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
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, 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.[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]
In flight
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.[5]
Suits on the Moon
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.[6][7]
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.[8][9]
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
- ↑ 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.
- ↑ 2.0 2.1 2.2 2.3 F. H. Samonski Jr. and E. M. Tucker, Apollo Experience Report: Command and Service Module Environmental Control System, NASA TN D-6718 (1972).
- ↑ 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).
- ↑ NASA Office of the Chief Health and Medical Officer, Apollo Lunar Lander technical brief (2023).
- ↑ J. C. LeBlanc, Apollo 13 LiOH Canister Breakthrough Test, NASA TM X-68571 (1970).
- ↑ Hamilton Standard, Apollo Portable Life Support and Oxygen Purge Systems (early backpack: 1.12 lb oxygen, 2.85 lb purge oxygen).
- ↑ Hamilton Standard, Portable Life Support System (J-mission backpack: 1.75 lb at 1,410 psi).
- ↑ Apollo 11 final EVA procedures, cartridge change and oxygen fill.
- ↑ Eric M. Jones, Apollo PLSS Images, Apollo Lunar Surface Journal. Recharge between walks.