Apollo 13 article

[Harry]

There is an interesting article about Apollo 13 at IEEE Spectrum:
http://www.spectrum.ieee.org/WEBONLY/wonews/apr05/0405napola.html

Harald

[TERRIER]

I thought this was a bit of an 'eye opener';

Still, the Apollo 13 service module probably could have gotten away with using the underrated switches, if not for an accident that occurred in 1968. Along with its twin, oxygen tank one, oxygen tank two was installed in the service module that would fly as part of the Apollo 10 mission. Because of an unrelated problem, oxygen tanks one and two in Apollo 10 were swapped out for another pair, and the originals went into Apollo13. But during the swap, oxygen tank two was dropped about 5 centimeters.

The tank was inspected and appeared to be fine, and so it was installed in Apollo 13's service module. During a test about a month before Apollo 13's launch in 1970, the tank was filled with liquid oxygen. All went well until gas was pumped into the tank to empty it by forcing the liquid oxygen out under pressure. Oxygen tank two failed to empty. The official investigation into the Apollo 13 crisis later surmised that the 5- centimeter drop in 1968 had knocked a fitting loose inside the tank, preventing normal emptying.

Stuck with a tank full of liquid oxygen, the test engineers eventually decided to turn on the heaters built into the tank to boil off the oxygen. The heaters were connected to the 65-volt ground-power supply, which fused the thermostatic switches shut. Instead of tripping open when temperatures in the tank reached 27 ?°C, the switches allowed the heaters to keep running as temperatures inside the tank soared to more than 500 ?°C. This high temperature damaged the Teflon insulation coating the wires near the heaters and left exposed wiring within the tank. Later, when the spacecraft was already on its way to the moon, it was this wiring that sparked when the tank fans were turned on, which in turn led to the Teflon insulation igniting. The resulting fire sent pressures within the tank through the roof, and the tank blew up.

It seems to me that it was known there was some sort of problem with one of the oxygen tanks before the launch, but they still went ahead and used it.

[Matt McIrvin]

It seems to me that it was known there was some sort of problem with one of the oxygen tanks before the launch, but they still went ahead and used it. 8O

It wasn't thought to be a problem that would have any effect on space operations. But combined with several other minor mishaps that fell exactly the wrong way, it did.

I first read about this chain of events in Chaikin's book A Man on the Moon. It's always struck me as an excellent cautionary tale, because it's so characteristic of the way that engineering catastrophes almost always happen: A collection of minor oversights and accidents, each of which individually would be harmless and are therefore allowed to slide, just happen to compound in a way that causes disaster. In this case, some of the events in the chain weren't even known to have happened until it was too late.