Apollo 13 was scheduled to be the 4th Apollo mission in less than a year. Already the magic of landing on the moon had worn off the general public. Apollo 13 was going to be entered into the historic books as the third manned mission to land on the Moon and become an asterisk. Instead it became one of the most daring, dangerous and memorable missions. A movie with an all-star cast would be made about this mission.
Apollo 13 was going to be exploring an area of the moon that might give a better insight for scientists into the age of the moon. The first 2 missions had sights that were selected because of their even terrain so that the landing surface would not be a major obstacle in getting down safely. Those missions proved that the commander of the space craft could maneuver the craft with some skill. This gave the site selection planners some leeway in where they could go and Apollo 13's landing site was selected because it had plenty of craters, and highlands with exposed bedrock.
After the excitement of the fiery liftoff, separation of the different stages and the Earth orbit designed to line up the spacecraft precisely to rendezvous with the moon several days later. Apollo 13 was on its way to the moon with its 3 men on board making the most of the quiet time between leaving Earth's orbit and landing on the moon. It was during this quiet time that the entire mission changed from a routine trip to the moon to a mission that they called "a successful failure."
There was some joking about the mission's numerical designation: unlucky 13, but to the world of science and physics, 13 was just a number. However, Apollo 13, about 9:00 p.m. Houston time, on the 13th day of the month, at fifty-five hours, fifty-five minutes into the mission, all 3 members of the Apollo crew felt what they called "a pretty large bang". Over the next few minutes they and the ground controllers began to assess the situation. Something had happened that wasn't planned, and nobody knew what exactly had happened.
What they could determine is that 2 of the 3 fuel cells in the Service Module were dead. The problems were serious and as they soon became aware, the situation had become a life and death situation for the crew.
The fortunate piece of the problem was that whatever happened, happened early in the flight plan. If the "big bang" had happened on their way back to Earth, after the Lunar Module and all of its supplies and fuel had been used up on the lunar surface, it is unlikely the three astronauts would have survived the trip back. As it was there was enough time to implement procedures to conserve fuel, come up with a way of filtering out the carbon dioxide that would quickly poison the air and work out a way of sending an electrical current from the Lunar Module to the Command Module so they could make necessary adjustments so that when the time came, they could make flight adjustments that would allow them to safely re-enter the Earth's atmosphere.
There would be 2 major rocket burns required to get the men home safely. The first was about 5 hours after the accident. This burn was required to put them into the proper trajectory so that the moon's gravity would grab hold of their craft and swing them around and back towards Earth. The second major burn was after the lunar swing. This burn gave them a much needed acceleration boost for their return trip home. This boost would take about 9 hours after their total flight time.
On April 17, 1970 at 12:07 p.m. Central Standard Time, Apollo 13 splashed down just 4 miles away from the prime recovery ship the USS Iwo Jima.
It would take almost 2 months before engineers had a good grasp of what happened on the mission. Like most accidents, it wasn't just one thing that went wrong, but a series of events that led up to the accident.
During the flight the oxygen in the tanks tended to stratify and give a false reading as to quantity of oxygen in the tanks. By turning on the fans, the liquid oxygen was stirred creating a more homogenized tank.
It was during this stirring process that a spark caused one of the tanks to explode and damage another one. Why the spark occurred required extensive research and testing.
Five years before the flight, back in 1965, design engineers changed the standard power supply from 28 to 65 volts. This change would cause other changes to be made as designers adapted components to the new operating environment. However, the people building Service Module oxygen tanks somehow never became aware of that change. Each of the oxygen tanks contained a stirring fan, a heating element, and a temperature-sensitive switch designed to shut everything off if the element got hotter than about 80 F. None of these components were ever re-designed to accommodate the higher voltage.
NASA might have gotten away with this built-in flaw (as it had on Apollo's 7 through 12 ) if one of the oxygen tanks destined to fly on Apollo 13 hadn't been damaged in 1968.
That particular oxygen tank had originally been installed in the Apollo 10 Command Module, but prior to that mission the tank was removed for modification. During this modification process the tank was accidently dropped and because of its thin walls, suffered some damage requiring a replacement be installed in Apollo 10.
The damaged tank was set aside for repair, after which, it was installed in the Apollo 13 spacecraft. What was not known was that this oxygen tank was fitted with obsolete thermostatic switch that was designed for the 28 Volt system. Tests indicated proper function, however, in the weeks preceding Apollo 13's launch, ground crews were having problems draining the tank.
At this point NASA should have taken a hard look at the tank, given the tank's history, but everyone, including the crew agreed that the problem was not serious. Replacing the tank would have caused at least a month's delay; at the time, it seemed acceptable to try emptying the tank by running the internal heater for several hours. No one imagined just how serious a problem implementing that procedure would cause.
The temperature-sensitive switch was not designed to operate at 65 volts; it was designed to be operated at 28 volts. During normal operations, the heater was on for only brief periods and the switch never opened. However, during what proved to be a lengthy process of emptying the tank using the internal heater, the switch opened briefly, but then immediately welded itself shut by an electric arc caused by the higher voltage. Indications that the switch had closed were missed.
After this event, whenever the Command Module was powered up, the heaters went into operation without the protection normally provided by the switch. At some point during pre-launch activities, the whole assembly reached a temperature of over 1000 F. This temperature was high enough to cause severe damage to the Teflon insulation protecting the fan-motor wiring and, as the Apollo 13 Review Board later concluded, "from that time on the oxygen tank was in a hazardous condition when filled with oxygen and electrically powered." The stage was set for the accident.
At that moment, 55 hours and 55 minutes into the mission, at a quiet time during the mission, during the simple start up of the fan, the wires arced once again and this time, for whatever reason, the insulation caught fire.
The heat of the fire began boiling the liquid oxygen in the tank. Within 30 seconds, the pressure was too high for the tank's thin walls and it burst. The explosion wreaked havoc throughout the innards of the Service Module, rupturing the other oxygen tank and blowing out the side of the spacecraft. After that moment the mission to the moon became a mission for survival.
James A. Lovell Jr., Commander
Fred W. Haise Jr., Lunar Module Pilot
John L. Swigert Jr., Command Module Pilot
April 11, 1970; 1:13 p.m. CST
Launch Pad 39A
April 17, 1970
Recovery Ship: USS Iwo Jima