The goal was to improve the range of activity and decrease the energy cost of work associated with wearing conventional gas filled pressure suits. The suit would also be half the weight, simpler, and not endanger the astronaut's life if punctured.
The human skin was almost an ideal pressure suit. It had a high tensile strength, almost no gas permeability, and very good water retention characteristics. The skin required only an applied pressure equal to the pressure of the breathing gas to function normally. A second skin was conceived, capable of augmenting the biological skin using Mechanical Counter Pressure (MCP) to mimic a pressurized environment. An MCP suit would not only require less energy by the astronaut for motion, but would be safer and more reliable than full pressure suits since suit rupture would not mean loss of life supporting gas pressure. If designed properly, a small tear in an MCP suit would only expose a local body region to reduced pressures. This exposure would cause the wearer discomfort, and possibly pain, but would allow them to return to safety without major injury. Tests of the Space Activity Suit directly exposed areas of skin as large as 1 square millimeter to vacuum without problems.
The life support system was greatly simplified since body cooling was accomplished by the simple and natural method of sweating. The suit was porous, and the sweat simply evaporated through the second skin. Tests of the suit in vacuum chambers showed no signs of excessive fluid loss, or freezing of the skin. They also demonstrated that, at such a scale, the skin withstood the tensile loads. Without the need for thermal control, the life support system became a tank of oxygen with pressure regulators and a carbon dioxide scrubber.
In the 1971 program ten successively refined prototypes of the suit were built, including 13 hours of wear by test subjects in vacuum chambers of up to 2 hours 45 minutes duration. The tests showed no significant problems in comparison to conventional suits. But whereas the energy cost of simple tasks in the Gemini or Apollo suits was typically 2.26 times that for a nude subject, in the SAS it was 1.64 times. And the mass of the suit, was half that of the Apollo suit.
But the tests also showed the major detailed technical challenges of completing development of such a suit. Most important was swelling/edema in parts of the body. To prevent blood pooling, the pressure across these regions needed to remain smooth. The most difficult areas to accomplish this in the crotch. There was also swelling of the hands, but this was reduced by using palm pads. This problem was seen as primarily mechanical in nature, and could be solved by further development, improved design, and use of new stretch materials.
Another issue major issue was the need to tailor each suit precisely to the form of each subject. In the prototype suit this required extended suit donning and doffing times. A production suit would have to use different techniques to ensure easier donning and fitting. The prototype suit had to be tailored to individual subjects and consisted of no fewer then eight garments that had to each be fitted and adjusted precisely and donned in the correct order:
Gross mass: 24 kg (52 lb).