How do EVA suits manage water excretion?

Question

This answer immediately sparked the question in my mind of how space suits manage water excretion from the person wearing them. I am aware that the astronauts had to wear diapers since one may need to pee during an hours long EVA. However this only manages the liquids exiting the person.

If during the course of a long and strenuous EVA astronauts lose several pounds of liquid to sweat and breathing, how does the EVA suit manage all this water? They may have a thermal management system to reduce sweating, but that doesn't solve the breathing issue. I can't imaging they let water/water vapor build up inside the suit, it would make the suit environment extremely uncomfortable.

Answer 1

The shuttle (and ISS) EMU (Extravehicular Mobility Unit) has a condensing heat exchanger as part of its ventilation loop. The condensate is stored, used for cooling, and the excess is drained after each EVA (Extravehicular Activity).

Reference: Shuttle Crew Operations Manual: https://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf page 2.11-7

The water produced by perspiration and breathing is withdrawn from the oxygen supply by being condensed in the sublimator and is carried by the condensate circuit. The water is then sent to the water - storage tanks of the feedwater circuit and added to their supply for eventual use in the sublimator. In this manner, the PLSS is able to maintain suit cooling for a longer period than would be possible with just the tank’s original water supply.

From Suited for Spacewalking https://www.nasa.gov/pdf/143159main_Suited_for_Spacewalking.pdf

PLSS = Portable Life Support System

Answer 2

I'm sure there are different techniques for different space suits, but here's an example of how it is done for the ISS suits:

The key to handling body heat and sweat is the Liquid Ventilation Garment, or LVC. This is essentially what looks like a full body thermal underwear, but it is lined with tubes that pass water through them. If you heat up, cold water is passed through the tubes, cooling you down. The idea is to keep you cool enough that you don't sweat much.

After the water is heated, it passes through a heat exchanger to cool it down again, then it's recirculated. If necessary, the water can be passed into a sublimator, where it passes though small holes into the vacuum of space, where it sublimates away, carrying heat with it.

If a person does still sweat, the LVC has a wicking layer that will absorb the sweat, and through some mechanism the sweat is collected and added back to the cooling water supply. The problem here is that as you work harder, the temperature of the water in the cooling tubes has to be lower to transport enough heat, which can make it feel cold. Add in a damp undergarment, and astronauts can feel very uncomfortable with this system. Also, you are losing a lot of water to space through the sublimator - maybe a kg of water for a four hour EVA. That's not only expensive, but it limits EVA time.

There is a new LVC being developed for the next gen of suits which will have porous micro-tubules stitched into it that can actually absorb the sweat, the transport it back to the cooling system for recycling. That will allow astronauts to use sweating as their natural heat control.

In the future, mechanical counterpressure suits that don't need to be pressurized will have semi-permeable materials that allow sweat to pass through and sublimate away all across the surface of the suit, which would allow for more passive cooling and a more natural feeling environment.

Link to a paper on the new generation of LVC garments being considered:

Multi-Functional Cooling Garment for Spacesuit Environmental Control

And a paper from 1969 describing the old system for cooling:

Regulation of Thermal Sweating in EVA Space Suits

Answer 3

We have an answer for Shuttle, and an answer for ISS. Here is the answer for Apollo.

1. The main strategy was to prevent sweating in the first place. The astronauts wore a Liquid Cooling Ventilation Garment (LCVG), which was essentially long underwear with closed tubes that circulated cooling water. Heat was discarded through the sublimator on the suit's backpack. This was enough to keep the temperature low enough to prevent sweating:

   The large sensible capacity of water permits the 1.8 kg/min (4 lb/min) to carry the maximum design load of 586 watts (2000 Btu/hr) with a temperature rise of only 4.6 K (8.3° F). The temperature of the crewman's skin can be held sufficiently low to inhibit sweating.

   Apollo Experience Report: Development of the Extravehicular Mobility Unit, p. 47

   The system had enough capacity to remove the heat of moderate exercise:

   This system consisted of plastic cooling tubes on the inside of an undergarment. The garment could suppress sweating at work rates as high as 1670 x 10$^3$ J/hr ($\approx$ 400 kcal/hr) and allowed sustained operation at rates as high as 2090 x 10$^3$ J/hr ($\approx$ 500 kcal/hr).

   Biomedical Results of Apollo, p. 116

2. If the astronaut does produce sweat, the next step is to wick away the moisture from the surface of the skin. This allows the water in the sweat to evaporate into the air of the suit. Evaporation absorbs heat -- which is the reason why humans sweat! -- further cooling the astronaut.

   The LCVG, the constant wear garment (essentially a cotton pantsuit worn over the LCVG), comfort gloves, and socks were all designed to wick away sweat. As the LCVG did not extend to the hands, those astronauts who chose not to wear the comfort gloves often complained about having sweaty hands.

3. The moisture of the evaporated sweat was carried away in the suit airflow. Expelled air was processed by the PLSS "backpack", where moisture was condensed out and stored:

   Oxygen from the circuit enters the suit ventilation distribution system at a minimum flow of 0.156 m$^3$/min (5.5 acfm) and absorbs heat, moisture, and metabolic byproduct contaminants as it passes through the suit adjacent to the crewman's body. The warm, moist, contaminated gas is then returned to the PLSS where it is transported to the contaminant control package, which consists of an activated charcoal bed that absorbs trace contaminant gases and an LiOH bed that reacts with CO$_2$ to form lithium carbonate. Byproducts of this chemical reaction, heat and moisture, are added to the heat and moisture already carried by the recirculating gas stream. The gas stream then enters the heat exchanger (sublimator) where the heat is given up, and the excess moisture in the stream is condensed. When it leaves the heat exchanger, the entrained free water is removed from the gas stream by an elbow wick-type water separator and transferred to the back side of the feedwater reservoir bladder.

   Apollo Experience Report: Development of the Extravehicular Mobility Unit, p. 36

   Cooling by airflow goes back to aircraft, Mercury, and Gemini. Apollo was the first program to use a LCVG:

   Traditionally, personnel in aircraft have been cooled by gas ventilation systems, which carry heat from the generating source to the rejection device, through a rise in temperature of the ventilating gas (sensible means) or through an increase in the absolute humidity of the ventilating gas caused by evaporation of available moisture (latent means). This approach was used on the Mercury, Gemini, Apollo CM [in cabin], and Apollo LM [in cabin] vehicles and was considered appropriate for the original PLSS.

   Apollo Experience Report: Development of the Extravehicular Mobility Unit, p. 45

The suits were also tested for the potential effects of sweat. They were placed in a thermovacuum chamber for up to 115 hours at elevated temperature and 95$\pm$5% humidity. A separate "salt fog" test was conducted for 48 hours. No degradation was observed in either tests.