Apollo 13: How a Thermostat Error Led to the Oxygen Tank Blast

Apollo 13 lifted off on April 11, 1970, with Jim Lovell, Jack Swigert and Fred Haise aboard. At 55 hours and 53 minutes into the flight, after Houston asked the crew to stir the oxygen tanks, a tank exploded. The cause, the video explains, began five years earlier with a thermostat the size of a fingernail.

A crew changed three days out

72 hours before launch, NASA pulled Ken Mattingly from the mission. A backup astronaut had caught German measles from his son, and Mattingly had never had measles, so doctors feared he could fall sick during the flight. Jack Swigert took his seat. Mattingly never got sick.

The chain of small errors

Oxygen tank two's heaters were designed to run on 28 volts. In 1965, NASA changed the rules so the heaters would get 65 volts on the launch pad to warm the tanks faster. Everyone updated their parts except a thermostat meant to cut off the heater if it got too hot. Nobody noticed, and the same switch sat inside every Apollo service module.

In October 1968, the tank, first built for Apollo 10, was lifted out of the ship with one bolt still in place. The shelf slipped and the tank dropped about 2 inches. It was inspected, tested and approved, but a small tube inside had probably been knocked loose.

In March 1970, two weeks before launch, a routine test on the pad left the tank full, and the oxygen would not drain. Engineers turned on the heaters to boil it away and left them running for 8 hours. At 65 volts the thermostat welded shut, and temperatures inside climbed to around 1,000 degrees F, hot enough to burn insulation off the wires. The temperature gauge only went up to 80 degrees, so it kept showing the tank as fine. Replacing the tank would have taken nearly two days, so it was kept.

The bang

When the fans in tank two spun up, their power ran through the damaged wires. Bare wires touched and sparked, the insulation burned in pure oxygen, pressure soared and the tank top blew open. The blast tore off an outer panel of the service module and broke the lines of oxygen tank one. The fuel cells starved of oxygen began to die.

In Houston, flight director Gene Kranz told controllers to stay calm and not make things worse by guessing. Turning around directly would have needed the main engine next to the explosion, and nobody knew if it worked. Houston chose the longer route around the Moon, about four days.

The lifeboat

Lovell and Haise brought the lunar module Aquarius to life. Before Odyssey went dark, its navigation data was copied by hand into Aquarius. Aquarius had been designed to keep two men alive for about two days; now it had to support three for four. The cabin fell to about 38 degrees F, water dripped down the walls, and the crew lost more than 30 lb between them. Haise developed an infection and high fever and kept working.

Carbon dioxide rose because Aquarius's filters were round and Odyssey's spares were square. Engineers read instructions over the radio, and the crew built a device of tape and plastic they called the mailbox. Carbon dioxide began to fall.

On the night of April 14, the ship passed behind the Moon, out of radio contact for about 25 minutes, about 248,000 miles from Earth. The Aquarius landing engine was fired to speed the trip home. For a later burn with the computer off, Lovell steered by hand, keeping the edge of the Earth lined up in his window while Swigert timed it with a watch.

Restarting Odyssey

In the simulator, Mattingly and engineers rewrote the restart procedure to fit a tiny amount of battery and read it to the crew for hours. Condensation covered the inside of Odyssey, but there were no sparks or fire. The wiring had been sealed and redesigned three years earlier after the Apollo 1 fire killed three astronauts.

On April 17, the crew released the service module and saw an entire side gone, then released Aquarius. Odyssey fell toward Earth at almost 25,000 mph. Radio silence during reentry lasted six minutes, longer than the usual four, before Swigert's voice came through. Three parachutes opened and Odyssey splashed down near Samoa, a few miles from the recovery ship.

The investigation found the chain: the wrong thermostat, the dropped tank, the 8 hours of heating and the gauge that could not show the danger. NASA redesigned the tanks, added a third one, and flew again less than a year later.