Apollo fuel cells

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The Apollo fuel cells turned the service module's hydrogen and oxygen into electricity and into the command module's drinking water. Three of them, in the service module, carried the ship until the command module separated for entry. The oxygen they burned is the same supply described on Apollo oxygen.

The plants

The 1969 command-module handbook calls them Bacon-type fuel cell power plants. They supply primary direct-current power "until CSM separation." Each one normally delivers from 400 to 1,420 watts, at 31 to 27 volts measured at the terminals. All three run in a normal flight. Two are enough to finish the mission. If two fail, the third can still bring the crew home, once the loads are cut to what one plant can carry. Fuel cell 1 feeds main bus A. Fuel cell 2 feeds both main buses. Fuel cell 3 feeds main bus B.

A 1973 experience report, written after the flights through Apollo 12, describes the same machines from the design side. The system was qualified for about 2 kilowatts across a 14-day mission. Each module's normal range in that report is 563 to 1,420 watts at 27 to 31 volts. A module is 44 inches high, 22.5 inches in diameter, and weighs about 245 pounds. The stack inside is 31 alkaline cells in series. The two documents do not open the normal band at the same wattage. They agree on the top of it, 1,420 watts, and on three plants fed by cryogenic hydrogen and oxygen.

The report states the job in one sentence: the electrochemical reaction "produces electricity and water. The water is a byproduct and is used for the environmental control system for cooling and is consumed by the crewmembers."

The reaction

The handbook's account of one plant:

The power plants generate d-c power on demand through an exothermic chemical reaction. The by-product water is fed to a potable water storage tank in the CM where it is used for astronaut consumption and for cooling purposes in the ECS. The amount of water produced is equivalent to the power produced which is relative to the reactant consumed.

Cryogenic oxygen arrives from the service-module tanks at a normal 865 to 935 psia, the same band as the 900 psia supply on Apollo command module oxygen. It picks up heat in the lines and in a preheater and reaches the regulator as a gas warmer than 100 degrees Fahrenheit. The regulator then holds the oxygen at the stack at a nominal 62.5 psia. Hydrogen arrives at about 245 psia and is regulated to a nominal 61.5 psia.

Inside each of the 31 cells the handbook splits the chemistry in two. At the oxygen electrode, oxygen, water, and electrons from the circuit produce hydroxyl ions. At the hydrogen electrode, hydrogen and those ions produce water, electrons, and heat. The nickel electrodes are the catalyst. The electrons leave through the load. What remains, once the ions are cancelled, is hydrogen and oxygen in and water out.

The electrolyte is potassium hydroxide in water. The experience report calls it an 80 percent solution and gives the cell conditions as 380 to 460 degrees Fahrenheit and about 60 psia. The handbook is more particular about the fill. The initial charge is 83 percent potassium hydroxide and 17 percent water. In operation the water share runs between 23 and 28 percent. At that mix the electrolyte has a critical temperature of 300 degrees Fahrenheit, and it solidifies near 220 degrees Fahrenheit. The reaction becomes effective at the critical temperature. Ground equipment brings a plant up to that point. The spacecraft cannot. Once a load is on the plant, the reaction holds the temperature. With no load, an in-line heater holds the plant at 385 degrees Fahrenheit. The handbook's normal operating band for the skin of the plant is 385 to 450 degrees Fahrenheit, and the maximum skin temperature is 500 degrees Fahrenheit.

Figure 2.6-5 of the 1969 Block II handbook. Hydrogen and oxygen enter the power plant, the stack is the block on the left, and the water line leaves toward the potable tank. The drawing is the handbook figure, not a photograph.

Water does not drip out of the stack as a liquid stream. It forms as vapor in the hydrogen side. A pump moves that wet hydrogen through a condenser, which gives the waste heat to the glycol loop, and a separator pulls the liquid water off the gas. The hydrogen goes back around. The experience report notes that the separator leaves water free of gas except for the hydrogen that water naturally absorbs at the cell's pressure and temperature.

Impurities collect in the reactants, and the crew purges them. An oxygen purge takes 2 minutes and can add as much as 0.6 pounds an hour to the oxygen flow while it runs. A hydrogen purge takes 80 seconds.

How much oxygen

The handbook prints a table for one power plant. The oxygen, the hydrogen, and the water all rise in step with the current.

Load	Oxygen	Hydrogen	Water
20 amperes	0.408 lb/hr	0.051 lb/hr	0.459 lb/hr
50 amperes	1.020 lb/hr	0.129 lb/hr	1.149 lb/hr
100 amperes	2.040 lb/hr	0.257 lb/hr	2.297 lb/hr

The crew's breathing budget on the oxygen page is 1.8 pounds per man per day. Three men at that rate use 5.4 pounds a day, which is 0.225 pounds an hour. One power plant at 50 amperes takes 1.02 pounds of oxygen an hour, between four and five times the oxygen the whole crew was budgeted to breathe. Three plants at that load take about fourteen times the breathing budget. That is why the oxygen article says most of the oxygen went to the fuel cells. The table is a handbook rate at a stated current, not a claim that every hour of every flight sat at 50 amperes.

The water

A fuel-cell condenser on an Apollo system test rig at Lewis Research Center, 1964. The insulated vessel and the gauges are ground equipment. NASA GRC-1964-C-68129.

The potable-water report states the arrangement from the drinking end. "The inflight water supply for the command module is a byproduct of the operation of the fuel cells designed for power generation in the spacecraft." The tank is filled from a ground supply before launch and then topped up in flight. The lunar module has no fuel cells, so every pound of its water is loaded before lift-off.

The same report describes the cells as two chambers separated by porous nickel electrodes, with concentrated potassium hydroxide between them, oxygen on one side and hydrogen on the other, both held at 60 psi. "Extremely pure water is a byproduct of the chemical reaction." The cells run at approximately 410 degrees Fahrenheit. Water production is a nominal 1.2 pounds an hour and can reach 2.0 pounds an hour for brief periods, following the power drawn. Before the water is sent to the command module it is cooled to 74 degrees Fahrenheit and the pressure is reduced to 25 psi. Chemically it is as pure as distilled water, and it is saturated with hydrogen. Dissolved solids average 0.73 milligrams per liter. The average pH is 5.6.

The hydrogen is the problem at the tap. As the water falls from 60 psi at the cell toward the 5 psi cabin, dissolved hydrogen comes out of solution and makes bubbles large enough to spoil a drink or a food bag. Bagging and spinning the water did not separate the gas in weightlessness. Apollo 11 flew a hydrophobic and hydrophilic separator "with reasonable success." Apollo 12 and the later flights also used a palladium-silver hydrogen separator. The tubes vent the hydrogen overboard.

The command module's biocide was chlorine, added as sodium hypochlorite, with sodium dihydrogen phosphate as a buffer. That combination attacked the aluminum tubing, and sodium nitrate was added as a corrosion inhibitor. The lunar module used iodine instead. NASA's standard for the water was sterility: no viable organisms.

When the oxygen stopped

Apollo 13, 17 April 1970, photographed from the command module after the service module was jettisoned. The panel over sector 4 is gone. Two of the three fuel cells are in the opening, above the oxygen-tank bay. NASA AS13-58-8458.

The plants sit in sector 4 of the service module with the hydrogen tanks and the oxygen tanks. They have no other oxygen. On Apollo 13, oxygen tank 2 failed. The review board's narrative of the next minutes says that Fred Haise checked the fuel cells and found that cells 1 and 3 "were showing no hydrogen or oxygen flow and no electrical output," and that fuel cell 2 was carrying the command module through bus A. Bus B was dead. The cell with no output was taken off the bus, and then the other dead cell was taken off as well. For several minutes an entry battery was tied to bus A so that fuel cell 2 would not have to carry the ship alone.

The cells did not restart. The oxygen they required was gone. The crew lived on the lunar module's batteries and water for the trip home, and the command module's entry batteries were saved for the last hours. That is the same dependence the handbook states in the other direction: the plants run until the service module is cast off, and entry is on batteries.

See also

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