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The answer to this question included this image from a book by Wernher von Braun he wrote in 1960. enter image description here

This space suit design misses a micro meteroite protection as well as liquid cooling to remove excess heat and a scrubber to remove exhaled CO2.

But what are helium tanks used for? Diluting oxygen with an inert gas like nitrogen or helium is only necessary for a very high suit pressure in the range from about 0.5 to several bar. Using helium instead of nitrogen to avoid nitrogen narcosis may be necessary for pressures about 5 bar and more.

For a suit used in a vaccuum environment a pure oxygen atmosphere with a low pressure of about 0.4 bar is much better when using regular gloves. A suit with low pressure is much more flexible.

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    $\begingroup$ This looks a lot like copy and paste from common diving suites, so he used the same Oxygen-Helium mix. If you vent gasses you don't need a scrubber (not a good idea due to condensation and large resource consumption, but possible). $\endgroup$ – asdfex Jun 11 at 11:28
  • $\begingroup$ @asdfex A mix of oxygen and helium is needed only for deep diving, deeper than about 40 m. $\endgroup$ – Uwe Jun 11 at 12:32
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    $\begingroup$ Although Heliox had been tested by 1960 it wasn't widely used in diving until much later @asdfex. In any case it only has value at higher air pressures, you wouldn't use a helium breathing mix in a spacesuit. $\endgroup$ – GdD Jun 11 at 12:46
  • $\begingroup$ What about mass? Helium weighs less than nitrogen. Perhaps he envisioned that the spacecraft would use it to cut down on as much mass as possible? $\endgroup$ – Michael Stachowsky Jun 11 at 21:41
  • $\begingroup$ @MichaelStachowsky The ballasted shoes indicate that the suit is to be used on a small body like the moon. Such small bodies could not hold an atmosphere. For a space suit used in a vaccuum the solution with lowest mass is the use of a pure oxygen atmosphere with a pressure of about 0.4 bar and no helium at all. $\endgroup$ – Uwe Jun 11 at 22:40
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A first guess would be that in the author's idea, Oxygen would be used to pressurize the helmet and meet the Astronaut's metabolic needs, and Helium to pressurize the spacesuit. That would be a terrible design though, at least concerning insulating properties : thermal conductivity of Helium is very high (152 mW/m/K) compared to Oxygen (23,4 mW/m/K) or some rare gases used in technical diving like Argon (17,7 mW/m/k). Conducting Extra-Vehicular activities in such conditions would be extremely demanding for the temperature control system which would be easily overtaxed.

As you raised it in your question I suppose that you are familiar with Lorrain-Smith hyperoxia-toxicity effect : an extended exposure to an oxygen partial pressure over 0,5 Bar leads to pulmonary inflammatory lesions. It is possible that the author, although he was familiar with this very specific effect, was not familiar with the possibility to conduct EVAs with a spacesuit pressurized under the atmospheric standard pressure. Although this is unlikely, this misconception could have led him to imagine the use of Helium to reduce ppO2 below 0,5 Bar in the spacesuit.

Here is my suggestion : pressurizing the suit with Helium is a way to get rid of the pre-breath protocol currently used by our Astronauts in ISS sas QUEST. Explanations :

  1. As you know, ISS has a fairly Earth-like, sea-level atmosphere: 21% Oxygen - 79% Nitrogen. During EVAs, the EMU or ORLAN suits of Astronauts are approximately pressurized at 1/3 of an atmosphere with 100% Oxygen. As they suit on and begin the EVA, the Nitrogen desaturation gradient would therefore be very high between their body saturated at 0,8 Bar of ppN2 and the 0 Bar ppN2 pure-Oxygen they breath.
  2. This is not a problem if the total pressure is high enough (you have no decompression sickness on earth if you begin to breath pure oxygen at the surface), but as soon as the spacesuit is only pressurized at 1/3 of an atmosphere, conditions are met for an explosive desaturation of Nitrogen. This is why Astronauts breath pure oxygen on the ISS for 2 hours and 20 minutes before the EVA, with short periods of high intensity exercises, in order to wash out Nitrogen before any exposure to a low pressure environment.
  3. That being said, you have two options to avoid decompression sickness during EVAs: either you perform this cumbersome pre-breath protocole, or you perform the EVA with a 1 Bar-pressurized spacesuit. For the second option, if you use pure oxygen, ppO2 will be more than 0,5 and you will be exposed to oxygen toxicity (Lorrain-Smith effect). You can use then either Nitrogen (the mix would then be close to air), Helium or other gas as a trade-off between a totale pressure high enough to avoid decompression sickness, and a ppO2 low enough to avoid toxicity.
  4. Why Helium ? You could use Argon as it is a much better insulator, but it is far too dense and can lead to asphyxiation. This lets us with Nitrogen or Helium. My opinion is that Helium is more future-like, sounds like technology and this is why the author chose it.

Please note that this is only a mental experiment: a spacesuit pressurized at 1 bar would lead to numerous other problems, for example the stiffness of the joints.

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  • $\begingroup$ A suit pressure of 1 bar would explain the use of the tentacle gloves for most of the time and the classic gloves for only very short time. But to put on a tentacle glove on one hand using the other hand in classic gloves would be at least very difficult. $\endgroup$ – Uwe Jun 12 at 21:18

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