Physiological Experience During Shuttle EVA 951592
To date, 59 man-EVA's have been conducted in the Shuttle Program with minimum physiological problems or limitations. The physiological requirements for life support in the Shuttle EVA include pressure, gas composition, inspired CO2 pressure, heat- removal capability, in-suit water replacement, and caloric replacement. These requirements and their basis in verification testing or analysis are reviewed. The operational measures are identified.
The suit pressure in combination with a gas composition of at least 92 percent assures that sufficient O2 pressure is available to the crewmember. The nominal suit pressure of 4.3 psi±0.1 psi was maintained during all 59 man-EVA's. The contingency suit pressure was never required to be used. The suit pressure in combination with the cabin pressure and pre-EVA denitrogenation procedures minimize the risk of altitude decompression sickness. There has been no incidence of decompression sickness during Shuttle EVA. The inspired CO2 is maintained below hypercapnic levels by 6CFM ventilation directed to the helmet of the suit in combination with CO2 scrubbing in the environmental control system. There has been no objective or subjective indication of hypercapnia in any of these EVA's. The Shuttle EVA suit limits heat leak into and out of the suit by using multi layer insulation. The heat balance system in the suit, including a liquid-cooled garment, was designed to handle high work rates. The average work rate of the 59 man-EVA's was very moderate with modest peak work rates. There have been instances of overcooling which are discussed. These may be due in part to thermal capacitance in the cooling system that limits cooling control.
The in-suit drink bags have been utilized to different extents by different crew-members, some drinking all of the fluid and some drinking none. In no case was there significant indication of dehydration which is consistent with analysis of water requirements.
The experience with Shuttle EVA provides a high degree of confidence in the capability of EVA crewmembers in the EVA suit to perform International Space Station (ISS) construction and maintenance EVA's with minimal physiological problems. There is some evidence that, as our experience with EVA increases and training is improved, EVA work rates (key to many of the physiological limitations) are more moderate.