A pressure suit is a protective suit worn by high-altitude pilots who may fly at altitudes where the air pressure is too low for an unprotected person to survive, even when breathing pure oxygen at positive pressure. Such suits may be either full-pressure (e.g., a space suit) or partial-pressure (as used by aircrew). Partial-pressure suits work by providing mechanical counter-pressure to assist breathing at altitude.
Background The region from sea level to around 3,000 m (10,000 ft) is known as the physiological-efficient zone. Oxygen levels are usually high enough for humans to function without supplemental oxygen and decompression sickness is rare. The physiological-deficient zone extends from 3,600 m (12,000 ft) to about 15,000 m (50,000 ft). There is an increased risk of problems such as hypoxia, trapped-gas dysbarism (where gas trapped in the body expands), and evolved-gas dysbarism (where dissolved gases such as nitrogen may form in the tissues, i.e. decompression sickness). Above approximately 4,267 m (14,000 ft) oxygen-rich breathing mixture is required to approximate the oxygen available in the lower atmosphere, while above 12,000 m (40,000 ft) oxygen must be under positive pressure. Above 15,000 m (49,000 ft), respiration is not possible because the pressure at which the lungs excrete carbon dioxide (approximately 87 mmHg) exceeds outside air pressure. Above 19,000 m (62,000 ft), also known as the Armstrong limit, fluids in the throat and lungs will boil away. Generally, 100% oxygen is used to maintain an equivalent altitude of 3,000 m (10,000 ft).
Methods of operation Generally, pressure suits work by either indirectly compressing the human body, or directly compressing it.
Indirect compression
Indirect compression is typically done by enclosing the body in a gas envelope. For this type, design effort focuses on compressing and containing the gas, at an equal pressure around the body as the wearer moves, and not having the gas pressure or the enclosing suit envelope restricting body movement of the wearer. Maintaining constant gas pressure as the wearer moves is difficult, because the internal volume of a simple construction inflatable suit will change when body joints are flexed. The gas pressure constantly tries to push the wearer's body into a position where the suit has been inflated to its maximum volume. Moving against this gas pressure can be very difficult, and be very exhausting for the suit wearer, limiting the amount of work that can be performed using the suit. Indirect compression suits generally require complex ribbed mechanical structures at the joints, which create flexible but inelastic folds or pockets in the skin of the suit that act to maintain a constant volume in the suit as the wearer moves. These pockets exist on both sides of a flexible joint and are designed to work together in tandem, so that when a joint is flexed, the folds on one side of the joint will compress and shrink in volume, while the folds on the opposite side will relax and expand in volume. The ribbed structures are usually braced with wire cables or cloth straps to limit their motion and prevent unusual flexing modes that may chafe against the user's body. The wire hinge cables also restrain the complex folds, which if released could unfurl and extend to be more than a meter longer than the wearer's body. These constant-volume joint structures greatly reduce fatigue of the wearer so that they do not have to constantly struggle against the suit pressure.
Direct compression
Direct compression involves applying pressure directly to the human body using the suit material, usually without any additional gas envelope around the wearer, which is instead provided by an outer rigid cabin structure enclosing the person. One method used for this is known as a capstan suit, which uses a compressible inflatable tube known as the capstan, enclosed by alternating fabric strips that wrap around the air tube and are attached to an inelastic fabric that closely fits the shape of the wearer's body. To provide a custom tight form-fit to the wearer's body, there are groups of laces along the length of each limb. Zippers may also run the length of a limb to allow for room to get into the suit. To apply pressure, the capstan tube is pressurized which expands in diameter and applies pressure to the fabric strips. The strips then pull the suit material laterally tighter around the wearer's body. A problem with this design is that the suit fabric is unable to directly provide pressure to the human body in areas with a surface that curves inward, away from the suit fabric. Locations with concave skin surfaces are in the armpit, behind the knees, the front and back of the crotch region, and along the spine. Inflatable air bladder structures or molded rigid expanded foam may be used, which fit into these cavity spaces to provide direct skin pressure where the suit material is unable to provide that contact directly.
Types Partial pressure suits only pressurize certain parts of the body. They can only provide protection up to a certain altitude. They do not provide protection for extended periods of time at low ambient pressure. Full pressure suits pressurize the entire body. These suits have no altitude limit.
Exposure to space without a spacesuit
The human body can briefly survive the hard vacuum of space unprotected, despite contrary depictions in much popular science fiction. Human flesh expands to about twice its size in such conditions, giving the visual effect of a body builder rather than an overfilled balloon. Consciousness is retained for up to 15 seconds as the effects of oxygen starvation set in. No snap freeze effect occurs because all heat must be lost through thermal radiation or the evaporation of liquids, and the blood does not boil because it remains pressurized within the body. The greatest danger is in attempting to hold one's breath before exposure, as the subsequent explosive decompression can damage the lungs. These effects have been confirmed through various accidents (including in very high altitude conditions, outer space and training vacuum chambers). Human skin does not need to be protected from vacuum and is gas-tight by itself. Instead it only needs to be mechanically compressed to retain its normal shape. This can be accomplished with a tight-fitting elastic body suit and a helmet for containing breathing gases, known as a space activity suit.
History
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