Apollo lunar 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 lunar module oxygen system took warm compressed oxygen from the descent stage, dropped it to about 900 psi, and then to cabin pressure or suit pressure. The cabin was about 4.8 psia while the crew lived in it. For a walk it was dumped, and the suits were held near 3.8 psia. Getting into the lunar module, and getting back to the command module, used the command module's oxygen through the docking tunnel.

A shorter account of both spacecraft is on Apollo oxygen. The command-module plumbing is on Apollo command module oxygen.

The supply

The lunar module had no fuel cells and no cryogenic tanks, so it had no glycol warmer. One descent-stage tank held about 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.

A high-pressure regulator in the descent stage dropped that supply 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 regulators

Apollo 17 Challenger, close-out. The oxygen control module. The two knobs are regulators A and B: cabin, egress, direct oxygen, or close. Apollo Lunar Surface Journal, LM12-co38.

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. The crew sets both handles to cabin after landing, and lives at about 4.8 psia of oxygen. For a walk both handles go to egress first, so the suits are already on the lower setting before the cabin is opened to space.

Leaving the command module

The tunnel between the two spacecraft was not left full of air. Before the first trip across, the command-module pilot checked the difference across the forward hatch and, if the lunar module was soft, vented the tunnel until that difference was large enough. The Apollo 15 systems checklist says to put the tunnel vent valve to the lunar-module/command-module difference at least two hours before the transfer, and to verify more than 2.7 psi. If the difference is smaller, the valve is set to vent until it is.

At least 30 minutes before the hatch comes off, the same checklist has the direct oxygen valve opened until the command-module cabin reads 5.7 psia, then closed. That extra fraction of a psi is what the equalization uses. The pressure equalization valve is in the command module's forward hatch. Opening it lets command-module oxygen flow through the tunnel and through the lunar module's overhead hatch, which is left so that it can pass the gas.

If the difference across the hatch is already under 2.4 psi, the checklist opens the equalization valve and leaves it open until the difference is about zero and the cabin is still near 5 psia. If the difference is 2.4 psi or more, the steps are slower, so the command module is not emptied into the tunnel:

1. Open the equalization valve until the difference is about 2 psi, then close it.
2. Watch the gauge for 3 minutes. A steady reading means the tunnel and the lunar module are not leaking.
3. Open the valve again. When the command-module cabin falls to 4.0 psia, open the repressurization oxygen valve and bring the cabin back up, cycling between 4.0 and 5.7 psia as the oxygen crosses over.

Apollo 17 flew that second case. Ronald Evans called the difference at 3.6 psi, above the 3.1 the checklist wanted, and then: "we'll open the pressure equalization valve and when the cabin pressure gets to 4.0, hit the Repress O₂." With the difference gone, the forward hatch, the probe, and the drogue come out, and the lunar-module hatch is opened. The lunar module is then at the command module's oxygen pressure. Its own regulators take over once the crew is living there.

Before a walk

The backpacks do not feed the suits until the cabin is on its way to vacuum. First the lunar module itself is set so the suits will not follow the cabin down.

After landing, the suit gas diverter is pulled to egress and the cabin gas return is set to egress. Those two close the path between the suit loop and the cabin. The pressure regulators are set to egress, so the suit hoses are held near 3.8 psia instead of the cabin's 4.8. The cabin repressurization valve is verified automatic, and then, when the dump starts, its circuit breaker is opened and the valve is closed, so the cabin is not refilled while it is being emptied.

Cabin relief and dump valve, one in each hatch. The outside handle only dumps. The inside handle selects close, automatic, or open. Drawing from the 1969 lunar module news reference, Apollo Lunar Surface Journal.

A cabin relief and dump valve was installed in each hatch, overhead and forward. The 1969 lunar module news reference describes it as a differential-pressure poppet. In automatic, the servo opens it when the cabin is about 5.4 psi above the pressure outside, and vents oxygen overboard. At 5.8 psi of difference, one fully open valve can dump 11.1 pounds of oxygen a minute. The dump position unseats the poppet by hand. With no bacteria filter, either valve can take the cabin from 5.0 psia to 0.08 psia in 180 seconds. Both open, the time is 90 seconds. Apollo 11 flew a filter on the forward valve, which stretched one-valve dump to 310 seconds. The closed position keeps the valve shut if the servo fails.

Overhead dump valve in the Kennedy Space Center lunar-module simulator. The striped handle is the one the crew reaches. Detail of NASA KSC-72PC-540.

The Apollo 17 checklist dumps in two moves, and it does not start until Houston has given a go. The cabin repressurization circuit breaker is open and the valve is closed. The overhead or the forward dump valve is opened, then put back to automatic when the cabin is at 3.5 psia. The crew verifies the cabin is at 3.5, the suit circuit has locked up at 4.3 psia, and the suit cuff gauge has not fallen below 4.6. The same valve is then opened again and left open. The cabin goes to vacuum. The checklist has them partially open the forward hatch once the tone and the water flag show the suit is on the backpack, then open the hatch the rest of the way.

The cuff-gauge check is the stop. If the suit pressure falls with the cabin, the dump valve goes back to automatic. The backpack oxygen is already on before the dump, and the crew has watched it hold the suit for a minute with the bottle shut, so a decaying suit is a leak and not the plan.

After a walk

The refill is the descent-stage oxygen, through the cabin repressurization valve, not a second stored atmosphere.

The forward hatch is closed and locked. Both dump valves go to automatic, so they are no longer held open. The checklist warns that the backpack oxygen and pressure flags may come on during the refill. If the backpack bottle is under 10 percent, the crew repressurizes by hand, with the circuit breaker left open, so the suit stays above the rising cabin. Otherwise the cabin repressurization valve is set to automatic and its circuit breaker is closed. The master alarm and the cabin warning light come on. The crew verifies that cabin pressure is increasing, and sets both demand regulators back to cabin. Backpack oxygen is turned off once the cabin is above 2.5 psia. The warning light goes out when the cabin is stable at 4.6 to 5 psia. A purge valve on the suit lets the crew soften the suit as the cabin catches up.

Then the paths between the suit loop and the cabin are opened again. Cabin gas return goes to automatic. Suit circuit relief is verified automatic. The suit gas diverter is pushed to cabin. The crew takes off the gloves and helmets, takes the safety off the dump valve, and reconnects the lunar-module oxygen and water hoses in place of the backpack. Apollo 12 called the cabin at 4.6 over the radio as that refill finished, and Houston read the same number.

Returning to the command module

The last crossing is the same tunnel, used in the other direction. The Apollo 15 checklist starts the final transfer with the lunar module's forward dump valve verified automatic, so that cabin is not being dumped. The crew equalizes the command module and the lunar module, takes the tunnel hatch off, checks that at least three docking latches are engaged, and moves the probe and drogue. Equipment comes across. The lunar-module hatch is closed from inside the command module. The direct oxygen valve is closed. The tunnel hatch goes back on.

The hatch integrity check is the proof that the command module is a cabin again. The lunar-module hatch is closed and its dump valve is automatic. Command-module cabin pressure is verified at 4.7 to 5.3 psi. The tunnel vent valve is held in vent until the difference across the forward hatch is more than 3 psi, about eight and a half minutes of venting. That difference has to stay within 0.2 psi for 2 minutes, and the oxygen flow meter must not rise. Before undocking, the tunnel is vented another 10 minutes, until the gauge is pegged above 4 psi. The tunnel is then empty, and the lunar module can leave.

See also

• Apollo oxygen
• Apollo command module oxygen
• Pounds per square inch