I seem to be condemned to posting wildly off-topic material, but nonetheless,
I'll bite the bullet and do it again.

Thirty years ago today, on April 13th, 1970, a liquid oxygen (LOX) bottle on
the side of the Apollo 13 Service Propulsion Stage exploded when the
spacecraft was 2/3rds the way to the Moon and almost killed the three
astronauts on board.

If you've had a chance to see the move, Apollo 13, you already know the
story. "Apollo 13" may be one of the most technically accurate movies ever
made. It is an excellent recounting of the history of an event that began
today.

I didn't work the actual mission. I was back in graduate school for the
spring semester, but I did work the engineering redesign and test phase that
had to be completed before Apollo14 was certified to fly. In 1970, even
though we were going to Moon every six months (that's hard to imagine now,
isn't it?), NASA was in state of collapse. Almost all of its budget had been
cut and people were leaving in droves, although very few wanted to. Because
of that, and because NASA was a fairly class-oriented organization, and
because I was just about finished with a Master's degree in engineering, I
was put in charge of testing the design solution that had been quickly
deduced to prevent another Apollo 13-like incident, even though I was only 25
at the time.

Apollo 13 "exploded" because of a teflon-coated heater wire inside the LOX
bottle had a crimp in it (at least, that was the best guess). Finding
non-metallic LOX-compatible materials is extremely difficult. At the time of
Apollo 13, only five non-metals were certified as LOX-compatible (the grease
from your fingers in the presence of LOX is more explosive than nitroglycerin
-- and more unstable). Teflon had been one of those NASA certified materials
and the heater wire insulation on the Apollo 13 spacecraft was made of teflon.

It is now presumed that during the "stirring" of the LOX tanks necessary to
get internal gaseous oxygen pressure back up to normal levels, the heater
wire in the LOX tank came to be sufficiently warm that it began a slow
oxidation event within the teflon itself. When the pressure built to the
appropriate levels, the control circuitry turned the electricity off to the
tank heater, just as it should -- but by now, the teflon was "burning",
eventually causing a sufficient pressure in the tank to cause it to explode
and blow most of the side of the SPS off.

Electricity, water and oxygen were supplied by the combination of four LOX
and LH2 bottles feeding two the fuel cells that produced electricity (Main
Bus A and B circuits) and water. When the LOX tank blew, it took out the Main
Bus B fuel cell immediately ("Houston, We have a problem. We have a Main Bus
B undervolt.") and punctured the A system oxygen lines, causing a relatively
slow venting of all of the remaining oxygen on board the SPS into space.

If the explosion had been any worse, it would have been immediately fatal.

The solution to the LOX tank heater wire redesign problem was amazingly
simple. It was merely to get rid of the telfon insulation and replace the
wire with an aluminum wire coated with a heavy coating of aluminum oxide (a
substance already at its maximum state of oxidation). Aluminum oxide is also
a relatively good electrical insulator.

As a result of the redesign, Johnny Russell, a good friend, and I spent the
summer of 1970 in the desert, behind sand bags, using remote manipulators,
dressed in rubber explosion suits, in 110 degree heat, trying our best to see
if we could cause the new LOX tank heater wire design to cause a similar
explosion. The most likely hypothesis for re-creating an explosive oxidation
event was a dielectric breakdown of the aluminum oxide coating, thereby
causing the exposure of pure aluminum to the LOX environment, augmented by a
high-energy spark.

No matter how hard we tried, we couldn't get this arrangement to ever
indicate any form of excessive oxidation to the heater wire -- and thus the
redesigned LOX bottles were certified safe for Apollo 14's flight.

I put up some NASA-White Sands Test Facility pictures that I have left over
from my report 30 years ago on the web. The URL is:

     http://aics-research.com/apollo13.html

including the next day's page from the NY Times and a Time Magazine cover
from that period (the hand in one of the test fixtures' pictures is Johnny
Russell's). The page is heavy on graphics, so be warned, it will be a little
slow to load.

Wirt Atmar