Extravehicular Activity Mobility Units

The EMUs provide the necessities for life support, such as oxygen, carbon dioxide removal, a pressurized enclosure, temperature control and meteoroid protection during EVA.

The EMU space suit comes in various sizes so that flight crew members can pick their suits before launch. Components are designed to fit men and women from the 5th to the 95th percentiles of body size.

The self-contained life support system contains seven hours of expendables, such as oxygen, a battery for electrical power, water for cooling, lithium hydroxide for carbon dioxide removal and a 30-minute emergency life support system during an EVA.

The airlock adapter plate in the airlock also provides a fixed position for the EMUs to assist the crew member during donning, doffing, checkout and servicing. The EMU weighs approximately 225 pounds, and its overall storage envelope is 26 by 28 by 40 inches. For launch and entry, the lower torso restraint, a cloth bag attached to the airlock adapter plate with straps, is used to hold the lower torso and arms securely in place.

The EMU is pressurized to 4 psid. It is designed for a 15-year life with cleaning and drying between flights. The EMU consists of a hard upper torso, lower torso assembly, gloves, helmet and visor assembly, communications carrier assembly, liquid cooling and ventilation garment, urine collection device and operational bioinstrumentation system. The upper torso, including arms, is that portion of the pressure suit above the waist, excluding the gloves and helmet. It provides the structural mounting for most of the EMU-helmet, arms, lower torso, portable life support system, display and control module and electrical harness. The arm assembly contains the shoulder joint and upper arm bearings that permit shoulder mobility as well as the elbow joint and wrist bearing.

The PLSS is made of fiberglass and provides a mounting for other EMU components. It includes oxygen bottles; water storage tanks; a fan, separator and pump motor assembly; a sublimator; a contaminant control cartridge; various regulators, valves and sensors; communications; bioinstrumentation; and a microprocessor module. The secondary oxygen pack attaches to the bottom of the PLSS. The PLSS expendables include 1.2 pounds of oxygen pressurized to 850 psia in the primary bottles, 2.6 pounds of oxygen at 6,000 psia in the secondary pack, 10 pounds of water for cooling in three bladders and lithium hydroxide in the contaminant control cartridge.

The primary oxygen system and water bladders provide enough of these expendables for seven hours inside the EMU, including 15 minutes for checkout, six hours of EVA, 15 minutes for EMU doffing and 30 minutes for reserve. The SOP will supply oxygen and maintain suit pressure for 30 minutes in the event of a failure in the primary system or depletion of the primary oxygen system.

The lower torso assembly is that portion of the EMU below the waist, including boots. It consists of pants and hip, knee and ankle joints. The lower torso comes in various sizes and connects to the hard upper torso by a waist ring. It is composed of several layers, beginning with a pressure bladder of urethane-coated nylon, a restraining layer made of Dacron, an outer thermal garment made of neoprene-coated nylon, four layers of aluminized Mylar and a surface layer of Gortex and Nomex. The foot section consists of specialized socks that contain return air ports. The EVA crew members' feet are fitted with boot inserts that fit into the boots.

The gloves contain the wrist connection, wrist joint and insulation padding for palms and fingers. They connect to the arms and are available in 15 sizes.

The helmet is a clear polycarbonate bubble with a neck disconnect and ventilation pad that provides pressurization for the head. An assembly that goes over the helmet contains visors that are manually adjusted to shield the EVA crew members' eyes from micrometeoroids and from ultraviolet and infrared radiation from the sun. Two EVA lights are attached on each side of the helmet. A TV camera can also be attached to the helmet.

A cap, known as the Snoopy cap, is worn under the EMU helmet. It fits over the crew member's head and is held in place by a chin guard. It contains a microphone and headphones for two-way communications and receiving caution and warning tones.

The liquid cooling and ventilation garment worn by the EVA crew member under the pressure suit has sewn-in tubes. It provides circulation of cooling water and pickup of vent flow at the extremities. It is a mesh one-piece suit made of spandex and has a zipper in the front for entry. It has 300 feet of plastic tubing that carries cooling water at a rate of 240 pounds per hour. It is controlled by a valve on the display and control module. Ducting along the garment's arms and legs directs oxygen and carbon dioxide from the suit to the life support system for purification and recirculation. The garment weighs 6.5 pounds and provides cooling to maintain desired body temperature and physical activity that nominally generates 1,000 Btu per hour and can generate up to 2,000 Btu per hour, which is considered extremely vigorous.

The urine collection device collects urine. It can store approximately 1 quart of urine. It consists of adapter tubing, a storage bag and disconnect hardware for emptying after an EVA to the orbiter waste water tank.

The bioinstrumentation system monitors the EVA crew member's heart rate (electrocardiogram) during an EVA.

An in-suit drink bag stores approximately 0.5 of a quart of drinking water in the upper torso. A tube from the upper hard torso to the helmet permits the EVA crew member to drink water while suited.

The life support system consists of the portable life support system, display and control module, contaminant control cartridge, battery, secondary oxygen pack, and EVA communicator and EMU antenna. The PLSS is also referred to as a backpack. The PLSS normally provides the EVA crew member with oxygen for breathing, ventilation and pressurization and water for cooling.

The contaminant cartridge consists of lithium hydroxide, charcoal and filters to remove carbon dioxide, odors, particulates and other contaminants from the ventilation circuit. It is replaceable upon completion of an EVA.

A silver-zinc battery provides all electrical power used by the EMU and life support system. It is stored dry, filled, sealed and charged before flight. It is rechargeable upon completion of an EVA and is rated at 17 volts dc.

The SOP provides oxygen for breathing, ventilation, pressurization and cooling in the event of a PLSS malfunction. It is mounted at the base of the PLSS and contains a 30-minute oxygen supply, a valve and a regulator assembly.

The EVA communicator and EMU antenna provide EVA communications via its transceiver and antenna between the suited crew member and the orbiter. In addition, the crew member's electrocardiogram is telemetered through the communicator to the orbiter. It is a separate subassembly that attaches to the upper portion of the life support system at the back of the hard upper torso. The controls are located on the display and control module mounted at the front of the upper torso.

The radios for space walk communications have two single-UHF-channel transmitters, three single-channel receivers and a switching mechanism. In addition, telemetry equipment is included so that ground personnel can monitor the astronaut's heart beat. These backpack radios have a low-profile antenna, a 1-foot-long rectangular block fitted to the top of the packs. The radios weigh 8.7 pounds and are 12 inches long, 4.3 inches high and 3.5 inches wide.

The EMU electrical harness provides biomedical instrumentation and communications connections to the PLSS. The harness connects the communications carrier assembly and the biomedical instrumentation subsystem to the hard upper torso, where internal connections are routed to the EVA communicator. The cable routes signals from the electrocardiogram sensors, which are attached to the crew member, through the bioinstrumentation system to the EVA communicator. It also routes caution and warning signals and communications from the communicator to the crew member's headset.

The DCM is an integrated assembly that attaches directly to the front of the hard upper torso. The module contains a series of mechanical and electrical controls, a microprocessor, and an alphanumeric LED display easily seen by a crew member wearing the space suit. It contains the displays and controls associated with the operation of the EMU.

The function of the display and control module is to enable the crew member to control the PLSS and the secondary oxygen pack. It also indicates the status of the PLSS, the suit and, the manned maneuvering unit (when it is attached) visibly and audibly.

The mechanical controls consist of a suit purge valve, the liquid cooling and ventilation garment cooling valve, and the oxygen actuator control, which has four positions: off , iv (which turns primary oxygen on to a 0.5-psid suit pressure setting), press (which turns primary oxygen on to a 4.1-psid suit pressure setting), and ev (which leaves primary oxygen on the 4.1-psid setting and turns the secondary oxygen pack on). The electrical controls include a voice communications mode switch, dual volume controls, push-to-talk switches, a power mode switch, feedwater and C/W switches and the LED display brightness control. The displays on the module are a 12-digit LED display, a built-in test equipment indicator and an analog suit pressure gauge.

The display and control module is connected to the hard upper torso and to the PLSS by both internal and external hookups. A multiple-function connector links the display module to the service and cooling umbilical, thus enabling the use of the display module controls during suit checkout inside the airlock station.

The display module interacts with a microprocessor in the PLSS that contains a program that enables the crew member to cycle the display through a series of systems checks and thereby determine the condition of a variety of components. The microprocessor monitors oxygen pressure and calculates the time remaining at the crew member's present use rate. It signals an alarm at high oxygen use in the primary oxygen tanks. It also monitors water pressure and temperature in the cooling garment. The carbon dioxide level is monitored and an alarm is signaled when it reaches high concentrations in the suit. The microprocessor monitors the power consumed and signals at high current-drain rates and also when an estimated 30 minutes of battery power is left. All the warnings are displayed on the LED display.

The display module also has a fiber-optic cable that is used when the MMU is connected to the EMU. The fiber-optic cable connects the display unit to the MMU. A fiber-optic cable is more reliable and more covenient and safer to use than an electrical connector for extravehicular applications. The MMU is mounted on the back of the portable life support system. When the MMU is connected, the display module also provides a cycled readout of propellant pressures, temperatures, and battery condition (in the MMU) and an audible thruster cue. The C/W system warns of low propellant, low battery, and failed components.

Oxygen from the system enters the suit at the helmet and flows from behind the head down through the suit. Oxygen and carbon dioxide are removed from the suit through the liquid cooling and ventilation garment at ports near the crew member's wrists and feet. Return air goes first through the contaminant control cartridge, where activated charcoal and lithium hydroxide beds remove carbon dioxide, odors and dust. From there the return air goes through a water separator, where moisture from exhalation and the lithium hydroxide and carbon dioxide reaction is removed. The oxygen then goes through the fan, which maintains air flow at 6 cubic feet per minute. It is then routed through the sublimator, where it is cooled to 85º F, and then passes through a vent and flow detector and back to the suit. Oxygen for the air system is fed from the primary oxygen containers through regulators that maintain suit pressure at 4.1 psid.

The system is protected from suit overpressure, primary oxygen supply depletion or mechanical failure by regulators, sensors and the secondary oxygen pack. The secondary oxygen pack can maintain suit pressure at 3.45 psid. A purge valve on the display and control module allows a crew member to completely replace system oxygen in the suit if, for instance, the carbon dioxide level rises too high too quickly.

The cooling water system takes the warm water from the cooling garment and divides it into two loops. One loop goes to the sublimator, where the water in that loop is cooled and sent back to the cooling control valve. The other loop goes directly back to the cooling control valve, where the loops are recombined and full flow goes back to the cooling garment. Thus, the cooling garment has a constant flow of cooling water at a temperature set by the crew member using the cooling control valve. During the process, the full flow from the cooling garment goes through a gas separator, where gas is removed from the loop, and then through a pump that maintains a flow of 260 pounds per hour. Another side loop circulates 20 pounds per hour through the contaminant control cartridge to cool the lithium hydroxide canister since the lithium hydroxide and carbon dioxide reaction produces heat and needs to be kept cool for an efficient reaction.

Since the system is a closed-loop design, water from the water separator is fed back to the water system, and air from the gas trap is fed back to the oxygen system. Water from the water tanks is also fed, through regulators, into the cooling system. However, the primary purpose of the water tanks is to feed water to the sublimator. The sublimator works on the principle of sublimation, that is, the process by which a solid turns directly into a vapor, bypassing the liquid phase. In this case, ice is formed on the sublimator evaporator sieve and is allowed to vaporize to space, removing heat with it. Air and cooling water are passed through fins in the sublimator, which extracts heat from each system.

The PLSS sensors detect system air flow, air pressure, water flow, water pressure, differential water pressure (between the circulating system and the water tanks), water temperature and carbon dioxide content in the return air. In addition, there are a number of crew-selectable valves, including a purge valve, a cooling control valve (infinitely variable), oxygen supply and a direct-reading air pressure gauge. The sensors supply information to the display and control module, where a microprocessor maintains an automatic watch over system integrity.

Normally, the day before an EVA, the orbiter crew compartment cabin pressure is allowed to decrease from 14.7 psia to 12.5 psia through metabolic usage. One hour before depressurizing the crew compartment from 12.5 psia to 10.2 psia, the EVA crew member and prebreathes 100-percent oxygen for 45 minutes. There are two PEAPs in the airlock. The crew compartment is then depressurized from 12.5 psia to 10.2 psia and remains at this pressure until after the EVA is completed. This is necessary to remove nitrogen from the EVA crew member's blood before the EVA crew member works in the pure oxygen environment of the EMU. Without the prebreathing, bends can occur. When an individual fails to reduce nitrogen levels in the blood before working in a pressure condition, it can result in nitrogen coming out of solution in the form of bubbles in the bloodstream. This condition results in pain in the body joints, possibly because of restricted blood flow to connective tissues or because of the extra pressure caused by bubbles in the blood in the joint area.

In preparation for an EVA, the crew member dons the liquid cooling and ventilation garment first, enters the airlock and dons the lower torso assembly. The crew member then squats under the hard upper torso mounted on the airlock adapter plate and slides up into the upper torso. The upper and lower torsos are connected with a waist ring. The gloves and helmet are then put on, and the EMU is disconnected from the AAP.

The orbiter provides electrical power, oxygen, liquid cooling and ventilation garment cooling and water to the EMUs in the airlock via the service and cooling umbilical for EVA preparation and after EVA operations.

The service and cooling umbilical contains communication lines, electrical power, water, water drain line and oxygen recharge lines. The umbilical permits the EVA crew member to check out the suit in the airlock without using the EMU supply of water, oxygen and battery power.

The SCU is launched with the orbiter end fittings permanently connected to the appropriate ECLSS panels in the airlock and the EMU connected to the airlock adapter plate stowage connector. It allows all supplies (oxygen, water, electrical and communication) to be transported from the airlock control panels to the EMU before and after EVA without using the EMU expendable supplies of water, oxygen and battery power that are scheduled for use in the EVA. The SCU also provides EMU recharge. The SCU umbilical is disconnected just before the crew member leaves the airlock on an EVA and is reconnected when he returns to the airlock. Each SCU is 144 inches long, 3.5 inches in diameter and weighs 20 pounds. Actual usable length after attachment to the control panel is approximately 7 feet.

The airlock has two display and control panels. The airlock control panels are basically split to provide either ECLSS or avionics operations. The ECLSS panel provides the interface for the SCU waste and potable water, liquid cooling and ventilation garment cooling water, EMU hardline communications, EMU power and oxygen supply. The avionics panel includes the airlock lighting, airlock audio system and EMU power and battery recharge controls. The avionics panel is located on the right side of the cabin airlock hatch and the ECLSS panel is on the left side. The airlock panels are designated AW18H, AW18D and AW18A on the left side and AW82H, AW82D and AW82B on the right side. The ECLSS panel is divided into EMU 1 functions on the right side and EMU 2 functions on the left.

Airlock communications are provided with the orbiter audio system at airlock panel AW82D, where connectors for the headset interface units and the EMUs are located at airlock panel AW18D, the airlock audio terminal. The HIUs are inserted in the crew member communications carrier unit connectors on airlock panel AW82D. The CCUs are also known as the Snoopy caps. The adjacent two-position switches labeled CCU1 and CCU2 power enable transmit functions only, as reception is normal as soon as the HIUs are plugged in. The EMU 1 and EMU 2 connectors on the panel to which the SCU is connected include contacts for EMU hardline communications with the orbiter before EVA. Panel AW18D contains displays and controls used to select access to and control the volume of various audio signals. Control of the airlock audio functions can be transferred to the middeck ATUs on panel M042F by placing the control knob to the middeck position.

During EVA, the EVA communicator is part of the same UHF system that is used for air-to-air and air-to-ground voice communications between the orbiter and landing site control tower. The EVA communicator provides full duplex (simultaneous transmission and reception) communications between the orbiter and the EVA crew members. It also supplies continuous data reception of electrocardiogram signals from each crew member by the orbiter and processing by the orbiter and relay of electrocardiogram signals to the ground. The UHF airlock antenna in the forward portion of the payload bay provides the UHF EVA capability.

Panel AW18H in the airlock provides 17 volts dc, plus or minus 0.5 volt dc, at 5 amperes at both EMU electrical connector panels on panel AW82D and in EVA preparation. Main bus A or B can be selected on the bus select switch; then the mode switch is positioned to power . The bus select switch provides a signal to a remote power controller that applies 28 volts dc from the selected bus to the power and battery recharger. The mode switch in the power position makes the power available at the SCU connector and also closes a circuit that provides a battery feedback voltage charger control that inhibits EMU power when any discontinuity is sensed in the SCU/EMU circuitry. The mode switch in the power position also applies power through the SCU for the EMU microphone amplifiers for hardline communication. When the SCU umbilical is disconnected for EVA, the EMU operates on its self-contained battery power. After EVA, when the SCU is reconnected to the EMU, selecting a bus and the charge position on the mode switch charges the PLSS battery at 1.55 amps, plus or minus 0.05 amp. When the battery reaches 21.8 volts dc, plus or minus 0.1 volt dc, or the charging circuit exceeds 1.55 amps, plus or minus 0.05 amp, a solenoid-controlled switch internal to the battery charger removes power to the charging circuitry.

Cooling for flight crew members before and after the EVA is provided by the liquid-cooled garment circulation system via the SCU and LCG supply and return connections on panel AW82B. These connections are routed to the orbiter LCG heat exchanger, which transfers the collected heat to the orbiter Freon-21 coolant loops. The nominal loop flow of 250 pounds per hour is provided by the EMU and PLSS water loop pump. The system circulates chilled water at 50º F maximum to the liquid cooling and ventilation garment inlet and provides a heat removal capability of 2,000 Btu per hour per crew member. When the SCU is disconnected, the PLSS provides the cooling. Upon return from the EVA, the PLSS is reconnected to the SCU, and crew member cooling is as it was in the EVA preparation.

With the suit connected to the SCU, oxygen at 900 psia, plus or minus 500 psia, is supplied through airlock panel AW82B from the orbiter's oxygen system when the oxygen valve is in the open position on the airlock panel. This provides the suited crew member with breathing oxygen and prevents depletion of the PLSS oxygen tanks before the EVA. Before the crew member seals the helmet, an oxygen purge adapter hose is connected to the airlock panel to flush nitrogen out of the suit.

When the SCU is disconnected, the PLSS provides oxygen for the suit. When the EVA is completed and the SCU is reconnected, the orbiter's oxygen supply begins recharging the PLSS, assuming that the oxygen valve on panel AW82B is open. Full oxygen recharge takes approximately one hour (allowing for thermal expansion during recharge), and the tank pressure is monitored on the EMU display and control panel as well as on the airlock oxygen pressure readout.

The EMU water supply and waste valves are opened during the EVA preparation by switches on panel AW82D. This provides the EMU, via the SCU, access to the orbiter's potable water and waste water systems. The support provided to the EMU PLSS is further controlled by the EMU display and control panel. Potable water (supplied from the orbiter at 16 psi, plus or minus 0.5 psi; 100 to 300 pounds per hour; and 40º to 100º F) is allowed to flow to the feedwater reservoir in the EMU that provides pressure, which would top off any tank not completely filled. Waste water condensate developed in the PLSS is allowed to flow to the orbiter waste water system via the SCU whenever the regulator connected at the bacteria filters (airlock end of the SCU) detects upstream pressure in excess of 16 psi, plus or minus 0.5 psi.

When the SCU is disconnected from the EMU, the PLSS assumes its functions. When the SCU is reconnected to the EMU upon completion of the EVA, it performs the same functions it did before the EVA except that the water supply is allowed to continue until the PLSS water tanks are filled, which takes approximately 30 minutes.

In preparation for the EVA, the airlock hatch to the orbiter crew cabin is closed and depressurization of the airlock begins.

Airlock depressurization is accomplished in two stages by a three-position valve located on the ECLSS panel AW82A in the airlock. The airlock depressurization valve is covered with a pressure and dust cap. Before the cap is removed from the valve, it is necessary to vent the area between the cap and valve by pushing the vent valve on the cap. In flight the pressure and dust cap is stored next to the valve. The airlock depressurization valve is connected to a 2-inch-inside-diameter stainless steel overboard vacuum line. The airlock depressurization valve controls the rate of depressurization by varying the valve's diameter. Closing the valve prevents any air flow from escaping to the overboard vent system.

When the crew members have completed the 40-minute prebreathe in the EMUs, the airlock is depressurized from 10.2 psia to 5 psia by moving the airlock depressurization valve to the 5 position, which opens the depressurization valve and allows the pressure in the airlock to decrease at a controlled rate. The airlock depressurization valve must be closed to maintain 5 psia. During depressurization, pressure can be monitored by the delta pressure gauge on either airlock hatch. A delta pressure gauge is installed on each side of both airlock hatches.

At this time, the flight crew performs an EMU suit leak check, electrical power is transferred from the umbilicals to the EMU batteries, the umbilicals are disconnected, and the suit oxygen packs are brought on-line.

The second stage of airlock depressurization is accomplished by positioning the airlock depressurization valve to 0 , which increases the valve's diameter and allows the pressure in the airlock to decrease from 5 psia to zero psia. The suit sublimators are activated for cooling, EMU system checks are performed, and the airlock and payload bay hatch can be opened. The hatch is capable of opening against a 0.2 psia differential maximum.

Hardware provisions are installed in the orbiter payload bay for use by the crew member during the EVA.

Handrails and tether points are located on the payload bulkheads, forward bulkhead station Xo 576 and aft bulkhead station Xo 1307 along the sill longeron on both sides of the bay to provide translation and stabilization capability for EVA crew members and facilitate movement in the payload bay. The handrails are designed to withstand a load of 200 pounds, or 280 pounds maximum, in any direction. Tether attach points are designed to sustain a load of 574 pounds, 804 pounds maximum, in any direction.

The handrails have a cross section of 1.32 inches by 0.75 of an inch. They are made of aluminum alloy tubing and are painted yellow. The end braces and side struts of the handrails are constructed of titanium. An aluminum alloy end support standoff functions as the terminal of the handrail. Each end support standoff incorporates a 1-inch- diameter tether point.

A 25-foot safety tether is attached to each crew member at all times during an EVA.

The tether consists of a reel case with an integral D-ring, a reel with a light takeup spring, a cable and a locking hook. The safety tether hook is locked onto the slidewire before launch, and the cable is routed and clipped along the left and right handrails to a position just above the airlock and payload bay hatch. After opening the airlock hatch but before leaving the airlock, the crew member attaches a waist tether to the D-ring of the safety tether to be used. The other end of the waist tether is hooked to a ring on the EMU waist bearing. The crew member may select either the left or the right safety tether. With the selector on the tether in the locked position, the cable will not retract or reel out. Moving the selector to the unlocked position allows the cable to reel out and the retract feature to take up slack. The cable is designed for a maximum load of 878 pounds. The routing of the tethers follows the handrails, which allows the crew member to deploy and restow his tether during translation.

The two slidewires, approximately 46.3 feet long, are located in the longeron sill area on each side of the payload bay. They start approximately 9.3 feet aft of the forward bulkhead and extend approximately 46.3 feet down the payload bay. The slidewires withstand a tether load of 574 pounds with a safety factor of 1.4 or 804 pounds maximum.

EVA support equipment may consist of a small work station, tool caddies and equipment tethers. The work station contains a universal attachment tether for crew member restraint and a carrying location for the tool caddies. The caddies hold the tools and provide tethers for them when they are not in use.

A cargo bay stowage assembly installed in the orbiter payload bay contains miscellaneous tools for use in the payload bay during an EVA. The CBSA is approximately 42 inches wide, 24 inches deep and 36 inches high. The CBSA weighs 573 pounds.

The airlock and cabin hatch has two pressure equalization valves that can be operated from both sides of the hatch to repres surize the airlock volume. Each valve has three positions- closed , norm (normal) and emerg (emergency)-and is protected by a debris pressure cap on the intake (high-pressure) side of the valve. The pressure cap on the outer hatch must be vented for removal. The caps are tethered to the valves and also have small Velcro spots that allow them to be stored temporarily on the hatch. The exit side of the valve contains an air diffuser to provide uniform flow out of the valve.

Through the use of the equalization valves, the airlock is initially pressurized to 5 psia, and the space suit is connected to the umbilical in the airlock and electrical power is transferred back to umbilical power. After the airlock is pressurized to the 10.2-psia cabin pressure, the EVA crew members remove and recharge their EMUs. Shortly thereafter, the crew compartment cabin is pressurized from 10.2 psia to 14.7 psia.

The orbiter can accommodate three six-hour EVAs by two crew members per flight at no weight or volume cost to the payload. Two of the EVAs are for payload support; the third is reserved for orbiter contingency. Additional EVAs can be considered with consumables charged to payloads.

When fitted with a tunnel adapter, hatches, tunnel extension and tunnel, the middeck airlock permits flight crew members to transfer from the orbiter middeck into the Spacelab pressurized modules, where they can work in a pressurized shirt-sleeve environment. The airlock, tunnel adapter and hatches also permit EVA flight crew members to transfer into the payload bay from the tunnel adapter in the space suit assembly without depressurizing the spacecraft crew cabin and Spacelab.

The tunnel adapter is located in the payload bay and is attached to the airlock at orbiter station Xo 576 and to the tunnel extension at Xo 660, thus attaching it to the Spacelab tunnel and Spacelab. The tunnel adapter has an inside diameter of 63 inches at the widest section and tapers in the cone area at each end to two 40-inch-diameter D-shaped openings, 36 inches across. An identical D-shaped opening is located at the top of the tunnel adapter. Two pressure-sealing hatches are located in the tunnel adapter: one at the upper area of the tunnel adapter and one at the aft end of the tunnel adapter. The tunnel adapter is constructed of 2219 aluminum and is a welded structure with 2.4- by 2.4-inch exposed structural ribs on the exterior surface and external waffle skin stiffening.

The hatch located in the aft end isolates the tunnel adapter and airlock from the extension tunnel and Spacelab. This hatch opens into the tunnel adapter. The hatch located in the tunnel adapter at the upper D-shaped opening isolates the airlock and tunnel adapter from the unpressurized payload bay when closed and permits EVA crew members to exit the airlock and tunnel adapter to the payload bay when open. This hatch opens into the tunnel adapter.

The two hatches in the tunnel adapter are installed to open toward the primary pressure source and the orbiter crew cabin to achieve pressure-assist sealing when closed.

Each hatch has six interconnected latches (with the exception of the aft hatch, which has 17) with a gearbox and actuator, window, hinge mechanism and hold-open device, differential pressure gauge on each side and two equalization valves.

The window in each hatch is 4 inches in diameter. The window is used for crew observation from the cabin and airlock, tunnel adapter to tunnel, and tunnel adapter to payload bay. The dual window panes are made of polycarbonate plastic and are mounted directly to the hatch using bolts fastened through the panes. Each hatch window has dual pressure seals with seal grooves located in the hatch.

Each hatch has dual pressure seals to maintain pressure integrity. One seal is mounted on the hatch and the other on the structure. Leak check quick disconnects are installed between the hatches and the pressure seals to verify the hatches' pressure integrity before flight.

The gearbox with latch mechanisms on each hatch allows the flight crew to open or close the hatch during transfers and EVA operation. The gearbox and the latches are mounted on the low-pressure side of each hatch and a gearbox handle is installed on both sides to permit operation from either side of the hatch.

The aft hatch is hinged to be first pulled into the tunnel adapter and then pulled forward at the bottom. The top of the hatch is rotated toward the tunnel and downward until the hatch rests with the Spacelab side facing the tunnel adapter's floor. The linkage mechanism guides the hatch from the close/open, open/close position with friction restraint throughout the stroke. The hatch is held in the open position by straps and Velcro.

The upper (EVA) hatch in the tunnel adapter opens and closes to the left wall of the tunnel adapter. The hatch is hinged to be pulled first into the tunnel adapter and then pulled forward at the hinge area and rotated down until it rests against the left wall of the tunnel adapter. The linkage mechanism guides the hatch from the close/open, open/close position with friction restraint throughout the stroke. The hatch is held in the open position by straps and Velcro. The hatches can be removed in flight from the hinge mechanism via pip pins, if required.

When the airlock hatch is opened on orbit, a duct is connected to the cabin air system to provide conditioned air to the airlock, tunnel adapter and tunnel during non-EVA-operation periods. The duct must be disconnected before the airlock hatch is closed for entry.

For an EVA during a mission with pressurized Spacelab modules, all hatches are closed and depressurization of the airlock tunnel adapter begins. The prebreathe requirements, lowering of cabin pressure to 10.2 psia and space suit preparations, etc., remain the same as for an EVA from the airlock. The difference is that the EVA crew member enters the payload bay from the upper tunnel adapter hatch.

Hardware is installed in the orbiter payload bay and in the tunnel adapter, tunnel and Spacelab for use by the crew member during the EVA.

When EVA is completed, the crew member enters the upper tunnel adapter hatch and closes it. The airlock and tunnel adapter are repressurized in the same manner as when the airlock alone is used.