Putting on the EMU
Putting on a Shuttle EMU is a relatively simple operation that can be accomplished in a matter of about 15 minutes. However, the actual process of preparing to go EVA takes much longer. When working in the Shuttle cabin, crew members breathe a normal atmospheric mix of nitrogen and oxygen at 101 kilopascals. The suits atmosphere is pure oxygen at 29.6 kilopascals. A rapid drop from the cabin pressure to the EMU pressure could result in a debilitating ailment that underwater divers sometimes experience- the bends. The bends, also known as caisson disease, are produced by the formation and expansion of nitrogen gas bubbles in the bloodstream when a person breathing a normal air mixture at sea-level pressure is exposed to a rapid drop in external pressure. In severe cases, the bends are characterized by pains in the joints, cramps, paralysis, and eventual death if not treated by gradual recompression. To prevent an occurrence of the bends, crew members intending to go EVA spend a period of time prebreathing pure oxygen. During that time, nitrogen gas in the bloodstream is replaced by pure oxygen.
Prior to prebreathing, the atmospheric pressure of the entire orbiter cabin is depressed from the normal 101 kilopascals to 70.3 pascals while the percentage of oxygen is slightly increased. Prebreathing begins when the crew members who plan to go EVA don a mask connected to an oxygen supply. A short hose permits them to continue their EVA preparations during this period. The length of time for cabin decompression and the time for prebreathing is related. Without any cabin decompression, prebreathing must last at least four hours. A prebreathe of 30 minutes is safe providing cabin decompression takes place at least 24 hours before the exit into space.
By now, much of the dissolved nitrogen gas has been cleared from the EVA crew members, and they can remove their helmets. Later, when they don their spacesuits and seal the helmets, an additional 30 to 40 minutes of pure oxygen prebreathing takes place before the suits are lowered to their operating pressure of 29.6 kilopascals.
Most of the EMU-donning process takes place inside the airlock. The airlock is a cylindrical chamber located on the orbiters mid-deck. One hatch leads from the middeck into the airlock, and a second hatch leads from the airlock out to the unpressurized payload bay.
Before entering the hatch, but following their initial prebreathing, the crew members put on the maximum absorbency garment (MAG). The MAG is an adult-size diaper.
Next, several simple tasks are performed. Antifog compound is rubbed on the inside of the helmet. A wrist mirror and a small spiral-bound 27-page checklist are put on the left arm of the upper torso. The wrist mirror was added to the suit because some of the knobs on the front of the displays and control module are out of the vision range of the crew member. The mirror permits the knob settings to be read. Setting numbers are written backwards for ease of reading in the mirror.
Another task at this time is to insert a food bar and a water-filled In-Suit Drink Bag (IDB) inside the front of the HUT. The food bar of compressed fruit, grain, and nuts is wrapped in edible rice paper, and its upper end extends into the helmet area near the crew members mouth. When hungry, the crew member bites the bar and pulls it upward before breaking off a piece to chew. In that manner, a small piece of the bar remains extended into the helmet for the next bite. It is necessary to eat the entire bar at one time, because saliva quickly softens the protruding food bar, making it mushy and impossible to break off. The IDB is placed just above the bar. Two sizes of bags are available and the one chosen is filled with water from the water supply of the orbiter's galley prior to entry into the airlock. The largest bag contains nearly one liter of water. A plastic tube and valve assembly extends up into the helmet so that the crew member can take a drink whenever needed. Both the food bar and drink bag are held in place by Velcro attachments.
During EVAs, the crew members may need additional lighting to perform their tasks. A light-bar attachment (helmet-mounted light array) is placed above the helmet visor assembly. Small built-in flood lamps provide illumination to places that sunlight and the regular payload bay lights do not reach. The EVA light has its own battery system and can be augmented with a helmet-mountable television camera system with its own batteries and radio frequency transmitter. The cameras lens system is about the size of a postage stamp. Through this system, the crew remaining inside the orbiter and the mission controllers on Earth can get an astronauts eye view of the EVA action. During complicated EVAs, viewers may be able to provide helpful advice for the tasks at hand.
Next, the Communications Carrier Assembly (CCA), or "Snoopy cap," is connected to the EMU electrical 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 the suit are completed, the lower torso, or suit pants, are pulled on. The lower torso comes in various sizes to meet the varying size requirements of different astronauts. It features pants with boots and joints in the hip, knee, and ankle, and a metal body-seal closure for connecting to the mating half of the ring mounted on the hard upper torso. The lower torsos waist element also contains a large bearing. This gives the crew member mobility at the waist, permitting twisting motions when the feet are held in workstation foot restraints.
Joints for the lower and upper torsos represent an important advance over those of previous spacesuits. Earlier joint designs consisted of hard rings, bellows-like bends in the pressure bladder, or cable- and pulley-assisted fabric joints. The Shuttle EMU joints maintain nearly constant volume during bending. As the joints are bent, reductions in volume along the inner arc of the bend are equalized by increased volume along the outer arc of the bend.
Long before the upper half of the EMU is donned, the airlocks Service and Cooling Umbilical (SCU) is plugged into the Displays and Control Module Panel on the front of the upper torso. Five connections within the umbilical provide the suit with cooling water, oxygen, and electrical power from the Shuttle itself. In this manner, the consumables stored in the Primary Life-Support System will be conserved during the lengthy prebreathing period. The SCU also is used for battery and consumable recharging between EVAs.
The airlock of the Shuttle orbiter is only 1.6 meters in diameter and 2.1 meters high on the inside. When two astronauts prepare to go EVA, the space inside the airlock becomes crowded. For storage purposes and as an aid in donning and doffing the EMU, each upper torso is mounted on airlock adapter plates. Adapter plates are brackets on the airlock wall for supporting the suits upper torsos.
With the lower torso donned and the orbiter providing consumables to the suits, each crew member "dives" with a squirming motion into the upper torso. To dive into it, the astronaut maneuvers under the body-seal ring of the upper torso and assumes a diving position with arms extended upward. Stretching out, while at the same time aligning arms with the suit arms, the crew member slips into the upper torso. As two upper and lower body-seal closure rings are brought together, two connections are made. The first joins the cooling water-tubing and ventilation ducting of the LCVG to the Primary Life-Support System. The second connects the biomedical monitoring sensors to the EMU electrical harness that is connected to the PLSS. Both systems are turned on, and the crew member then locks the two body-seal closure rings together, usually with the assistance of another crew member who remains on board.
One of the most important features of the upper half of the suit is the HUT, or Hard Upper Torso. The HUT is a hard fiberglass shell under the fabric layers of the thermal-micrometeoroid garment. It is similar to the breast and back plates of a suit of armor. The HUT provides a rigid and controlled mounting surface for the Primary Life-Support System on the back and the Displays and Control Module on the front.
In the past, during the Apollo Moon missions, donning suits was a very lengthy process because the life-support system of those suits was a separate item. Because the Apollo suits were worn during launch and landing and also as cabin-pressure backups, a HUT could not be used. It would have been much too uncomfortable to wear during the high accelerations and decelerations of lift-off and reentry. The life-support system had to be attached to the suit inside the lunar module. All connections between PLSS and the Apollo suit were made at that time and, with two astronauts working in cramped quarters, preparing for EVA was a difficult process. The Shuttle suit HUT eliminates that lengthy procedure because the PLSS is already attached. It also eliminates the exposed and vulnerable ventilation and life-support hoses of earlier EMU designs that could become snagged during EVA.
The last EMU gear to be donned includes eyeglasses if needed, the communications carrier assembly (CCA), comfort gloves, the helmet with lights and optional TV, and EVA gloves. The two gloves have fingertips of silicone rubber that permit some degree of sensitivity in handling tools and other objects. Metal rings in the gloves snap into rings in the sleeves of the upper torso. The rings in the gloves contain bearings to permit rotation for added mobility in the hand area. The connecting ring of the helmet is similar to the rings used for the body-seal closure. Mobility is not needed in this ring because the inside of the helmet is large enough for the crew members head to move around. To open or lock any of the connecting rings, one or two sliding, rectangular-shaped knobs are moved to the right or the left. When opened, the two halves of the connecting rings come apart easily. To close and lock, one of the rings slides part way into the other against an O-ring seal. The knob is moved to the right, and small pins inside the outer ring protrude into a groove around the inside ring, thereby holding the two together.
All suit openings have locking provisions that require a minimum of three independent motions to open. This feature prevents any accidental opening of suit connections.
With the donning of the helmet and gloves, the spacesuits are now sealed off from the atmosphere of the air lock. The crew members are being supported by the oxygen, electricity, and cooling water provided by the orbiter. A manual check of suit seals is made by pressurizing each suit to 29.6 kilopascals d. (The "d" stands for differential, meaning greater than the air lock pressure.) Inside the air lock, the pressure is either 70.3 or 101 kilopascals. The suits pressure is elevated an additional 29.6 kilopascals, giving it a pressure differential above the air lock pressure. Once pressure reaches the desired level, the oxygen supply is shut off and the digital display on the chest-mounted control module is read. To assist in reading the display, an optional Fresnel lens inside the space helmet may be used to magnify the numbers. Some leakage of spacesuit pressure is normal. The maximum allowable rate of leakage of the Shuttle EMU is 1.38 kilopascals per minute, and this is checked before the suit is brought back down to air lock pressure.
As the suit pressure is elevated, crew members may experience discomfort in their ears and sinus cavities. They compensate for the pressure change by swallowing, yawning, or pressing their noses on an optional sponge mounted to the left on the inside of the helmet ring. Attempting to blow air through the nose when pressing the nose on the sponge forces air inside the ears and sinus cavities to equalize the pressure.
During the next several minutes the two spacesuits are purged of any oxygen/nitrogen atmosphere remaining from the cabin; this is replaced with pure oxygen. Additional suit checks are made while the final oxygen prebreathe takes place.
The inner door of the air lock is sealed, and the air lock pressure bleed-down begins. A small depressurization valve in the air lock latch is opened to outside space, permitting the air lock atmosphere to escape. While this is taking place the EMU automatically drops its own pressure to 66.9 kilopascals and leak checks are conducted. Failure of the leak test would require repressurizing the air lock, permitting the EVA crew to reexamine the seals of their suits.
Final depressurization is begun by opening the air lock depressurization
valve. The outer air lock hatch is then opened and the suited astronauts
prepare to pull themselves out into the payload bay. As a safety measure,
they tether themselves to the orbiter to prevent floating away as they
move from place to place by hand holds. It is at this point that they
disconnect the orbiter Service and Cooling Umbilical from the EMU. The
PLSS begins using its own supply of oxygen, cooling water, and electricity.
The astronauts pull themselves through the outer air lock hatch, and the