The PLSS from the Apollo A7L suit, with its outer cover removed

A Primary (or Portable) Life Support System/Subsystem, or PLSS, is the "backpack" of a spacesuit. It provides most of the functions of a spacecraft life support system, in a smaller package. The functions performed by the PLSS include:

• regulating suit pressure
• providing breathable oxygen
• removing carbon dioxide, humidity, odors, and contaminants from breathing oxygen
• cooling and recirculating oxygen and water through the Liquid Cooling and Ventilation Garment, or LCVG.
• sensing and reporting suit parameters
• providing communications for the suit occupant

The air handling function of a PLSS is similar to that of a diving rebreather, in that exhaled gases are recycled into the breathing gas in a closed loop.

Apollo OPS

The breathing set used in the Apollo lunar landing missions had an emergency bailout in case the main supply failed. This bailout was provided by an oxygen purge system (OPS) similar to an open-circuit scuba system. Like the PLSS, the OPS also maintained suit pressure and removed heat and water vapor. When activated, the OPS provided oxygen to a separate inlet on the pressure suit. A vent valve on a separate suit outlet was manually opened to establish a steady, once-through flow to space, removing CO₂, heat and water vapor. Like scuba, the OPS used gas far less efficiently than the PLSS, but as an emergency backup simplicity and reliability were paramount. The OPS was a separate unit mounted on top of the PLSS backpack, immediately behind the astronaut's helmet. The OPS was also used as a backup on tethered EVAs where a spacecraft provided oxygen to the astronaut through an umbilical cable.

Similar systems are now used by Space Shuttle and International Space Station astronauts.

Space Shuttle/International Space Station PLSS

The PLSS for the EMU suit currently used on the Space Shuttle and International Space Station is manufactured by Hamilton Sundstrand. It is mounted to the back of the Hard Upper Torso (HUT) assembly.

Oxygen (O₂), carbon dioxide (CO₂) and water vapor are drawn from the extremities of the suit by the LCVG, which sends the gas to the PLSS. When gas enters the PLSS, activated charcoal removes odors and lithium hydroxide (LiOH) removes carbon dioxide. Next, the gas passes through a fan which maintains a flow rate of about six cubic feet per minute. A sublimator then condenses water vapor, which is removed by a "slurper" and a rotary separator. The removed water is stored and used to supplement the water supply used in the LCVG. The sublimator also cools the remaining oxygen to about 55 °F (~12.8 °C). A flow sensor monitors the flow rate.

Extra oxygen is added to the flow from a storage tank as necessary, downstream of the flow sensor. The oxygen is then returned to the suit at the back of the head, where it flows down over the astronaut's face. By delivering oxygen to the helmet and drawing gas from the extremities, the suit is designed to ensure that the suit occupant breathes the freshest oxygen possible.

The operating pressure of the suit is maintained at 4.3 psi during extravehicular operations, and 0.7 psi relative to external pressure while in intravehicular mode, i.e., before and after extra-vehicular activity (EVA).

Developing technology

Technologies being considered for application in future PLSSs include Pressure Swing Adsorption (PSA), a process by which CO₂ can be separated from gas more efficiently, and through a repeatable process, as opposed to the current LiOH canisters, which become saturated with each use, and are limited to around 8 hours.^[1] By regenenerating the sorbent during EVA, the size and weight of the sorbent canister can greatly reduced. PSA accomplishes this by venting CO₂ and water vapor into space.^[2]

See also

• Gas separation
• Rebreather
• Carbon dioxide scrubber

References

it:Primary Life Support System