the space shuttle extravehicular mobility unit (emu)

Suited for Spacewalking An Activity Guide for Technology Education, Mathematics, and Science, EG-1998-03-112-HQ • 1 7

As NASA changed from launching astronautson expendable rockets to the Space Shuttle systemwith its reusable orbiter and solid rocket boosters,spacesuit engineers began deve l o pment of areusable EMU. Previously, all spacesuits were one-time garments. Spacesuits were custom-built toeach astronaut’s body size. In the Apollo program,for example, e a ch astronaut had three customsuits—one for flight, one for training, and one forflight backup. Shuttle suits, however, are tailoredfrom a stock of standard-size parts to fit astronautswith a wide range of measurements.

In con s t ructing the Shuttle spacesuit, d eve l o p-ers were able to con c e n t rate all their designs tow a rda single function — going EVA . Suits from earl i e rmanned spaceflight pro g rams had to serve mu l t i p l ef u n c t i on s . T h ey had to provide backup pre s s u re incase of cabin pre s s u re failure and, on Gemini m i s s i on s , p ro t e c t i on if ejection became necessaryd u ring launch .T h ey also had to provide an env i ron-ment for EVA in micro g ra v i ty and in low gra v i tywhile walking on the Moon (Ap o llo mission s ) . Su i t sw e re worn during lift off and re e n t ry and had to bec om f o rtable under the high-g forces experi e n c e dd u ring accelera t i on and decelera t i on . Shuttle suitsa re worn on ly when it is time to ve n t u re outside theo rbiter ca b i n . At other times, c rew members wearc om f o rtable shirts and slack s , or short s . For launch

The Space Shuttle Extravehicular Mobility Unit (EMU)

and reentry, special orange-colored flight suits withhelmets are worn.

Many Layers

The Shuttle EMU has 14 layers to pro t e c ta s t ronauts on EVA s . The inner layers com p rise the l i q u i d - c o o l i n g - a n d - ve n t i l a t i on garm e n t . First com e sa liner of Nyl on tricot over which is a layer of span-dex fabric laced with plastic tubing. Next comes the

p re s s u re bladder layer of urethane-coated nyl onand fabric layer of pre s s u re - re s t raining Dacron ® .A b ove the bladder and re s t raint layer is a liner ofNe o p rene coated Nyl on Ripstop. This is foll ow e dby a seve n - l ayer thermal microm e t e o roid garm e n tof aluminized Myl a r ® , laminated with Dacron ®s c ri m . The outer layer of the suit is made of Ort h o -Fa b ric which consists of a blend of Gort e x ® ,K evl a r ® , and Nomex® materi a l s .

1 8 • Suited for Spacewalking An Activity Guide for Technology Education, Mathematics, and Science, EG- 1 9 9 8 - 0 3 - 1 1 2 - H Q

1. Liquid Cooling and VentilationGarment Liner (Nylon tricot)

2 . Liquid Cooling and Ve n t i l a t i onG a rment Liner Outer Laye r( Nyl on / Sp a n d e x )

3 . Liquid Cooling and Ve n t i l a t i onG a rment Liner Water Tra n s p o rtTu b i n g

4 . Pre s s u re Garment Bladder ( Urethane Coated Nyl on )

5 . R e s t raint (Dacron®)

6 . T h e rmal Microm e t e o ro i dG a rment Liner (Ne o p re n eCoated Nyl ong Ripstop)

7–13. ThermalMicrometeoroidGarment Liner(Multi-layeredInsulation—AluminizedMylar®)

14. ThermalMicrometeoroidGarment Cover(Ortho-Fabric)

Shuttle EMU End Items

The Shuttle extra vehicular mobility unit(EMU) consists of 18 separate items. Fully assem-bled, the Shuttle EMU becomes a nearly completeshort-term spacecraft for one person. It providespressure, thermal and micrometeoroid protection,oxygen, cooling water, drinking water, food, wastecollection,(including carbon dioxide removal),elec-trical power, and communications.The EMU lacksonly maneuvering capability, but this capability canbe added by fitting a gas jet-propelled SimplifiedAid for Extravehicular Activity Rescue (SAFER)over the EM U’s pri m a ry life - s u p p o rt sys t e m . O nE a rt h , the suit and all its part s , f u lly assembled butwithout SAFER, weighs about 113 kilogra m s .O rbiting above Earth it has no weight at all . It does,h ow eve r, retain its mass in space, which is felt asresistance to a change in motion.

1. Primary Life-Support System (PLSS)Self-contained backpack unit containing an oxygensupply, carbon-dioxide-removal equipment, cautionand warning system,electrical power, water-coolingequipment, ventilating fan, machinery, and radio.

2 . D i s p l a ys and Control Module (DC M )Chest-mounted con t rol module containing allc on t ro l s , a digital display, the external liquid, g a s ,and electri cal interf a c e s . The DCM also has thep ri m a ry purge valve for use with the Se c on d a ryOxygen Pa ck .

3 . EMU El e c tri cal Harness (EEH )A harness worn inside the suit to provide bioin-s t ru m e n t a t i on and com mu n i ca t i ons con n e c t i ons tothe PLSS.

4. Secondary Oxygen Pack (SOP)Two oxygen tanks with a 30-minute emergencysupply combined,valve, and regulators.The SOP isattached to the base of the PLSS. The SOP can beremoved from the PLSS for ease of maintenance.

5. Service and Cooling Umbilical (SCU)Connects the orbiter airlock support system to theEMU to support the astronaut before EVA and toprovide in-orbit recharge capability for the PLSS.


The SCU contains lines for power, communica-t i on s , oxygen and water re ch a r g e, and waterdrainage. The SCU conserves PLSS consumablesduring EVA preparation.

6.BatteryBattery that supplies electrical power for the EMUduring EVA.The battery is rechargeable in orbit.

7. Contaminant Control Cartridge (CCC)Cleanses suit atmosphere of contaminants with anintegrated system of lithium hydroxide, activatedcharcoal, and a filter contained in one unit. TheCCC is replaceable in orbit.

8. Hard Upper Torso (HUT)Upper torso of the suit, c omposed of a hard fiber-glass shell . It provides stru c t u ral support formounting the PLSS, DC M , a rm s , h e l m e t , I n - Su i tDrink Bag, EEH , and the upper half of the waistcl o s u re . The HUT also has prov i s i ons for mount-ing a mini-work s t a t i on tool ca r ri e r.

9. Lower TorsoSpacesuit pants, boots, and the lower half of theclosure at the waist.The lower torso also has a waistbearing for body rotation and mobility, and D ringsfor attaching a safety tether.

10. Arms (left and right)Shoulder joint and shoulder bearing, upper armbearings, elbow joint, and glove-attaching closure.

11. EVA Gloves (left and right)Wrist bearing and disconnect, wrist joint, and fin-gers.The gloves have loops for attaching tethers forrestraining small tools and equipment. Generally,crew members also wear thin fabric comfort gloveswith knitted wristlets under the EVA gloves.


Reference Numbers 8–11.

Hard UpperTorso under-neath fabric

Lower Torso

12.HelmetPlastic pressure bubble with neck disconnect ringand ventilation distribution pad. The helmet has abackup purge valve for use with the secondary oxy-gen pack to remove expired carbon dioxide.

1 3 . Liquid Cooling-and-Ve n ti l a tion Garm e n t(LCVG)Long underwear-like garment worn inside thepressure layer. It has liquid cooling tubes, gas venti-lation ducting, and multiple water and gas connec-tors for attachment to the PLSS via the HUT.

14.Maximum Absorption Garment (MAG)An adult-sized diaper with extra absorption mater-ial added for urine collection.

15. Extravehicular Visor Assembly (EVA)Assembly containing a metallic-gold-covered Sun-filtering visor, a clear thermal impact-protectivevisor, and adjustable blinders that attach over thehelmet. In addition, four small “head lamps” aremounted on the assembly; a TV camera-transmittermay also be added.

16.In-Suit Drink Bag (IDB)Plastic water-filled pouch mounted inside theHUT. A tube projecting into the helmet works likea straw.


17.Communications Carrier Assembly (CCA)Fabric cap with built-in earphones and a micro-phone for use with the EMU radio.

18.Airlock Adapter Plate (AAP)Fixture for mounting and storing the EMU insidethe airlock and for use as an aid in donning the suit.

Putting on the EMU

Putting on a Shuttle EMU is a relatively sim-ple operation that can be accomplished in a matterof about 15 minutes.However, the actual process ofpreparing to go EVA takes much longer. Whenw o rking in the Shuttle ca b i n , c rew membersbreathe a normal atmospheric mix of nitrogen andoxygen at 101 kilopascals. The suit ’s atmosphere ispure oxygen at 29.6 kilopascals. A rapid drop fromthe cabin pressure to the EMU pressure could resultin a debilitating ailment that underwater diverssometimes experience—the bends. The bends, also

known as caisson disease, are produced by the for-mation and expansion of nitrogen gas bubbles inthe bloodstream when a person breathing a normalair mixture at sea-level pressure is exposed to arapid drop in external pressure. In severe cases, thebends are characterized by pains in the joints,cramps, paralysis, and eventual death if not treatedby gradual recompression.To prevent an occurrenceof the bends, crew members intending to go EVAspend a period of time prebreathing pure oxygen.During that time, nitrogen gas in the bloodstreamis replaced by pure oxygen.

Prior to prebreathing, the atmospheric pressureof the entire orbiter cabin is depressed from thenormal 101 kilopascals to 70.3 pascals while thep e rcentage of oxygen is slightly incre a s e d .Prebreathing begins when the crew members whoplan to go EVA don a mask connected to an oxygensupply. A short hose permits them to continue theirEVA preparations during this period.The length oftime for cabin decompression and the time for prebreathing is related. Without any cabin decom-pression, prebreathing must last at least four hours.A prebreathe of 30 minutes is safe providing cabindecompression takes place at least 24 hours beforethe exit into space.

By now, much of the dissolved nitrogen gas hasbeen cleared from the EVA crew members,and theycan remove their helmets. Later, when they dontheir spacesuits and seal the helmets, an additional30 to 40 minutes of pure oxygen prebreathing takesplace before the suits are lowered to their operatingpressure of 29.6 kilopascals.

Most of the EM U - d onning process takesplace inside the airl o ck . The airl o ck is a cyl i n d ri ca lchamber located on the orb i t e r’s mid-deck . O n eh a t ch leads from the middeck into the airl o ck ,a n da second hatch leads from the airl o ck out to theu n p re s s u ri zed payload bay.

Before entering the hatch, but following theirinitial prebreathing, the crew members put on them a x i mum absorb e n cy garment (MAG ) . T h eMAG is an adult-size diaper.


Next comes the Liquid Cooling-and-Ve n t i l a t i onG a rment (LC VG ) . The LC VG has the genera la p p e a rance of long underw e a r. It is a one-piece suitwith a zippered fron t , made of stre t chable spandexf a b ric laced with 91.5 meters of plastic tubing. Wh e nthe EMU is com p l e t e ly assembled, cooling and ve n t i l a t i on become significant pro b l e m s . Body heat,c ontaminant gases, and perspira t i on —a ll waste

p ro d u c t s — a re contained by the insulation and pre s-s u re layers of the suit and must be re m ove d .C o o l i n gof the crew member is accomplished by circ u l a t i n gch i lled water through the tubes. C h i lling the wateris one of the functions of the Pri m a ry Life - Su p p o rtSystem (PLSS). The PLSS device for water coolingand the tubing system are designed to provide cool-ing for phys i cal activity that generates up to tw om i ll i on joules of body heat per hour, a rate that isc on s i d e red “e x t re m e ly vigo ro u s . ” ( Ap p rox i m a t e ly160 joules are released by burning a piece ofn ew s p rint one centimeter square.) Ducting attach e dto the LC VG ventilates the suit by drawing ve n t i-lating oxygen and expired ca rb on dioxide from thes u i t’s atmosph e re into the PLSS for puri f i ca t i on andre c i rc u l a t i on . Body perspira t i on is also drawn awayf rom the suit by the venting system and re cycled inthe water cooling sys t e m . The intakes are loca t e dnear the hands and feet of the suit. Du c t s , ru n n i n ga l ong the arms and legs on the back of the LC VGch a n n e l , the ve n t i l a t i on gases to a circular junctionon the back of the LC VG and into the torso ve n td u c t . From there, the gases are re t u rned to the PLSSvia the LCCGV multiple water con n e c t o r. Pu ri f i e doxygen from the PLSS reenters the suit thro u g hanother duct, mounted in the back of the helmet,that directs the flow over the astron a u t’s face toc omplete the circ u i t .

The EMU electrical harness is attached to theHUT and provides biomedical and communica-tions hookups with the PLSS. The biomedicalhookup monitors the heart rate of the crew mem-bers, and this information is radioed via a link withthe orbiter to Mission Control on Earth. Voicecommunications are also carried on this circuit.

Ne x t , s eve ral simple tasks are perf o rm e d .Antifog compound is rubbed on the inside of thehelmet. A wrist mirror and a small spiral-bound 27-page checklist are put on the left arm of theupper torso. The wrist mirror was added to the suitbecause some of the knobs on the front of the dis-plays and control module are out of the vision rangeof the crew member. The mirror permits the knob


Spacesuit technician prepares to put on the Space Shuttle EMU by first don-ning the Liquid Cooling-and-Ventilation garment.

settings to be read. Setting numbers are writtenbackwards for ease of reading in the mirror.

Another task at this time is to insert a food barand a water-filled In-Suit Drink Bag (IDB) insidethe front of the HUT. The food bar of compressedfruit,grain,and nuts is wrapped in edible rice paper,and its upper end extends into the helmet area nearthe crew member’s mouth. When hungry, the crewmember bites the bar and pulls it upward beforebreaking off a piece to chew. In that manner, a smallpiece of the bar remains extended into the helmetfor the next bite.It is necessary to eat the entire barat one time, because saliva quickly softens the pro-truding food bar, making it mushy and impossibleto break off. The IDB is placed just above the bar.Two sizes of bags are available and the one chosenis filled with water from the water supply of theorbiter's galley prior to entry into the airlock. Thelargest bag contains nearly 1 liter of water. A plastictube and valve assembly extends up into the helmetso that the crew member can take a drink whenev-er needed.Both the food bar and drink bag are heldin place by Velcro attachments.

During EVAs, the crew members may needadditional lighting to perform their tasks. A light-bar attachment (helmet-mounted light array) isplaced above the helmet visor assembly. Small built-in flood lamps provide illumination to places thatsunlight and the regular payload bay lights do notreach. The EVA light has its own battery systemand can be augmented with a helmet-mountabletelevision camera system with its own batteries andradio frequency transmitter. The camera’s lens sys-tem is about the size of a postage stamp. Throughthis system, the crew remaining inside the orbiterand the mission controllers on Earth can get anastronaut’s eye view of the EVA action. Duringcomplicated EVAs, viewers may be able to providehelpful advice for the tasks at hand.

Next, the Communications Carrier Assembly(CCA), or “Snoopy cap,” is connected to the EMUelectrical harness and left floating above the HUT.The CCA earphones and microphones are held by

a fabric cap. After the crew member dons the EMU,the cap is placed on the head and adjusted.

When the tasks preparatory to donning thesuit are completed,the lower torso, or suit pants,arepulled on.The lower torso comes in various sizes tomeet the varying size requirements of differentastronauts.It features pants with boots and joints inthe hip, knee, and ankle, and a metal body-seal clo-sure for connecting to the mating half of the ringmounted on the hard upper torso. The lower torso’swaist element also contains a large bearing. Thisgives the crew member mobility at the waist, per-mitting twisting motions when the feet are held inworkstation foot restraints.

Joints for the lower and upper torsos re p re s e n tan important advance over those of prev i o u ss p a c e s u i t s . E a rlier joint designs consisted of hardri n g s , b e ll ows-like bends in the pre s s u re bladder,or cable- and pull ey-assisted fabric joints. T h eShuttle EMU joints maintain nearly constant volume during bending. As the joints are bent,re d u c t i ons in volume along the inner arc of thebend are equalized by increased volume along theouter arc of the bend.

Long before the upper half of the EMU isd on n e d , the airl o ck ’s Se rvice and Cooling Umbilica l(SCU) is plugged into the Displays and Con t ro lModule Panel on the front of the upper torso. Fi vec on n e c t i ons within the umbilical provide the suitwith cooling water, ox y g e n , and electri cal pow e rf rom the Shuttle itself. In this manner, the con s u m-ables stored in the Pri m a ry Life - Su p p o rt Sys t e mw i ll be con s e rved during the lengthy pre b re a t h i n gp e ri o d . The SCU also is used for battery and con-sumable re charging between EVA s .

The airl o ck of the Shuttle orbiter is on ly 1.6meters in diameter and 2.1 meters high on the inside.When two astronauts pre p a re to go EVA , the spaceinside the airl o ck becomes crowd e d . For storage pur-poses and as an aid in donning and doffing theEM U, e a ch upper torso is mounted on airl o ckadapter plates. Adapter plates are bra ckets on the air-l o ck wall for supporting the suits’ upper torsos.


With the lower torso donned and the orbiterp roviding consumables to the suits, e a ch crewmember “dives” with a squirming motion into theupper torso.To dive into it,the astronaut maneuversunder the body-seal ring of the upper torso andassumes a diving position with arms extendedupward. Stretching out, while at the same timealigning arms with the suit arms, the crew memberslips into the upper torso. As two upper and lowerbody-seal closure rings are brought together, twoconnections are made. The first joins the coolingwater-tubing and ventilation ducting of the LCVGto the Primary Life-Support System. The secondconnects the biomedical monitoring sensors to theEMU electrical harness that is connected to thePLSS. Both systems are turned on, and the crewmember then locks the two body-seal closure ringstogether, usually with the assistance of another crewmember who remains on board.

One of the most important features of theupper half of the suit is the HUT, or Hard UpperTorso. The HUT is a hard fiberglass shell under thefabric layers of the thermal-micrometeoroid gar-ment. It is similar to the breast and back plates of asuit of armor. The HUT provides a rigid and con-trolled mounting surface for the Primary Life-Support System on the back and the Displays andControl Module on the front.

In the past, d u ring the Ap o llo Moon mission s ,d onning suits was a ve ry lengthy process because thel i fe - s u p p o rt system of those suits was a separate item.B e cause the Ap o llo suits were worn during launchand landing and also as ca b i n - p re s s u re back u p s , aHUT could not be used. It would have been mu chtoo uncom f o rtable to wear during the high accelera-t i ons and decelera t i ons of lift-off and re e n t ry. T h el i fe - s u p p o rt system had to be attached to the suitinside the lunar module. A ll con n e c t i ons betw e e nPLSS and the Ap o llo suit were made at that timea n d , with two astronauts working in cramped quar-t e r s , p re p a ring for EVA was a difficult pro c e s s . T h eShuttle suit HUT eliminates that lengthy pro c e d u reb e cause the PLSS is already attach e d . It also elimi-

nates the exposed and vulnerable ve n t i l a t i on and life -s u p p o rt hoses of earlier EMU designs that couldb e c ome snagged during EVA .

The last EMU gear to be donned includes eye-glasses if needed, the com mu n i ca t i ons ca r ri e rassembly (CCA), comfort gloves, the helmet withlights and optional TV, and EVA gloves. The twogloves have fingertips of silicone rubber that permitsome degree of sensitivity in handling tools andother objects. Metal rings in the gloves snap intorings in the sleeves of the upper torso. The rings inthe gloves contain bearings to permit rotation foradded mobility in the hand area. The connectingring of the helmet is similar to the rings used for thebody-seal closure. Mobility is not needed in thisring because the inside of the helmet is largeenough for the crew member’s head to movearound. To open or lock any of the connectingrings, one or two sliding, rectangular-shaped knobsare moved to the right or the left. When opened,the two halves of the connecting rings come aparteasily. To close and lock, one of the rings slides partway into the other against an O-ring seal.The knobis moved to the right, and small pins inside theouter ring protrude into a groove around the insidering, thereby holding the two together.

All suit openings have locking provisions thatrequire a minimum of three independent motionsto open.This feature prevents any accidental open-ing of suit connections.

With the donning of the helmet and gloves,the spacesuits are now sealed off from the atmos-phere of the air lock. The crew members are beingsupported by the oxygen, electricity, and coolingwater provided by the orbiter. A manual check ofsuit seals is made by pressurizing each suit to 29.6kilopascals d. (The “d” stands for differential,mean-ing greater than the air lock pressure.) Inside the airlock, the pressure is either 70.3 or 101 kilopascals.The suit’s pressure is elevated an additional 29.6kilopascals, giving it a pressure differential abovethe air lock pressure. Once pressure reaches thedesired level, the oxygen supply is shut off and the


digital display on the chest-mounted control mod-ule is read. To assist in reading the display, anoptional Fresnel lens inside the space helmet maybe used to magnify the numbers. Some leakage ofspacesuit pressure is normal. The maximum allow-able rate of leakage of the Shuttle EMU is 1.38kilopascals per minute, and this is checked beforethe suit is brought back down to air lock pressure.

As the suit pre s s u re is elev a t e d ,c rew members maye x p e rience discom f o rt in their ears and sinus ca v i t i e s .T h ey compensate for the pre s s u re change by swall ow-i n g, y a w n i n g, or pressing their noses on an option a ls p onge mounted to the left on the inside of the helmetri n g. Attempting to blow air through the nose whenp ressing the nose on the sponge forces air inside the earsand sinus cavities to equalize the pre s s u re .

During the next several minutes the two space-suits are purged of any oxygen/nitrogen atmosphereremaining from the cabin;this is replaced with pureoxygen. Additional suit checks are made while thefinal oxygen prebreathe takes place.

The inner door of the air lock is sealed,and theair lock pre s s u re bleed-down begins. A smalldepressurization valve in the air lock latch is openedto outside space, permitting the air lock atmosphereto escape. While this is taking place the EMU auto-matically drops its own pressure to 66.9 kilopascalsand leak checks are conducted. Failure of the leaktest would require repressurizing the air lock, per-mitting the EVA crew to reexamine the seals oftheir suits.

Final depressurization is begun by opening theair lock depressurization valve. The outer air lockhatch is then opened and the suited astronauts pre-pare to pull themselves out into the payload bay. Asa safety measure, they tether themselves to theorbiter to prevent floating away as they move fromplace to place by hand holds. It is at this point thatthey disconnect the orbiter Service and CoolingUmbilical from the EMU. The PLSS begins usingits own supply of oxygen, cooling water, and elec-tricity. The astronauts pull themselves through theouter air lock hatch, and the EVA begins.

The Primary and Secondary

Life-Support Systems

A s t ronauts experienced their first real fre e d omwhile wearing spacesuits during the Ap o ll oM o onwalk EVA s , b e cause of a portable life - s u p-p o rt system worn on their back s . A ll other EVA sup to that time were tied to the spacecra ft by theu m b i l i cal-tether line that supplied oxygen and keptc rew members from dri fting away. In one sense, t h etether was a leash, b e cause it limited move m e n t sa w ay from the spacecra ft to the length of the teth-e r. On the Moon , h ow eve r, a s t ronauts were noth a m p e red by a tether and, in the later mission s ,w e re permitted to dri ve their lunar rovers up to 10k i l ometers away from the lander. (That distancelimit was imposed as a safe ty measure . It was deter-mined that 10 kilometers was the maximum dis-tance an astronaut could walk back to the lander ifa lunar rover ever broke dow n . )

Space Shuttle astronauts have even gre a t e rf re e d om than the Ap o llo lunar astronauts beca u s etheir EVAs take place in the micro g ra v i ty env i ron-ment of space. T h ey do employ tethers when EVA scenter in and about the Sh u t t l e’s payload bay, b u tthose tethers act on ly as safe ty lines and do notp rovide life support . Fu rt h e rm o re, the tethers ca nbe moved from one loca t i on to another on theo rbiter along a slide wire, p e rmitting even gre a t e rdistances to be cove re d .

The fre e d om of movement afforded toShuttle astronauts on EVAs is due to the Pri m a ryL i fe - Su p p o rt System (PLSS) ca r ried on theirb a ck s . The PLSS, an advanced ve r s i on of theAp o llo sys t e m , p rovides life support , voice com-mu n i ca t i on s , and biom e d i cal telemetry for EVA slasting as long as seven hours. Within its dimen-s i ons of 80 by 58.4 by 17.5 centimeters, the PLSSc ontains five major groups of com p onents for lifes u p p o rt . Those are the ox y g e n - ve n t i l a t i n g, c on-d e n s a t e, fe e d w a t e r, liquid tra n s p o rt , and pri m a ryoxygen circ u i t s .


The oxygen-ventilating circuit is a closed-loopsystem. Oxygen is supplied to the system from theprimary oxygen circuit or from a secondary oxygenpack that is added to the bottom of the PLSS foremergency use. The circulating oxygen enters thesuit through a manifold built into the Hard UpperTorso. Ducting carries the oxygen to the back of thespace helmet,where it is directed over the head andthen downward along the inside of the helmetfront. Before passing into the helmet, the oxygenwarms sufficiently to prevent fogging of the visor.As the oxygen leaves the helmet and travels into therest of the suit, it picks up carbon dioxide andh u m i d i ty from the crew member’s re s p i ra t i on .More humidity from perspiration, some heat fromphysical activity, and trace contaminants are also

picked up by the oxygen as it is drawn into theducting built into the Liquid Cooling-and-Ventilation Garment. A centrifugal fan, running atnearly 20,000 rpm, draws the contaminated oxygenback into the PLSS at a rate of about 0.17 cubicmeters per minute, where it passes through theContaminant Control Cartridge.

Carbon dioxide and trace contaminants are fil-tered out by the lithium hydroxide and activatedcharcoal layers of the cartridge.The gas stream thentravels through a heat exchanger and sublimator forremoval of the humidity. The heat exchanger andsublimator also chill water that runs through thetubing in the Liquid Cooling and Ve n t i l a t i onGarment (LCGV).The humidity in the gas streamcondenses out in the heat exchanger and sublima-


tor. The relatively dry gas (now cooled to approxi-mately 13 degrees Celsius) is directed through aca rb on dioxide sensor before it is re c i rc u l a t e dthrough the suit. Oxygen is added from a supplyand regulation system in the PLSS as needed. Inthe event of the failure of the suit fan, a purge valvein the suit can be opened. It initiates an open looppurge mode in which oxygen is delivered from boththe primary and secondary oxygen pack. In thismode, moisture and the carbon dioxide-rich gas aredumped outside the suit just before they reach theContaminant Control Cartridge.

One of the by-products of the ox y g e n - ve n t i l a t-ing circuit is moisture . The water produced by perspi-ra t i on and breathing is withdrawn from the ox y g e ns u p p ly by being condensed in the sublimator and isca r ried by the condensate circ u i t . (The small amountof oxygen that is also ca r ried by the condensate circ u i tis re m oved by a gas separator and re t u rned to the ox y-g e n - ventilating system.) The water is then sent to the

w a t e r - s t o rage tanks of the feedwater circuit andadded to their supply for eventual use in the sublima-t o r. In this manner, the PLSS is able to maintain suitcooling for a longer period than would be possiblewith just the tank’s original water supply.

The function of the feedwater and the liquidtransport circuits is to cool the astronaut. Using thepressure of oxygen from the primary oxygen circuit,the feedwater circuit moves water from the storagetanks (three tanks holding a total of 4.57 kilogramsof water) to the space between the inner surfaces oftwo steel plates in the heat exchanger and sublima-tor. The outer side of one of the plates is exposeddirectly to the vacuum of space.That plate is porousand, as water evaporates through the pores, thetemperature of the plate drops below the freezingpoint of water. Water still remaining on the insideof the porous plate freezes, sealing off the pores.Flow in the feedwater circuit to the heat exchangerand sublimator then stops.


Astronaut Joe Tanner works on the Hubble Space Telescope during the STS-82 mission.The curvature ofEarth and the Sun are over his left shoulder.

On the opposite side of the other steel plate is as e c ond chamber through which water from the liq-uid tra n s p o rt circuit passes. The liquid tra n s p o rt cir-cuit is a closed-loop system that is connected to theplastic tubing of the LC VG . Water in this circ u i t ,d ri ven by a pump, a b s o rbs body heat. As the heatedwater passes to the heat exchanger and sublimator,heat is tra n s fe r red through the aluminum wall to thechamber with the porous wall .The ice formed in thep o res of that wall is sublimated by the heat dire c t lyinto gas, p e rmitting it to tra vel through the pore sinto space. In this manner, water in the tra n s p o rtc i rcuit is cooled and re t u rned to the LC VG . T h ecooling rate of the sublimator is determined by thew o rkload of the astron a u t . With a greater work l o a d ,m o re heat is released into the water loop, causing iceto be sublimated more ra p i dly and more heat to beeliminated by the sys t e m .

The last group of components in the PrimaryLife-Support System is the primary oxygen circuit.Its two tanks contain a total of 0.54 kilograms ofoxygen at a pressure of 5,860.5 kilopascals, enoughfor a normal seven-hour EVA. The oxygen of this

circuit is used for suit pressurization and breathing.Two regulators in the circuit step the pressure downto usable levels of 103.4 kilopascals and 29.6 kilo-p a s ca l s . Oxygen coming from the 103.4-kilopas-cal regulator pre s s u ri zes the water tanks, and ox y-gen from the 29.6-kilopascal regulator goes to theventilating circ u i t .

To insure the safe ty of astronauts on EVA s , aSe c on d a ry Oxygen Pa ck (SOP) is added to the bot-t om of the PLSS. The two small tanks in this sys t e mc ontain 1.2 kilograms of oxygen at a pre s s u re of41,368.5 kilopasca l s . The Se c on d a ry Oxygen Pa ckcan be used in an open-loop mode by activating apurge valve or as a backup supply should the pri m a rys ystem fall to 23.79 kilopasca l s . The supply autom a t-i ca lly comes on line whenever the oxygen pre s s u reinside the suit drops to less than 23.79 kilopasca l s .

If the Displays and Control Module (DCM)purge valve (discussed below) is opened, used-oxy-gen contaminants and collected moisture dumpdirectly out of the suit into space.Because oxygen isnot conser ved and recycled in this mode, the largequantity of oxygen contained in the SOP is con-sumed in only 30 minutes.This half-hour still givesthe crew member enough time to return to theo rb i t e r’s airl o ck . If ca rb on dioxide con t rol isrequired, the helmet purge valve may be openedinstead of the DCM purge valve. That valve has alower flow rate than the DCM valve.

Displays and Control Module

The PLSS is mounted directly on the back ofthe Hard Upper Torso, and the controls to run it aremounted on the front. A small, irregularly-shapedbox, the Displays and Control Module (DCM),houses a variety of switches, valves, and displays.Along the DCM top are four switches for power,feedwater, communications mode selection, andcaution and warning. A suit-pressure purge valveprojects from the top at the left. It is used fordepressurizing the suit at the end of an EVA andcan be used in an emergency to remove heat and



humidity when oxygen is flowing from both theprimary and secondary oxygen systems. Near thefront on the top is an alpha-numeric display. Amicroprocessor inside the PLSS permits astronautsto monitor the condition of the various suit circuitsby reading the data on the display.

Stepped down from the top of the DCM, on asmall platform to the astronaut’s right, is a ventila-tion-fan switch and a push-to-talk switch. The

a s t ronaut has the option of having the radio channel open at all times or only when needed.

On a second platform, to the left, is an illumi-nated mechanical-suit pressure gauge. At the bot-tom, on the front of the DCM, are additional con-trols for communications volume, display lightingintensity, temperature control, and a four positionselector for controlling suit pressure in differentEVA operating modes.

the space shuttle extravehicular mobility unit (emu)

Documents