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The Primary and Secondary Life-Support Systems

Astronauts experienced their first real freedom while wearing spacesuits during the Apollo Moonwalk EVAs, because of a portable life-support system worn on their backs. All other EVAs up to that time were tied to the spacecraft by the umbilical-tether line that supplied oxygen and kept crew members from drifting away. In one sense, the tether was a leash, because it limited movements away from the spacecraft to the length of the tether. On the Moon, however, astronauts were not hampered by a tether and, in the later missions, were permitted to drive their lunar rovers up to 10 kilometers away from the lander. (That distance limit was imposed as a safety measure. It was determined that 10 kilometers was the maximum distance an astronaut could walk back to the lander if a lunar rover ever broke down.)

primary life support system illustration
Space Shuttle astronauts have even greater freedom than the Apollo lunar astronauts because their EVAs take place in the microgravity environment of space. They do employ tethers when EVAs center in and about the Shuttle's payload bay, but those tethers act only as safety lines and do not provide life support. Furthermore, the tethers can be moved from one location to another on the orbiter along a slide wire, permitting even greater distances to be covered.

The freedom of movement afforded to Shuttle astronauts on EVAs is due to the Primary Life-Support System (PLSS) carried on their backs. The PLSS, an advanced version of the Apollo system, provides life support, voice communications, and biomedical telemetry for EVAs lasting as long as seven hours. Within its dimensions of 80 by 58.4 by 17.5 centimeters, the PLSS contains five major groups of components for life support. Those are the oxygen-ventilating, condensate, feedwater, liquid transport, and primary oxygen circuits.

The oxygen-ventilating circuit is a closed-loop system. Oxygen is supplied to the system from the primary oxygen circuit or from a secondary oxygen pack that is added to the bottom of the PLSS for emergency use. The circulating oxygen enters the suit through a manifold built into the Hard Upper Torso. Ducting carries the oxygen to the back of the space helmet, where it is directed over the head and then downward along the inside of the helmet front. Before passing into the helmet, the oxygen warms sufficiently to prevent fogging of the visor. As the oxygen leaves the helmet and travels into the rest of the suit, it picks up carbon dioxide and humidity from the crew member's respiration. More humidity from perspiration, some heat from physical activity, and trace contaminants are also picked up by the oxygen as it is drawn into the ducting built into the Liquid Cooling-and-Ventilation Garment. A centrifugal fan, running at nearly 20,000 rpm, draws the contaminated oxygen back into the PLSS at a rate of about 0.17 cubic meters per minute, where it passes through the Contaminant Control Cartridge.

Carbon dioxide and trace contaminants are filtered out by the lithium hydroxide and activated charcoal layers of the cartridge. The gas stream then travels through a heat exchanger and sublimator for removal of the humidity. The heat exchanger and sublimator also chill water that runs through the tubing in the Liquid Cooling and Ventilation Garment (LCGV). The humidity in the gas stream condenses out in the heat exchanger and sublimator. The relatively dry gas (now cooled to approximately 13 degrees Celsius) is directed through a carbon dioxide sensor before it is recirculated through the suit. Oxygen is added from a supply and regulation system in the PLSS as needed. In the event of the failure of the suit fan, a purge valve in the suit can be opened. It initiates an open loop purge mode in which oxygen is delivered from both the primary and secondary oxygen pack. In this mode, moisture and the carbon dioxide-rich gas are dumped outside the suit just before they reach the Contaminant Control Cartridge.

One of the by-products of the oxygen-ventilating circuit is moisture. 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 small amount of oxygen that is also carried by the condensate circuit is removed by a gas separator and returned to the oxygen-ventilating system.) 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.
description at right Astronaut Joe Tanner works on the Hubble Space Telescope during the STS-82 mission. The curvature of the Earth and the Sun are over his left shoulder.
The function of the feedwater and the liquid transport circuits is to cool the astronaut. Using the pressure of oxygen from the primary oxygen circuit, the feedwater circuit moves water from the storage tanks (three tanks holding a total of 4.57 kilograms of water) to the space between the inner surfaces of two steel plates in the heat exchanger and sublimator. The outer side of one of the plates is exposed directly to the vacuum of space. That plate is porous and, as water evaporates through the pores, the temperature of the plate drops below the freezing point of water. Water still remaining on the inside of the porous plate freezes, sealing off the pores. Flow in the feedwater circuit to the heat exchanger and sublimator then stops.

On the opposite side of the other steel plate is a second chamber through which water from the liquid transport circuit passes. The liquid transport circuit is a closed-loop system that is connected to the plastic tubing of the LCVG. Water in this circuit, driven by a pump, absorbs body heat. As the heated water passes to the heat exchanger and sublimator, heat is transferred through the aluminum wall to the chamber with the porous wall. The ice formed in the pores of that wall is sublimated by the heat directly into gas, permitting it to travel through the pores into space. In this manner, water in the transport circuit is cooled and returned to the LCVG. The cooling rate of the sublimator is determined by the workload of the astronaut. With a greater workload, more heat is released into the water loop, causing ice to be sublimated more rapidly and more heat to be eliminated by the system.

illustration of secondary oxygen pack The last group of components in the Primary Life-Support System is the primary oxygen circuit. Its two tanks contain a total of 0.54 kilograms of oxygen at a pressure of 5,860.5 kilopascals, enough for a normal seven-hour EVA. The oxygen of this circuit is used for suit pressurization and breathing. Two regulators in the circuit step the pressure down to usable levels of 103.4 kilopascals and 29.6 kilopascals. Oxygen coming from the 103.4-kilopascal regulator pressurizes the water tanks, and oxygen from the 29.6-kilopascal regulator goes to the ventilating circuit.

To insure the safety of astronauts on EVAs, a Secondary Oxygen Pack (SOP) is added to the bottom of the PLSS. The two small tanks in this system contain 1.2 kilograms of oxygen at a
pressure of 41,368.5 kilopascals. The Secondary Oxygen Pack can be used in an open-loop mode by activating a purge valve or as a backup supply should the primary system fall to 23.79 kilopascals. The supply automaticaly comes on line whenever the oxygen pressure inside the suit drops to less than 23.79 kilopascals.

If the Displays and Control Module (DCM) purge valve (discussed below) is opened, used-oxygen contaminants and collected moisture dump directly out of the suit into space. Because oxygen is not conserved and recycled in this mode, the large quantity of oxygen contained in the SOP is consumed in only 30 minutes. This half-hour still gives the crew member enough time to return to the orbiter's airlock. If carbon dioxide control is required, the helmet purge valve may be opened instead of the DCM purge valve. That valve has a lower flow rate than the DCM valve.

Displays and Control Module

The PLSS is mounted directly on the back of the Hard Upper Torso, and the controls to run it are mounted on the front. A small, irregularly-shaped box, the Displays and Control Module (DCM), houses a variety of switches, valves, and displays. Along the DCM top are four switches for power, feedwater, communications mode selection, and caution and warning. A suit-pressure purge valve projects from the top at the left. It is used for depressurizing the suit at the end of an EVA and can be used in an emergency to remove heat and humidity when oxygen is flowing from both the primary and secondary oxygen systems. Near the front on the top is an alpha-numeric display. A microprocessor inside the PLSS permits astronauts to monitor the condition of the various suit circuits by reading the data on the display.

Stepped down from the top of the DCM, on a small platform to the astronaut's right, is a ventilation-fan switch and a push-to-talk switch. The astronaut has the option of having the radio channel open at all times or only when needed.

On a second platform, to the left, is an illuminated mechanical-suit pressure gauge. At the bottom, on the front of the DCM, are additional controls for communications volume, display lighting intensity, temperature control, and a four position selector for controlling suit pressure in different EVA operating modes.

display and control module illustration

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