Apollo's Fuel Cells: Electricity from Hydrogen and Oxygen

The Command and Service Module ran on fuel cells—not because they were the simplest power source (batteries were simpler) or the lightest for short missions (batteries won there too), but because the CSM had to stay powered for up to 14 days, and no battery system of the 1960s could store enough energy for two weeks of continuous operation at the CSM’s power demands. Fuel cells could. They consumed hydrogen and oxygen—which the CSM already carried for other purposes—and produced electricity, heat, and water. The water was drinkable. The electricity ran every system on the spacecraft. The heat was a byproduct that the thermal control system had to manage. Three fuel cells, each producing about 1,420 watts at peak output, powered the CSM from the moment the Saturn V’s instrument unit handed off to the spacecraft’s internal power until the CM’s entry batteries took over for reentry.

The Electrochemistry: How Fuel Cells Work

A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy, without combustion. The Apollo fuel cells used the reaction between hydrogen and oxygen—the same reaction that occurs in burning hydrogen, but performed electrochemically across a membrane rather than thermally in a flame.

At the anode (fuel electrode), hydrogen gas was split into protons and electrons:

2 H2 → 4 H+ + 4 e-

The electrons traveled through an external circuit (doing useful work as electrical current), while the protons migrated through the electrolyte to the cathode (oxygen electrode). At the cathode, oxygen combined with the protons and electrons to form water:

O2 + 4 H+ + 4 e- → 2 H2O

The overall reaction was simply:

2 H2 + O2 → 2 H2O + electrical energy + heat

The Apollo fuel cells used a concentrated potassium hydroxide (KOH) solution as the electrolyte—an alkaline fuel cell design. The electrodes were porous sintered nickel, impregnated with platinum catalyst to promote the electrochemical reactions. The KOH electrolyte was held in an asbestos matrix between the electrodes, forming a “bacon cell” configuration (named after Francis Bacon, the British engineer who developed the alkaline fuel cell technology).

Each fuel cell stack contained 31 individual cells connected in series, producing a nominal output voltage of 27-31 volts DC (matching the spacecraft’s 28-volt bus) at currents up to about 50 amperes. The three fuel cells were designated FC-1, FC-2, and FC-3, and were mounted in the Service Module’s Sector 4 bay.

Reactant Supply: Cryogenic Tanks

The hydrogen and oxygen consumed by the fuel cells were stored in cryogenic tanks in the Service Module. The oxygen was shared with the Environmental Control System—the same oxygen that pressurized the cabin and provided breathing gas also fed the fuel cells. The hydrogen was dedicated to the fuel cells; no other system used it.

The CSM carried two oxygen tanks and two hydrogen tanks. Each oxygen tank held approximately 320 pounds of supercritical oxygen at about -297°F and 900 psi. Each hydrogen tank held approximately 28 pounds of supercritical hydrogen at about -423°F and 245 psi. “Supercritical” meant the fluid was above its critical point—neither liquid nor gas, but a uniform, dense fluid that behaved predictably during consumption regardless of the tank fill level.

The cryogenic tanks required active thermal management. The supercritical fluids slowly warmed from heat leaking through the tank insulation, increasing the tank pressure. Heaters inside the tanks, controlled by the spacecraft’s thermal management system, periodically warmed the fluid to maintain the pressure and density within operating limits. Fans inside the tanks—small electric motors with paddle blades—stirred the fluid to prevent stratification and ensure uniform density readings from the quantity gauges.

It was one of these fans—in oxygen tank number 2—that initiated the Apollo 13 disaster. The fan’s electrical wiring, damaged during a ground test incident months earlier, short-circuited when the fan was activated for a routine stir, igniting the Teflon insulation inside the tank and causing the tank to rupture.

Water Production: A Valuable Byproduct

The fuel cells produced water at a rate of approximately 1 pound per hour of combined operation—about 0.33 pounds per hour per fuel cell at nominal load. Over a 10-day mission, the three fuel cells produced roughly 240 pounds of water. This water was potable—pure H2O with trace KOH electrolyte removed by a separator—and was used by the crew for drinking and food preparation, and by the Environmental Control System’s evaporator for supplemental cabin cooling.

The water production rate was tied to the electrical load. Higher power demand meant higher reactant consumption, which meant more water. Lower power demand meant less water. The water balance—the relationship between water production, crew consumption, and cooling system demand—was managed as part of the mission’s consumables budget. On some missions, excess water was dumped overboard through a waste water vent; on others, the water supply was tight enough that dump events were minimized.

The dual-purpose nature of the cryogenic oxygen—shared between the fuel cells and the cabin atmosphere—created a coupling between the electrical and life support systems that had profound consequences. The Apollo 13 oxygen tank rupture didn’t just affect the fuel cells; it vented the oxygen that the crew needed to breathe. And the loss of fuel cell power didn’t just darken the spacecraft; it stopped the water production that the crew needed to drink. The cascading failure—one event taking out power, water, and atmosphere simultaneously—was the nightmare scenario that the redundant tank design was supposed to prevent, and did prevent for single-tank failures, but could not prevent when the rupture of one tank damaged the plumbing of the other.

The fuel cells required a warmup period after activation. During the countdown, the fuel cells were connected to the reactant supply and brought to operating temperature—approximately 400°F—before the spacecraft switched from ground power to internal power. The warmup took roughly an hour, during which the cells were gradually loaded and their output voltage was monitored.

Once operating, the three fuel cells shared the spacecraft’s electrical load. The load sharing was managed by the Electrical Power System’s diode isolation circuits and by crew configuration of the fuel cell bus connections. Each fuel cell could be connected to either or both of the main DC buses (Main Bus A and Main Bus B). The normal configuration connected all three fuel cells to both buses, providing maximum redundancy—the loss of any single fuel cell would reduce total power capacity by one-third but would not cause the loss of either bus.

The crew could also isolate fuel cells from specific buses for troubleshooting or to manage a fuel cell anomaly. On Apollo 13, the crew isolated fuel cells as they failed, attempting to preserve bus voltage as long as possible before the cascading oxygen loss made all three cells inoperable.

Fuel cell shutdown was a controlled process: the reactant valves were closed, the cell cooled, and the residual water was removed. Shutdown was normally performed only at the end of the mission, just before CM/SM separation, when the CM switched to its internal entry batteries for the reentry phase. An uncontrolled shutdown—a fuel cell stopping due to reactant loss, overtemperature, or internal failure—was an emergency that required the crew to reconfigure the electrical system to maintain bus voltage on the remaining cells.

Thermal Management: Dealing with Waste Heat

The fuel cells were approximately 50% efficient—half the energy from the hydrogen-oxygen reaction became electricity, and the other half became heat. At full load, each fuel cell generated roughly 1,400 watts of waste heat that had to be removed. The total waste heat from three fuel cells at full power was about 4,200 watts—a significant thermal load in a spacecraft with limited heat rejection capability.

The fuel cells were cooled by the SM’s primary water-glycol coolant loop—the same loop that cooled the cabin air and the avionics. The coolant circulated through heat exchangers on each fuel cell stack, picking up waste heat and carrying it to the spacecraft’s radiators on the Service Module’s outer surface. The radiators—panels of aluminum tubes bonded to the SM skin—rejected heat to space by infrared radiation.

If the coolant loop failed or the radiators were obstructed (by spacecraft attitude holding one side continuously in sunlight, for instance), the fuel cell temperatures would rise. Each fuel cell had thermal shutdown limits—if the stack temperature exceeded approximately 500°F, the cell would be shut down automatically to prevent damage. Managing the thermal environment of the fuel cells was one of the reasons the spacecraft performed the Passive Thermal Control “barbecue roll”—the slow rotation that distributed solar heating evenly and ensured the radiators alternated between sunlit and shaded orientations.

Redundancy and the Two-Cell Minimum

The CSM carried three fuel cells, but could operate on two. The loss of one fuel cell reduced the total power capacity from roughly 4,260 watts to 2,840 watts. The crew and Mission Control would respond by shedding non-essential electrical loads—turning off experiments, reducing communications bandwidth, dimming cabin lighting—to keep the total demand within the capacity of the remaining two cells.

The loss of two fuel cells was a mission abort scenario. A single remaining fuel cell could produce roughly 1,420 watts—barely enough to maintain the essential systems (guidance, communications, life support) needed for an emergency return to Earth. The crew would shut down virtually everything except the AGC, one communications link, and the minimum ECS configuration. A two-cell-failed condition would trigger an immediate evaluation of abort options—a direct return trajectory if the CSM hadn’t yet reached the Moon, or an emergency TEI if already in lunar orbit.

The loss of all three fuel cells was what happened on Apollo 13. With no fuel cells, the CSM had no electrical power, no water production, and (because the oxygen was also lost) no atmosphere supply. The CM’s entry batteries—designed for the reentry phase—provided limited backup power, but they were sized for hours, not days. The crew evacuated to the LM, whose batteries and oxygen supply became the mission’s survival margin.

The Power That Made Apollo Possible

The fuel cell technology that Pratt & Whitney developed for Apollo was not new—Francis Bacon had demonstrated alkaline fuel cells in the 1930s, and NASA had used Gemini-era fuel cells from General Electric on earlier programs. But the Apollo fuel cells pushed the technology further: higher power density, longer operational life, better reliability, and integration with the spacecraft’s life support system through the shared oxygen supply and the potable water byproduct.

The fuel cells ran for up to 295 hours continuously on the longest Apollo missions (Apollo 17). They produced approximately 1,600 kilowatt-hours of electricity over the course of the program—enough to power a typical American home for about two months. They generated thousands of pounds of drinking water. They never failed due to an internal defect—the only fuel cell losses were caused by external events (the Apollo 13 oxygen supply rupture).

Three cylindrical stacks, each about 44 inches tall and 22 inches in diameter, tucked into a bay of the Service Module, quietly converting hydrogen and oxygen into the electricity that ran every computer, every radio, every fan, every heater, and every light on the spacecraft. The fuel cells didn’t fly the mission. They powered everything that did.