Airlocks

 

It’s difficult to believe that something as simple as getting dressed could be dangerous, but it is.

A hard hat diver has someone to assist them with suiting up, sealing the suit and helmet and managing the air supply. One of the risks during this period is that the face plate is the last thing to be closed and sealed. Because the diver is wearing about 200 pounds of gear and possibly sitting on a rocking boat, the diver is vulnerable until the face plate is closed and sealed.

When suiting up for an off-world excursion outside the habitat or transportation vehicle the “colonist” will be vulnerable until the air is turned on and the face shield sealed. What could happen during this time is an airlock seal could fail or an electrical spark could "ignite" the pure O2 used in the common spacesuit. 

Oxygen itself does not burn, but it does enhance combustion. In the confined space of a hyperbaric chamber, though, fires can be catastrophic, experts say. “We know that an increase in pressure and oxygen concentration can result in increased burning and burning rates of material,”[1]

 All experts agree that in a high-pressure system containing elevated concentrations of oxygen, a standard of cleanliness called "oxygen clean" is required

The "oxygen clean" concept centers on the possibility of oxygen-enriched air enhancing combustion in a closed pressurized environment that is contaminated with things that will burn. Add an ignition source and the outcome would be classed as a deflagration, which literally means, "to burn violently". This situation could cause serious injury or death if there was a sudden depressurization that resulted from a breach in the pressure containment system. (This would be like an explosion without the causative detonation.)

Astronauts usually go through a nitrogen purge cycle which reduces the blood nitrogen levels to help avoid what is called the bends. The Bends is described as an illness that arises from the rapid release of nitrogen gas from the bloodstream and is caused by bubbles forming in the blood when a diver ascends to the surface of the ocean to rapidly. It is also referred to as Caisson sickness, decompression sickness (DCS), and Divers' Disease[2]

Astronauts go through this purge cycle before suiting up so that they can operate their space suit at reduced pressure well below the 14.5 PSI we generally have here on Earth. This is a time consuming period during which the astronaut is breathing pure O2.

In 2001, the Quest Joint Airlock built by Boeing for NASA was launched for attachment to the ISS. This airlock has two segments, one for equipment and space suits and the other for exiting to space. The Quest module provides an environment where astronauts can "camp out" before a spacewalk for a prolonged period purging the nitrogen from their blood stream.

(Quest was built to accommodate both Russian and American space suit designs (size). Previous airlocks could not – This image shows Quest being maneuvered into position for attachment to the ISS)

This scenario works when your external work frequency is not very high. In a colonial setting the probability of going outside more frequently is a realistic  expectation, so this prolonged purge period would be an issue.

The questions then are these:

a.      Do we continuously operate the habitat / transport vehicle at reduced pressures?

b.      Or, do we operate them at earth pressure and do prolonged purges for each exit?

c.      Or, do we design a suit capable of withstanding the pressure differential of 14.5 psi? (Mars for example has an atmospheric pressure of just 0.095 psi)

The answer to these changes the lockout chamber design as well as suit design, but does not alter the hazards of cabin / suit differential pressure and the problems with that.

In an emergency such as an airlock breach or seal loss, the need to protect the colonist from the affects of the rapid pressure change becomes paramount and immediate. Possible solutions to this issue would include:

a.      A suit and helmet design permitting the colonist to seal their own suit quickly

b.      The ability to quickly re-pressurize a suit sealed after the incident using an external source

c.      A dual door airlock which has two exterior doors. At worst, the colonist would be rapidly exposed to the interior cabin pressure.

 The best possible design configuration to reduce or eliminate the risk of suit pressure issues includes all three. In this case, the “emergency life support” is built into the design reducing or eliminating the risk. 

[1] https://www.nfpa.org/News-and-Research/Publications-and-media/NFPA-Journal/2017/January-February-2017/Features/Hyperbaric-chambers

[2] https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Equilibria/Heterogeneous_Equilibria/The_Bends

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