Apollo command module oxygen

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

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

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.

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.

Coolant control panel on a Block II environmental control unit. The oxygen capillaries were wound on a line in this coolant system. They are not visible here. Photograph by Steve Jurvetson, 2020.

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, stenciled as the oxygen demand regulator. The handle selects BOTH or OFF. Above it is the suit test valve. Photograph from the 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.

A relief valve in the same assembly vents the suits if they rise to between 2 and 9 inches of water above the cabin. The flight report's table says the regulator supplies oxygen when the suit circuit is 3.0 inches of water, within 0.5 inch, below cabin pressure, and relieves when the suit is 2 inches of water above the cabin. 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.

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

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. With the cabin sealed, this connection is why suit pressure and cabin pressure stay together. Photograph from the spaceaholic collection, 2010.

Suit oxygen return. Gas coming back from the suits passed the debris-trap screen before the compressors and the lithium hydroxide. Photograph from the spaceaholic collection, 2010.

The two suit-circuit compressors, cover off. One circulated the oxygen loop at a time. Photograph from the 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.

Lithium hydroxide 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. Photograph from the 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.

See also

• Apollo oxygen
• Moon landings