About cabin pressure :
- The SoyuzTMA settings are sea level atmosphere :
760 mmHG
21% Oxygen (partial pressure 160 mmHg)
78% Nitrogen (partial pressure 594 mmHg)
1% CO2 and H2O.
- Say we use for Lunar EVA an Orlan-KM suit pressurized at 300 mmHg of
total pressure, with a 100% oxygen atmosphere inside the suit. We have two problems :
1) We need to bring the crew to that pressure of 300mmHg.
2) We need to change the atmosphere composition to 100% oxygen.
- According to NASA charts, 300mmHg and 100% oxygen atmosphere is too rich in oxygen, but astronauts can breath it 72 hours without showing hyperoxia symptoms. The LM will supply oxygen for only 36 hours, so that's not a problem, we're go.
- We now have to deal with decompression sickness. We can use the LM as an "airlock" and apply there the pre-breath protocol, quite short with the Orlan suit :
The Russian EVA preparation protocol includes a 30-minute oxygen pre-breathe in the Orlan spacesuit at a pressure of 73 kPa (10.6 psia) to partially wash out nitrogen from crew members’ blood and tissues (Barer and Filipenkov, 1994)
Literature from the Russian program shows that of approximately 114 EVAs that had been performed in the Russian spacesuit, including 18 EVAs from the ISS, crew members showed no signs of DCS (Malkin, 1994; Davis et al., 1977; Fulton, 1951).
The drawback of the Orlan higher pressure is that once on the lunar ground, our cosmonauts will get tired faster. But the Soyuz and the Orlan were designed to work together, so it seems to make sense.
- When the LM undocks after 90 minutes (if we apply the depressurization rates of the Space Shuttle, a 460 mmHg pressure drop should last no less), the crew is ready for EVA and can continue the mission without having to worry with pressurization. Also, if there was a leak in the LM hull, they would only have to put their helmet on and to switch the O2 supply to their suits to be safe.
- The decompression of the LM on the lunar surface causes no problem : they put their helmet on, and switch the O2 supply on the suit life support system.
- The invert process is repeated on the ascent phase ; once the LM docked to the SoyuzTMA, they take several hours to get the pressure and atmosphere back to the nominal levels, then open the hatch and crawl back in the Soyuz BO.
