Apollo command module oxygen

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Apollo oxygen

Apollo oxygen (main article)

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Command Module • Lunar Module • PLSS

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Apollo oxygen chart • Commander Davenport oxygen tanks

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The command module oxygen system took oxygen from the service module at about 900 psi, warmed it, stored a small reserve in the command module, and stepped the pressure down to about 5 psi for the crew. The tanks themselves stayed in the service module. What follows is the plumbing on the command-module side of that line: the capillaries on the water-glycol line, the surge tank, the regulators, the suit loop, and the lithium hydroxide beds.

A shorter account of the spacecraft as a whole, including the lunar module and the backpacks, is on Apollo oxygen.

The pipeline

The Block II handbook and the flight report describe the same path, in this order.

1. Service-module tanks delivered oxygen at about 900 psi.
2. Two inlet assemblies, each a filter, a capillary, and a check valve, limited the flow and warmed the oxygen on the hot water-glycol line.
3. Downstream of those capillaries the line was still the high-pressure supply. It filled a surge tank and a repressurization package, both near 900 psi.
4. The main regulator, a dual unit on panel 351, reduced that supply to 85 to 110 psi.
5. Two cabin pressure regulators held the cabin near 5 psi. An emergency regulator could flood the cabin if a leak opened.
6. The oxygen demand regulator on panel 380 fed the suit loop, and let excess gas out so the suits stayed soft.

From the service module

Service module cryogenic oxygen tank, serial 10024XTA0051, in its shipping container at the Stafford Air and Space Museum. This is the tank the command module drew from. The surge tank inside the command module is a smaller bottle. Photo: Mike Jetzer/heroicrelics.org.

The oxygen subsystem shared its supply with the fuel cells. The 1969 handbook states it this way:

The oxygen subsystem shares the oxygen supply with the electrical power system. Approximately 640 pounds of oxygen is stored in two cryogenic tanks located in the service module. Heaters within the tanks pressurize the oxygen to 900 psig for distribution to the using equipment.

Flights through Apollo 13 carried two tanks. Apollo 14 and the later landings carried three. Each tank held about 330 pounds in 4.75 cubic feet. The oxygen was supercritical, denser than a gas and not a boiling liquid. The environmental-control share of a 14-day, three-man design mission was about 173 pounds. Crew use was figured at 1.8 pounds per man per day. Most of the oxygen went to the fuel cells.

The tanks were filled before launch and were not changed in flight. The command module had no spare tank to bolt in.

Warming on the glycol line

The water-glycol loop, what it cooled, and the radiators on the service module are on Apollo command module cooling.

Oxygen crosses into the command module through two capillary restrictors. The same capillaries are marked on the water-glycol line that runs to the radiators. That line is the heater. Apollo Operations Handbook, spacecraft 012, 12 November 1966.

Oxygen entered the command module on two lines. The 1969 handbook describes the hardware at the inlet:

Oxygen is delivered to the command module through two separate supply lines, each of which enters at an oxygen inlet restrictor assembly. Each assembly contains a filter, a capillary line, and a spring-loaded check valve. The filters provide final filtration of gas entering the CM. The capillaries which are wound around the hot glycol line, serve two purposes; they restrict the total O₂ flow rate to 7.5 pounds per hour maximum, and they heat the oxygen to prevent it from entering the CM in a liquid state. The check valves serve to isolate the two supply lines.

The spacecraft 012 handbook, three years earlier, says the same thing and gives the flow per restrictor rather than for the pair:

To assure uniform flow, the capillary restrictors are coiled around a warm water-glycol line to increase the oxygen temperature. Each restrictor allows a maximum flow of 4.5 pounds per hour into the ECS.

The 1972 flight report matches that second number, and it adds the temperature the hardware was built to hold:

Oxygen delivery to the ECS at 900 ± 35 psia is limited to 4.5 lb/hr from each tank by the combination flow restrictors and heat exchangers, which ensure a minimum temperature of 0°F at the maximum flow rate.

Coolant control panel on a Block II environmental control unit. The capillaries were wound on a line in this system. Photograph by Steve Jurvetson, 2020.

A third document, the Apollo 14 command-module news reference, sets the pair at 9 pounds an hour, to prevent starvation of the fuel cells, and again says the capillaries heat the oxygen so it does not enter as a liquid. The three sources agree on the method. They do not agree on the flow number: 7.5 pounds an hour for the pair, 4.5 pounds an hour from each tank, or 9 pounds an hour for the pair.

The capillaries are the restriction that chills the gas as it expands, and the glycol line under the coil is what puts the heat back. The spacecraft 012 handbook is explicit about which glycol line, and about what happens if that line is cold during a cabin refill:

To insure proper operation of the oxygen supply restrictors, in the line between the cryogenic O₂ storage in the S/M to the surge tanks in the C/M during cabin repressurization, full water-glycol flow through the line to the space radiators is required. Sufficient heat must be available to prevent cryogenic oxygen entering the C/M oxygen system and preclude the possibility of freezing the water-glycol. To achieve this, the mixing valve must be manually placed to the full closed position 15 to 30 minutes before repressurization and remain closed until the surge tank returns to maximum pressure after repressurization of the C/M.

Closing the mixing valve sends warm coolant, rather than coolant just back from the radiators, through the line the capillaries are wound on. The water-glycol hardware lived in the environmental control unit in the left-hand equipment bay, together with the suit loop. The panel in the photograph sets primary and secondary glycol to the suit heat exchangers, and it sets the glycol evaporator. No public photograph shows the capillary coil itself. It is drawn on the handbook schematic above, and it is described in the sentences quoted here. The round vessels on the same unit are part of that environmental control package. The 900 psi oxygen surge tank was a separate bottle beside the unit.

High-pressure oxygen in the command module

After the check valves, the two inlet lines joined. The 1969 handbook sends that single line to the oxygen service-module supply valve on panel 326, which was closed before the command module separated from the service module. From there the oxygen was still near 900 psi. It went, in parallel, to the surge tank and to the main regulator.

The surge tank stores approximately 3.7 pounds of oxygen at 900 psig for use during entry, and for augmenting the SM supply when the operational demand exceeds the flow capacity of the inlet restrictors.

A relief and shutoff valve for that tank was on panel 375. The flight report gives the same 3.7 pounds at 900 psia, and a repressurization pack of 3 pounds at 900 psia. Either or both could be isolated so the oxygen was still there for entry. The handbook splits that pack into hardware:

The package consists of three one-pound capacity oxygen tanks connected in parallel; a toggle-type fast acting REPRESS O₂ valve on panel 601 for dumping oxygen into the cabin at very high flowrates; a toggle valve and regulator on panel 600 for supplying oxygen to the emergency O₂ face masks; a relief and shut-off valve on panel 602 to protect the package against overpressurization; and a direct-reading pressure gauge on panel 602.

Opening the repress valve with the supply valve in FILL dumped both the package and the surge tank at a rate that will pressurize the command module from 0 to 3 psia in one minute. With the valve in ON, the package tanks added to the surge tank for entry and emergencies. The crew filled the package by watching the surge-tank pressure. The handbook says that when the indicator read 900 psi, within 35 psi, both the surge tank and the package tanks were full.

The drop to about 100 psi

The main regulator is the step from the 900 psi line down to the pressure the cabin and suit hardware were built for.

The main regulator reduces the supply pressure to 85-110 psig for use by the subsystem components. The regulator assembly is a dual unit which is normally operated in parallel. Two toggle valves at the inlet to the assembly provide a means of isolating either of the units in case of failure, or for shutting them both off.

The flight report states the same regulator as 100 psig, within 10 psi. That 85 to 110 psi manifold then fed the cabin regulators, the emergency cabin regulator, the demand regulator, the direct oxygen valve, and a smaller regulator that kept pressure on the water tanks and the glycol reservoir. That last one reduced the oxygen again, to about 20 psi above cabin pressure.

Cabin pressure

On the pad the cabin was about 60 percent oxygen and 40 percent nitrogen. After launch a bleed overboard, plus makeup from the oxygen regulators, left a pure-oxygen cabin. In flight the handbook's operating description is:

During normal space operations, the cabin pressure is maintained at a nominal 5 psia by the cabin pressure regulator, at flowrates up to 1.4 pounds of oxygen per hour. In the event a high leak rate develops, the emergency cabin pressure regulator will supply oxygen at high flow rates to maintain the cabin pressure above 3.5 psia for more than 5 minutes, providing the leak is effectively no larger than a 1/2-inch hole.

The same section says the cabin regulator is two absolute-pressure regulators in parallel, plus a manual cabin repress valve. The flight report's table gives the control range as 5.0 psia, within 0.2 psi, at up to 0.7 pound an hour from each regulator, which is the 1.4 pounds an hour of the handbook. The manual repress valve flows a minimum of 6 pounds an hour. If cabin pressure falls to 3.5 psia, the cabin regulator shuts itself off so it does not empty the tanks into a leak.

The emergency regulator is a separate pair of valves on the same panel. The handbook says the valve starts to open when cabin pressure decreases to 4.6 psia, and at 4.2 psia it is full open. It can supply at least 0.67 pound per minute, enough to hold the cabin above 3.5 psia for five minutes against a leak the size of a half-inch hole. It was used in shirtsleeves, to give the crew time to get into the suits. During suit operations it was shut off, so a dumped cabin would not pour oxygen overboard. The flight report lists the same valve as opening at a cabin pressure of 4.4 psia, within 0.2 psi, at 0.67 pound per minute.

The suit loop

Panel 380, the oxygen demand regulator. The handle selects BOTH or OFF. Spaceaholic collection, 2010.

The demand regulator sits on panel 380, on the environmental control unit. It is the regulator the crew could reach. The handbook puts it there, and the stencil on the hardware says the same thing.

The O₂ demand regulator supplies oxygen to the suit circuit whenever the suit circuit is isolated from the cabin (return air shutoff valve closed), and during depressurized operations. It also relieves excess gas to prevent overpressurizing the suits.

Each half is an aneroid and a diaphragm. Suit pressure is sensed on one side of the diaphragm. A bleed of oxygen through a reference chamber puts cabin pressure on the other side, as long as the cabin is above about 3.75 psia. When the crew exhales, the carbon dioxide and water are taken out of the loop, the suit pressure falls, and the diaphragm opens the demand valve:

When the pressure drops approximately 3-inch H₂O pressure below cabin, the diaphragm will open the demand valve.

The same corner: demand regulator at the top, cabin return in the middle, coolant panel below. Photograph by Steve Jurvetson, 2020.

A relief valve in the same assembly vents the suits if suit pressure rises to between 2 and 9 inches of water column above cabin pressure, about 0.07 to 0.33 psi. The flight report's table says the regulator supplies oxygen when suit pressure is 3.0 inches of water column, within 0.5 inch, below cabin pressure, and relieves when suit pressure is 2 inches of water column above cabin pressure. The suits stay just under cabin pressure, so they do not balloon.

If the cabin itself is taken below 3.5 psia, the cabin regulator has already shut off, and the demand regulator holds the suits:

When performing depressurized operations the suit circuit pressure is maintained above 3.5 psia by the O₂ demand regulator; the cabin pressure regulator shuts off automatically to prevent wasting oxygen.

The flight report gives that suited case as 3.75 psia, within 0.25 psi, at flow rates up to 0.67 pound per minute. Before entry the crew closed the suit-circuit return valve, so the demand regulator, not the cabin, held suit pressure for the descent.

While the cabin was sealed and the return valve was open, makeup for the suit loop could come from the cabin, and the cabin regulator made that up. The handbook says makeup comes from the demand regulator when the return valve is closed, and from the cabin when the return valve is open.

Two compressors circulated the loop. One ran at a time. Gas from the suits came back through the return manifold, through a debris screen, through a compressor, through the lithium hydroxide, and back to the suits.

Cabin return inlet and the suit-circuit return isolation valve. Spaceaholic collection, 2010.

Suit oxygen return, ahead of the debris screen. Spaceaholic collection, 2010.

Suit-circuit compressors, cover off. One ran at a time. Spaceaholic collection, 2010.

Carbon dioxide

The hydroxide does not make oxygen. The reaction is 2 LiOH + CO₂ → Li₂CO₃ + H₂O. It takes up carbon dioxide and leaves water and heat. The tanks replaced the oxygen the crew used and the oxygen that leaked.

Two beds sat in parallel. Each was sized for 1.5 man-days. The crew changed them alternately, one every 12 hours, so the carbon dioxide did not jump when a fresh bed went in. An 8- to 10-day mission carried 20 beds. The holder could be opened without dumping the suit loop, including with the cabin already at vacuum. The carbon dioxide limit was 7.6 mmHg. Readings usually stayed near 3 mmHg.

Canisters A and B. The crew changed one at a time. Photograph by Steve Jurvetson, 2020.

One canister open. The round element is the lithium hydroxide and charcoal bed. Spaceaholic collection, 2010.

The lunar module used round cartridges. The command-module cartridges were square and did not fit those holders. On Apollo 13 the lunar-module beds would not last the trip home. The crew ran cabin air through a square command-module cartridge with a suit hose, a bag, cardboard, and tape.

The mailbox prototype in Mission Control, 15 April 1970, with a command-module lithium hydroxide canister on a suit hose. NASA S70-35013.

Return to sea level

The stored oxygen did not bring the cabin up to sea level. Outside air did. The cabin stayed near 5 psia through the hot part of entry. As the spacecraft fell into thicker air, the outside pressure caught the cabin and then passed it. The relief valve opened inward, and Earth's air flowed in. Cabin pressure then rose with the outside air, all the way down.

Cabin pressure relief valve. Two outflow poppets share one discharge into the steam duct. On the way down, outside air pushes a poppet open. Handbook drawing, spacecraft 012, 12 November 1966, figure 2.7-5.

Before entry the suit-circuit return valve was closed, so the demand regulator held the suits. The 1969 handbook then describes the cabin on the way down:

Cabin pressure is maintained during the descent by the cabin pressure regulator until the ambient pressure rises to a maximum of 0.9 psi above cabin pressure. At that point the cabin relief valve will open, allowing ambient air to flow into the cabin. As the cabin pressure increases, the O₂ demand regulator admits oxygen into the suit circuit to maintain the suit pressure slightly below the cabin, as measured at the suit compressor inlet manifold.

That 0.9 psi is about 25 inches of water column. The spacecraft 012 handbook gives the same limit, and it says where the valve sits:

The dual cabin pressure relief valve provides positive and negative pressure relief for the cabin throughout the entire mission. When the cabin pressure becomes higher than the external ambient pressure, the relief valves limit the differential pressure to 6 psi, within 0.2 psi on the high side and 0.4 psi on the low side. When the cabin pressure becomes lower than the external ambient pressure (as during descent), the relief valves limit the differential pressure to a maximum of 25 inches of water. The assembly is located in the steam duct overboard line which provides the means of venting the positive and negative cabin pressures.

The 6 psi figure is the ascent case: on the way up the cabin stays that far above the thinning outside air, and the extra gas goes overboard. The same two poppets let air in on the way down, once outside pressure overcomes the spring. Cables from panel 307 can close both valves, or open one. The 1972 flight report describes the same descent as repressurizing the cabin when ambient pressure exceeds cabin pressure by about 1 psi.

The Apollo 15 entry checklist is the crew version of that valve. At 90,000 feet both cabin pressure relief valves are set to BOOST/ENTRY. That is the position that lets Earth's air in once the outside pressure is above the cabin. At 23,500 feet the checklist calls for the cabin pressure to be increasing. If it is not increasing by 17,000 feet, the right-hand valve is set to DUMP. At 10,000 feet, with the main parachutes out, the checklist marks the cabin at about 10 psia. If the crew is out of the suits, the surge-tank oxygen valve and the repressurization package are closed, and the direct oxygen valve is opened.

Under the mains both valves are closed while the reaction-control propellant is dumped. At 3,000 feet the right-hand valve goes to DUMP again, so the cabin can finish rising with the outside air. At 800 feet both valves are closed and latched off. The opening that just let the air in is shut before the spacecraft meets the water.

After splashdown the cabin is ventilated. It is not filled from the oxygen tanks. The 1969 handbook:

After spacecraft landing, the cabin is ventilated with ambient air by postlanding ventilation fan and valves. When the CM is floating upright in the water, the POST LANDING VENT switch is placed in the HIGH (day) or LOW (night) position. Either of these positions will supply power to open both vent valves and start the fan. In the HIGH position, the fan will circulate 150 cubic feet per minute; LOW, 100 cubic feet per minute.

The inlet duct, its shutoff valve and fan, and the outlet duct are on the forward tunnel cover. Lockpins hold the vent valves shut in flight. The placard says to pull the pin before operating the fan. If the command module is upside down, a pendulum switch closes those valves so they do not take in water. The float bags bring it upright first. The Apollo 15 checklist does the rest after the spacecraft is upright: close the vent circuit breaker, unlock the vent valve, take off the exhaust cover, and set the vent to HIGH or LOW. A distribution duct is installed in the cabin. The fan is turned off before the side hatch is opened.

See also

• Apollo command module cooling
• Apollo oxygen
• Apollo lunar module oxygen
• Pounds per square inch
• Moon landings