Apollo oxygen

Apollo oxygen is the breathing supply for the Apollo command module, the lunar module, and the suits worn on the Moon. The service-module tanks also fed the fuel cells. Carbon dioxide was removed with lithium hydroxide. The crews changed those cartridges on a schedule. The oxygen tanks themselves were filled before launch and were not replaced in flight.

Command module and lunar module

Cutaway of a service-module oxygen tank, drawn for the Apollo 14 redesign. The fans were removed and a third heater was added after Apollo 13. NASA S71-16745.

The command module drew oxygen from cryogenic tanks in the service module. Flights through Apollo 13 carried two. Apollo 14 and the later landings carried three, with the extra tank plumbed so a failed tank could be shut off. Each tank held about 330 pounds in 4.75 cubic feet, kept near 900 psi by electric heaters. The oxygen was supercritical: denser than a gas, and not a boiling liquid. A surge tank of 3.7 pounds and a repressurization bottle of 3 pounds, both in the command module, covered short peaks in demand.

Most of the oxygen went to the three fuel cells, which produced electricity and water. The environmental-control budget for a 14-day, three-man design mission was about 173 pounds, including cabin leakage and repressurization. Crew use was figured at 1.8 pounds per man per day. In flight the cabin was held at 5 psi of pure oxygen. On the pad the mix was about 60 percent oxygen and 40 percent nitrogen, then bled down after launch.

The lunar module had no fuel cells and no cryogenic tanks. One descent-stage tank held 48 pounds of oxygen at about 3,000 psi. Two ascent-stage tanks held 2.4 pounds each and were saved for the climb back to orbit. The descent tank also refilled the backpacks. Cabin pressure was about 4.8 psi, pure oxygen.

A fan circulated the air through lithium hydroxide and activated charcoal. The hydroxide reaction is 2 LiOH + CO₂ → Li₂CO₃ + H₂O. It takes up carbon dioxide and leaves water and heat. It does not restore oxygen, so the tanks made up what the crew used and what leaked. The carbon dioxide limit was 7.6 mmHg. Command-module readings usually stayed near 3 mmHg.

Pressure reduction

The tanks were not at cabin pressure. Letting 900 psi oxygen expand without adding heat would chill it, far enough to arrive as liquid and to freeze the coolant it passed. The command module warmed it on purpose. The lunar module and the backpacks started with warm compressed gas, so they did not use that warmer.

Command module

Oxygen left the service-module tanks at about 900 psi. It entered the command module through two inlet assemblies, each a filter, a capillary, and a check valve. The capillaries are wound around the hot water-glycol line, the same coolant that is pumped out to the space radiators. Squeezing the flow through the capillary is what drops the pressure, and that expansion is what cools the oxygen. The glycol line under the coil puts the heat back. The handbook gives the coil two jobs: limit the flow, rated there at 7.5 pounds an hour for the pair, and heat the oxygen so it does not enter the command module as a liquid. The flight report rates each tank's restrictor at 4.5 pounds an hour and says the restrictors and their heat exchangers keep the oxygen at least 0 degrees Fahrenheit at that flow.

Oxygen from the cryogenic tanks crosses into the command module through two capillary restrictors, just inside the service-module line. The same capillaries are the part marked on the water-glycol line to the radiators. That line is what warms them. Apollo Operations Handbook, spacecraft 012, 12 November 1966.

A full cabin refill is the hard case, because the flow is high and the oxygen is still cold. The capillaries sit on the glycol line that runs to the radiators. The handbook has the crew close the glycol mixing valve 15 to 30 minutes beforehand, so that line is carrying warm coolant instead of coolant just back from the radiators, and leave it closed until the surge tank is full again. That keeps cryogenic oxygen out of the command-module plumbing and keeps the glycol from freezing.

Downstream of the capillaries the line is still the high-pressure supply. It feeds the surge tank, still near 900 psi, and the main regulator, which drops the line to about 100 psi. Two cabin regulators on that manifold hold the cabin at 5.0 psi, within 0.2 psi. Each can flow up to 0.7 pounds an hour. If the cabin falls to about 4.4 psi, an emergency valve opens a much larger flow.

Lunar module

Apollo 17 lunar module Challenger. The oxygen control module. Regulators A and B are the knobs marked cabin, egress, direct oxygen, and close. Those two take the 900 psi supply down to cabin pressure. Close-out photo from the Apollo Lunar Surface Journal.

The lunar module did not carry cryogenic oxygen, so it had no glycol warmer. The descent tank was compressed gas near 2,800 psi. A high-pressure regulator in the descent stage dropped that to about 900 psi before the line came up into the cabin. That 900 psi line refilled the backpacks. It is also the supply the cabin regulators used.

The step from 900 psi to cabin pressure is two oxygen demand regulators in the oxygen control module. The module is on the suit-circuit package, at the right rear of the lunar module pilot's station. Besides the regulators it holds the descent and ascent shutoff valves, the backpack fill valve, and a cabin repressurization valve.

Each regulator has one handle: cabin, egress, direct oxygen, or close. A sense line brings suit-loop pressure back to an aneroid bellows, and the bellows moves a poppet against a spring. Cabin and egress are the same valve with a different spring. Cabin holds the loop at 4.8 psi, within 0.2 psi. Egress holds it at 3.8 psi, within 0.2 psi. Direct oxygen opens the poppet all the way. Close shuts that regulator off. Either one can feed the loop by itself. At 900 psi and 70 degrees Fahrenheit, one of them can flow about 7 pounds an hour.

While the cabin is sealed, the suit loop and the cabin are connected, so the pressure the regulators hold in the loop is the cabin pressure. For a moonwalk the crew sets both handles to egress, closes the cabin return, and dumps the cabin. The suits stay at 3.8 psi. Relief valves on the hatches open if the cabin rises about 5.6 psi above the pressure outside.

Suits

Command-module suits took oxygen that had already been warmed and dropped to about 100 psi. Demand regulators fed that loop only when it sat a few inches of water below cabin pressure, and they vented it if it rose above the cabin, so the suits stayed soft. If the cabin was depressurized below 3.5 psi, the same regulators held the suit loop at 3.75 psi, within 0.25 psi.

Lunar-module suits used the demand regulators above: 4.8 psi in the cabin, 3.8 psi once the cabin was dumped for a walk.

On the walk the backpack did the drop in one step, from a bottle of warm compressed gas rather than from the cold service-module supply. A single-stage regulator took the bottle, several hundred psi up to about 1,400 psi depending on the flight, down to about 3.85 psi in the suit. The purge bottle on top of the pack had its own single-stage regulator and held the suit near 3.7 psi if the backpack failed. An early purge-system design 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 it, and the flight purge system flew without that heater.

In flight

Apollo 13 lunar module. A square command-module lithium hydroxide canister, hosed in so it could scrub lunar-module air. The lunar-module holders took round cartridges. NASA AS13-62-8929.

Oxygen left the tanks through valves as the fuel cells and the cabins called for it. There was no in-flight procedure for changing a tank.

The job the crews did repeat was the carbon-dioxide cartridges. In the command module two cartridges sat in parallel in the suit loop. Each was sized for 1.5 man-days. The crew changed them alternately, one every 12 hours, so the carbon dioxide level did not jump when a fresh bed was installed. An 8- to 10-day mission carried 20 cartridges. The holder let a cartridge be changed without dumping the suit loop, including while the cabin was depressurized.

The lunar module used round cartridges. The primary was rated for 41 man-hours. A smaller secondary cartridge, the backpack type, rated for 14 man-hours, carried the cabin while the primary was out. The primary could be changed with the cabin already at vacuum, as long as the suits were still on the lunar-module hoses.

Command-module cartridges were square and did not fit the lunar-module holders. On Apollo 13 the lunar-module beds would not last the trip home. The crew ran cabin air through a square cartridge using a suit hose, a bag, cardboard, and tape. Houston built the same rig, measured how long each cartridge lasted, and then read the steps up.

Suits on the Moon

Portable life support system with the cover off, 1968. The backpack held the oxygen supply, the carbon-dioxide cartridge, and the cooling water. NASA S68-34582.

On a moonwalk the suit was a closed loop inside the backpack. A fan pulled gas from the suit, through a lithium hydroxide and charcoal cartridge, through a cooler, and back to the helmet. The primary oxygen bottle added only makeup gas: oxygen the crewman consumed, plus leakage. The bottle on the earlier flights held 1.12 pounds at 850 psi. The backpacks on Apollo 15, 16, and 17 held 1.75 pounds at 1,410 psi. After reserves, about 1.3 pounds remained for the walk, which covered excursions planned up to about seven hours. The cartridge on those flights held about 3.1 pounds of lithium hydroxide.

A second bottle, the oxygen purge system, was mounted on top of the pack. It held 2.85 pounds at about 5,880 psi and was open-loop: oxygen passed through the suit and out a purge valve. It was the backup if the backpack failed. It was not used to extend a normal walk.

Packs for the first walk were filled before launch. Between later walks, inside the lunar module, each crewman serviced his own pack. He removed the spent hydroxide cartridge, locked in a fresh one, and exchanged the battery. He connected the descent-stage oxygen hose and filled the backpack bottle, then let it cool and topped it off, because compression heats the gas and the pressure falls as the bottle cools. Cooling water was refilled from the descent tank, and wastewater was drained. The work took about half an hour a suit.

If a backpack failed away from the lunar module, that crewman opened the purge bottle. The working backpack could share cooling water through a buddy hose. Walks were planned so the ride back still fit that emergency oxygen.

See also

• Moon landings
• Commander Davenport oxygen tanks