Spaceflight Research Goal #2: Cryogenic CO2 Removal

For PHEnOM, we have been assigned to propose our own research goals for our suborbital spaceflights. Before the automated ASL translation project came to my attention, I chose something that I have been thinking about since 2011 and working on since January 2015.

Space life support is one of my major areas of interest. It covers many things – CO2 removal, humidity and temperature control, and waste management. I first got interested in the problem of CO2 removal back in 2011, when I was reading about the Apollo primary life support system (PLSS).

I was surprised to find that our current PLSSes, used on the Shuttle and ISS spacesuits, were no more advanced than from the Apollo days. They still ran the astronaut’s breath through a lithium hydroxide (LiOH) canister to remove the CO2, and looped it back into the breathing circuit. The chemical reaction generates lithium carbonate and water as follows:

2LiOH + CO2 → Li2CO3 + H2O

While mature and reliable, this method has a drawback in that the LiOH canister soon turns into lithium carbonate and water – an irreversible reaction. These canisters cannot be reused. What’s more, they take up a significant cross-section inside the breathing loop, so the PLSS must employ fans to force the air through the porous material in order to keep the astronaut breathing comfortably. That’s one reason why these backpacks are so bulky.

And heavy, too. Scuba rebreathers, which use something similar, weigh about 10 kg (22 lb). The additional air tanks will increase this to about 30 kg (66 lb).

They’re good for short spaceflights and backup duty, but the high mass of these systems demands a better way. Better, as in less mass and operations downtime.

Apollo A7 Primary Life Support System (PLSS), outer cover removed. The horizontal grey canister in the middle is the LiOH canister where CO2 is scrubbed out of the breathing loop.

After much research, I settled on a possible alternative. Liquid oxygen (LOX) is used in aerospace as a rocket oxidizer and concentrated oxygen source for life support, since one volume of it equals 700 volumes of gaseous oxygen (GOX). It only exists under -183 degrees C (90 K), which is well below CO2’s freezing point of -78 C. This gave rise to the thought that LOX could freeze CO2 out of an astronaut’s breath with a suitable heat exchanger, with a second heat exchanger rewarming the freshened air before it gets re-breathed. The LOX is naturally warmed as it chills the breathing gas, so it would make up for the oxygen used during respiration. There would be no need for heavy pressurized tanks – a small, light, vacuum-jacketed bottle could hold the same amount of oxygen.

Oceaneering Space Systems recently developed and tested a cryogenic scrubber for that purpose. According to their paper, they meant to trap CO2 on a spiral heat exchanger with smooth surfaces, so the ice would fall into a collection box as the unit was shaken. While they did trap ice, it stuck too well to the interior surfaces and plugged the scrubber up. This unit had a duration of only 4 hours before it got too full – not quite long enough to use on a spacewalk.

I realized that if the ice-stick problem could be solved, the LOX method could significantly reduce PLSS mass and volume, because it only has one consumable – LOX, which can be used as both the coolant and breathing gas source – killing two birds with one stone.

What would such a PLSS look like? It would consist of only a vacuum-jacketed LOX bottle, similar to a thermos, and a couple of heat-exchanger tubes. One tube would be used to remove the water vapor from the user’s breath, and another would take care of the CO2. There would be a thin tube to supply makeup oxygen into the breathing loop from the bottle.

With suitable insulation, the whole thing would be little bigger than a kid’s lunchbox (although with stainless steel construction, it’d be quite a bit heavier).

This could be easily tested on a suborbital flight on Blue Origin’s New Shepard vehicle, which allows for a few minutes of weightlessness. The other ways to experience microgravity on Earth do not come close – a Vomit Comet flight only gives 25-30 seconds of free-fall.

I have a few ideas on how this ice-stick problem could be solved, but whether they can be affected by microgravity remains to be seen. That’s the joy of research!




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