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What was the pressure and oxygen concentration used in Apollo space suits on Earth before launch and after closing the suit with helmet and gloves during the transfer of the astronauts to the launch pad and before the suits were connected to the suit circuit of the capsule?

Was a high oxygen flow used to reduce the remaining nitrogen in the suit as much as possible?

What suit pressure was used to avoid a very stiff and inflexible suit when walking or climbing into the capsule? Suit pressure should be a little more than ambient air pressure, but a pressure difference as high as about 0.26 bar as used during EVA was not neccessary.

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tl;dr In the ascent config, the suits were pressurized with pure oxygen to slightly above the cabin pressure. During ascent, the cabin pressure dropped from ~ sea level pressure to ~ 6 psia. The suit pressure control system vented gas to maintain the same slight positive pressure.


Before sealing up the suits, the crew performed a "prebreathe" to flush nitrogen out of their bodies. When the crew was suited up in the Suit Lab before launch, their suits were at 100% oxygen.

enter image description here

The hand-carried "ventilators" maintained a positive pressure of ~0.4 psi in the suits while traveling from the Suit Lab to the capsule. (source)

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(Cropped and annotated from here)

In the capsule, pressure in the Pressure Suit Circuit (PSC) was controlled to slightly above cabin pressure (+.07 psi) by the O2 demand regulator.

Before the hatch was closed, the cabin atmosphere was sea level air. After the crew was 'installed' and the hatch closed, the cabin was maintained at slightly above ambient pressure. The crew was suited and breathing pure oxygen, the flow into the PSC was set to maintain a slight overpressure above the cabin pressure so that flow would only be out of the suit circuit into the cabin.

During ascent, when ambient pressure decreased below 6 psia, the cabin positive pressure relief valve opened and the cabin atmosphere started to flow out through it. The PSC bled down to maintain the slight overpressure through a valve in the O2 demand regulator.

All cabin atmosphere makeup gas was pure oxygen, so after most of the sea level atmosphere was relieved out during ascent, the cabin atmosphere would trend more and more toward pure oxygen. Eventually it would be ~5 psi of pure oxygen + metabolic contaminants.

Helmets and gloves were removed ~ 15 minutes after launch.

This environmental system schematic is still busy but it's better than the one in the Operations Handbook!

enter image description here

(Drawing from Apollo ECLSS Experience Report)

Suit absolute pressures:

  • Traveling from Suit Lab to capsule: Ambient pressure (~15 psi) + ~0.4 psi
  • At liftoff: Ambient pressure + slight cabin pressure increment + slight PSC increment
  • After cabin blowdown during ascent: Cabin pressure (~6 psia) + slight PSC increment

Source: Apollo Operations Handbook

Some info from Crew Systems Division Apollo 11 postflight report

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The answers below are from Apollo Experience Report: Development of the Extravehicular Mobility Unit, NASA Tech Note D-8093.

  1. The air inside the suits was always 100% oxygen. Yes, even after the Apollo 1 fire! Because of the pure oxygen environment, current-limiting resistors were added to the electrical connections inside the suit (namely, the Snoopy Cap and the biomedical sensors).

  2. The following graph shows the pressure in the oxygen tank, before going through the regulator.

Apollo spacesuit oxygen

  1. The oxygen supply was regulated to 26 $\pm$ 1.7 kN/m$^2$ (3.75 $\pm$ 0.25 psid). This is close to the partial pressure of oxygen in Earth air. Lower than that, and the astronaut blacks out; higher than that can cause lung damage. The flow rate is whatever is needed to maintain this pressure; however, there is a warning light if the flow is too small, and there is an oxygen purge system that can provide oxygen regardless of pressure.

  2. Thus the total pressure in the suit was about 0.26 atmospheres. Higher pressures were found to cause the suit to balloon too much and become inflexible. Thus, the suit air was simply the oxygen, with no inert gas to bring it to 1 atm.

  3. Prior to launch, the astronauts pre-breathed pure oxygen from the portable oxygen ventillator, a suitcase-sized unit:

    The Apollo portable oxygen ventilator (POV) (fig. 50) is designed primarily to maintain a crewman or a test subject in a preoxygenated state before launch or altitude testing. Because of the decompression involved, nitrogen must be purged from the subject to avoid the bends, and it is imperative that the subject be maintained in a 100-percent-oxygen environment for several hours before decompression. The POV is a hand-carried, self-contained, life-support unit capable of performing this function while providing some degree of cooling.

    [...]

    The operation of the unit is relatively simple. The LOX or the liquid air is stored in a Dewar flask. A buildup valve allows some liquid to boil, maintaining the pressure at 1034 to 1103 kN/m$^2$ (150 to 160 psig). Opening the supply valve allows this pressure to force liquid out the bottom of the Dewar flask into a heat exchanger. The liquid boils and absorbs heat in a series of heat exchangers. A diverter valve alters flow through a heat exchanger to regulate the gas temperature. The gas then is routed to the diffusion pump and exhausted through an ejector into the suit loop. The gas, having achieved a high velocity at the ejector, impinges on the gas in the suit loop, providing the force for a ventilation flow of at least 0.28 m3/min (10.0 scfm).

    (p. 66)

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  • $\begingroup$ I've added a link to the source so that every reader does not have to do their own google search, the same way that it was done for you here. The plot of the pressure in the tank doesn't really help, can you note the location in the 79 page document that specifically answer the following? "What was the pressure and oxygen concentration used in Apollo space suits before launch and after closing the suit with helmet and gloves?" Was it really 3.75 ± 0.25 psid of pure oxygen, absolute? I don't see how that is possible. $\endgroup$ – uhoh Jul 18 at 1:31
  • $\begingroup$ Or was it ~15 psi (ambient pressure)? Or was it perhaps ambient plus 3.75 psi? $\endgroup$ – uhoh Jul 18 at 1:41
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    $\begingroup$ I was asking for suit pressure on Earth before launch, it could not be less than air pressure at sea level. $\endgroup$ – Uwe Jul 18 at 13:53

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