There is an important question that was asked here many times. That question is: "Why did the Oxygen Tank blow up?" I hope I'm able to cover this extensive problem in the words to follow.
In the Apollo 13 Service Module rests four liquid-specific tanks in Bay 4. The first two are the Oxygen Tanks. They're spherical (to withstand high containment pressure without distortion) worse yet, they're right next to each other. The third and fourth are Hydrogen Tanks. These tanks are located at the bottom of the Service Module if you were to stand the SM on it's end. The valve modules combine the Hydrogen and Oxygen to form water (as one of many functions of the tanks).
Perhaps this picture will speak a thousand words:
In each of the O2 Tanks is mounted several instruments critical to the proper operation of the tank. You have a Thermostat, Temperature Sensor, Fan Motors (two), Capacity Gauge, Heater, and Teflon Insulation that lines the wires and the inside of the tank. All of these instruments were mounted on two long bars that ran down the center of the tank.
Again, Picture = Thousand Words.
This was not the first action that the O2 Tank 2 (the tank that ruptured) had seen. Originally, it was installed in Apollo 10 and then promptly removed for modification. The modification that occurred was a change in the voltage that the equipment inside the tank was supposed to handle. Everything on the craft was designed to run at 28 volts DC, However the ground power at Kennedy Space Center was changed to 65 volts DC. For what reason I do not know. NASA notified the component makers (like Rockwell, which made the O2 tanks) of the change in specifications, and that all components were to now accept 65 volts DC as well as 28 volts DC.
This is the reason behind the removal of the tank from Apollo 10. There was one problem however; while the tank was out of the craft, during moving or transporting it, the tank was dropped a mere 2 inches off of the ground. I remind you that these tanks are spherical except at the ends, so the shape of the tank absorbed the impact (even though it was a little one) However the components inside the tank did not take it so well. The external portions of the tank were inspected, deemed fine, and the tank still sat aside. NASA choose to put a separate tank in the Apollo 10 craft, and the Apollo 10 mission went without a hitch (Thankfully)
What NASA didn't know at the time was that the Internal Fill Line in the O2 Tank had been jarred slightly, just enough to give everyone problems. When the O2 Tank was installed In Apollo 13, during normal preflight testing, It's filled half way with Liquid Oxygen, and then emptied. The filling portion of the test went without a problem. However whenever the ground crew attempted to empty the tank, the Liquid Oxygen refused to leave the tank. On the ground emptying the tank was done by forcing Oxygen Gas into the tank, forcing the Liquid Oxygen to leave the tank. However, they had problems with this method as well. It was decided to use the internal heaters in the tank to "boil off" the excess Liquid Oxygen.
Normal temperatures inside either O2 Tank is anywhere between -300 and -100F. The Heaters are designed only for short term usage, to increase the temperature, which increases the pressure. The idea is that an increase in pressure will help force Liquid O2 out, to maintain a steady flow for things like Water and Power generation in the craft. The heater has a thermostatic switch designed to open when the temperature inside the tank reaches 80F; this is done to cut the heater off. These switches were overlooked in the adaptation of components for 65 volts.
Ground power at Kennedy Space Center was 65 volts. It's no surprise that the heater inside the tank was powered by 65 volts, but the heater was designed to support the 65 volts. The problem was with the thermostatic switches, which, under 2.32 times the voltage, welded shut - not allowing the heater to cut off when temperatures inside the tank exceeded 80F.
This was the primary flaw. No one knew that the temperature inside the tank had exceeded 80F, the temperature gauges were designed to read from -300F to 80F, no higher. The tank isn't supposed to get any hotter, right? The ground crew ran the heater in the tank for a total of 8 Hours, and nobody knew that the temperature in the tank exceeded 1000F. It accomplished their goal, however, the tank was emptied. For the flight, the tank was refilled, and deemed useable (no need to empty the tank in space, right?)
What the ground crew didn't know was that the Teflon insulation on the wires inside the tank had pretty much melted away, as well as a lot of Teflon on the inside walls of the tank. Unfortunately these wires that had lost their insulation were apart of the Fan Motors used to do house cleaning tasks. This was called a "Cryo-Stir". In the zero gravity of space, the liquid oxygen tends to separate, forming a slushy cloud. The crew hits two switches to "Stir the Tanks" which cuts the fans on. This makes a good portion of the Liquid O2 condense so that more accurate Quantity readings can be taken from the tank.
56 Hours into the mission, April 14th, 10:06PM EST; a crew member of the Apollo 13 team was instructed to do just that, stir the tanks. Following his orders, he hit the switch that ended the mission. The fan motors cut on for a second, shorted out, and in an oxygen rich environment a slow burning fire began in the tank. The pressure in the tank went off scale low, meaning the pressure sensor had failed.
O2 Tank 2 really didn't explode as the media liked to say. It pretty much just popped, like what would happen if you put an unopened beer can in the oven. Flying fragments of metal in Bay 4 hit stuff on the O2 Tank 1 (right next to Tank 2) damaging its valve allowing Oxygen to escape from that tank. The bay cover blew off when the pressure differential between the empty ness of space and the inside of the bay varied greatly. The bay wasn't sealed, but the pressure difference grew so quickly that the cover blew off the bay with force.
Apollo 13 was a sign to NASA that the Apollo missions had to cease before something really bad had happened. I think we should still be out there today, with an active base on the moon. However, the general public doesn't care about NASA because they believe nothing NASA does affects their lives (although this is far from the truth). It's been 28 years since we've been to the moon, don't you think we ought to go back?
And a vast array of other sources picked up here and there over the past few years. Sorry I can't remember them all.