The Apollo 13 crew the day before launch
NASA
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Forty-six hours and 43 minutes into the flight of Apollo 13, Capcom Joe Kerwin had a mission update for commander Jim Lovell: “The spacecraft is in real good shape as far as we are concerned. We’re bored to tears down here.” Nine hours later, things got a lot less boring. A routine stir of the cryogenic oxygen tanks to stop the super chilled gas from stratifying caused a wire in tank 2 to arc. It ruptured, taking oxygen tank 1 with it. Bleeding this vital gas, Apollo 13 lost its electricity, light, and water supply was lost. Of course, no one on board or in Houston knew exactly what had happened. So at what point did it finally become clear to the crew that Apollo 13 wouldn’t be landing on the Moon?

The loss of an oxygen tank was crippling to an Apollo spacecraft because the oxygen tanks powered the fuel cells that powered the spacecraft. Three fuel cells, each containing 31 separate but connected cells, were stored in the service module. The electrochemical reaction of combining cryogenic hydrogen and oxygen produced electricity, heat, and potable water as byproducts. This electric power was ultimately transferred into the main command module through an umbilical running along the side of the mated spacecraft.

Under normal flight condition, each fuel cell could produce between 400 and 1420 watts at 31 to 27 volts direct current (DC). And though they fed off the same oxygen and hydrogen tanks, the fuel cells weren’t connected to one another, a decision that brought some safety into the system; the failure of one wouldn’t mean a total loss of power.

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The products of fuel cells

North American personnel demonstrate the water and electricity produced by the fuel cells that powered the Apollo spacecraft.

The fuel cells were tied to two buses that distributed the produced power through the spacecraft systems. Fuel cell 1 was connected to main DC bus A, fuel cell 2 connected to DC main buses A and B, and fuel cell 3 connected to main DC bus B. Solid state inverters converted the DC power into alternating current (AC) power for distribution into the spacecraft’s electrical systems through two main buses.

But these connections between fuel cells and buses weren’t permanent. There were switches inside the command module that would allow the crew to manually isolate one fuel cell and control the direction of power flow should some malfunction damage one of the cells. Losing a fuel cell didn’t mean the spacecraft was dead. Similarly, one of the AC inverters could provide the spacecraft with its basic electricity needs while the other two served as backups, and each bus was powered by its own inverter so that one could be isolated if it failed.

So there were a number of redundancies built into the power system on Apollo. A crew could isolate any failed element in the whole system, and NASA had mission rules in place outlining what would happen should some element of the power system fail in flight.

According to the mission rules published before Apollo 11’s launch, a fuel cell was considered lost when its output fell below 5 amps, an oxygen tank was considered lost when its pressure fell below 150 psi, and a hydrogen tank was considered lost when its pressure fell below 100 psi. The course of action after losing one or more elements of the power system in flight varied depending on the mission phase — during launch, the lunar landing phase, or cruise.

Losing all three fuel cells during launch didn’t mean an abort; the charged batteries in the command module would give the crew 4.75 hours of power to troubleshoot the problem in orbit before they would be forced to reenter the atmosphere. Only loss of three fuel cells and one of the reentry batteries during launch would require an abort.

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An unflown Apollo fuel cell

An unflown Apollo fuel cell on display at the NASM.

A loss of two or three fuel cells in lunar orbit meant cancelling the landing. The crew would need to retain the lunar module to harvest its power and consumables to get them home safely, and might even need its big descent engine to perform the transearth injection burn to get them on a path home. The same fuel cell loss during lunar landing meant a NO GO for lunar stay. In this case, the goal was to get the spacecraft docked as quickly as possible, retaining LM consumables and getting the crew headed home.

Losing two or three fuel cells and any reentry batteries at any point in the mission demanded an emergency shutdown to conserve all onboard power for the trip home and reentry through the atmosphere.

Losing one fuel cell was the only failure that didn’t mean a bad day. With one lost fuel cell, the crew would attempt to restore the cell then reconfigure the spacecraft to direct power from the two remaining cells to one bus each, restoring some redundancy. From there, NASA would look at how other systems were performing and consider what phase the mission was in before making the decision on whether to continue with the scheduled mission. By the time Apollo 13 flew, the mission rules stated that you couldn’t land on the Moon with two fuel cells.

So when oxygen tank 2 ruptured on Apollo 13, the crew proceeded with mission rules. Readouts in the spacecraft and in Houston said the oxygen tank and fuel cells 1 and 3 had failed. Thinking that many failures might be some kind of instrumentation problem, capcom Jack Lousma had the crew try to reconnect fuel cell 1 to Main A and fuel cell 3 to Main B, but this didn’t solve the problem.

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A mockup of the Apollo service module

Above the engine, the hydrogen tank is on the lowest shelf, the oxygen tanks in the centre, and the fuel cells on top.

In the movie Apollo 13, Houston has Tom Hanks as Lovell shut down the Reac valve on fuel cells 1 and 3 in an attempt to stop the oxygen leak, conserving what oxygen was left on the one working fuel cell. It’s a drawn-out, quiet moment in the movie with the poignant line from Hanks, “we just lost the Moon.”

The crew actually knew they weren’t going to be landing before long before this call came up. The call to close the reactant valves came one hour and two minutes into the crisis. Lousma called up saying that Apollo 13 was losing oxygen through fuel cell 3. “So, we want you to close the Reac valve on fuel cell 3. It looks like fuel cell 1 and 2 are trying to hold up okay.” Lunar module pilot Fred Haise responded, “Are you saying fuel cell 1 and 2 – 1 and 2 are trying to hold up but we’re leaking O2 out of fuel cell 3? And you want me to shut the Reac valve on fuel cell 3? Did I hear you right?” Lousy confirmed, “That’s affirmative. Close the Reac valve on fuel cell 3.”

That might have been the decision point where the lunar landing was lost, but at least Haise knew he wouldn’t be walking on the Moon before then. A little more than five minutes after the explosion, Haise called down to Houston that he tried to reset everything but fuel cells 1 and 3 still showed no power flow. Haise called down to Houston, “I’m showing – I tried to reset and fuel cell 1 and 3 are both showing gray flags, but they are both showing zip on the flows.” That, he said in the mission debriefing, was the moment he knew they weren’t going into lunar orbit and wouldn’t be landing on the Moon. “I looked at fuel cell 3, and its flows were showing full-scale low. This meant that this fuel cell wasn’t carrying any load. That meant the whole bus was gone. I admitted that LOI was going to be NO GO about now.”

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Apollo 13’s damaged service module

The extent of the damage to Apollo 13’s service module wasn’t known until the crew took this picture not long before reentry.

Want to read the Coles Notes version of Apollo 13? I’ve Storify-ed my live tweet from last year!

Sources: Apollo Expeditions to the Moon; the Apollo Lunar Flight Journal, specifically this page wit the transcript at the moment of the O2 tank rupturing; Apollo Operations Handbook, Block II spacecraft; Apollo mission rules, all of which can be found here; Apollo 13.

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