Primary Life Support System
Encyclopedia
Extra-vehicular activity
Extra-vehicular activity is work done by an astronaut away from the Earth, and outside of a spacecraft. The term most commonly applies to an EVA made outside a craft orbiting Earth , but also applies to an EVA made on the surface of the Moon...
with maximum freedom, independent of a spacecraft's life support system
Life support system
In human spaceflight, a life support system is a group of devices that allow a human being to survive in space.US government space agency NASA,and private spaceflight companies...
. The PLSS is generally worn like a backpack. The functions performed by the PLSS include:
- regulating suit pressure
- providing breathable oxygenOxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
- removing carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, humidityHumidityHumidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
, odors, and contaminants from breathing oxygen - cooling and recirculating oxygen through the pressure garment, and water through a Liquid Cooling and Ventilation GarmentLiquid Cooling and Ventilation GarmentA Liquid Cooling Garment is a form-fitting garment that is used to remove body heat from the wearer in environments where evaporative cooling from sweating and open air convection cooling does not work, or the wearer has a biological problem that hinders self-regulation of body temperature.A...
or Liquid Cooling Garment. - two-way voice communication
- display and/or telemetryTelemetryTelemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure...
of suit health parameters - telemetry of an indicator of the wearer's immediate health (e.g. heart rate)
The air handling function of a PLSS is similar to that of a diving rebreather
Rebreather
A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where...
, in that exhaled gases are recycled into the breathing gas in a closed loop.
When used in a microgravity environment, a separate propulsion system is generally needed for safety and control, since there is no physical connection to a spacecraft.
Apollo PLSS
Lithium hydroxide
Lithium hydroxide is an inorganic compound with the formula LiOH. It is a white hygroscopic crystalline material. It is soluble in water and slightly soluble in ethanol...
to remove the carbon dioxide from the breathing air, and circulated water in an open loop through a Liquid Cooled Garment
Liquid Cooling and Ventilation Garment
A Liquid Cooling Garment is a form-fitting garment that is used to remove body heat from the wearer in environments where evaporative cooling from sweating and open air convection cooling does not work, or the wearer has a biological problem that hinders self-regulation of body temperature.A...
, expelling the water into space, where it turned to ice crystals. Some of the water was also used to remove excess heat from the astronaut's breathing air, and collected for dumping into the spacecraft
Apollo Lunar Module
The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...
's wastewater tank after an EVA. The PLSS also contained a radio transceiver and antenna for communications, which were relayed through the spacecraft's communication system to Earth. PLSS controls were provided in the Remote Control Unit (RCU) mounted on the astronaut's chest. Oxygen and water were rechargeable for multiple EVA's from the spacraft's environmental control system.
Lunar surface EVA times for the first four missions (Apollo 11 through 14) were limited to 4 hours, with oxygen stored at 1020 pound per square inches (7 MPa), 3 pounds (1.4 kg) of lithium hydroxide, 8.5 pounds (3.9 liters) of cooling water, and a 279 watt-hour battery. For the extended missions of Apollo 15 through 17, the EVA stay time was doubled to 8 hours by increasing oxygen to 1430 pound per square inches (9.9 MPa), lithium hydroxide to 3.12 pounds (1.4 kg), cooling water to 11.5 pounds (5.2 liters), and battery capacity to 390 watt-hours.
An emergency backup was provided in case the main system failed, by a separate unit called the Oxygen Purge System (OPS), mounted on top of the PLSS, immediately behind the astronaut's helmet.
The OPS maintained suit pressure and removed carbon dioxide, heat and water vapor through a continuous, one-way air flow vented to space.
When activated, the OPS provided oxygen to a separate inlet on the pressure suit, once
a vent valve on a separate suit outlet was manually opened.
The OPS provided a maximum of about 30 minutes of emergency oxygen for breathing and cooling. This could be extended to 75 to 90 minutes by employing a "buddy system", using the other astronaut's functional PLSS.
The PLSS was 26 inches (66 cm) high, 18 inches (45.7 cm) wide, and 10 inches (25.4 cm) deep. It was tested in space for the first time by Russell Schweickart
Rusty Schweickart
Russell Louis "Rusty" Schweickart aka Schweikart is an American former astronaut, research scientist, US Air Force fighter pilot, business and government executive...
in a stand-up EVA in Earth orbit on Apollo 9
Apollo 9
Apollo 9, the third manned mission in the American Apollo space program, was the first flight of the Command/Service Module with the Lunar Module...
. His PLSS weighed 84 pounds (38.1 kg) on Earth, which translated to a weight of only 14 pounds (6.4 kg) on the Moon. The OPS weighed 41 pounds (18.6 kg) on Earth (6.8 pounds (3.1 kg) on the Moon).
The OPS was also used as a backup on tethered in-space EVAs where a spacecraft provided oxygen to the astronaut through an umbilical hose.
Space Shuttle/International Space Station PLSS
Similar systems have been used by Space ShuttleSpace Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
astronauts, and are currently used by International Space Station
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...
crews.
The Primary Life Support System for the EMU
Extravehicular Mobility Unit
The Space Shuttle/International Space Station Extravehicular Mobility Unit is an independent anthropomorphic system that provides environmental protection, mobility, life support, and communications for a Space Shuttle or International Space Station crew member to perform extra-vehicular activity...
suit used on the Space Shuttle and International Space Station is manufactured by Hamilton Sundstrand
Hamilton Sundstrand
Hamilton Sundstrand, is a global corporation that manufactures and supports aerospace and industrial products for worldwide markets. It was formed from the merger of Hamilton Standard and Sundstrand Corporation in 1999. A subsidiary of United Technologies Corporation, HS is headquartered in Windsor...
. It is mounted to the back of the Hard Upper Torso
Hard Upper Torso
A Hard Upper Torso Assembly, or HUT, is a central component of several space suits, notably Roscosmos' Orlan and NASA's Extravehicular Mobility Unit . The fiberglass HUT forms a rigid enclosure about the upper body of the occupant, providing pressure containment for this part of the body...
(HUT) assembly.
Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O2), carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2) and water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
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
Activated carbon
Activated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.The word activated in the name is sometimes replaced...
removes odors and lithium hydroxide
Lithium hydroxide
Lithium hydroxide is an inorganic compound with the formula LiOH. It is a white hygroscopic crystalline material. It is soluble in water and slightly soluble in ethanol...
(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
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
, 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
Fahrenheit
Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit . Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees...
(~12.8 °C
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
). A flow sensor
Flow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
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
Astronaut
An astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft....
'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
Extra-vehicular activity
Extra-vehicular activity is work done by an astronaut away from the Earth, and outside of a spacecraft. The term most commonly applies to an EVA made outside a craft orbiting Earth , but also applies to an EVA made on the surface of the Moon...
operations, and 0.7 psi relative to external pressure while in intravehicular mode, i.e., before and after extra-vehicular activity (EVA).
Developing technologies
Technologies being considered for application in future PLSSs include Pressure Swing AdsorptionPressure swing adsorption
Pressure swing adsorption is a technology used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperatures and so differs from cryogenic distillation...
(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. By regenenerating the sorbent
Sorbent
A sorbent is a material used to absorb liquids or gases. Examples include:*A material similar to molecular sieve material. It has a large internal surface area and good thermal conductivity. It is typically supplied in pellets of 1 mm to 2 mm diameter and roughly 5 mm length or as...
during EVA, the size and weight of the sorbent canister can be greatly reduced. PSA accomplishes this by venting CO2 and water vapor into space.