Apollo lunar module oxygen
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. 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 psi, regulated to 900 psi before it reaches the cabin. The same report's design history sizes the descent tank at approximately 48 pounds at 3,000 psi, and each ascent tank at approximately 2.4 pounds at 900 psi.[1] 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.[2]
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 psia and 70 degrees Fahrenheit. The residual it prints is 0.84 pound in each tank at 50 psia and the same temperature. Burst pressure is 4,500 psi. Each ascent tank holds 2.43 pounds at 840 psia and 70 degrees Fahrenheit, with 0.14 pound left at 50 psia, and a burst pressure of 1,500 psia.[3] Those three documents do not use the same pressure for a full tank. The experience report says about 3,000 psi and a 2,800 psi supply. The handbook stores the descent oxygen at 2,690 psia. The 1970 vessel survey lists a normal operating pressure of 2,869 psia for the descent tank and 850 for each ascent tank.[4]
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.[5]
What the tanks were
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 psia limit, a 3,990 psia proof, and a 4,500 psia burst. The two ascent vessels are 14.500 by 11.968 inches, wall 0.029 inch, Inconel 718, with a 1,000 psia limit, a 1,330 psia proof, and a 1,500 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.[4] 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.[4]
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.[6]
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.[7] 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.[3]
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.[8] 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.[3]
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 psi.[1] The LM-10 handbook's regulator holds 875 to 960 psia. A bypass relief protects the descent tanks. Its cracking pressure is 2,875 psig at most, at 75 degrees Fahrenheit, and full flow is at least 3,030 psig. Downstream, an overboard relief cracks at 1,025 psig. If that is not enough, a burst disk vents the high-pressure assembly and reseats near 1,000 psig, so one cabin refill is still left.[3]
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.[3]
The 900 psi descent line is what the earlier vehicles used to refill the backpacks.[9] From LM-10, tank number 2 has a separate regulator for that job. The handbook drops tank 2 from about 2,690 psia to about 1,400 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 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.[3]
The regulators
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.[10]
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.[10][1]
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.[11]
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:
- Open the equalization valve until the difference is about 2 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 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.[11]
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."[12] 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.[13]
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.[14]
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.[13]
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.[13]
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.[15]
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.[11]
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.[11]
See also
References
- ↑ 1.0 1.1 1.2 R. J. Gillen, J. C. Brady, and F. Collier, Apollo Experience Report: Lunar Module Environmental Control Subsystem, NASA TN D-6724 (1972).
- ↑ NASA TN D-7140, Apollo Experience Report: Consumables Budgeting, table VI, lunar module environmental control system oxygen data.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 NASA, Apollo Operations Handbook, Lunar Module, LM 10 and Subsequent, Volume I, Subsystems Data, LMA790-3-LM, 1 April 1971, section 2.6.
- ↑ 4.0 4.1 4.2 MSC Apollo 13 Investigation Team, Reaction Processes in High Pressure Fluid Systems, Panel 7 final report, May 1970, table A-I.
- ↑ MSC Apollo 13 Investigation Team, Panel 6, related systems evaluation, volume 2, lunar module.
- ↑ Glenn M. Ecord, Apollo Experience Report: Pressure Vessels, NASA TN D-6975 (September 1972).
- ↑ Lunar Module structures study guide, course 30915, quadrant descriptions.
- ↑ Lunar module news reference, aft equipment bay.
- ↑ Oxygen supply and cabin pressure control, Apollo Lunar Surface Journal, from the lunar module news reference.
- ↑ 10.0 10.1 Oxygen supply and cabin pressure control equipment, Apollo Lunar Surface Journal, from the lunar module news reference.
- ↑ 11.0 11.1 11.2 11.3 NASA, Apollo 15 CSM Systems Checklist, 15 March 1971, pages 2-1 through 2-11.
- ↑ Apollo Lunar Surface Journal, Apollo 17 Flight Journal, Day 3, entering the lunar module, 039:38:52 and 039:45:48.
- ↑ 13.0 13.1 13.2 NASA, Apollo 17 LM Lunar Surface Checklist, cabin depressurization at 116:37 and post-EVA configuration at 123:40.
- ↑ Cabin relief and dump valves, Apollo Lunar Surface Journal, from the 1969 Grumman lunar module news reference.
- ↑ Apollo Lunar Surface Journal, Apollo 12, post-EVA in the lunar module, 119:09:18.