A mechanical counterpressure (MCP) suit, partial pressure suit, direct compression suit, or space activity suit (SAS) is an experimental spacesuit which applies stable pressure against the skin by means of skintight elastic garments. The SAS is not inflated like a conventional spacesuit: it uses mechanical pressure, rather than air pressure, to compress the human body in low-pressure environments. Development was begun by NASA and the Air Force in the late 1950s and then again in the late 1960s, but neither design was used. Research is under way at the Massachusetts Institute of Technology (MIT) on a "Bio-Suit" System which is based on the original SAS concept.
Background The human body can briefly survive exposure to the hard vacuum of space unprotected, despite contrary depictions in some popular science fiction. Human skin does not need to be protected from vacuum and is gas-tight by itself. Human flesh expands to about twice its size in such conditions, giving the visual effect of a body builder rather than an overfilled balloon. This can be counteracted through mechanical counter-pressure from a suitably designed garment. Consciousness is retained for up to 15 seconds as the effects of oxygen starvation set in. Counteracting this requires a helmet to contain breathing gases and protect the ears and eyes. These effects have been confirmed through various accidents in very high altitude conditions, outer space, and training vacuum chambers.
Cooling Cooling of the astronaut with an SAS is generally achieved with evaporation from body perspiration which is emitted from the suit in all directions. Water, salts, and proteins can deposit on optics and other sensitive surfaces causing damage or degradation. This can limit the usefulness of an SAS. For the inflated spacesuits used on the Space Shuttle, International Space Station, and the Apollo program, cooling was achieved in the Primary Life Support System by sublimation of water in a vacuum.
Designs
Mauch In 1959 Hans Mauch was working on "breathable" undergarments for the Mercury space suit when he came up with the idea of a way to build a mechanical counterpressure design. The Mauch team noticed that closed-cell foams, which trap gas within their structure, expand when outside pressure is lowered. By containing the foam within a non-expanding outer layer, it would place increasing pressure on the body as the pressure lowered. This appeared to allow for a design that would offer far better mobility than the almost-rigid Mercury design. Late in 1959 Mauch Laboratories was granted a contract by the US Air Force to develop a working model, as part of the Air Force's secret X-20 Dynasoar efforts. The program ran until 1962, during which time NASA had joined the effort. The suit was built with a layer of foam sandwiched between two layers of fabric, the inner against the wearer's skin (or undergarments) to provide mechanical support, and the outer providing containment. A separate, and bulky, helmet provided pressure and breathing gases. Like the undergarments that Mauch was developing for Mercury, thermal control was provided by direct sweat transpiration through the fabric. The resulting suit was about as bulky as the original Mercury design, excluding the large helmet. Extended vacuum testing was carried out successfully, but the suit proved to have less mobility than expected and further development was dropped.
Webb
The introduction of improved fabrics led to Paul Webb's concept for a new way to build an SAS. Further work was contracted in order to test various design concepts. Between 1968 and 1971 ten designs of increasing sophistication were built, leading eventually to a series of successful tests in vacuum chambers. The longest test was two hours and forty-five minutes. The tests were successful: the practicality of a mechanical counter pressure spacesuit was demonstrated conclusively. The energy needed to move about was considerably less than conventional designs, which was a major improvement for long-duration spacewalks. Tests of punctures showed that up to a square millimeter of skin could be directly exposed to vacuum for extended periods with no permanent effect. A similar puncture in a conventional suit would result in a loss of pressure and breathing air. It weighed half as much as the primary pressure suit worn by NASA astronauts for Project Apollo, the A7L. A number of problems also turned up, primarily related to the problem of keeping the suit in strong mechanical contact at every point on the body. Concavities or small folds in the fabric could lead to fluid pooling in the gaps; the groin area proved extremely difficult to tailor successfully. To correct this, small pads of polyurethane foam were inserted into concavities and were successful in most problem areas. The suits had to be tailored to each individual, although the same was true of all space suits of the era. The largest difficulty was donning and removing the suit. In order to effectively provide the minimum pressure of 0.3 bars (4.4 psi) necessary for human physiology, the suit had to be extremely tight-fitting, making donning and doffing a highly strenuous task. In 1971, Webb, along with James F. Annis, published their findings in a report. The report remained positive, and the researchers felt that further improvements were possible. Quoting the Report:
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