The nominal 100% oxygen pressure inside the SPORE working area is 48 kPa.
- SPORE maximum pressure
- 70.11 kPa (1 Standard Lunar Homestead Atmosphere)
- SPORE nominal operating pressure
- 48 kPa
- SPORE minimum safe pressure (“Go/No Go”)
- 36 kPa
- SPORE minimum pressure
- 24.82 kPa
Check out Atmospheric Pressure and Composition for more detailed information on the Lunar Homestead atmosphere.
Figuring out atmospheric pressure inside the SPORE working area is pretty straight forward. We just need to establish the boundaries.
Minimal safe operating pressure
According to NASA, the minimum allowed oxygen partial pressure is 24.82 kPa (3.6 psia) with 100% oxygen. This is the minimum pressure necessary to maintain alveolar oxygen partial pressure to provide sea level oxygen equivalent. No mission has ever flown with this breathing gas mix at this pressure
I’m going to consider this the bare minimum requirement for Homesteaders. This atmosphere presents significant health risks and would not be ideal for long-term use. Obviously, Homesteaders would only breath it if something went wrong with the SPORE atmosphere system, their breathing apparatus wasn’t working, and they were on their way back to the habitat.
If the pressure falls below this level then Homesteaders will have to come back in pressure suits to deal with the problem.
Maximum safe operating pressure
The maximum safe operating pressure would be the operating pressure of the Homestead’s habitats. I can’t think of a reason why they would ever want to go over this. I also can’t think of a reason why they would want to operate at this pressure either.
This pressure presents some serious risks to the Homesteaders operating in the SPORE environment.
- Fire – The Atmospheric Pressure and Composition page has a detailed section on the fire risk so I won’t repeat it here. It’s pretty obvious that a 100% oxygen atmosphere at 101.325 kPa (1 Standard Atmosphere), or even 70.11 kPa (1 Standard Lunar Homestead Atmosphere) would be pretty flammable.
- Health risks – Even though Homesteaders will have to wear a self-contained breathing system (not yet designed at this point), the constant exposure to this atmosphere could pose health risks.
- Increased atmosphere loss – While the rate of SPORE atmosphere loss through the regolith is still a big unknown (and will probably be highly variable anyway), it’s common sense that increasing the pressure will increase the rate of loss.
For our purposes we’re going to assume that the Homestead is operating at One Standard Lunar Homestead Atmosphere (1 SLHA) at 70.11 kPa.
No decompression injury risk pressure
The SPORE work area should have a lower atmospheric pressure than the habitats for the reasons listed above. However, if the pressure difference is too great the Homesteaders run the risk of suffering a decompression injury (DCI) when they leave the habitat and enter SPORE. This is what happens when scuba, or commercial, divers come up from depth too fast.
Everyone breathing a nitrogen dominated atmosphere has nitrogen gas dissolved in their blood and tissues. Normally this isn’t a problem. However, when the pressure on the body is reduced the nitrogen comes out of solution and forms bubbles. If the pressure change is is minor (or slow) then the bubbles remain small and the excess nitrogen can be safely breathed out. If the pressure change is significant (or fast) then large nitrogen bubbles can form. These large bubbles can get lodged in blood vessels and tissues, causing all sorts of problems. Severe pain (that’s why it’s called the “bends”), strokes, and death can occur. I had a DCI in my left elbow flying home from a diving trip and it was very unpleasant. Fortunately, it resolved itself once the pressure increased when we landed.
Through a lot of experimentation, and some deaths, humans have figured out formulas that determine how long a diver can stay at a certain depth, at what depths (and for how long) they need to stop to allow their body to safely get rid of nitrogen, and how long they need to stay on the surface before they can safely dive again. These are called dive, or decompression, tables. Dive computers use algorithms that do the same thing as the tables.
How does this apply to space? Here’s an example. Every time an astronaut needs to go outside the ISS they have to breathe pure oxygen for several hours to flush the nitrogen out of their blood. The ISS is operated at 101.325 kPa and American space suits are at 34.47 kPa. That big of a pressure difference is going to cause a decompression injury if someone suited up and went outside without purging nitrogen. We don’t want to have our Homesteaders spending time sitting around sucking oxygen while they wait to enter the SPORE work area.
Fortunately, there is a simple way to avoid (or at least minimize) decompression injuries without time consuming oxygen pre-breathing. Again, through a lot of experimentation on animals and humans, we’ve figured out the we can half the pressure without causing a DCI. So, if the habitat is operating at 1 Standard Lunar Homestead Atmosphere (70.11 kPa) then the minimum “no decompression injury risk” pressure would be 35.055 kPa. If the SPORE work area is at, or above, 35.055 kPa then Homesteaders can free move between the work area and the habitat. Want to eat lunch? No problem. Desperately need the loo? A problem but at least you can run back in without worrying about having to repeat the nitrogen purge when you are done.
SPORE’s minimum safe pressure (35.055 kPa) is higher than what was used for all of the Mercury, Gemini, and Apollo missions (34.47 kPa) and for ISS space suits (29.6 kPa). So we know people can deal with it for at least a week or more.
Going from a low pressure area to a high pressure one isn’t a problem. Homesteaders working in SPORE could go back to the habitat at any time without concern for getting a DCI. This is a great safety feature for when things go wrong (and they eventually will).
Finally, I’m going to use 36 kPa as the “Go/No Go” threshold. This means two things. First, if the Homesteaders can’t establish a stable pressure above this level in the SPORE work area then they’ll have to put on pressure suits before they can fix the problem. The suits may not be absolutely necessary but it’s better to have an abundance of caution. Second, if the pressure inside the SPORE work area falls to this level, and they can’t seem to fix the problem, then everyone has to immediately return to the habitat. Again, better to lose some time and productivity than get someone injured or killed.
SPORE nominal operating pressure
Alright, we’ve got out upper and lower limits. We’ve got a minimum safe limit. Now we need to add in some padding to allow for pressure fluctuations. We don’t want to evacuate the SPORE work area every time a Homesteader hits a void of fissure and the pressure dips.
This is something we’ll probably need to determine through experimentation and trial and error. Homesteads will probably set their own number based on their tolerance for risk and personal experiences.
I’m going to set the nominal pressure to 48 kPa for research purposes. It gives us a 12 kPa buffer.
Fire science is pretty complex and I don’t claim to understand most of it. So this is going to be quick and dirty.
Let’s look at the three things we need for a fire:
- An oxidizing agent – This is our 100% oxygen at 48 kPa atmosphere.
- NASA has flown many missions (all of the Mercury, Gemini, and Apollo) with a 100% oxygen atmosphere at 5 psi (34.47 kPa).
- Oxygen concentration (volume percent) is the primary driver for fire risk, followed by total pressure (11, pg 11). We’re not changing the % concentration.
- We are slightly increasing the pressure, so the fire risk will go up.
- A heat source
- Sparks caused by iron tools impacting iron-rich rocks will probably be a common occurrence.
- Other, to be determined, mining techniques could be heat sources as well.
- We can reduce this risk by including it in the equipment design.
- The SPORE work area is composed of metal and rock, neither of which should be flammable under these conditions.
- People, their clothes, and equipment could be fuel sources. We can mitigate this by designing protective clothing that isn’t flammable. Maybe made from basalt fibers. The equipment can be designed to minimize flammable material as well.
- While it’s possible that Homesteaders will hit a pocket of flammable gas while mining, it’s highly unlikely. We’ll have to come up with strategies to mitigate this as well.
Here’s my thinking. The SPORE atmosphere is only 13 kPa more than the Apollo capsule atmosphere. That increases the fire risk. However, the SPORE work environment won’t have much in the way of flammable fuels.
Obviously, there is a lot of research that needs to be done so we can make the SPORE work area a safe environment. But for now, the fire risk seems manageable. Of course, I could be completely wrong and this could be a deal-breaker.
I’ve pulled the data in this page from the “Atmospheric Pressure and Composition” page.
- Bounding the Spacecraft Atmosphere Design Space for Future Exploration Missions (spaceflightsystems.grc.nasa.gov/repository/NRA/cr-2005-213689.pdf)