How will the Polaris Dawn cabin pressure and oxygen partial pressure dovetail with that of their EVA suits? (100% oxygen?)

Question

CNN's September 10, 2024 SpaceX’s Polaris Dawn mission just made history. But the riskiest part is still to come says:

The Polaris Dawn crew’s pre-breathe routine, however, will last about 45 hours, (SpaceX Engineer Sarah) Gillis told CNN, as the oxygen content in the cabin slowly increases while the pressure decreases.

and a little further down (after the 'can of soda-pop' analogy):

By lowering the pressure inside Crew Dragon, Gillis said, and putting on their spacesuits just as the ambient pressure equals the suit pressure*...

At the same time, Wikipedia's Polaris Dawn;p Mission says:

Over three days, the cabin pressure will gradually decrease from 14.5 to 8.65 pounds per square inch (100.0 to 59.6 kPa) while oxygen levels increase.

and further down:

Flight day three is dedicated to the first-ever extravehicular activity (EVA) on a commercial spaceflight mission. After extensive preparations, all four crew members will don their EVA suits, which are pressurized with 100% oxygen at 5.1 pounds per square inch (35 kPa).

These blurbs have a discontinuity in both total pressure and oxygen partial pressure, probably because there's a discontinuity in time.

But I'm wondering, will the capsule atmosphere match 5.1 psi at 100% oxygen(!) when they don their suits and close them up?

1. It's been a while since crewed capsules have had 100% oxygen; hasn't it?
2. Is this a fire safety issue? Does the lower total pressure decrease the danger?
3. Do I totally misunderstand what will actually happen?

Answer 1

Partial answer to

Is this a fire safety issue? Does the lower total pressure decrease the danger?

I have no detailed (or even sketchy) knowledge about how SpaceX does it but...

There are definite flammability concerns when depressurizing the cabin which are caused by the need to keep ppO2 high enough for the crew to breathe while reducing the total pressure. "lower total pressure" increases the danger because the percent O2 has to increase. Sea level O2 concentration is ~20%, the flammability limit used in shuttle was 28.5%

Shuttle would reduce the cabin pressure to 10.2 psi from the normal 14.7 psi a day or so before EVAs were performed. To do that, they would reduce the cabin pressure by venting the cabin overboard, but the ppO2 and total cabin pressure had to be maintained in a "band" to keep the O2 concentration below the 28.5% O2 flammability limit while keeping the ppO2 high enough. If the ppO2 got too low, they had to stop the depress and continue adding O2. If they got too close to the flammability limit, they had to stop the depress, stop adding O2, and add N2.

This chart of total pressure vs. ppO2 was used.

It's busy, but you can see the diagonal desired band in the middle, the "approach zone", and the flammability limit to the right. "SYS 2 - OP" means O2 is being added, "SYS 1 - OP" means N2 is being added.

Here is an example of a real 10.2 depress from STS-088. You can see how the ppO2 got too low, so the crew had to close the depress valve and let the ppO2 build up until they got into the "approach zone", then start the depress again.

You ended up in the little box at the bottom, which had to be manually controlled to, because the Orbiter environmental control system didn't automatically do 10.2. Once or twice a day, the crew had to do "10.2 maintenance" ops.

Sources:

• EVA Checklist
• Personal notes

Answer 2

Wow, 3 Questions in one... but all nice ones!

Let's begin with the first one:

The last crewed vehicle with a 100% oxygen atmosphere was the Apollo Spacecraft during the Apollo Soyuz Test Project in July 1975 . Apollo had basically what resembles an Airlock attached to let the crews transfer between the pure oxygen low pressure Apollo Spacecraft and the full pressure, nitrogen oxygen atmosphere of Soyuz.

Second: Isn't the 100% oxygen atmosphere a safety issue?

No. Fire hazard is a function of Partial Pressure . Therefore a pure oxygen atmosphere at 5.1 psi is about the same as a 35% Oxygen Atmosphere at 14.5 psi. There is a slightly increased "burnability" but that's no issue in a controlled environment like a crew capsule. (Turns out this is complete hogwash)

3. No, but let me add. The difference you see in the suit pressure of 5.1psi and 8.65 psi to which they decrease the cabin pressure. Our body can take some amount of change in pressure without starting to "bubble up" (like in diving, you can almost always just surface from a depth of 10m (30ft) where you have twice the pressure compared to the surface, without adverse effect (there might be edge cases where this isn't true but that's not the discussion here). So 8.65 psi is probably a pressure that's still comfortable to live in. Keep in mind that lower pressure has a few side effects like less heat transfer, reduced transmission of sound and probably others. But if you're already in 8.65 psi you can probably go down to 5.1 psi almost instantly without adverse effects.

While the pressure in the cabin is above 5.1 psi, the pressure inside the suit will be the same. Only when the pressure in the cabin is lowered below 5.1 psi towards basically zero, the suit should hold the pressure at 5.1. What I don't know is how exactly those pressures will relate.

I see a few different possibilities:

• Keep the suit pressure equal to cabin pressure until cabin pressure goes below 5.1 psi (probably easiest from a technical standpoint)
• Keep the suit pressure at 8.65 psi until the difference between cabin and suit is bigger than 5.1psi, than keep the difference constant (would allow for leak checks earlier in the process)
• linearly reduce pressure in the suit from 8.65 psi to 5.1psi while cabin pressure goes from 8.65 to nearly zero (probably the most comfortable for the people inside the suit)