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The '''Apollo PLSS oxygen''' system is the breathing supply in the backpack worn on the Moon. PLSS is the portable life support system. A fan recirculated the gas already in the suit. A high-pressure bottle added only the oxygen the crewman used and the oxygen that leaked. A second unit, the oxygen purge system, sat on top of the pack and was the backup if that loop failed. The same service-module and lunar-module supplies are on [[Apollo oxygen]].

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[[File:Apollo 11 Aldrin PLSS AS11-40-5875 small.jpg|link=File:Apollo 11 Aldrin PLSS AS11-40-5875 full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">Apollo 11, 20 July 1969. Edwin Aldrin beside the flag. The backpack is the portable life support system. The box on top of it is the oxygen purge system. Hasselblad frame AS11-40-5875, [https://tothemoon.ser.asu.edu/gallery/apollo/11/7#AS11-40-5875 March to the Moon], [https://images.nasa.gov/details/as11-40-5875 NASA].</div>
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== The loop ==
The J-mission backpack manual calls the recirculating path the oxygen ventilating circuit. It is a contaminant-control assembly, a section of the sublimator, a water separator, a fan, a flow sensor, a back-flow check valve, and the oxygen connectors into the suit. The manual's account of one pass:

<blockquote>The oxygen ventilating circuit circulates cool, reconditioned oxygen through the PGA. The oxygen enters the suit ventilation distribution system and picks up heat, moisture, and metabolic byproduct contaminants as it passes through the suit adjacent to the man's body. This warm, moist, contaminated gas then is transported to a contaminant control package, which consists of an activated charcoal bed for absorption of trace contaminant gasses, and a lithium hydroxide bed, which reacts with carbon dioxide to form lithium carbonate. By-products of this chemical reaction are heat and moisture.<ref name="jman">Hamilton Standard, [https://ibiblio.org/apollo/Documents/apollo_portable_life_support_systems.pdf ''Portable Life Support System''], the J-mission backpack manual. Oxygen ventilating circuit, section 1.2, and primary oxygen subsystem, section 1.5.</ref></blockquote>

The gas then gives up that heat in the sublimator. Condensed water is pulled out by an elbow separator and sent to the feedwater side of the cooling loop. A centrifugal fan, run from the silver-zinc battery, pushes the dried gas back into the suit. The fan is what moves the ventilation flow. The oxygen bottle is not.<ref name="jman" />

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[[File:Apollo PLSS interior S68-34580 small.jpg|link=File:Apollo PLSS interior S68-34580 full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">Portable life support system with the cover off, 1968. The oxygen bottle, the cartridge, the fan, and the cooling hardware are in the chassis. NASA S68-34580.</div>
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A 1972 Hamilton Standard paper, written after the early flights and before the last three landings, prints the ventilation requirement as 5.5 cubic feet a minute for the liquid-cooled packs. The suit pressure requirement on that table is 3.85 [[Pounds per square inch#Absolute pressure|psia]]. The carbon-dioxide limit on the same table is 15 mm Hg. An earlier gas-cooled design had been held to 7.6 mm Hg, which is the cabin limit on [[Apollo oxygen]]. The packs that flew were the liquid-cooled ones.<ref name="beggs">J. C. Beggs and F. H. Goodwin, [https://ntrs.nasa.gov/citations/19720019459 ''Apollo PLSS: Environmental Control of the Smallest Manned Space Vehicle''], Hamilton Standard (1972). Table 3.1 and the system description.</ref>

The cartridge specification says lithium hydroxide removes carbon dioxide and charcoal removes odors. The cartridge assembly is specified at 5.0 pounds. The Apollo Lunar Surface Journal puts the hydroxide charge at 3.12 pounds on the later flights, up from 3.0 pounds. The 1972 paper says that increase was revised packing of the cartridge.<ref name="jman" /><ref name="alsj">Apollo Lunar Surface Journal, [https://apollojournals.org/alsj/plss.html ''Apollo PLSS Images'']. PLSS improvements for Apollo 15 through 17.</ref><ref name="beggs" /> The hydroxide does not make oxygen. The bottle makes up what the crewman consumes and what leaks. The same reaction is written out on [[Apollo oxygen]].

The chest unit, the remote control unit, shows primary oxygen quantity, which is a reading of bottle pressure. It sounds a tone for low feedwater pressure, low ventilation flow, low suit pressure, and high oxygen flow.<ref name="beggs" />

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== The bottle ==
The primary oxygen subsystem is one bottle, a fill connector, a regulator, a flow sensor, and the pressure switches. The manual states the job in one sentence. "Pure gaseous oxygen is stored in the primary bottle and admitted to the suit to make up for leakage losses and metabolic consumption." The single-stage regulator "maintains a suit pressure level of approximately 1/4 atmosphere." Recirculation "is limited to that required for proper carbon dioxide purging from the helmet and defogging of the visor." The subsystem is rechargeable.<ref name="jman" />

The early component specification, for the bottle used before the J missions, calls it a cylinder with approximately hemispherical ends, one port, and two mounting bosses, built of 302 stainless steel. Specified weight 3.50 pounds. Capacity 1.12 pounds of oxygen at 850 [[Pounds per square inch#Gauge pressure|psig]] and 78 degrees Fahrenheit. Proof pressure 1,665 [[Pounds per square inch#Gauge pressure|psig]].<ref name="early">Hamilton Standard, [https://ibiblio.org/apollo/Documents/apollo_portable_life_support_and_oxygen_purge_systems.pdf ''Apollo Portable Life Support and Oxygen Purge Systems'']. Primary oxygen bottle, item 271, and the purge-system bottle, item 511.</ref> The requirements table in the 1972 paper lists the charge pressure for the packs on Apollo 11 through 14 as 1,020 [[Pounds per square inch#Absolute pressure|psia]] at 70 degrees Fahrenheit. That is a higher charge than the 850 [[Pounds per square inch#Gauge pressure|psig]] point in the capacity line. The table's charge pressure for Apollo 15, 16, and 17 is 1,410 [[Pounds per square inch#Absolute pressure|psia]].<ref name="beggs" />

The J-mission bottle specification is the later hardware. Construction is 301 stainless steel, cryoformed. Specified weight 3.90 pounds. Capacity 1.75 pounds of oxygen at 1,410 [[Pounds per square inch|psi]] and 70 degrees Fahrenheit. External leakage 2 standard cubic centimeters an hour maximum at 1,500 [[Pounds per square inch#Absolute pressure|psia]].<ref name="jman" />

The regulator is one spring-loaded poppet, a restrictor upstream of the poppet, and a manual shutoff. Handle up is off. The shutoff is closed when the pack is not in use and while the bottle is being filled. The restrictor limits the flow if the regulator fails.<ref name="jman" /> With lunar vacuum as the reference pressure, the early specification holds the outlet at 3.75 to 4.05 [[Pounds per square inch|psi]] differential for any oxygen flow from 0 to 2.0 pounds an hour. The J-mission specification holds the same 3.75 to 4.05 band for flows from 0 to 3.8 pounds an hour. On the Moon that differential is the suit pressure. It is the same 3.85 [[Pounds per square inch#Absolute pressure|psia]] the requirements table names.<ref name="early" /><ref name="jman" /><ref name="beggs" />

The regulator's own flow capacity is the makeup rate, not the purge rate. The early specification lists 0.360 pounds an hour at an inlet of 85 [[Pounds per square inch#Gauge pressure|psig]]. The J-mission specification lists 0.355 pounds an hour minimum at 145 [[Pounds per square inch#Absolute pressure|psia]] inlet, and 0.38 pounds an hour maximum at 1,500 [[Pounds per square inch#Absolute pressure|psia]] inlet.<ref name="early" /><ref name="jman" />

The paper plots how long the expendables last after the pre-egress checks, on worst-case leakage and heat leak. For the packs through Apollo 14, oxygen and feedwater are the two that run out first. If the specified suit leakage were zero, the oxygen at an average of 1,200 Btu an hour would last about 0.8 hours longer. The duration requirement on the table is 4 hours at 1,200 Btu an hour for Apollo 11 through 14, and 3 hours at 1,600 Btu an hour. For Apollo 15 through 17 the table says 6 hours at 1,200 Btu an hour and 5 hours at 1,600. The same paper, describing the modified pack, says it was changed to support lunar surface missions of up to 8 hours. The extra time came from the higher oxygen charge pressure, a larger feedwater supply, a larger battery, and more lithium hydroxide in the same style of cartridge.<ref name="beggs" />

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[[File:Apollo PLSS primary oxygen schematic small.jpg|link=File:Apollo PLSS primary oxygen schematic full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">Figure 1-9 of the J-mission backpack manual. The primary bottle, the fill connector, the regulator with its shutoff and its flow-limiting orifice, and the suit-pressure switch. The drawing is the manual figure, not a photograph.</div>
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== The purge system ==
The oxygen purge system is a separate supply mounted on top of the pack. The manual and the 1972 paper describe it the same way: a pair of spherical tanks holding 5.8 pounds of oxygen at 5,800 [[Pounds per square inch|psi]], a shutoff worked by a cable from the chest, and a single-stage regulator that holds suit pressure at about a quarter of an atmosphere. A gauge reads tank pressure. A second, low-range gauge and a fixed orifice are the regulator test, and both are checked before a walk. The communications antenna folds down from the top of the unit.<ref name="jman" /><ref name="beggs" />

The component specification is written for one of those spheres. Inconel 718, specified weight 9.5 pounds, operating pressure 0 to 6,750 [[Pounds per square inch#Absolute pressure|psia]]. Volume 2.85 pounds of oxygen at 5,880 [[Pounds per square inch#Absolute pressure|psia]] and 70 degrees Fahrenheit. Two such bottles are the 5.8 pound pair in the system description. The two documents do not print the same fill pressure. One says 5,880 [[Pounds per square inch#Absolute pressure|psia]] for the bottle, the other 5,800 [[Pounds per square inch|psi]] for the pair.<ref name="early" /><ref name="beggs" />

The requirements table gives the purge system a suit pressure of 3.7 [[Pounds per square inch#Absolute pressure|psia]], a maximum flow of 8 pounds an hour, and a duration of 30 minutes. The paper is more particular about the valve on the suit. Full open is an 8 pound an hour leak, and that flow is the minimum 30 minutes. The low setting is 4 pounds an hour, "at least 1 hr of emergency get-back capability" when the cooling water is still available. An early purge system heated the gas before the regulator, to keep what reached the suit above 30 degrees Fahrenheit. Manned tests showed the flow and the suit stayed warm enough without the heater, and the flight unit flew without it.<ref name="beggs" />

The paper lists three ways the purge oxygen is used:

# It can replace the primary oxygen supply while the rest of the backpack keeps running.
# With the suit purge valve on low, 4 pounds an hour goes through the suit to carry off carbon dioxide and humidity, and the cooling water comes from the working backpack or from the buddy hose.
# With the purge valve on high, 8 pounds an hour goes through the suit and does both the breathing and the cooling, with no dependence on the backpack.<ref name="beggs" />

There is no requirement to refill the purge system from the lunar module. It flies with the charge it was given before launch.<ref name="beggs" /> The Apollo 11 contingency checklist checks that charge in the cabin: oxygen pressure 5,380 to 6,380 [[Pounds per square inch#Absolute pressure|psia]], shutoff on, regulator 3.4 to 4.0 [[Pounds per square inch#Gauge pressure|psig]], then shutoff off and the regulator gauge under 2.5 [[Pounds per square inch#Gauge pressure|psig]].<ref name="a11">[https://www.apollojournals.org/alsj/a11/a11FinalEVAProc_HSI-38438.pdf Apollo 11 final EVA procedures], HSI-38438. PLSS recharge, section 3.4, and the oxygen purge system check in the contingency EVA preparation.</ref>

The buddy hose is cooling water, not oxygen. If one backpack loses its water cooling, the two crewmen share the transport water from the backpack that is still running. The man on the failed pack still breathes from the purge bottles.<ref name="beggs" />

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[[File:Apollo PLSS oxygen purge schematic small.jpg|link=File:Apollo PLSS oxygen purge schematic full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">Figure 2-1 of the J-mission manual. Two purge bottles, the shutoff, the regulator, and the hose to the suit connector beside the purge valve. The drawing is the manual figure, not a photograph.</div>
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== Refilling the bottle ==
The first walk used packs that were filled before launch. Between later walks the crew refilled them inside the lunar module, from the descent-stage oxygen. That line is the 900 [[Pounds per square inch|psi]] supply on [[Apollo lunar module oxygen]]. The purge bottles are not part of the refill.<ref name="beggs" />

The Apollo 11 checklist does the cartridge first, with the backpack oxygen shutoff verified off, handle up. The crew equalizes the oxygen loop by depressing the valve in the red hose nozzle, then pulls the spent cartridge and locks in a fresh one. The oxygen fill itself starts only after three checks: the shutoff still off, the pack in the recharge station, and the lunar module oxygen quantity above 35 percent. The vehicle oxygen line goes on the backpack fill fitting. The fill valve is opened. The checklist allows about 2 minutes for a partial charge, about three-quarters full, or 70 minutes for a full charge. The valve is then closed, the line comes off, and the dust caps go back on.<ref name="a11" />

The journal's account of the later flights adds the pause the long charge is there to cover. A first charge brought the bottle to about 90 percent. After a few minutes, so the bottle could cool, the crew topped it off to about 95 to 98 percent. Compression heats the gas, and the pressure falls as the bottle cools, so a reading taken hot is not the full charge.<ref name="alsj" />

== See also ==
* [[Apollo oxygen]]
* [[Apollo lunar module oxygen]]
* [[Apollo command module oxygen]]
* [[Pounds per square inch]]
* [[Moon landings]]

== References ==
<references />

[[Category:Apollo program]]
[[Category:Apollo evidence]]
[[Category:Moon]]
[[Category:Spaceflight]]