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Apollo oxygen

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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

Main articles: Apollo command module oxygen, Apollo lunar module oxygen
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.[1] 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.[2]

Most of the oxygen went to the three fuel cells. Those plants turned hydrogen and oxygen into electricity and drinking water. The cells, the reaction, and the water path are on Apollo fuel cells. 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.[2] 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.[3][4]

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.[2]

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

Service-module oxygen arrived at about 900 psi. Capillaries wound on the hot water-glycol line warmed it and limited the flow. A surge tank in the command module stayed near 900 psi. The main regulator dropped the line to 85 to 110 psi, and the cabin regulators held about 5 psi. The suit regulator held the suits just under cabin pressure, or near 3.75 psi if the cabin was dumped. The handbook wording, the valves, and photographs of the hardware are on Apollo command module oxygen.

Lunar module

The lunar module stored warm compressed oxygen. A descent-stage regulator dropped about 2,800 psi to about 900 psi. Two demand regulators then held the cabin near 4.8 psi, or the suits near 3.8 psi for a walk. Before a moonwalk the crew dumped the cabin. After the walk they filled it again from the descent tank. The trips between the command module and the lunar module used the command module's oxygen through the docking tunnel. The valves, the checklists, and the photographs are on Apollo lunar module oxygen.

Suits

Command-module suits took oxygen that had already been warmed and dropped to about 100 psi. Demand regulators fed that loop only when suit pressure sat a few inches of water column below cabin pressure, a fraction of a psi, and they vented the suits if suit pressure rose above cabin pressure, 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.[2]

Lunar-module suits used the demand regulators on Apollo lunar module oxygen: 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.[5]

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.[2]

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.[3] 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.[6]

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.[7][8]

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.[9][10]

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

References

  1. ↑ R. K. M. Seto, C. E. Barton, and J. E. Cunningham, Apollo Cryogenic Integrated Systems, TRW for NASA Manned Spacecraft Center (1971, changed 1972). Table 2.1: 330.1 lb stored, 323.45 lb usable, 4.75 cubic feet, about 900 psi.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 F. H. Samonski Jr. and E. M. Tucker, Apollo Experience Report: Command and Service Module Environmental Control System, NASA TN D-6718 (1972).
  3. ↑ 3.0 3.1 R. J. Gillen, J. C. Brady, and F. Collier, Apollo Experience Report: Lunar Module Environmental Control Subsystem, NASA TN D-6724 (1972).
  4. ↑ NASA Office of the Chief Health and Medical Officer, Apollo Lunar Lander technical brief (2023).
  5. ↑ J. C. Beggs and F. H. Goodwin, Apollo PLSS: Environmental Control of the Smallest Manned Space Vehicle, Hamilton Standard (1972).
  6. ↑ J. C. LeBlanc, Apollo 13 LiOH Canister Breakthrough Test, NASA TM X-68571 (1970).
  7. ↑ Hamilton Standard, Apollo Portable Life Support and Oxygen Purge Systems (early backpack: 1.12 lb oxygen, 2.85 lb purge oxygen).
  8. ↑ Hamilton Standard, Portable Life Support System (J-mission backpack: 1.75 lb at 1,410 psi).
  9. ↑ Apollo 11 final EVA procedures, cartridge change and oxygen fill.
  10. ↑ Eric M. Jones, Apollo PLSS Images, Apollo Lunar Surface Journal. Recharge between walks.
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