4 29.4 29.4 PRESS 14.0 3 29.4 29.4 29.4 dP/dT-EQ +.00 +.000 AC 02 CONC VOLT φA 118 118 117 PPO2 3.00 3.00 φB 117 117 118 FAN ∆P 5.00 φC 117 117 118 HX OUT T 46 AMPS φA 4.3 6.3 2.1 O2 FLOW 0.0 0.0 φB 5.5 6.6 2.2 N2 FLOW 0.0 0.0 φC 3.1 5.0 3.2 IMU FAN A B C FUEL CEL ∆V FC1 FC2 FC3 AMPS 180 232 146 SS1 22 21 22 REAC VLV OP OP OP SS2 22 22 23 STACK T +202 +206 +200 SS3 23 21 21 EXIT T 150 152 149 TOTAL AMPS 557 COOL P 61 60 61 KW 17 PUMP

243

SM SYS SUMM 1 (DISP 78)

FUEL CELL START/STOP Switches and READY FOR LOADand COOLANT PUMP P Talkback Indicators on Panel R1

Electrical Power Distribution and Control

The electrical power distribution and controlsubsystem controls and distributes ac and dcelectrical power to the orbiter subsystems, thesolid rocket boosters, the external tank, andpayloads. The 28 volts dc generated by each ofthe three fuel cells is distributed to a main dcbus. The three main dc buses (MN A, MN B,and MN C) are the prime sources of power forthe vehicle's dc loads. Each of the three dc mainbuses supplies power to three solid-state (static),single-phase inverters, each of which powersone three-phase alternating-current bus; thus,the nine inverters convert dc power to 117 +3. -1volt rms, 400-hertz ac power for distribution tothree ac buses (AC 1, AC 2, and AC 3) for thevehicle's ac loads.

Bus System

The three main dc buses are main A (MN A),main B (MN B), and main C (MN C). Three acbuses, AC 1, AC 2, and AC 3, supply ac power

to the ac loads. Three essential buses, ESS1BC,ESS2CA, and ESS3AB, supply dc power toselected flight crew controls and electrical loadsthat are deemed essential. Nine control buses -CNTL AB 1, 2, 3; CNTL BC 1, 2, 3; and CNTLCA 1, 2, 3 - are used to supply control power toflight crew controls. Two preflight buses,PREFLT 1 and PREFLT 2, are used only duringground operations.

Depending on the criticality of orbiter electricalequipment, some electrical loads may receiveredundant power from two or three main buses.If an electrical load receives power from two orthree sources, it is for redundancy only and notfor total power consumption.

DC Power

Main Buses

The main buses distribute dc electrical powerfrom the fuel cells to locations throughout theorbiter. Distribution assemblies handle therouting of main bus power.

IDCA1 (2, 3)

Inverters(9)

Forwardcontroller

assemblies1 (2, 3)

Forwardmotor

controllerassemblies

1 (2, 3)

Panel buses

Flight deck

Middeck

Forwardpower

controllerassemblies

1 (2, 3)

Fuel cells1 (2, 3)

Main dcdistributionassemblies

1 (2, 3)

Mid powercontroller

assemblies1 (2, 3)

Mid motorcontroller

assemblies(1, 2, 3, 4)

Alt powercontroller

assemblies1 (2, 3)

Aft motorcontroller

assemblies1 (2, 3)

Externaltank

Aft sectionMidsectionForward section

GSE

Left SRBRight SRB

Panel buses

Alt powercontroller

assemblies4 (5, 6)

Solid rocketbooster

Alt loadcontroller

assemblies1 (2, 3)

335.cvs

Electrical Power Distribution Block Diagram

AC Bus 1

FLCA 1

FMCA 1

336.cvs

AC Bus 2

FLCA 2

FMCA 2

AC Bus 3

FLCA 3

FMCA 3

FPCA 1

FPCA 2

FPCA 2

DA 1

MPCA 1

FuelCell 1

MMCA 1, 3

DA 2

MPCA 2

FuelCell 2

MMCA 1, 2, 3, 4

DA 3

MPCA 3

FuelCell 3

MMCA 2, 4

APCA 4

APCA 1

ALCA 1

AMCA 1

APCA 5

APCA 2

ALCA 2

AMCA 2

APCA 6

APCA 3

ALCA 3

AMCA 3

Ground SupportEquipment

GSE

GSE

Solid RocketBoostersExternalTank

SRBs

ET

SRBs

ET

DA - Distribution AssemblyFLCA - Forward Load Controller AssemblyFMCA - Forward Motor Controller AssemblyFPCA - Forward Power Controller AssemblyMPCA - Mid Power Controller AssemblyMMCA - Mid Motor Controller AssemblyAPCA - Aft Power Controller AssemblyALCA - Aft Load Controller AssemblyAMCA - Aft Motor Controller Assembly

Direct Current Power Distribution Diagram

CONTROL BUS Circuit Breakers on Panel R15

Essential Buses

Essential buses supply power to switches thatare necessary to restore power to a failed maindc or ac bus and to essential electrical loads andswitches. In some cases, essential buses areused to power switching discretes tomultiplexers/demultiplexers. Examples of theselected flight crew switches and loads are theEPS switches, GPC switches, TACAN, radaraltimeter and microwave scan beam landingsystem power switches, the caution andwarning system, emergency lighting, audiocontrol panel, and master timing unit.

Three essential buses, ESS1BC, ESS2CA, andESS3AB receive power from three redundantsources. For example, ESS1BC receives dcpower from fuel cell 1 through the ESS BUSSOURCE FC 1 switch on panel R1 via a fusewhen the switch is positioned to ON, and frommain dc buses B and C through remote powercontrollers (RPC) when the ESS BUS SOURCEMN B/C switch on panel R1 is positioned toON. Electrical power is then distributed from theessential bus in distribution assembly 1 throughfuses to the corresponding controller assembliesand to the flight and middeck panels.

Control Buses

Nine control buses supply control power to thedisplay and control panel switches on the flightdeck and in the middeck area. A control busdoes not supply operational power to anysystem loads. Each control bus receives powerfrom each main dc bus for redundancy. Themain buses identified in the control bus nameare connected through RPCs and diodes. Forexample, CNTL BC1 is powered by MN B andMN C through remote power controllers. The"unnamed bus", which in this example is MN A,provides power to the control bus through acircuit breaker and a fuse. The number in eachcontrol bus name does not refer to a powersource for the bus; it is merely a numericdesignator for the bus.

The control buses are enabled by the CONTROLBUS PWR MN A, MN B, and MN C switches onpanel R1 and the MN A CONTROL BUSBC1/2/3, the MN B CONTROL BUS CA1/2/3and the MN C CONTROL BUS AB1/2/3 circuitbreakers on panel R15.

Bus Control Switches and Talkbackson Panel R1

The RPCs are powered continuously unless oneof the CONTROL BUS PWR MN A, MN B, MN Cmomentary switches on panel R1 is positionedto the RESET position, which turns thecorresponding RPCs power off and resets theRPC if it has been tripped off. An SM ALERTlight is illuminated if the control bus voltage isless than 24.5 volts dc and a fault message issent to the CRT. The Mission Control Centercan monitor the status of each RPC.

Payload Buses

Payload buses are provided to supply dcelectrical power to the payloads. Fuel cell 3 maybe connected to the primary payload bus bypositioning the PRI FC3 momentary switch onpanel R1 to the ON position. The talkbackindicator above the switch will indicate ONwhen fuel cell 3 is connected to the PRI PL bus.The PRI PL bus is the prime bus for supplyingpower to the payloads. Fuel cell 3 may bedisconnected from the payload bus bypositioning the PRI FC3 switch to the OFFposition. The talkback indicator above theswitch will indicate OFF.

A second source of electrical power for the PRIPL bus may be supplied from the MN B bus by

positioning the PRI MN B momentary switch onpanel R1 to the ON position. The talkbackindicator above this switch will indicate ON.The MN B bus may be removed from the PRI PLbus by positioning the switch momentarily toOFF. The talkback indicator will indicate OFF.A third possible source of electrical power forthe PRI PL bus may be supplied from the MN Cbus through the PRI MN C switch on panel R1,positioned momentarily to the ON position.The talkback indicator will indicate ON. TheMN C bus may be removed from the PRI PL busby positioning the switch momentarily to OFF.The talkback indicator will indicate OFF.

NOTE

Main buses B and C can be tied via thePRI PL bus. This is referred to as a"backdoor bus tie."

Two additional payload buses are located in theaft section of the payload bay at the aftbulkhead station. The aft payload B bus may bepowered up by positioning the AFT MN Bswitch on panel R1 to ON. The aft payload Cbus may be powered up by positioning the AFTMN C switch on panel R1 to ON. The OFFposition of each switch removes power from thecorresponding aft payload bus.

The AUX switch on panel R1 permits main busA and main bus B power to be supplied to theAUX PL A and AUX PL B buses when theswitch is positioned to ON. The auxiliarypayload buses can provide power foremergency equipment or controls associatedwith payloads. The OFF position removespower from the AUX PL A and PL B buses. Thetwo auxiliary payload buses may be diodedtogether to form one bus for redundancy.

The CABIN switch on panel R1 provides MN Aor MN B power to patch panels located behindthe payload specialist and mission specialiststations on the aft flight deck. These patchpanels supply power to the payload-relatedequipment located on panels at these stations.Two three-phase circuit breakers, AC 2PAYLOAD 3� and AC 3 PAYLOAD 3� on panelMA73C, provide ac power to the payload patchpanels.

AC Power Generation

Alternating-current power is generated andmade available to system loads by the electricalpower distribution and control subsystem usingthree independent ac buses, AC 1, AC 2, andAC 3. The ac power system includes the acinverters for dc conversion to ac and inverterdistribution and control assemblies containingthe ac buses and the ac bus sensors. The ac

power is distributed from the inverterdistribution and controller assemblies to theflight and middeck display and control panelsand from the motor controller assemblies to thethree-phase motor loads.

Each ac bus consists of three separate phasesconnected in a three-phase array. Static invert-ers, one for each phase, are located in theforward avionics bays. Each inverter has anoutput voltage of 116 to 120 volts rms at 400hertz, ±7 hertz.

Input power to the inverters is controlled by theINV PWR 1, 2, 3 switches on panel R1. INV PWR

1 routes power to AC 1 from MN A, INV PWR 2to AC 2 from MN B, and INV PWR 3 to AC 3from MN C. All three inverters of AC 1 receiveMN A bus power when the INV PWR 1 switch ispositioned to ON, and all three must be inoperation before the talkbacks above the switchesindicate ON. The indicators will show OFF whenmain bus power is not connected to the inverters.The INV/AC BUS 1, 2, 3 switches on panel R1are used to apply each inverter's output to its

respective ac bus. An indicator above eachswitch shows its status, and all three invertersmust be connected to their respective ac busesbefore the indicators show ON. The talkbackindicators will show OFF when any of the phasesare not connected to their respective ac bus.

The INV PWR and INV/AC BUS switches andtalkbacks must have control power from the ACCONTR circuit breakers on panel R1 in order tooperate. Once ac power has been established,these circuit breakers are opened to prevent anyinadvertent disconnection, whether by switchfailure or accidental movement of the INV PWRor INV/AC BUS switches.

AC2 Cabin PAYLOAD 3�� AND AC3 Cabin PAYLOAD 3�� Circuit Breakers on Panel MA73C

Electrical Power Distribution andControl Assemblies

Electrical power is controlled and distributed byassemblies. Each assembly (main distributionassembly, power controller assembly, load con-troller assembly, and motor controller assembly)is in an electrical equipment container.

INV PWR and INV/AC BUS Switches andTalkbacks, and AC CONTR Circuit Breakers

on Panel R1

Distribution Assemblies

The dc power generated by each of the fuel cellsis supplied to a corresponding distributionassembly (DA). Fuel cell 1 powers DA 1, fuelcell 2 powers DA 2, and fuel cell 3 powers DA 3.Each distribution assembly contains remotelycontrolled motor-driven switches called powercontactors used for loads larger than 125 amps.The power contactors are rated at 500 amps.They control and distribute dc power to a

corresponding mid power controller assembly,forward power controller assembly, and aftpower controller assembly.

The FC MAIN BUS A switch on panel R1positioned to ON connects fuel cell 1 to the MNA DA 1 and controllers and disconnects fuel cell1 from the MN A DA 1 and controllers whenpositioned to OFF. The talkback indicator nextto the FC MAIN BUS A switch will indicate ONwhen fuel cell 1 is connected to main bus Adistribution assembly and controllers, and OFFwhen fuel cell 1 is disconnected from main busA distribution assembly and controllers. The FCMAIN BUS B and C switches and talkbackindicators on panel R1 function in the samemanner. The No. 1 distribution assembly andall No. 1 controllers go with fuel cell 1 and MNA bus, all No. 2 controllers and DA 2 go withfuel cell 2 and MN B, and all No. 3 controllersand DA 3 go with fuel cell 3 and MN C.

Power Controllers

Power contactors are also located in the aft powercontroller assemblies to control and distributeground-supplied 28-volt dc power to the orbiterthrough the T-0 umbilical before the fuel cellstake over the supply of orbiter dc power.

Each of the mid, forward, and aft powercontroller assemblies supplies and distributes dcpower to a corresponding motor controller as-sembly and dc power to the orbiter through theT-0 umbilical before the fuel cells take over thesupply of orbiter dc power.

Each of the mid, forward, and aft powercontroller assemblies supplies and distributes dcpower to a corresponding motor controllerassembly and dc power to activate the corre-sponding ac power system.

Remote Power Controllers

Each power controller assembly contains RPCsand relays. The RPCs are solid-state switchingdevices used for loads requiring current in arange of 3 to 20 amps. The RPCs are currentprotected by internal fuses and also have thecapability to limit the output current to amaximum of 150 percent of rated value for 2 to3 seconds. Within 3 seconds, the RPC will tripout, removing the output current. To restore

power to the load, the RPC must be reset. Thisis accomplished by cycling a control switch. Ifmultiple control inputs are required before aRPC is turned on, hybrid drivers are usuallyused as a logic switch, which then drives thecontrol input of the RPC.

Load Controllers

Each load controller assembly contains hybriddrivers, which are solid-state switching devices(no mechanical parts) used as logic switches andfor low-power electrical loads of less than 5 amps.When the drivers are used as a logic switch,several control inputs are required to turn on aload. Hybrid drivers are also used in the midpower controller assemblies. The hybrid driversare current protected by internal fuses. Hybridrelays requiring multiple control inputs are usedto switch three-phase ac power to motors.

Relays are also used for loads between 20 ampsand 135 amps in power controller assembliesand motor controller assemblies.

In the midbody, there are no load controllerassemblies; therefore, the mid power controllerassemblies contain remote power controllers,relays, and hybrid drivers.

Aft power controller assemblies 4, 5, and 6 wereadded to accommodate additional electricalloads in the aft section. Assemblies 4, 5, and 6are powered by fuel cell 1/MN A, fuel cell2/MN B, and fuel cell 3/MN C respectively.

Motor Controllers

There are 10 motor controller assemblies usedon the orbiter: three are in the forward area,four are in the midbody area, and three are inthe aft area. Panel MA73C contains the controlsfor the motor controller assemblies. Their onlyfunction is to supply ac power to noncontinuousac loads for ac motors used for vent doors, airdata doors, star tracker doors, payload baydoors, payload bay latches, ET doors andlatches, RMS deploy motors and latches, andreaction control system/orbital maneuveringsystem motor-actuated valves. Each motorcontroller assembly contains main dc buses, acbuses, and hybrid relays that are remotelycontrolled for the application or removal of acpower to ac motors. The main dc bus is usedonly to supply control or logic power to thehybrid relays so the ac power can be switchedon or off. The hybrid relays permit majorelectrical power distribution buses to be locatedclose to the major electrical loads, which

Motor Controller Assembly Controls on Panel MA37C

minimizes use of heavy electrical feeders to andfrom the pressurized crew compartment displayand control panels. This reduces the amount ofspacecraft wiring and weight, and permits moreflexible electrical load management. If a mainbus is lost, the hybrid relays using that main buswill not operate. In some cases, the hybridrelays will use logic power from a switchinstead of the motor controller assembly bus.

The three forward motor controller assemblies(FMC 1, FMC 2, and FMC 3) correspond to MNA/AC 1, MN B/AC 2, and MN C/AC 3respectively. Each FMC contains a main bus, anac bus, and an RCS ac bus. The main bussupplies control or logic power to the relaysassociated with both the ac bus and RCS ac bus.The ac bus supplies power to the forward leftand right vent doors, the star tracker Y and Zdoors, and the air data left and right doors. TheRCS ac bus supplies power to the forward RCSmanifold and tank isolation valves.

The aft motor controller assemblies (AMC 1,AMC 2, and AMC 3) correspond to MN A/AC1, MN B/AC 2, and MN C/AC 3 respectively.Each aft motor controller assembly contains amain bus and its corresponding ac bus and amain RCS/OMS bus and its correspondingRCS/OMS ac bus. Both main buses are used forcontrol or logic power for the hybrid relays. Theac bus is used by the aft RCS/OMS manifold andtank isolation and crossfeed valves, the aft leftand right vent doors, and the ET umbilical doors.

The mid motor controller assemblies (MMC 1,MMC 2, MMC 3, and MMC 4) contain two maindc buses and two corresponding ac buses. MMC1 contains main bus A and B and theircorresponding buses, AC 1 and AC 2. MMC 2contains MN B and C and AC 2 and AC 3 buses.MMC 3 contains the same buses as MMC 1, andMMC 4 contains the same buses as MMC 2. Loadsfor the main buses/ac buses are vent doors,payload bay doors and latches, radiator paneldeployment actuator and latches, RMS deploymotors and latches, and payload retention latches.

Component Cooling

The electrical components in the midbody aremounted on cold plates and cooled by the Freoncoolant loops. The power controller assemblies,load controller assemblies, motor controller

assemblies, and inverters located in forwardavionics bays 1, 2, and 3 are mounted on coldplates and cooled by the water coolant loops.The inverter distribution assemblies in forwardavionics bays 1, 2, and 3 are air-cooled. Theload controller assemblies, power controllerassemblies, and motor controller assemblieslocated in the aft avionics bays are mounted oncold plates and cooled by the Freon systemcoolant loops.

Bus Tie

In the event of an electrical failure, or to loadshare between fuel cells, any main bus can beconnected to another main bus through the useof the MN BUS TIE switches on panel R1 andpower contactors in the distribution assemblies.For example, main bus A can be connected tomain bus B by positioning both the MN BUS TIEA and the MN BUS TIE B switch to ON. Thetalkback indicators next to the switches willindicate ON when main bus A is connected tomain bus B. Other indications of an effectivebus tie are matching voltages for the two buses.To disconnect main bus A from main bus B, theMN BUS TIE A and B switches must bepositioned to OFF; the talkback indicators nextto the switches will then indicate OFF.

NOTE

It is also possible to tie the main buses Band C together via the PRI switches. Thisis done by connecting each main bus tothe primary payload bus.

Instrumentation and System Monitoring

Main bus A, B, or C voltages can be displayedon the DC VOLTS meter on panel F9 byselecting MAIN VOLTS A, B, or C on the rotaryswitch beneath the meter. The MAIN BUSUNDERVOLT red caution and warning light onpanel F7 will be illuminated if main bus A, B, orC voltage is 26.4 volts dc, informing the crewthat the minimum equipment operating voltagelimit of 24 volts dc is being approached. ABACKUP C/W ALARM light will also beilluminated at 26.4 volts dc. An SM ALERT lightwill be illuminated at 27 volts dc or less, alertingthe flight crew to the possibility of a future low-voltage problem. A fault message also istransmitted to the CRT.

MN BUS TIE Switches on Panel R1

EPS Meters and Switches on Panel F9

The nominal fuel cell voltage is 27.5 to 32.5 voltsdc, and the nominal main bus voltage range is27 to 32 volts dc, which corresponds to 12- and2-kilowatt loads respectively. If buses are tied,they will indicate the same voltage.

The ESS bus voltage can be monitored on theDC VOLTS meter on panel F9 by selecting ESSVOLTS 1BC, 2CA, 3AB on the rotary switchbelow the meter. An SM ALERT light will beilluminated to inform the flight crew if theessential bus voltage is less than 25 volts dc. Afault message also is displayed on the CRT.

Each ac bus has a sensor, switch, and circuitbreaker for flight crew control. The AC 1, 2, and3 SNSR circuit breakers located on panel O13apply essential bus power to their respective ACBUS SNSR 1, 2, 3 switch on panel R1 andoperational power to the respective INV/ACBUS switch and indicator. The AC BUS SENSR1, 2, 3 switch selects the mode of operation ofthe ac bus sensor: AUTO TRIP, MONITOR, orOFF. The ac bus sensor monitors each ac phasebus for over- or undervoltage, and each phaseinverter for an overload signal. The overvoltagelimits are bus voltages greater than 123 to 127volts ac for 50 to 90 milliseconds. Theundervoltage limits are bus voltages less than102 to 108 volts ac for 6.5 to 8.5 milliseconds.An overload occurs when any ac phase current isgreater than 14.5 amps for 10 to 20 seconds, or isgreater than 17.3 to 21.1 amps for 4 to 6 seconds.

When the respective AC BUS SNSR switch is in theAUTO TRIP position, and an overload orovervoltage condition occurs, the ac bus sensor willilluminate the respective yellow AC OVERLOADor AC VOLTAGE caution and warning light onpanel F7 and trip off (disconnect) the inverter fromits respective phase bus for the bus/invertercausing the problem. There is only one ACVOLTAGE and one AC OVERLOAD caution andwarning light; as a result, all nine inverters/acphase buses can illuminate the lights. The CRT orthe AC VOLTS meter and rotary switch beside iton panel F9 would be used to determine whichinverter or phase bus caused the light to illuminate.The phase causing the problem would approach 0amps after the bus sensor disconnects the inverterfrom the bus. Because of the various three-phasemotors throughout the vehicle, there will be aninduced voltage on the disconnected phase if onlyone phase has loss of power.

AC 1, 2, 3 SNSR Circuit Breakers on Panel O13

AC BUS SNSR Switches and INV/AC BUTSwitches and Talkback Indicators on Panel R1

Before power can be restored to the tripped bus,the trip signal to the INV/AC BUS switch mustbe removed by positioning the AC BUS SNSRswitch to OFF, then back to the AUTO TRIPposition, which extinguishes the caution andwarning light. The INV/AC BUS switch is thenpositioned to ON, restoring power to the failedbus. If the problem is still present, the sensortrip sequence will be repeated.

If an undervoltage exists, the yellow ACVOLTAGE caution and warning light on panelF7 will be illuminated, but the inverter will notbe disconnected from its phase bus.

When the AC BUS SENSR 1, 2, 3 switches are inthe MONITOR position, the ac bus sensor willmonitor for an overload, overvoltage, andundervoltage and illuminate the applicablecaution and warning light; but it will not tripout the phase bus/inverter causing the problem.

NOTE

AC BUS SNSR switches are placed inMONITOR for ascent and entry to preventan AC bus problem from causing anAUTO-TRIP, which could result in theloss of SSME controllers on ascent, orlanding gear prox boxes on entry.

When the AC BUS SNSR switches are OFF, theac bus sensors are non-operational, and allhardware caution and warning and trip-outcapabilities are inhibited. The BACKUP C/WALARM light will be illuminated for overload orover- and undervoltage conditions. The SMALERT will occur for over- and undervoltageconditions. A fault message also is sent to theCRT.

Operations

Prelaunch

During prelaunch operations, the onboard fuelcell reactants (oxygen and hydrogen) aresupplied by ground support equipment toassure a full load of onboard reactants beforelift-off. At T minus 2 minutes 35 seconds, theground support equipment filling operation isterminated. The ground support equipmentsupplies hydrogen and oxygen to the PRSDmanifold at pressures greater than the tank

heater control range pressures (approximately280 psia for hydrogen and 960 psia for oxygen).This helps to prevent cryo tank heater cyclingduring powered flight. The ground supportequipment supply valves close automatically totransfer to onboard reactants.

The fuel cells will be on when the crew boardsthe vehicle. Until T minus 3 minutes and 30seconds, power to the orbiter is load sharedwith the fuel cells and ground supportequipment, even though the fuel cells are onand capable of supplying power. Main buspower is supplied through the T-0 umbilicals,MN A through the left-side umbilical and MN Band C through the right-side umbilical to aftpower controllers (APCs) 4, 5, and 6. FromAPCs 4, 5, and 6, the ground support equipmentpower is directed to the distribution assemblywhere the power is distributed throughout thevehicle. The power for the PREFLT 1 andPREFLT 2 test buses is also supplied throughthe T-0 umbilical. These test buses are scatteredthroughout the orbiter and are used to supportlaunch processing system control of criticalorbiter loads, although they also power up theessential buses in the APCs when on groundsupport equipment. As in the main busdistribution, essential bus power from the APCsis directed to the distribution assemblies andthen distributed throughout the vehicle. At Tminus 3 minutes 30 seconds, the ground turnsoff the ground support equipment power to themain buses, and the fuel cells automatically pickup the vehicle electrical loads.

Indication of the switchover can be noted on theCRT display and the DC AMPS meter. The fuelcell current will increase to approximately 220amps; the oxygen and hydrogen flow willincrease to approximately 4.6 and 0.6 pound perhour respectively; and the fuel cell stacktemperature will increase slightly. At T minuszero, the T-0 umbilical is disconnected with thepreflight test bus wires live.

Prelaunch and On-Orbit Tank HeaterManagement

Before lift-off, the O2 TK1 HEATERS A, O2 TK2HEATERS A, H2 TK1 HEATERS A, and H2 TK 2HEATERS A switches on panel R1 are set onAUTO. As part of the Post OMS 1 Burn proce-dures, all the switches on O2 and H2 tanks 1 & 2are positioned to AUTO, while the other tankheaters remain OFF. On orbit, the cryo tankheaters are managed to provide for tankquantity balancing. When hydrogen tanks 4 and5 are installed on OV-102, they are depleted firstdue to their common check valve. Because thetank 3 and 4/5 heater controller pressure limitsare higher than those of tanks 1 and 2, tanks 3and 4/5 supply the reactants to the fuel cellswhen all tank heaters are set to AUTO. For entry,tanks 1 and 2 supply the reactants to the fuel cells.

Fuel Cell Operating Modes

Fuel cell standby consists of removing theelectrical loads from a fuel cell but continuingoperation of the fuel cell pumps, controls,instrumentation, and valves, while electricalpower is supplied by the remaining fuel cells. Asmall amount of reactants is used to generatepower for the fuel cell internal heaters. If thetemperature in the fuel cell compartmentbeneath the payload bay is lower than 40° F, thefuel cell should be left in STANDBY instead ofbeing shut down to prevent it from freezing.

Fuel cell shutdown, after standby, consists ofstopping the coolant pump and hydrogenpump/water separator by positioning thatFUEL CELL START/STOP switch on panel R1 tothe STOP position.

Fuel cell safing consists of closing the fuel cellreactant valves and consuming the reactants insidethe fuel cell by either leaving the fuel cellconnected to its main bus or by keeping the pumppackage operating with the internal heaters.

EPS Caution and Warning Summary

� The SM ALERT will illuminate if a cryoheater current level sensor detects a short ona heater circuit.

� The yellow O2 PRESS and the red BACKUPC/W ALARM lights illuminate if oxygentank pressure is below 540 psia or above 985psia.

� The yellow H2 PRESS and the red BACKUPC/W ALARM lights illuminate if hydrogentank pressure is below 153 psia or above293.8 psia.

� When any fuel cell reactant valve is closed,the red FUEL CELL REAC light is illumi-nated, a tone is sounded and the BACKUPC/W ALARM is illuminated.

� The yellow FUEL CELL STACK TEMP lightwill illuminate if fuel cell and stack tempera-tures are below 170.7° F or above 241.9° F.The red BACKUP C/W ALARM and blue SMALERT will also light for similar limits.

� The yellow FUEL CELL PUMP light willilluminate if fuel cell 1, 2, or 3 coolant pumpdelta pressure is lost. If the coolant pump forfuel cell 1, 2, or 3 is off, the BACKUP C/WALARM will be illuminated.

� The yellow O2 HEATER TEMP light on panelF7 illuminates if the temperature sensorsenses the oxygen tank temperature is at orabove 344° F. Software checks the limit; if itis above 349°F, the BACKUP C/W ALARMlight illuminates as well.

� The red MAIN BUS UNDERVOLT light andthe BACKUP C/W ALARM illuminate if mainbus A, B, or C voltage is 26.4 volts dc.

� The yellow AC VOLTAGE light illuminatesfor an overvoltage or undervoltage conditionin the ac bus. The red BACKUP C/W ALARMwill also illuminate.

� The yellow AC OVERLOAD light illuminatesfor an overload condition in the ac bus.

CABIN ATM

FREON LOOP

AV BAY/CABIN AIR

IMU FWD RCS RCS JET

H O LOOP RGA/ACCEL AIR DATA LEFT RCS RIGHT RCS

LEFT RHC RIGHT/AFT RHC

PAYLOADWARNING

FCS SATURATION

OMS KIT

PAYLOADCAUTION

PRIMARY C/W FCS CHANNEL

APU TEMP

APU OVERSPEED

APU UNDERSPEED

HYD PRESS

(R)

(R)

(R)

(R)

(R)

OMS TVC

(R)

(R)

(R)

LEFT OMS RIGHT OMS

MPS(R)

2

GPC

(R)

O HEATER T E M P

2

O 2 P R E S S

B A C K U P C / WA L A R M (R)

H2 P R E S S FUEL CELLR E A C

FUEL CELLS T A C K T E M P

FUEL CELLP U M P

A CV O L T A G E

A CO V E R L O A D

M A I N B U SU N D E R V O L T

346.cvs

(R)

EPS Caution and Warning Lights on Panel F7

347.cvs

IF 5TH TK SET:

02/H2 HTR LOSS (ALL EXCEPT OV 102)

MNA DA1MNB DA2MNC DA3

PAIRED

TK1 TK2 TK3 TK4

PAIRED

A B A B A B A B

BUSLOSS

CNTL AB1AB2

CNTL BC1BC2BC3

CNTL CA1CA2CA3

ESS 1BC2CA3AB

A/M

AUTO A/MAUTOA/M

A/M A/MA/M

AUTO AUTOA/M

A/M

MANA/M

MANA/M

MANA/M

MANA/M

MAN MAN

MANA/M

MANA/M

A/M A/M

AUTO*AUTO

AUTO*AUTO

AUTOAUTO*

AUTOAUTO*

AUTO*

AUTO

AUTO*

AUTO

AUTO

AUTO*

AUTO

AUTO*

* Auto operation regained if both A and B HTR SW of other paired tank(s) are not in auto. A/M = Auto and manual loss.

TK5

A/M

AUTO A/M

MAN

A/M

MANA/M

AUTO AUTO

02/H2 HTR LOSS (OV 102 ONLY)

MNA DA1MNB DA2MNC DA3

PAIRED

TK1 TK2 TK3 TK4

PAIRED

A B A B A B O2 H2A

BUSLOSS

CNTL AB1AB2AB3

CNTL BC1BC2BC3

CNTL CA1CA2CA3

ESS 1BC2CA3AB

A/M

AUTO A/MAUTOA/M

A/M A/MA/M

AUTO AUTOA/M

AUTOA/M

MANA/M

MANA/M

MANA/M

MANA/M

MAN

MANA/M

MANA/M

MANA/M

A/M A/M

AUTO*AUTO

AUTO*AUTO

AUTOAUTO*

AUTOAUTO*

AUTO*

AUTO

AUTO*

AUTO

AUTO

AUTO*

AUTO

AUTO*

* Auto operation regained if both A and B HTR SW of other paired tank(s) are not in auto. TK1 paired with TK2, and TK3 paired with TK4/5. A/M = Auto and manual loss.

TK5

AUTO

AUTO*

AUTO

AUTO*

AUTO

AUTO*

AUTO

A/M A/M AUTOA/M

A/M

MAN MANAUTOMAN

A/M A/M

H2B O2 H2A H2B

MANA/M

PAIRED PAIRED

TK6 TK7 TK8 TK9

AUTO AUTO AUTOAUTO

AUTO*AUTOAUTO AUTO AUTO

MANA/M

MANA/M

MANA/M

MANA/M

MANA/M

MANA/M

MANA/M

MANA/M

A/M A/M A/M A/M

A B A B A B A B

SPLYVLVTO ECS

SYS 1MANFVLV

TK9

TK8

TK7

TK6

TK3

TK5

TK4

MANFVLV

SPLYVLVTO ECS

SYS 2

FC1

O2TK1

P MANF P

REACVLV

H2TK1

P MANF P

REACVLV

MANFVLV

TK9

TK8

TK7

TK6

TK3

FC2 TK2 P

MANF P

REACVLV

TK2 P

MANF P

REACVLV

MANFVLV

FC3 REACVLV

REACVLV TK5

TK4

OV103,OV104,OV105

OV103,OV104,OV105

OV102ONLY

A B

Cryogenic System

Panel R1

Panel R11U

Panel O2

Panel F9

2011/ /078 SM SYS SUMM 1 4 000/14:44:12 000/00:00:00SMOKE 1/A 2/B DC VOLTS 1/A 2/B 3/C CABIN 0.0 FC 30.6 30.1 31.0 L/R FD 0.0 0.0 MAIN 30.6 30.1 31.0 AV BAY 1 0.3 0.3 ESS 29.6 29.6 29.3 2 0.3 0.4 A B C A 3 0.3 0.3 CNTL 1 29.4 29.4 29.6 CABIN 2 29.4 29.4 29.4 PRESS 14.0 3 29.4 29.4 29.4 dP/dT-EQ +.00 +.000 AC 02 CONC VOLT φA 118 118 117 PPO2 3.00 3.00 φB 117 117 118 FAN ∆P 5.00 φC 117 117 118 HX OUT T 46 AMPS φA 4.3 6.3 2.1 O2 FLOW 0.0 0.0 φB 5.5 6.6 2.2 N2 FLOW 0.0 0.0 φC 3.1 5.0 3.2 IMU FAN A B C FUEL CEL ∆V FC1 FC2 FC3 AMPS 180 232 146 SS1 22 21 22 REAC VLV OP OP OP SS2 22 22 23 STACK T +202 +206 +200 SS3 23 21 21 EXIT T 150 152 149 TOTAL AMPS 557 COOL P 61 60 61 KW 17 PUMP

243

PASS SM SYS SUMM 1

0001/ /078 SM SYS SUMM 1 5 000/03:13:09 BFS 000/00:00:00SMOKE 1/A 2/B CABIN - 0.9 DC VOLTS 1/A 2/B 3/C L/R FD - 0.4 0.2 FC 31.1 31.1 31.1 AV BAY 1- 1.2 - 0.0 MAIN 31.0 31.1 31.0 2- 0.6 0.3 CNTL AB 29.3 29.3 29.3 3- 0.1 - 0.9 BC 29.3 29.3 29.3 CABIN CA 29.3 29.3 29.3 PRESS 14.7 ESS 29.8 29.8 29.8 dP/dT +.00 AC BU/EQ -.00 +.00 VOLT φA 117 117 117 PPO2 3.02 3.02 φB 117 117 117 FAN P 5.79 φC 117 117 117 HX OUT T 49 AMPS φA 4.4 4.1 2.7 φB 3.9 4.2 3.2 N2 FLOW 0.0 φC 2.4 3.2 4.8 IMU FAN ∆P 4.62 FUEL CELL PH ∆V FC1 FC2 FC3 AMPS 172 167 178 SS1 15 18 18 REAC VLV OP OP OP SS2 16 20 11 STACK T +204 +203 +203 SS3 22 26 26 EXIT T 150 150 151 TOTAL AMPS 510 COOL P 61 61 61 KW 15 PUMP 085

BFS SM SYS SUMM 1

2011/ /067 ELECTRIC 4 000/02:33:00 000/00:00:00 DC 1/A 2/B 3/C AC 1 2 3VOLT FC 31.1 31.1 31.1 V φA 117 117 117 MN 31.0 31.0 31.0 φB 117 117 117 PCA FWD 30.5 30.6 30.5 φC 117 117 117 AFT 30.7 30.7 30.7 AMP φA 4.5 4.2 2.8 ESS 29.8 29.8 29.8 φB 3.9 4.3 3.2 A B C A φC 2.5 3.2 4.7 CNTL 1 29.3 29.3 29.3 OVLD 2 29.3 29.3 29.3 O/U V 3 29.3 29.3 29.3AMPS FC 176 175 170 PAYLOAD DC AMPS FWD 86 64 114 AFT B 0.3 MID 4 35 9 C - 0.0L AFT + 35 + 24 + 17

TOTAL AMPS 521 KW 16.2

353

DISP 67 (ELECTRIC)

0001/ /079 SM SYS SUMM 2 5 008/23:29:22 BFS 000/00:00:00 CRYO TK 1 2 3 4 5 MANF1 MANF2H2 PRESS 208 208 206 206 206 208 207O2 PRESS 816 815 814 814 814 815 815HTR T1 -248 -248 -248 -248 -248 T2 -248 -248 -248 -248 -248 APU 1 2 3 HYD 1 2 3TEMP EGT 942 942 942 PRESS 3064 3064 3064 B/U EGT 942 942 942 ACUM P 3080 3080 3080 OIL IN 250 250 250 RSVR T 116 153 142 OUT 264 264 264 GG BED 511H 511H 511H QTY 72 74 71 INJ 1271 1271 1271SPEED % 99 102 101 W/BFUEL QTY 59 60 62 H2O QTY 78 73 78 PMP LK P 14 14 14 BYP VLV BYP BYP BYPOIL OUT P 42 42 41FU TK VLV A T 63 65 62 THERM CNTL 1 28 B T 63 65 62 H2O PUMP P 23 63 AV BAY 1 2 3 FREON FLOW 2384 2384TEMP 97 97 83 EVAP OUT T 38 38A4 14 27.439 27.435 26.324 31.873 18.48

051

BFS SM SYS SUMM 2

2011/ /068 CRYO SYSTEM 4 000/02:34:40 000/00:00:00 TK O2 1 2 3 4 5P 827 826 871 826 826TK P 828 828 871 828 828QTY 99 99 100 100 100T -249 -249 -249 -249 -249HTR T 1 -249 -249 -182 -249 -249 2 -249 -249 -182 -249 -249HTR 1ACUR 1B 4/5SNSR 2A 2B 4/5MANF P 871 871 VLV OP OP H2P 215 213 224 213 213TK P 215 214 224 214 214QTY 97 97 99 99 99T -417L -417L -417L -417L -417LHTR T -417L -417L -417L -417L -417LMANF P 222 222 VLV OP OP

313

DISP 68 (CRYO SYSTEM)

2011/ /069 FUEL CELLS 4 000/02:35:11 000/00:00:00 FC O2 1 2 3VOLTS 31.3 31.2 31.2 H2O RLF LINE T 72AMPS 161 164 163 NOZ T A 244 B 244FLOW O2 2.7 2.7 2.6 HTR SW A H2 0.5 0.5 0.5 PURGE LN O2 T 32REAC O2 OP OP OP H2 T1 40 H2 OP OP OP T2 40 H2O LINE PHSTACK T +204 +204 +204EXIT T 151 151 151COOL T 73 73 73 FC P 61 61 61 1 2 3 PUMP PHH2 PUMP 0.4 0.4 0.4 ∆V SS1 15 18 18 READY RDY RDY RDY SS2 16 20 11 SS3 22 26 26H2O ∆AMPS - 3 + 1 + 2PRI LN T 144 144 144 VLV T 93 93 93ALT LN T 79 79 79

328

DISP 69 (FUEL CELLS)

2011/ /168 CRYO PALLET 4 000/02:38:52 000/00:00:00 TK O2 6 7 8 9P 515L 515L 515L 515L PALLETTK P 0L 0L 0L 0L STBQTY - 5L - 5L - 5L - 5L STBT -417L -417L -417L -417L POPHTR T 1 -417L -417L -417L -417L POP 2 -417L -417L -417L -417LHTR 1A PALLETCUR 1B PPOSNSR 2A 2B H2P 145L 145L 145L 145LTK P 0L 0L 0L 0LQTY - 6L - 6L - 6L - 6LT -417L -417L -417L -417LHTR T -417L -417L -417L -417L

314

DISP 168 (CRYO PALLET)

EPS Summary Data

� The EPS, which operates during all flightphases, consists of the equipment and reac-tants that produce electrical power for distri-bution throughout the orbiter vehicle.

� The fuel cell picks up full power load supportafter ground equipment is turned off at Tminus 3 minutes 30 seconds, supportingpower requirements for the solid rocketbooster, orbiter, and some payloads.

� EPS subsystems are: power reactants storageand distribution, fuel cells, and electricalpower distribution and control.

� The power reactants storage and distributionsystem stores cryogenic hydrogen and oxy-gen and supplies them to the fuel cells. Italso supplies oxygen to the ECLSS. The com-ponents are located in the orbiter midbodyunderneath the payload bay.

� The fuel cell system (three fuel cells) trans-forms hydrogen and oxygen to electricitythrough a chemical reaction. The system alsosupplies potable water to the ECLSS. Itconsists of a power and an accessory section.

� The electrical power distribution and controlsystem distributes electrical power through-out the orbiter. It has five types of assem-blies: power control, load control, motorcontrol, main dc distribution, and ac distribu-tion and control.

� The EPS requires very little flight crewinteraction during nominal operations.

� The majority of the EPS controls are onpanels R1 and R11U. Power sources andcircuit breakers are distributed on panelsO13, O14, O15, O16, ML86B, R15, andMA73C. Heaters are controlled from panelR1, A11 and A15.

� Meters for monitoring EPS parameters arelocated on panels F9 and O2.

� CRTs that display EPS data are BFS SM SYSSUMM 1 & 2, DISP 67 (ELECTRIC), DISP 68(CRYO SYSTEM), DISP 69 (FUEL CELLS),PASS SM SYS SUMM 1 & 2, and DISP 168(CRYO PALLET).

EPS Rules of Thumb

� Never close a circuit breaker or reset an RPCfound out of configuration without MCCcoordination.

� Loss of cooling to a fuel cell requires crewaction within 9 minutes to prevent a cata-strophic loss of crew/vehicle due to possiblefuel cell fire and explosion.

� Fuel cell run limit prior to shutdown for lossof cooling is 9 minutes at a 7 kW nominalload. Run time is inversely proportional tofuel cell load.

� Any interruption of continuous ac powerduring ascent may result in the loss of mainengine controller redundancy. Reconfigura-tion of ac powered equipment prior to MECOshould be avoided.

� Never connect or reconnect power to aknown shorted or failed component; thisincludes switch throws, circuit protectiondevice resets, or bus ties.

� Three oxygen and hydrogen tanks are goodfor up to 8 days on orbit; five oxygen andhydrogen tanks are good for up to 12 days onorbit; eight oxygen and hydrogen tanks aregood for up to 18 days on orbit. Exact dura-tion varies with crew complement and powerload.

� A fuel cell hydrogen pump uses 0.3 amps/AC phase; a coolant pump uses 0.5 amps/ACphase.