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 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 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 to about . 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 oxygen-rich breathing mixture is required to approximate the oxygen available in the lower atmosphere, while above oxygen must be under positive pressure. Above , respiration is not possible because the pressure at which the lungs excrete carbon dioxide (approximately 87 mmHg) exceeds outside air pressure. Above , 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 .
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 air 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 air 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.