View source for Apollo lunar module oxygen

{{series apollo}}

The '''lunar module oxygen system''' took warm compressed oxygen from the descent stage, dropped it to about 900 [[Pounds per square inch|psi]], and then to cabin pressure or suit pressure. The cabin was about 4.8 [[Pounds per square inch#Absolute pressure|psia]] while the crew lived in it. For a walk it was dumped, and the suits were held near 3.8 [[Pounds per square inch#Absolute pressure|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 ==
<div style="float:right; width:280px; margin:0.15em 0 0.8em 1.15em; clear:right;">
[[File:Apollo LM oxygen supply schematic small.jpg|link=File:Apollo LM oxygen supply schematic full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">One descent tank and two ascent tanks. Drawing from the 1972 lunar-module environmental control report. The later landing vehicles added a second descent tank of the same capacity.</div>
</div>
The lunar module had no fuel cells and no cryogenic tanks, so it had no glycol warmer. The oxygen was high-pressure gas. The descent stage carried it for the landing, the stay, and the backpack refills. Two smaller tanks in the ascent stage were saved for the climb back to orbit.

The 1972 experience report, written around the Apollo 11 vehicle, stores that gas as one descent-stage tank and two ascent-stage tanks. The descent supply is given as 2,800 [[Pounds per square inch|psi]], regulated to 900 [[Pounds per square inch|psi]] before it reaches the cabin. The same report's design history sizes the descent tank at approximately 48 pounds at 3,000 [[Pounds per square inch|psi]], and each ascent tank at approximately 2.4 pounds at 900 [[Pounds per square inch|psi]].<ref name="lm">R. J. Gillen, J. C. Brady, and F. Collier, [https://ntrs.nasa.gov/citations/19720013195 ''Apollo Experience Report: Lunar Module Environmental Control Subsystem''], NASA TN D-6724 (1972).</ref> A consumables report for that same class of mission prints the loaded masses as 48.0 pounds in the descent tank and 2.4 pounds in each ascent tank. After residual, loading uncertainty, and leakage, it leaves 44.4 pounds and 2.0 pounds available. It calls the ascent allowance the exception to the usual unusable band, at 17 percent, because the tanks are small.<ref name="consumables">NASA TN D-7140, ''Apollo Experience Report: Consumables Budgeting'', table VI, lunar module environmental control system oxygen data.</ref>

The handbook for LM-10 and the vehicles after it, Apollo 15 and later, lists two descent tanks. Each holds 48.01 pounds of oxygen at 2,690 [[Pounds per square inch#Absolute pressure|psia]] and 70 degrees Fahrenheit. The residual it prints is 0.84 pound in each tank at 50 [[Pounds per square inch#Absolute pressure|psia]] and the same temperature. Burst pressure is 4,500 [[Pounds per square inch|psi]]. Each ascent tank holds 2.43 pounds at 840 [[Pounds per square inch#Absolute pressure|psia]] and 70 degrees Fahrenheit, with 0.14 pound left at 50 [[Pounds per square inch#Absolute pressure|psia]], and a burst pressure of 1,500 [[Pounds per square inch#Absolute pressure|psia]].<ref name="aoh">NASA, [https://www.ibiblio.org/apollo/Documents/LMA790-3-LM10-ApolloOperationsHandbookLunarModuleLM10AndSubsequent-Volume1-SubsystemsData.pdf ''Apollo Operations Handbook, Lunar Module, LM 10 and Subsequent'', Volume I, Subsystems Data], LMA790-3-LM, 1 April 1971, section 2.6.</ref> Those three documents do not use the same pressure for a full tank. The experience report says about 3,000 [[Pounds per square inch|psi]] and a 2,800 [[Pounds per square inch|psi]] supply. The handbook stores the descent oxygen at 2,690 [[Pounds per square inch#Absolute pressure|psia]]. The 1970 vessel survey lists a normal operating pressure of 2,869 [[Pounds per square inch#Absolute pressure|psia]] for the descent tank and 850 for each ascent tank.<ref name="panel7">MSC Apollo 13 Investigation Team, [https://ntrs.nasa.gov/api/citations/19710010491/downloads/19710010491.pdf ''Reaction Processes in High Pressure Fluid Systems''], Panel 7 final report, May 1970, table A-I.</ref>

An Apollo 13 structures survey says LM-8 and LM-9 have three oxygen tanks, two of them in the ascent stage and the remaining one in the descent stage. From LM-10 on, a second oxygen tank is added in the descent stage.<ref name="panel6">MSC Apollo 13 Investigation Team, Panel 6, [https://apollojournals.org/afj/ap13fj/pdf/a13-investigation-team-vol2-lm-19790075246.pdf related systems evaluation, volume 2, lunar module].</ref>

=== What the tanks were ===
<div style="margin:0.6em auto 1.1em; max-width:720px; text-align:center; clear:both;">
[[File:Apollo LM oxygen tank vessel table small.jpg|link=File:Apollo LM oxygen tank vessel table full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">Environmental-control rows from the May 1970 pressure-vessel survey. One descent oxygen tank, of D6AC steel, and two ascent tanks, of Inconel 718. The water tanks in the same table are a different system.</div>
</div>
The May 1970 survey lists one descent oxygen vessel, 23.522 by 21.722 inches, with a wall 0.118 inch thick, made of low-carbon steel D6AC. Its design pressures are a 3,000 [[Pounds per square inch#Absolute pressure|psia]] limit, a 3,990 [[Pounds per square inch#Absolute pressure|psia]] proof, and a 4,500 [[Pounds per square inch#Absolute pressure|psia]] burst. The two ascent vessels are 14.500 by 11.968 inches, wall 0.029 inch, Inconel 718, with a 1,000 [[Pounds per square inch#Absolute pressure|psia]] limit, a 1,330 [[Pounds per square inch#Absolute pressure|psia]] proof, and a 1,500 [[Pounds per square inch#Absolute pressure|psia]] burst. The same table predicts that the descent tank would fragment if it failed at its maximum design pressure, and that an ascent tank would leak.<ref name="panel7" /> The descent tank's interior is nickel plated and the exterior is painted, against corrosion. The survey calls combustion remote if that plating and paint stay intact, and it names crevice corrosion as a risk of D6AC steel that Apollo use had avoided.<ref name="panel7" />

A descent-stage oxygen vessel of that D6AC steel failed in acceptance testing, not in flight. The crack started in the radius of a mounting boss. The test cycled the vessel in a water bath, and D6AC steel grows flaws readily in water. The tank maker had been told not to use water as the pressurizing medium. The bath itself had not been called out, so the water was not caught before the failure. The boss had also not been inspected after machining. Water was then replaced with trichlorotrifluoroethane, the bosses were dye-checked and magnafluxed, and the domes were X-rayed before welding. Later tests qualified the vessels for the spacecraft.<ref name="ecord">Glenn M. Ecord, [https://ntrs.nasa.gov/api/citations/19720024230/downloads/19720024230.pdf ''Apollo Experience Report: Pressure Vessels''], NASA TN D-6975 (September 1972).</ref>

=== Where they sat ===
On the one-tank vehicles the gaseous oxygen tank is in quadrant 3 of the descent stage, the bay on the minus-Z, plus-Y side, with the descent engine's helium tanks. Its mount runs to the minus-Z beam. Quadrant 4, in that same description, carries the batteries.<ref name="structures">[https://www.ibiblio.org/apollo/Documents/lm_structures_study_guide.pdf Lunar Module structures study guide], course 30915, quadrant descriptions.</ref> The LM-10 handbook puts one descent tank in quadrant 3 and the other in quadrant 4. Each tank is filled through its own high-pressure port, then capped and lockwired before launch. A check valve in each feed line keeps one tank from emptying into the other.<ref name="aoh" />

The two ascent tanks are in the unpressurized aft equipment bay. The lunar module news reference secures them, with two gaseous helium tanks, to the truss between the midsection aft bulkhead and the equipment rack.<ref name="newsref">[https://www.apollojournals.org/alsj/LM04_Lunar_Module_ppLV1-17.pdf Lunar module news reference], aft equipment bay.</ref> The handbook says the same place: both tanks are in the aft equipment bay, and each has its own shutoff on the oxygen control module.<ref name="aoh" />

=== How the gas left the tanks ===
A high-pressure regulator in the descent stage drops the tank pressure to a level the cabin module can use. The experience report calls that outlet about 900 [[Pounds per square inch|psi]].<ref name="lm" /> The LM-10 handbook's regulator holds 875 to 960 [[Pounds per square inch#Absolute pressure|psia]]. A bypass relief protects the descent tanks. Its cracking pressure is 2,875 [[Pounds per square inch#Gauge pressure|psig]] at most, at 75 degrees Fahrenheit, and full flow is at least 3,030 [[Pounds per square inch#Gauge pressure|psig]]. Downstream, an overboard relief cracks at 1,025 [[Pounds per square inch#Gauge pressure|psig]]. If that is not enough, a burst disk vents the high-pressure assembly and reseats near 1,000 [[Pounds per square inch#Gauge pressure|psig]], so one cabin refill is still left.<ref name="aoh" />

The descent and ascent shutoff valves are in the cabin, and they are mechanically interlocked so the ascent tanks cannot be opened while the descent valve is open. If the descent valve will not close, an interlock-override pushbutton on the oxygen control module releases the ascent valves.<ref name="aoh" />

The 900 [[Pounds per square inch|psi]] descent line is what the earlier vehicles used to refill the backpacks.<ref name="lmoxy">[https://apollojournals.org/alsj/alsj-ECSFunDesOxy.html Oxygen supply and cabin pressure control], Apollo Lunar Surface Journal, from the lunar module news reference.</ref> From LM-10, tank number 2 has a separate regulator for that job. The handbook drops tank 2 from about 2,690 [[Pounds per square inch#Absolute pressure|psia]] to about 1,400 [[Pounds per square inch#Absolute pressure|psia]], and a high-pressure fill valve in the cabin connects that line to the backpack bottle. The performance table prints the refill as 1.60 pounds at 1,430 [[Pounds per square inch#Absolute pressure|psia]], and it says tank 2 provides four of those refills. The same table gives both descent tanks together four cabin repressurizations, 5.5 pounds each.<ref name="aoh" />

<div style="clear:both;"></div>

== The regulators ==
<div style="float:right; width:300px; margin:0.15em 0 0.8em 1.15em;">
[[File:Apollo LM oxygen control module small.jpg|link=File:Apollo LM oxygen control module full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">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.</div>
</div>
The step from 900 [[Pounds per square inch|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.<ref name="lmequip">[https://www.apollojournals.org/alsj/alsj-ECSEquipOxy.html Oxygen supply and cabin pressure control equipment], Apollo Lunar Surface Journal, from the lunar module news reference.</ref>

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 [[Pounds per square inch|psi]], within 0.2 [[Pounds per square inch|psi]]. Egress holds it at 3.8 [[Pounds per square inch|psi]], within 0.2 [[Pounds per square inch|psi]]. Direct oxygen opens the poppet all the way. Close shuts that regulator off. Either one can feed the loop by itself. At 900 [[Pounds per square inch|psi]] and 70 degrees Fahrenheit, one of them can flow about 7 pounds an hour.<ref name="lmequip" /><ref name="lm" />

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 [[Pounds per square inch#Absolute pressure|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 [[Pounds per square inch|psi]]. If the difference is smaller, the valve is set to vent until it is.<ref name="csm15">NASA, [https://www.ibiblio.org/apollo/Documents/Apollo15CSMSystemsChecklist.pdf ''Apollo 15 CSM Systems Checklist''], 15 March 1971, pages 2-1 through 2-11.</ref>

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 [[Pounds per square inch#Absolute pressure|psia]], then closed. That extra fraction of a [[Pounds per square inch|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 [[Pounds per square inch|psi]], the checklist opens the equalization valve and leaves it open until the difference is about zero and the cabin is still near 5 [[Pounds per square inch#Absolute pressure|psia]]. If the difference is 2.4 [[Pounds per square inch|psi]] or more, the steps are slower, so the command module is not emptied into the tunnel:

# Open the equalization valve until the difference is about 2 [[Pounds per square inch|psi]], then close it.
# Watch the gauge for 3 minutes. A steady reading means the tunnel and the lunar module are not leaking.
# Open the valve again. When the command-module cabin falls to 4.0 [[Pounds per square inch#Absolute pressure|psia]], open the repressurization oxygen valve and bring the cabin back up, cycling between 4.0 and 5.7 [[Pounds per square inch#Absolute pressure|psia]] as the oxygen crosses over.<ref name="csm15" />

Apollo 17 flew that second case. Ronald Evans called the difference at 3.6 [[Pounds per square inch|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 O<sub>2</sub>."<ref>Apollo Lunar Surface Journal, [https://www.nasa.gov/history/afj/ap17fj/08_day03_part2_enter_lm.html Apollo 17 Flight Journal, Day 3, entering the lunar module], 039:38:52 and 039:45:48.</ref> 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 [[Pounds per square inch#Absolute pressure|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.<ref name="a17">NASA, [https://www.apollojournals.org/alsj/a17/a17LMLunSurfChklst-HSI-481250.pdf ''Apollo 17 LM Lunar Surface Checklist''], cabin depressurization at 116:37 and post-EVA configuration at 123:40.</ref>

<div style="clear:both;"></div>
<div style="margin:0.6em auto 1.2em; max-width:480px; text-align:center;">
[[File:Apollo LM cabin dump valve drawing small.jpg|link=File:Apollo LM cabin dump valve drawing full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">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.</div>
</div>

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 [[Pounds per square inch|psi]] above the pressure outside, and vents oxygen overboard. At 5.8 [[Pounds per square inch|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 [[Pounds per square inch#Absolute pressure|psia]] to 0.08 [[Pounds per square inch#Absolute pressure|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.<ref name="dump">[https://www.apollojournals.org/alsj/DumpValve.html Cabin relief and dump valves], Apollo Lunar Surface Journal, from the 1969 Grumman lunar module news reference.</ref>

<div style="float:right; width:240px; margin:0.15em 0 0.8em 1.15em;">
[[File:Apollo LM overhead dump valve small.jpg|link=File:Apollo LM overhead dump valve full.jpg]]
<div style="font-size:13px; line-height:1.35; margin-top:6px;">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.</div>
</div>
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 [[Pounds per square inch#Absolute pressure|psia]]. The crew verifies the cabin is at 3.5, the suit circuit has locked up at 4.3 [[Pounds per square inch#Absolute pressure|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.<ref name="a17" />

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.

<div style="clear:both;"></div>
== 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 [[Pounds per square inch#Absolute pressure|psia]]. The warning light goes out when the cabin is stable at 4.6 to 5 [[Pounds per square inch#Absolute pressure|psia]]. A purge valve on the suit lets the crew soften the suit as the cabin catches up.<ref name="a17" />

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.<ref>Apollo Lunar Surface Journal, [https://apollojournals.org/alsj/a12/a12.posteva1.html Apollo 12, post-EVA in the lunar module], 119:09:18.</ref>

== 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.<ref name="csm15" />

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 [[Pounds per square inch|psi]]. The tunnel vent valve is held in vent until the difference across the forward hatch is more than 3 [[Pounds per square inch|psi]], about eight and a half minutes of venting. That difference has to stay within 0.2 [[Pounds per square inch|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 [[Pounds per square inch|psi]]. The tunnel is then empty, and the lunar module can leave.<ref name="csm15" />

== See also ==
* [[Apollo oxygen]]
* [[Apollo command module oxygen]]
* [[Pounds per square inch]]

== References ==
<references />

[[Category:Apollo oxygen]]
[[Category:Apollo evidence]]
[[Category:Moon]]
[[Category:Spaceflight]]